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Ground reaction forces during treadmill running in microgravity
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Ground reaction Forces during treadmill running in microgravity John K. De Witt, Lori L. Ploutz-Snyder
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S0021-9290(14)00255-3 http://dx.doi.org/10.1016/j.jbiomech.2014.04.034 BM6635
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Journal of Biomechanics
Accepted date: 17 April 2014 Cite this article as: John K. De Witt, Lori L. Ploutz-Snyder, Ground reaction Forces during treadmill running in microgravity, Journal of Biomechanics, http: //dx.doi.org/10.1016/j.jbiomech.2014.04.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
BM-D-14-0095 - Revision 1
Ground Reaction Forces during Treadmill Running in Microgravity
John K. De Witt1 & Lori L. Ploutz-Snyder2 1
Wyle Science, Technology and Engineering Group, Houston, TX, USA 2
Universities Space Research Association, Houston, TX, USA
Corresponding Author: John K. De Witt, PhD Wyle Science, Technology and Engineering Group 1290 Hercules, Ste. 120 Houston, TX 77058 Phone: (281) 483-8939 Fax: (281) 483-4181 E-mail: [email protected]
Address reprint requests to the corresponding author Keywords: spaceflight; gait; exercise; ground reaction forces; microgravity Word count: 4112
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Abstract
Astronauts perform treadmill exercise during long-duration space missions to counter the harmful effects of microgravity exposure upon bone, muscle, and cardiopulmonary health. When exercising in microgravity, astronauts wear a harness and bungee system that provides forces that maintain attachment to the treadmill. Typical applied forces are less than body weight. The decreased gravity-replacement force could result in differences in ground-reaction force at a given running speed when compared to those achieved in normal gravity, which could influence the adaptive response to the performed exercise. Seven astronauts (6m/1f) who completed approximately 6-month missions on the International Space Station (ISS) completed a preflight (1G) and multiple in-flight (0G) data collection sessions. Ground-reaction forces were measured during running at speeds of 8.0 kph and greater on an instrumented treadmill in the lab and on the ISS. Groundreaction forces in 0G were less than in 1G for a given speed depending upon the gravityreplacement force, but did increase with increased speed and gravity-replacement force. Ground-reaction forces attained in 1G during slower running could be attained by increasing running speed and/or increasing gravity-replacement forces in 0G. Loading rates in 1G, however, could not be replicated in 0G. While current gravity-replacement force devices are limited in load delivery magnitude, we recommend increasing running speeds to increase the mechanical loads applied to the musculoskeletal system during 0G treadmill exercise, and to potentially increase exercise session efficiency.
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1. Introduction
The effect of long-term exposure to microgravity upon the muscle, bone, and cardiopulmonary health of humans has been well-documented through the experiences obtained during missions completed on the Mir and International Space Station (ISS) (Lang et al., 2004; Sibonga et al., 2008; Smith et al., 2012; Smith et al., 2005). While health changes vary across individuals, in general, crewmembers lose muscle strength, bone strength and bone mineral density, and aerobic fitness (Gopalakrishnan et al., 2010; Stein, 2012). Although the time-course of the losses are not well-established, postflight medical tests suggest that long-term musculoskeletal unloading, such as that occurring during typical 4 to 6-month missions, has detrimental physiologic effects that can last for years after the return to Earth (Grigoriev et al., 1998; Sibonga et al., 2008). Crewmembers perform regular exercise as a countermeasure to the effects of microgravity exposure (Cavanagh et al., 2010; Smith et al., 2012). Exercise prescriptions completed by each crewmember are typically evidence-based using ground research. For treadmill locomotion, crewmembers may perform sessions that include long-duration running at moderate speeds, or interval training at high speeds. During ground-based training studies, these exercise prescriptions have been shown to be effective at increasing cardiopulmonary fitness. Furthermore, the mechanical loading applied to the musculoskeletal system can be quantified using the ground-reaction forces (GRF) developed, and increases in mechanical loading have been shown to be beneficial for bone growth (Rubin and Lanyon, 1984; Turner, 1998; Gibala and McGee, 2008). During treadmill exercise, astronauts wear a harness that is attached to the treadmill base using bungee cords (Novotny et al., 2013). The external force created by
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the bungees acts as a ‘gravity replacement force’, but due to bungee characteristics and other factors, the bungee force (BF) is typically less than the astronaut’s body weight (De Witt et al., 2014). Therefore the crewmember performs exercise with the same body mass but lower effective body weight (BW) than on Earth. In addition, impact loads applied to the treadmill surface during running exercise could cause harmful forces to be applied to the ISS, which can disrupt sensitive experiments or damage the station structure. For this reason, the treadmill is vibration isolated from the ISS using passive spring dampers. The vibration isolation results in a treadmill surface that is capable of slight roll and pitch motions. These aforementioned factors can influence the biomechanics of treadmill exercise and potentially cause long-term training effects in microgravity (0G) that differ from similar exercise programs performed in normal gravity (1G). Previously there have been some evaluations of treadmill biomechanics in 0G, but these investigations have been performed using ground-based 0G analogs or during parabolic flight (Schaffner et al., 2005; Genc et al., 2006; De Witt et al., 2010, Gosseye et al., 2010). These investigations demonstrated that peak GRF in 0G are less than those that occur at the same speed in 1G. While these experiments have provided useful data to understand how treadmill running in reduced gravity compared to on Earth, each has the limitation of the analog on which the study was performed. Two prior studies presented GRF data during running on the ISS, but these studies were limited because of subject size and because all exercise was performed on the treadmill with vibration isolation and stabilization (TVIS), which is no longer used by U.S. astronauts (Cavanagh et al., 2010; Genc et al., 2010). The common message in all of these studies is that the reduced mechanical loading that
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occurs in 0G may explain the loss of bone and muscle health that occurs during spaceflight. The second generation treadmill (T2) was installed on the ISS in 2009 and increased the exercise capabilities offered to the astronauts both in maximal speed (19.9 kph) and in the ability to directly measure GRF during locomotion. The treadmill offers the opportunity to capture GRF during normal exercise sessions without the need for additional data collection hardware and allows astronauts to exercise at speeds previously unattainable. Our study is the first to perform a comprehensive evaluation of treadmill running biomechanics during multiple ISS missions using multiple astronauts performing exercise sessions that include high speed running. The purpose of this evaluation was to quantify how speed influences GRF while running in 0G on the ISS and in 1G. This paper reports a subset of data from a larger study during which biomechanical data were collected from astronauts before and during spaceflight while performing typical treadmill exercise. The results will be used to better understand the mechanical loading provided during running exercise to allow for more effective exercise prescriptions to be completed.
2. Methods
Seven astronauts (6m/1f; age 48.9 ± 3.4 yr; ht 178.6 ± 8.8 cm; wt 82.6 ± 13.1 kg) (mean ± 1 SD) who flew long-duration missions on the ISS (mean mission duration: 158.7 ± 24.4 days) participated in this study. The methodology of this investigation was reviewed and approved by the NASA Institutional Review Board. Each subject provided written informed consent before data collection, and was free to withdraw from the study at any time. Ground-based trials before flight were conducted at the Exercise Physiology
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and Countermeasures Laboratory at NASA Johnson Space Center, and in-flight trials were conducted on the ISS. Data were collected once before flight and 4 to 6 times during flight for each astronaut. The number of sessions obtained for each astronaut was dependent on the ISS in-flight schedule and hardware availability Preflight trials were conducted 3 to 6 months before each astronaut’s expedition. Vertical GRF data were collected during the testing trials with a force-measuring treadmill (Kistler Gaitway, Amherst, NY). The treadmill was equipped with 2 force plates beneath the running tread arranged so 1 plate rested in the front and 1 in the rear of the locomotion area. Each plate contained 4 piezoelectric uniaxial load cells that measured vertical GRF. For the first astronaut, load cell data were collected at 100 Hz using software provided by the treadmill manufacturer. For the remaining astronauts, hardware became available that allowed load cell data to be collected at 1000 Hz simultaneously with additional data not reported here. Load cell voltage data were converted to GRF during post-processing using calibration factors obtained before data collection. In addition, data were collected as the astronaut stood stationary on the front and read force plate to verify BW and calibration (front plate error of actual BW= 0.9 ± 0.7%, rear plate error of actual BW = 1.9 ± 0.7%, mean ± SD). Communications between the investigators and training personnel occurred before the 1G data collection session to determine the probable maximal speed that each astronaut would run during their spaceflight. Based on these discussions, the maximal speed for ground data collection was determined. Data were collected for 15 s at each speed from 2.4 kph to the maximal speed in 0.8 kph increments. One astronaut’s maximal speed was 9.6 kph; all other astronauts’ maximal speeds were 12.8 to 15.3 kph. Subjects
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walked at speeds less than 8 kph; these trials were used as a warm-up and, although data were collected, they are not reported in this paper. Data collection ensued after the astronaut achieved steady state locomotion at a given speed. Subjects were allowed to rest at any time and the treadmill force platforms were reset after every 3 trials to ensure that sensor drift did not affect results. Custom software written in MATLAB (R2012a, R2012b, and R2013b; The MathWorks, Inc.; Natick, MA) converted the output from each force sensor to net vertical GRF. Data collected at 1000 Hz was smoothed with a fourth-order low pass Butterworth filter at a cutoff frequency of 75 Hz; GRF data collected at 100 Hz were not smoothed due to concern that the impact peak could be eliminated. The instance of heel strike for each stride was found using GRF data using the criterion of Chang et al., 2000. During data processing, some footfall GRF trajectories were found to exhibit anomalous shapes. GRF trajectories when the foot was in contact with the front or rear plate only appeared accurate. Subsequent analyses revealed that the anomalous data were developed during the period of foot-ground contact where the foot was in contact with the front and rear plate simultaneously. Because of the accurate calibration, we chose to use only the data collected when the foot was in contact with the front plate to report peak impact forces and loading rate. Therefore we wrote an algorithm to detect footfalls where a front plate strike occurred at least 50 ms before the initial foot contact with the rear plate. Contact with each plate was determined by examining if the net GRF was greater or less than 50 N (Cavanagh & LaFortune, 1980). In-flight data were collected during each astronaut’s assigned treadmill exercise session for a given day. To be least obtrusive on each crewmember’s exercise
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prescription, subjects completed their typical assigned exercise session according to their normal schedule. To maximize the collection of useful data, each in-flight session was designed to include as many unique speeds as possible. Typical sessions had 4 to 6 unique speeds performed. Three-dimensional GRF and speed data were collected at 250 Hz by the T2 throughout the exercise session by 4 load cells that were built within the treadmill frame corners. An exercise session prescription was programmed into the treadmill using ground-based software that allows an entire session to be created as a series of stages. A stage consisted of a specific speed and duration. An exercise session could consist of 1 or more stages and the software initiated GRF data collection for 60 s at the beginning of a stage. For all data collection sessions, protocols were designed to consist of a minimum of 6 stages, allowing GRF data to be collected at least 6 times during a session. Instantaneous speed data were rounded to the nearest 0.8 kph for comparison with 1G treadmill speeds. GRF data were smoothed at 20 Hz with a fourth-order low pass Butterworth filter before analysis to remove excessive noise. The cutoff frequency was selected during preliminary analysis as the threshold at which excessive noise was removed but the shape of the GRF was maintained. Before data collection for a given session, the crewmember stood in a static position while wearing the harness-bungee configuration for a given day and the GRF was measured and used to determine the static BF. Raw load cell data were down linked from the ISS and captured on servers located at NASA JSC. Once down linked, custom software was used to convert sensor data into GRF. Standard calibration coefficients relating sensor voltages to force were
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used in the conversion. During processing for all trials, the data stream was found to always exhibit a variable offset force during periods when neither foot was in contact with the treadmill belt that varied with each footfall (129.5 ± 28.2 N). Before flight, the load cells were calibrated successfully. There is no method currently available to verify the load cell accuracy on the T2 after installation on the ISS. Comparison of offset load to treadmill speed and peak GRF revealed decreases in mean offset force with increased speed and increased GRF, but with consistent offset force variations. On a typical force platform, the measured forces are corrected for the offset force. However, because the offset force on the T2 was variable from step to step, we felt that the force was not due to a preload, but instead may have been induced by other factors related to the inertia of the vibration isolation system (VIS). Because we have no way of verifying the source of the residual force and because the peak GRF and static BF measures were similar to expected magnitudes we did not correct our GRF for the offset force. Visual inspection of each footfall was used to ensure that there were no anomalous data. An adjusted approach similar to Chang et al. (2000) was used to determine heel strike, with thresholds adjusted to 3500/s when the GRF magnitude was greater than 1.5 times the minimum GRF for the trial. This method was decided upon after visually inspecting predicted heel strike times computed with various thresholds. Toe off was found by determining the last sample at the end of a footfall trajectory where the change in vertical GRF was less than –2000 N/s. An automated algorithm examined each footfall for both 1G and 0G trials and found the location and magnitude of the peak impact force (first local maxima) and loading rate. Loading rate was the rate of GRF application in the region from 25% of the
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peak impact force to 75% of the peak impact force. For 0G trials only, peak propulsive force (second local maxima occurring at least 20% of the foot-ground contact time after the peak impact force) as determined. For the 0G trials, BF levels as a percentage of BW were collapsed in to bins of <49% BW, 50% to 59% BW, 60% to 69% BW, 70% to 79% BW, and 80% to 89% BW. For comparison purposes, peak propulsive force data from Munro et al. (1987) were used. Microsoft Excel (Microsoft Excel for Mac 2011 Version 14.3.9 (131030), Redmond, WA) was used to find a best-fit line relating peak propulsive force to running speed. The line with the equation peak propulsive force = -0.0038 × speed2 + 0.1564 × speed + 1.2635 was found to relate peak propulsive force in BW to speed in kph with r2=0.998 and was used to interpolate 1G estimates of peak propulsive force at the speeds that were tested in this study. Effect sizes (Cohen’s d) were computed for peak impact force and loading rate between all speeds in 0G and 1G for each BF level to determine if there existed loading equivalent speeds between conditions. Effect size magnitudes between -0.35 and 0.35 were considered to indicate small to no difference between conditions (Cohen, 1992). All statistical analyses were completed using RStudio Version 0.7.551 and R version 3.0.1 (2013-05-16).
3. Results
In 1G, we collected data from 2385 footfalls across all subjects, and included 1521 in our final analysis. Only trials during which at least 10 usable footfalls occurred were included. Due to this criterion, we excluded all but one 1G trial for a single subject, and had to exclude 2 trials from a second subject, but were able to use data from all other
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trials. In 0G, our analyses were completed on 77,651 footfalls across all subjects (Table 1). Only speed and BF levels containing at least 10 footfalls were used for analyses. In addition, and speed and BF conditions for specific subjects with less than 10 footfalls were removed. Figure 1 shows typical 0G vertical GRF data from a single trial at 16.9 ± 0.8 kph. Subjects completed trials at speeds ranging from 8.0 to 20.1 kph in 0G. The BF for each session depended upon the specific crewmember (Table 2). Typical BF during exercise across astronauts was 52% to 80% BW. INSERT TABLES 1 and 2 & FIGURE 1 ABOUT HERE Across all subjects in both gravitational conditions, peak impact force increased with increasing speed (Figure 2 and Table 3). The addition of BF tended to increase the peak impact force at all speeds except at the lowest BF level (<49% BW). At any given speed, the peak impact force was lower in 0G than in 1G when expressed in BW. However, mean peak impact forces attained at speeds of between 8.0 to 14.5 kph in 1G could be attained in 0G by increasing running speed. INSERT TABLE 3 and FIGURE 2 ABOUT HERE Loading rate increased with increasing speed (Figure 3 and Table 4). However, loading rates attained in 1G could not be attained in 0G, regardless of speed or BF. INSERT TABLE 4 and FIGURE 3 ABOUT HERE Similar to peak impact force, mean peak propulsive forces increased with increasing exercise speed and BF level (Figure 4 and Table 5). However, when compared to 1G mean values reported by Munro et al., 1986, t There does not appear to be any speed or BF level combination where 1G loading levels can be attained in 0G. INSERT TABLE 5 and FIGURE 4 ABOUT HERE
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Figures 5 and 6 depict the effect size for a given gravitational comparison for the mean peak impact force and mean loading rate at each speed comparison condition. For peak impact force generation, the development of forces in 0G equivalent to or greater than those attained in 1G are attained by greater BF loading, running at higher speeds, or a combination of the 2 approaches. Peak impact forces attained in 1G are possible in 0G even with BF levels of 50% BW as long as exercise speed is of sufficient magnitude. With regards to loading rate, however, 1G values are unattainable in 0G regardless of the BF level or exercise speed. Loading rate in 0G did increase with increasing speed and BF, as evidenced by the lowest effect size magnitudes at higher speeds. INSERT FIGURES 5 and 6 ABOUT HERE
4. Discussion
The purpose of this investigation was to quantify the peak impact forces, propulsive forces, and loading rates obtained in 1G and 0G during treadmill exercise. The overall objective is the better understand how exercise prescriptions for astronauts can be modified to increase the applied mechanical load to better address health losses that occur during long-duration spaceflight. A unique feature of this experiment was the inclusion of subject-specific exercise protocols rather than the use of a standard test protocol during 0G data collection sessions. We realize that our approach makes statistical analyses of our data impossible due to the lack of common testing conditions across subjects but felt that obtaining data in actual exercise conditions was more critical to optimize results interpretation. Should we have developed a specific protocol for testing, we would either not have been able to
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collect data from the pool of subjects because of the potential that certain subjects could not complete the required protocol, or we would have had to develop a protocol that could be completed by all subjects, and this protocol could potentially not reflect a true exercise session. In addition, allowing the astronauts to complete their own exercise prescription gave us the opportunity to collect data at higher speeds, which has a direct bearing on the results presented in this paper. Peak impact and propulsive force magnitudes were less in 0G than in 1G for most subjects at all speeds. Although we did not collect propulsive forces in 1G, the propulsive forces in 0G were less than those reported by Munro et al. (1986). This finding was not unexpected, and agrees with the findings during past evaluations of running in microgravity (Cavanagh et al., 2010; De Witt et al., 2010; Genc et al., 2010, Gosseye et al., 2010). A major difference, however, between past research and our study was that past data were collected during short durations of locomotion, or during running at lower speeds (i.e., less than 11.2 kph). These studies were limited in determining if the generation of lower GRF was due to lack of subject adaptation, or in quantifying the GRF developed during faster running speeds. Our study has been able to demonstrate that in absolute terms, the vertical GRF at a given speed will be less than in 1G, and that the effect is relatively consistent. For both peak impact and peak propulsive forces, the loss of mechanical loading in 0G compared to 1G can be compensated for by exercising at higher speeds. Nilsson and Thorstensson (1989) demonstrated that in 1G, GRF are directly related to running speed. Our data indicate a similar response in GRF relative to speed in 0G. While this result is not surprising, the implications, especially when taken together with the GRF
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response at different BF levels are important. Even at BF levels less than 1 BW, the mechanical loading during treadmill running at lower speeds in 1G can be attained in 0G. Cavanagh et al., (2010) have suggested that increasing mechanical loading experienced by astronauts is vital if goals are to reduce the loss in bone health during spaceflight. Although our data do not include pre or post-flight health measures so we cannot comment on if the higher GRF attained by those astronauts who ran at higher speeds was beneficial, we can speculate that due to increased GRF and increased stride frequency at higher speeds, mechanical loading levels, and opportunities were greater. Therefore the use of high speed running as an exercise prescription should be highly considered when creating programs designed to protect astronaut health. Interestingly, although peak impact force and peak propulsive force magnitudes in 1G could be attained in 0G by running faster, loading rates in 0G were always less than those occurring in 1G regardless of BF level or running speed. Gosseye et al. (2010) found loading rates in 0G during parabolic flight to be similar to those occurring in 1G. Although we cannot confirm, we speculate that the decrease in 0G loading rate occurs due to the VIS upon which the treadmill resides. The treadmill on which Gosseye et al. (2010) collected their data from was hard-mounted to the deck of the parabolic aircraft. The purpose of the VIS is to reduce the force transmission from the treadmill to the ISS structure, and is a common feature of all exercise equipment. Our data suggest that in addition to reducing the vibrations transmitted to the ISS, the VIS also reduces the rate of force application to the musculoskeletal system, and potentially adversely affects the mechanical loading experienced by the astronauts. The loss in loading rate during 0G treadmill exercise may be an important factor when explaining how effective a
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countermeasure treadmill running may be for bone health, as rate of force application (Turner, 1998; Turner, Owan & Takano, 1995). Researchers who are attempting to quantify mechanical loading in 0G and comparing their data to 1G should consider accounting for the loss in loading rate in their approaches. Loading rate did increase with increasing speeds in 0G. While the rates were less than those attained in 1G at a given speed, loading rates in 0G at 20.1 kph approached those attained in 1G at 8 kph, giving further support to exercising at higher speeds to better attain beneficial bone loading stimuli. Increased BF levels were also beneficial to increasing GRF parameters at all speeds. The current harness-bungee system is limited by bungee force-length characteristics. Because BF is related to length, and bungee length will change during the gait cycle due to vertical oscillation of the subject, the applied BF during the gait cycle is not constant, which may in turn influence the resulting GRF magnitude. Although higher gravity-replacement forces are possible by placing multiple bungees in parallel on either side of the astronaut, this approach greatly increases the bungee stiffness and could cause comfort issues (De Witt et al., 2014). A subject loading system (SLS) designed by the European Space Agency is currently being assessed and is planned for installation on the ISS. When the SLS becomes operational, it will be possible for crewmembers to exercise with gravity replacement forces that are much higher than currently possible with the bungee system. It is possible that 1G like mechanical loading at a given speed could be replicated in 0G with greater gravity replacement forces (Gosseye et al., 2010). However, it is also possible that because the weight of the entire body is being replaced by bilateral force vectors attached to the hip and shoulders, that running motions and comfort could
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be influenced. It is important that we understand how running with higher gravity replacement force could influence exercise performance, and not extrapolate the results found in this experiment with the limited BF levels currently available. It is highly possible, based on experience during parabolic flight, that subject comfort, biomechanics, and exercise efficacy is influenced by higher EL. These influences should be examined given the opportunity. In summary, GRF attained during steady state exercise in 0G are similar in profile but less in magnitude to those attained in 1G at similar speeds. However, increasing exercise speed in 0G can compensate for the loss of peak impact and peak propulsive force magnitudes. Loading rates, on the other hand, cannot be compensated for with the increase in speed. When attempting to maximize mechanical loading, high speed running should be considered when creating training programs in 0G, with the knowledge that on the current treadmill on the ISS, loading rates will always be less than those occurring in 1G.
Conflict of interest statement
None of the authors had any financial or personal conflict of interest with regard to this study.
Acknowledgements We thank Roxanne Buxton, Melissa Scott-Pandorf, and Renita Fincke for assistance in collecting and processing data and creating analysis software. We also thank Peter Cavanagh for his advice in the processing of the GRF data. We would like to thank NASA’s ISS Medical Projects Group (ISSMP) for their assistance in implementing this
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project. Thanks specifically to Mr. Steve Hing of ISSMP as the primary contact. This study was funded by NASA’s Human Research Program.
References
Cavanagh, P.R., Genc, K.O., Gopalakrishnan, R., Kuklis, M.M., Maender, C.C., Rice, A.J., 2010. Foot forces during typical days on the international space station. J. Biomechanics. 43 (11), 2182-2188. Cavanagh, P.R., LaFortune, M.A., 1980. Ground reaction forces in distance running, Journal of Biomechanics, 13, 397-406. Chang, Y.H., Huang, H.W., Hamerski, C.M., Kram, R., 2000. The independent effects of gravity and inertia on running mechanics. Journal of Experimental Biology. 203 (2), 229238. Cohen J., 1992. A power primer. Psychological Bulletin. 12 (1), 155-159. De Witt, J.K., Perusek, G.P., Lewandowski, B.E., Gilkey, K.M., Savina, M.C., Samorezov, S., Edwards, W.B., 2010. Locomotion in simulated and real microgravity: Horizontal suspension vs. parabolic flight. Aviation, Space, and Environmental Medicine. 81 (12), 1092-1099. De Witt, J.K., Schaffner, G., Ploutz-Snyder, L.L., 2014, Bungee force level, stiffness, and variation during treadmill locomotion in simulated microgravity. Aviation, Space, and Environmental Medicine, 85, 1-7. Genc, K.O., Gopalakrishnan, R., Kuklis, M.M., Maender, C.C., Rice, A.J., Bowersox, K.D., Cavanagh, P.R., 2010. Foot forces during exercise on the International Space Station. Journal of Biomechanics. 43 (15), 3020-3027.
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Genc, K.O., Mandes, V.E., Cavanagh, P.R., 2006. Gravity replacement during running in simulated microgravity. Aviation, Space, and Environmental Medicine. 77 (11), 11171124. Gibala, M.J., McGee, S.L., 2008. Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain?. Exercise and Sport Sciences Reviews. 36 (2), 58-63. Gopalakrishnan, R., Genc, K.O., Rice, A.J., Lee, S., Evans, H.J., Maender, C.C., Cavanagh, P.R., 2010. Muscle volume, strength, endurance, and exercise loads during 6month missions in space. Aviation, Space, and Environmental Medicine. 81 (2), 91-104. Gosseye, T.P., Willems, P.A., Heglund, N.C., 2010. Biomechanical analysis of running in weightlessness on a treadmill equipped with a subject loading system. European Journal of Applied Physiology. 110 (4), 709-728. Grigoriev A.I., Oganov V.S., Bakulin A.V., Poliakov V.V., Voronin L.I., Morgun V.V., Shnaider V.S., Murashko L.V., Novikov V.E., LeBlanc A., Shakleford L., 1998. [Clinical and physiological evaluation of bone changes among astronauts after long-term space flights]. Aviakosm Ekolog Med. 32, 21-25. Lang T, LeBlanc A, Evans H, Lu Y, Genant H, Yu A., 2004. Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight. Journal of Bone and Mineral Research. 19 (6), 1006–12. Munro, C.F., Miller, D.I., Fuglevand, A.J., 1987. Ground reaction forces in running: a reexamination. Journal of Biomechanics. 20 (2), 147-155. Nilsson, J., & Thorstensson, A., 1989. Ground reaction forces at different speeds of human walking and running. Acta Physiologica Scandinavica. 136 (2), 217-227.
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Novotny, S.C., Perusek, G.P., Rice, A.J., Comstock, B.A., Bansal, A., Cavanagh, P.R., 2013. A Harness for Enhanced Comfort and Loading during Treadmill Exercise in Space. Acta Astronautica. 89, 205-214. Rubin, C.T., Lanyon, L.E., 1984. Regulation of bone formation by applied dynamic loads. The Journal of Bone & Joint Surgery, 66 (3), 397-402. Turner C.H., 1998. Three rules for bone adaptation to mechanical stimuli. Bone. 23, 399407. Schaffner, G., DeWitt, J., Bentley, J., Yarmanova, E., Kozlovskaya, I., Hagan, D., 2005. Effect of load levels of subject loading device on gait, ground reaction force, and kinematics during human treadmill locomotion in a weightless environment. National Aeronautics and Space Administration NASA/TP-2005-213169. Sibonga, J.D., Cavanagh, P.R., Lang, T.F., LeBlanc, A.D., Schneider, V.S., Shackelford, L.C., Smith, S.M., Vico, L., 2008. Adaptation of the skeletal system during long-duration spaceflight. Clinical Reviews in Bone and Mineral Metabolism. 5 (4), 249–261. Smith, S.M., Wastney, M.E., O'Brien, K.O., Morukov, B.V., Larina, I.M., Abrams, S.A., Davis-Street, J.E., Oganov, V., Shackelford, L.C., 2005. Bone markers, calcium metabolism, and calcium kinetics during extended-duration space flight on the Mir space station. Journal of Bone and Mineral Research. 20 (2): 208–218. Smith, S.M., Heer, M.A., Shackelford, L.C., Sibonga, J.D., PloutzǦSnyder, L., Zwart, S.R., 2012. Benefits for bone from resistance exercise and nutrition in longǦduration spaceflight: Evidence from biochemistry and densitometry. Journal of Bone and Mineral Research. 27 (9), 1896-1906.
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Stein, T.P., 2013. Weight, muscle and bone loss during space flight: another perspective. European Journal of Applied Physiology. 113 (9), 2171-2181. Turner, C.H., Owan, I., Takano, Y., 1995. Mechanotransduction in bone: Role of strain rate. American Journal of Physiology-Endocrinology and Metabolism. 269 (3), E438E442.
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Table 1. Number of footfalls analyzed at each speed in 1G and each speed and BF level in 0G across all subjects. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
144 125 146 170 177 178 175 146 147 113
0G 40-49% BW
50-59% BW
60-69% BW
678
1073 1108 4236 1926 5061 2655 4791 822 564 407 435 95 104 47 70 62
353 1429 6932 5224 3313 2159 1250 2626 1112 388 257 221 88 131 81 110
628 666 566 511
70-79% BW
80-89% BW
534 1884 7387 3074 1466 1328 983 2167 288 534 119 150
360 777 2316 1206 257 578 64
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Table 2. Absolute and relative bungee forces used during exercise sessions for all astronauts expressed in newtons (N) and relative to 1G BW (%BW).
Sessi on
1 N
1 2 3 4 5 6
Mea n± SD
530 .5 508 .2 552 .8 552 .8 499 .3 548 .3 532 .0 ± 23. 5
2 %B W 56 % 54 % 58 % 58 % 53 % 58 % 56 ±2
N 481 .4 512 .6 517 .1 534 .9 543 .8 557 .2 524 .5 ± 26. 8
Subject 4
3 %B W 69 % 74 % 74 % 77 % 78 % 80 % 75 ±4
N 490 .4 601 .8 606 .3 641 .9 637 .5 641 .9 603 .3 ± 58. 2
%B W 52 % 64 % 64 % 68 % 68 % 68 % 64 ±6
N 378 .9 289 .8 387 .8 387 .8 405 .7 405 .7 375 .9 ± 43. 6
%B W 52 % 40 % 53 % 53 % 56 % 56 % 52 ±6
5 N 481. 4 490. 4 477. 0 539. 4 DN C* DN C 497. 0± 28.8
6 %B W 77 % 79 % 77 % 87 % DN C DN C 80 ±5
N 468 .1 548 .3 548 .3 624 .1 615 .2 DN C 560 .8 ± 63. 0
7 %B W 53 % 62 % 62 % 70 % 69 % DN C 63 ±7
N 441 .3 459 .1 481 .4 477 .0 526 .0 552 .8 489 .6 ± 42. 0
%B W 52 % 54 % 56 % 56 % 62 % 65 % 58 ±5
*DNC = did not complete session due to scheduling issues
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Table 3. Peak impact force relative to 1G BW at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
1.36 ± 0.17 1.38 ± 0.17 1.44 ± 0.18 1.61 ± 0.24 1.64 ± 0.20 1.76 ± 0.25 1.85 ± 0.27 1.82 ± 0.25 1.91 ± 0.27 2.09 ± 0.29
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
0.71 ± 0.07
0.78 ± 0.12 0.81 ± 0.15 0.88 ± 0.18 1.01 ± 0.24 0.94 ± 0.21 0.95 ± 0.14 0.95 ± 0.15 1.18 ± 0.26 1.35 ± 0.26 1.34 ± 0.27 1.26 ± 0.27 1.34 ± 0.28 1.74 ± 0.10 1.80 ± 0.09 1.77 ± 0.09 1.78 ± 0.10
1.00 ± 0.11 1.10 ± 0.10 1.06 ± 0.12 1.13 ± 0.10 1.20 ± 0.12 1.24 ± 0.15 1.20 ± 0.12 1.23 ± 0.13 1.29 ± 0.12 1.50 ± 0.18 1.55 ± 0.19 1.73 ± 0.18 1.86 ± 0.17 1.83 ± 0.12 1.86 ± 0.15 1.88 ± 0.13
1.09 ± 0.08 1.17 ± 0.10 1.21 ± 0.12 1.33 ± 0.14 1.43 ± 0.12 1.45 ± 0.11 1.49 ± 0.10 1.46 ± 0.10 1.52 ± 0.11 1.60 ± 0.10 1.65 ± 0.10 1.67 ± 0.09
1.24 ± 0.08 1.27 ± 0.07 1.32 ± 0.08 1.38 ± 0.09 1.48 ± 0.07 1.50 ± 0.07 1.47 ± 0.08
0.71 ± 0.06 0.74 ± 0.07 0.80 ± 0.10 0.75 ± 0.06 0.76 ± 0.06
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Table 4. LR expressed relative to 1G BW in BW/s at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
47.70 ± 13.81 50.20 ± 16.43 52.19 ± 17.88 69.19 ± 23.14 72.00 ± 18.50 81.24 ± 20.24 88.98 ± 25.45 84.87 ± 23.57 94.39 ± 27.63 114.62 ± 27.25
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
14.30 ± 3.31
17.17 ± 3.79 17.67 ± 2.97 19.15 ± 3.69 21.29 ± 3.85 21.32 ± 3.55 21.47 ± 3.15 22.23 ± 2.98 23.51 ± 4.10 25.79 ± 4.49 26.41 ± 3.37 25.81 ± 4.31 27.25 ± 3.87 29.99 ± 2.98 31.72 ± 2.61 30.89 ± 2.47 31.30 ± 2.95
19.20 ± 3.45 22.83 ± 3.64 22.09 ± 3.91 23.66 ± 3.39 24.94 ± 3.69 26.59 ± 3.69 26.99 ± 3.00 27.16 ± 3.31 27.89 ± 3.50 29.31 ± 4.17 31.32 ± 3.03 30.58 ± 4.32 33.12 ± 4.43 32.14 ± 3.39 32.89 ± 3.94 33.37 ± 3.65
26.43 ± 3.91 27.14 ± 3.89 27.91 ± 4.74 32.12 ± 3.46 33.00 ± 4.57 33.50 ± 3.81 34.57 ± 3.85 34.37 ± 3.40 33.86 ± 3.91 35.48 ± 3.50 36.92 ± 3.01 31.46 ± 4.58
28.48 ± 2.80 29.83 ± 4.18 31.69 ± 3.34 32.89 ± 4.01 35.12 ± 3.29 36.02 ± 3.09 34.42 ± 2.14
16.38 ± 2.10 18.07 ± 2.39 19.22 ± 3.57 18.18 ± 2.03 18.76 ± 2.42
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Table 5. Peak propulsive force expressed relative to 1G BW at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. *1G data from Munro et al., 1987. Speed
1G*
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
2.27 2.35 2.42 2.49 2.55 2.60 2.65 2.69 2.73
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
0.82 ± 0.04
0.96 ± 0.10 1.02 ± 0.10 1.10 ± 0.12 1.18 ± 0.15 1.13 ± 0.13 1.18 ± 0.13 1.13 ± 0.12 1.30 ± 0.14 1.38 ± 0.13 1.39 ± 0.11 1.35 ± 0.08 1.36 ± 0.11 1.50 ± 0.05 1.52 ± 0.06 1.49 ± 0.05 1.52 ± 0.07
1.32 ± 0.12 1.34 ± 0.12 1.35 ± 0.10 1.36 ± 0.11 1.49 ± 0.12 1.44 ± 0.13 1.38 ± 0.08 1.40 ± 0.09 1.45 ± 0.08 1.59 ± 0.15 1.56 ± 0.15 1.72 ± 0.11 1.78 ± 0.11 1.76 ± 0.11 1.78 ± 0.11 1.78 ± 0.11
1.39 ± 0.05 1.48 ± 0.10 1.51 ± 0.10 1.58 ± 0.12 1.68 ± 0.11 1.68 ± 0.11 1.73 ± 0.09 1.67 ± 0.05 1.74 ± 0.05 1.73 ± 0.04 1.72 ± 0.04 1.76 ± 0.04
1.48 ± 0.03 1.53 ± 0.05 1.58 ± 0.08 1.67 ± 0.10 1.76 ± 0.10 1.82 ± 0.09 1.71 ± 0.04
0.86 ± 0.03 0.89 ± 0.04 0.92 ± 0.01 0.92 ± 0.03 0.93 ± 0.04
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Figure 1. Typical vertical GRF trajectories for multiple steps as a subject ran at 14.5 kph. Each trajectory is processed data from a single footfall. Multiple footfalls from a single trial are plotted indicating consistency in GRF shape.
Figure 2. Relative peak impact force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between peak impact force and running speed.
Figure 3. Relative loading rate for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed.
Figure 4. Relative peak propulsive force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed. Mean peak propulsive forces in 1G are estimated from Munro et al. (1986).
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Figure 5. Effect size (Cohen’s d) for mean peak impact force between 0G and 1G. Each panel is a scatterplot of ES versus treadmill speed in 1G for a specific speed in 0G. Speeds in 0G increase for each panel. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean peak impact forces in each gravitational condition. Positive effect size indicates that mean 1G peak impact forces are greater than 0G mean peak impact force for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating similarities in peak impact forces developed. Effect size comparisons within BF level indicate smaller effect sizes for a given speed in 0G at greater loading levels. These data indicate that running faster, if matching peak impact forces in 0G to 1G is the desired outcome, can compensate for lack of gravity replacement force. In addition, these data indicate that there are situations at higher speeds where peak impact forces in 0G are greater than those in 1G, as demonstrated by negative effect sizes.
Figure 6. Effect size (Cohen’s d) for mean loading rate between 0G and 1G. Speeds in 0G are depicted in each facet, and speeds in 1G are listed on the horizontal axis for each facet. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean loading rate in each gravitational condition. Positive effect size indicates that mean 1G loading rates are greater than 0G loading rates for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating the relative separation between 1G and 0G loading rates also decreases. However, because the effect size never becomes
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approximately 0, loading rates obtained in 1G are unattainable in 0G at the speeds and BF levels tested.
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Figure
Figure 1. Typical vertical GRF trajectories for multiple steps as a subject ran at 16.9 ± 0.8 kph. Each trajectory is processed data from a single footfall. Multiple footfalls from a single trial are plotted indicating consistency in GRF shape.
Figure 2. Relative peak impact force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between peak impact force and running speed.
Figure 3. Relative loading rate for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed.
Figure 4. Relative peak propulsive force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed. *Mean peak propulsive forces in 1G are estimated from Munro et al. (1986).
Figure 5. Effect size (Cohen’s d) for mean peak impact force between 0G and 1G. Each panel is a scatterplot of ES versus treadmill speed in 1G for a specific speed in 0G. Speeds in 0G increase for each panel. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean peak impact forces in each gravitational condition. Positive effect size indicates that mean 1G peak impact forces are greater than 0G mean peak impact force for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating similarities in peak impact forces developed. Effect size comparisons within BF level indicate smaller effect sizes for a given speed in 0G at greater loading levels. These data
indicate that running faster, if matching peak impact forces in 0G to 1G is the desired outcome, can compensate for lack of gravity replacement force. In addition, these data indicate that there are situations at higher speeds where peak impact forces in 0G are greater than those in 1G, as demonstrated by negative effect sizes.
Figure 6. Effect size (Cohen’s d) for mean loading rate between 0G and 1G. Speeds in 0G are depicted in each facet, and speeds in 1G are listed on the horizontal axis for each facet. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean loading rate in each gravitational condition. Positive effect size indicates that mean 1G loading rates are greater than 0G loading rates for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating the relative separation between 1G and 0G loading rates also decreases. However, because the effect size never becomes approximately 0, loading rates obtained in 1G are unattainable in 0G at the speeds and BF levels tested.
Ground reaction Forces during treadmill running in microgravity John K. De Witt, Lori L. Ploutz-Snyder
www.elsevier.com/locate/jbiomech
PII: DOI: Reference:
S0021-9290(14)00255-3 http://dx.doi.org/10.1016/j.jbiomech.2014.04.034 BM6635
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Journal of Biomechanics
Accepted date: 17 April 2014 Cite this article as: John K. De Witt, Lori L. Ploutz-Snyder, Ground reaction Forces during treadmill running in microgravity, Journal of Biomechanics, http: //dx.doi.org/10.1016/j.jbiomech.2014.04.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
BM-D-14-0095 - Revision 1
Ground Reaction Forces during Treadmill Running in Microgravity
John K. De Witt1 & Lori L. Ploutz-Snyder2 1
Wyle Science, Technology and Engineering Group, Houston, TX, USA 2
Universities Space Research Association, Houston, TX, USA
Corresponding Author: John K. De Witt, PhD Wyle Science, Technology and Engineering Group 1290 Hercules, Ste. 120 Houston, TX 77058 Phone: (281) 483-8939 Fax: (281) 483-4181 E-mail: [email protected]
Address reprint requests to the corresponding author Keywords: spaceflight; gait; exercise; ground reaction forces; microgravity Word count: 4112
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Abstract
Astronauts perform treadmill exercise during long-duration space missions to counter the harmful effects of microgravity exposure upon bone, muscle, and cardiopulmonary health. When exercising in microgravity, astronauts wear a harness and bungee system that provides forces that maintain attachment to the treadmill. Typical applied forces are less than body weight. The decreased gravity-replacement force could result in differences in ground-reaction force at a given running speed when compared to those achieved in normal gravity, which could influence the adaptive response to the performed exercise. Seven astronauts (6m/1f) who completed approximately 6-month missions on the International Space Station (ISS) completed a preflight (1G) and multiple in-flight (0G) data collection sessions. Ground-reaction forces were measured during running at speeds of 8.0 kph and greater on an instrumented treadmill in the lab and on the ISS. Groundreaction forces in 0G were less than in 1G for a given speed depending upon the gravityreplacement force, but did increase with increased speed and gravity-replacement force. Ground-reaction forces attained in 1G during slower running could be attained by increasing running speed and/or increasing gravity-replacement forces in 0G. Loading rates in 1G, however, could not be replicated in 0G. While current gravity-replacement force devices are limited in load delivery magnitude, we recommend increasing running speeds to increase the mechanical loads applied to the musculoskeletal system during 0G treadmill exercise, and to potentially increase exercise session efficiency.
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1. Introduction
The effect of long-term exposure to microgravity upon the muscle, bone, and cardiopulmonary health of humans has been well-documented through the experiences obtained during missions completed on the Mir and International Space Station (ISS) (Lang et al., 2004; Sibonga et al., 2008; Smith et al., 2012; Smith et al., 2005). While health changes vary across individuals, in general, crewmembers lose muscle strength, bone strength and bone mineral density, and aerobic fitness (Gopalakrishnan et al., 2010; Stein, 2012). Although the time-course of the losses are not well-established, postflight medical tests suggest that long-term musculoskeletal unloading, such as that occurring during typical 4 to 6-month missions, has detrimental physiologic effects that can last for years after the return to Earth (Grigoriev et al., 1998; Sibonga et al., 2008). Crewmembers perform regular exercise as a countermeasure to the effects of microgravity exposure (Cavanagh et al., 2010; Smith et al., 2012). Exercise prescriptions completed by each crewmember are typically evidence-based using ground research. For treadmill locomotion, crewmembers may perform sessions that include long-duration running at moderate speeds, or interval training at high speeds. During ground-based training studies, these exercise prescriptions have been shown to be effective at increasing cardiopulmonary fitness. Furthermore, the mechanical loading applied to the musculoskeletal system can be quantified using the ground-reaction forces (GRF) developed, and increases in mechanical loading have been shown to be beneficial for bone growth (Rubin and Lanyon, 1984; Turner, 1998; Gibala and McGee, 2008). During treadmill exercise, astronauts wear a harness that is attached to the treadmill base using bungee cords (Novotny et al., 2013). The external force created by
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the bungees acts as a ‘gravity replacement force’, but due to bungee characteristics and other factors, the bungee force (BF) is typically less than the astronaut’s body weight (De Witt et al., 2014). Therefore the crewmember performs exercise with the same body mass but lower effective body weight (BW) than on Earth. In addition, impact loads applied to the treadmill surface during running exercise could cause harmful forces to be applied to the ISS, which can disrupt sensitive experiments or damage the station structure. For this reason, the treadmill is vibration isolated from the ISS using passive spring dampers. The vibration isolation results in a treadmill surface that is capable of slight roll and pitch motions. These aforementioned factors can influence the biomechanics of treadmill exercise and potentially cause long-term training effects in microgravity (0G) that differ from similar exercise programs performed in normal gravity (1G). Previously there have been some evaluations of treadmill biomechanics in 0G, but these investigations have been performed using ground-based 0G analogs or during parabolic flight (Schaffner et al., 2005; Genc et al., 2006; De Witt et al., 2010, Gosseye et al., 2010). These investigations demonstrated that peak GRF in 0G are less than those that occur at the same speed in 1G. While these experiments have provided useful data to understand how treadmill running in reduced gravity compared to on Earth, each has the limitation of the analog on which the study was performed. Two prior studies presented GRF data during running on the ISS, but these studies were limited because of subject size and because all exercise was performed on the treadmill with vibration isolation and stabilization (TVIS), which is no longer used by U.S. astronauts (Cavanagh et al., 2010; Genc et al., 2010). The common message in all of these studies is that the reduced mechanical loading that
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occurs in 0G may explain the loss of bone and muscle health that occurs during spaceflight. The second generation treadmill (T2) was installed on the ISS in 2009 and increased the exercise capabilities offered to the astronauts both in maximal speed (19.9 kph) and in the ability to directly measure GRF during locomotion. The treadmill offers the opportunity to capture GRF during normal exercise sessions without the need for additional data collection hardware and allows astronauts to exercise at speeds previously unattainable. Our study is the first to perform a comprehensive evaluation of treadmill running biomechanics during multiple ISS missions using multiple astronauts performing exercise sessions that include high speed running. The purpose of this evaluation was to quantify how speed influences GRF while running in 0G on the ISS and in 1G. This paper reports a subset of data from a larger study during which biomechanical data were collected from astronauts before and during spaceflight while performing typical treadmill exercise. The results will be used to better understand the mechanical loading provided during running exercise to allow for more effective exercise prescriptions to be completed.
2. Methods
Seven astronauts (6m/1f; age 48.9 ± 3.4 yr; ht 178.6 ± 8.8 cm; wt 82.6 ± 13.1 kg) (mean ± 1 SD) who flew long-duration missions on the ISS (mean mission duration: 158.7 ± 24.4 days) participated in this study. The methodology of this investigation was reviewed and approved by the NASA Institutional Review Board. Each subject provided written informed consent before data collection, and was free to withdraw from the study at any time. Ground-based trials before flight were conducted at the Exercise Physiology
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and Countermeasures Laboratory at NASA Johnson Space Center, and in-flight trials were conducted on the ISS. Data were collected once before flight and 4 to 6 times during flight for each astronaut. The number of sessions obtained for each astronaut was dependent on the ISS in-flight schedule and hardware availability Preflight trials were conducted 3 to 6 months before each astronaut’s expedition. Vertical GRF data were collected during the testing trials with a force-measuring treadmill (Kistler Gaitway, Amherst, NY). The treadmill was equipped with 2 force plates beneath the running tread arranged so 1 plate rested in the front and 1 in the rear of the locomotion area. Each plate contained 4 piezoelectric uniaxial load cells that measured vertical GRF. For the first astronaut, load cell data were collected at 100 Hz using software provided by the treadmill manufacturer. For the remaining astronauts, hardware became available that allowed load cell data to be collected at 1000 Hz simultaneously with additional data not reported here. Load cell voltage data were converted to GRF during post-processing using calibration factors obtained before data collection. In addition, data were collected as the astronaut stood stationary on the front and read force plate to verify BW and calibration (front plate error of actual BW= 0.9 ± 0.7%, rear plate error of actual BW = 1.9 ± 0.7%, mean ± SD). Communications between the investigators and training personnel occurred before the 1G data collection session to determine the probable maximal speed that each astronaut would run during their spaceflight. Based on these discussions, the maximal speed for ground data collection was determined. Data were collected for 15 s at each speed from 2.4 kph to the maximal speed in 0.8 kph increments. One astronaut’s maximal speed was 9.6 kph; all other astronauts’ maximal speeds were 12.8 to 15.3 kph. Subjects
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walked at speeds less than 8 kph; these trials were used as a warm-up and, although data were collected, they are not reported in this paper. Data collection ensued after the astronaut achieved steady state locomotion at a given speed. Subjects were allowed to rest at any time and the treadmill force platforms were reset after every 3 trials to ensure that sensor drift did not affect results. Custom software written in MATLAB (R2012a, R2012b, and R2013b; The MathWorks, Inc.; Natick, MA) converted the output from each force sensor to net vertical GRF. Data collected at 1000 Hz was smoothed with a fourth-order low pass Butterworth filter at a cutoff frequency of 75 Hz; GRF data collected at 100 Hz were not smoothed due to concern that the impact peak could be eliminated. The instance of heel strike for each stride was found using GRF data using the criterion of Chang et al., 2000. During data processing, some footfall GRF trajectories were found to exhibit anomalous shapes. GRF trajectories when the foot was in contact with the front or rear plate only appeared accurate. Subsequent analyses revealed that the anomalous data were developed during the period of foot-ground contact where the foot was in contact with the front and rear plate simultaneously. Because of the accurate calibration, we chose to use only the data collected when the foot was in contact with the front plate to report peak impact forces and loading rate. Therefore we wrote an algorithm to detect footfalls where a front plate strike occurred at least 50 ms before the initial foot contact with the rear plate. Contact with each plate was determined by examining if the net GRF was greater or less than 50 N (Cavanagh & LaFortune, 1980). In-flight data were collected during each astronaut’s assigned treadmill exercise session for a given day. To be least obtrusive on each crewmember’s exercise
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prescription, subjects completed their typical assigned exercise session according to their normal schedule. To maximize the collection of useful data, each in-flight session was designed to include as many unique speeds as possible. Typical sessions had 4 to 6 unique speeds performed. Three-dimensional GRF and speed data were collected at 250 Hz by the T2 throughout the exercise session by 4 load cells that were built within the treadmill frame corners. An exercise session prescription was programmed into the treadmill using ground-based software that allows an entire session to be created as a series of stages. A stage consisted of a specific speed and duration. An exercise session could consist of 1 or more stages and the software initiated GRF data collection for 60 s at the beginning of a stage. For all data collection sessions, protocols were designed to consist of a minimum of 6 stages, allowing GRF data to be collected at least 6 times during a session. Instantaneous speed data were rounded to the nearest 0.8 kph for comparison with 1G treadmill speeds. GRF data were smoothed at 20 Hz with a fourth-order low pass Butterworth filter before analysis to remove excessive noise. The cutoff frequency was selected during preliminary analysis as the threshold at which excessive noise was removed but the shape of the GRF was maintained. Before data collection for a given session, the crewmember stood in a static position while wearing the harness-bungee configuration for a given day and the GRF was measured and used to determine the static BF. Raw load cell data were down linked from the ISS and captured on servers located at NASA JSC. Once down linked, custom software was used to convert sensor data into GRF. Standard calibration coefficients relating sensor voltages to force were
8
used in the conversion. During processing for all trials, the data stream was found to always exhibit a variable offset force during periods when neither foot was in contact with the treadmill belt that varied with each footfall (129.5 ± 28.2 N). Before flight, the load cells were calibrated successfully. There is no method currently available to verify the load cell accuracy on the T2 after installation on the ISS. Comparison of offset load to treadmill speed and peak GRF revealed decreases in mean offset force with increased speed and increased GRF, but with consistent offset force variations. On a typical force platform, the measured forces are corrected for the offset force. However, because the offset force on the T2 was variable from step to step, we felt that the force was not due to a preload, but instead may have been induced by other factors related to the inertia of the vibration isolation system (VIS). Because we have no way of verifying the source of the residual force and because the peak GRF and static BF measures were similar to expected magnitudes we did not correct our GRF for the offset force. Visual inspection of each footfall was used to ensure that there were no anomalous data. An adjusted approach similar to Chang et al. (2000) was used to determine heel strike, with thresholds adjusted to 3500/s when the GRF magnitude was greater than 1.5 times the minimum GRF for the trial. This method was decided upon after visually inspecting predicted heel strike times computed with various thresholds. Toe off was found by determining the last sample at the end of a footfall trajectory where the change in vertical GRF was less than –2000 N/s. An automated algorithm examined each footfall for both 1G and 0G trials and found the location and magnitude of the peak impact force (first local maxima) and loading rate. Loading rate was the rate of GRF application in the region from 25% of the
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peak impact force to 75% of the peak impact force. For 0G trials only, peak propulsive force (second local maxima occurring at least 20% of the foot-ground contact time after the peak impact force) as determined. For the 0G trials, BF levels as a percentage of BW were collapsed in to bins of <49% BW, 50% to 59% BW, 60% to 69% BW, 70% to 79% BW, and 80% to 89% BW. For comparison purposes, peak propulsive force data from Munro et al. (1987) were used. Microsoft Excel (Microsoft Excel for Mac 2011 Version 14.3.9 (131030), Redmond, WA) was used to find a best-fit line relating peak propulsive force to running speed. The line with the equation peak propulsive force = -0.0038 × speed2 + 0.1564 × speed + 1.2635 was found to relate peak propulsive force in BW to speed in kph with r2=0.998 and was used to interpolate 1G estimates of peak propulsive force at the speeds that were tested in this study. Effect sizes (Cohen’s d) were computed for peak impact force and loading rate between all speeds in 0G and 1G for each BF level to determine if there existed loading equivalent speeds between conditions. Effect size magnitudes between -0.35 and 0.35 were considered to indicate small to no difference between conditions (Cohen, 1992). All statistical analyses were completed using RStudio Version 0.7.551 and R version 3.0.1 (2013-05-16).
3. Results
In 1G, we collected data from 2385 footfalls across all subjects, and included 1521 in our final analysis. Only trials during which at least 10 usable footfalls occurred were included. Due to this criterion, we excluded all but one 1G trial for a single subject, and had to exclude 2 trials from a second subject, but were able to use data from all other
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trials. In 0G, our analyses were completed on 77,651 footfalls across all subjects (Table 1). Only speed and BF levels containing at least 10 footfalls were used for analyses. In addition, and speed and BF conditions for specific subjects with less than 10 footfalls were removed. Figure 1 shows typical 0G vertical GRF data from a single trial at 16.9 ± 0.8 kph. Subjects completed trials at speeds ranging from 8.0 to 20.1 kph in 0G. The BF for each session depended upon the specific crewmember (Table 2). Typical BF during exercise across astronauts was 52% to 80% BW. INSERT TABLES 1 and 2 & FIGURE 1 ABOUT HERE Across all subjects in both gravitational conditions, peak impact force increased with increasing speed (Figure 2 and Table 3). The addition of BF tended to increase the peak impact force at all speeds except at the lowest BF level (<49% BW). At any given speed, the peak impact force was lower in 0G than in 1G when expressed in BW. However, mean peak impact forces attained at speeds of between 8.0 to 14.5 kph in 1G could be attained in 0G by increasing running speed. INSERT TABLE 3 and FIGURE 2 ABOUT HERE Loading rate increased with increasing speed (Figure 3 and Table 4). However, loading rates attained in 1G could not be attained in 0G, regardless of speed or BF. INSERT TABLE 4 and FIGURE 3 ABOUT HERE Similar to peak impact force, mean peak propulsive forces increased with increasing exercise speed and BF level (Figure 4 and Table 5). However, when compared to 1G mean values reported by Munro et al., 1986, t There does not appear to be any speed or BF level combination where 1G loading levels can be attained in 0G. INSERT TABLE 5 and FIGURE 4 ABOUT HERE
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Figures 5 and 6 depict the effect size for a given gravitational comparison for the mean peak impact force and mean loading rate at each speed comparison condition. For peak impact force generation, the development of forces in 0G equivalent to or greater than those attained in 1G are attained by greater BF loading, running at higher speeds, or a combination of the 2 approaches. Peak impact forces attained in 1G are possible in 0G even with BF levels of 50% BW as long as exercise speed is of sufficient magnitude. With regards to loading rate, however, 1G values are unattainable in 0G regardless of the BF level or exercise speed. Loading rate in 0G did increase with increasing speed and BF, as evidenced by the lowest effect size magnitudes at higher speeds. INSERT FIGURES 5 and 6 ABOUT HERE
4. Discussion
The purpose of this investigation was to quantify the peak impact forces, propulsive forces, and loading rates obtained in 1G and 0G during treadmill exercise. The overall objective is the better understand how exercise prescriptions for astronauts can be modified to increase the applied mechanical load to better address health losses that occur during long-duration spaceflight. A unique feature of this experiment was the inclusion of subject-specific exercise protocols rather than the use of a standard test protocol during 0G data collection sessions. We realize that our approach makes statistical analyses of our data impossible due to the lack of common testing conditions across subjects but felt that obtaining data in actual exercise conditions was more critical to optimize results interpretation. Should we have developed a specific protocol for testing, we would either not have been able to
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collect data from the pool of subjects because of the potential that certain subjects could not complete the required protocol, or we would have had to develop a protocol that could be completed by all subjects, and this protocol could potentially not reflect a true exercise session. In addition, allowing the astronauts to complete their own exercise prescription gave us the opportunity to collect data at higher speeds, which has a direct bearing on the results presented in this paper. Peak impact and propulsive force magnitudes were less in 0G than in 1G for most subjects at all speeds. Although we did not collect propulsive forces in 1G, the propulsive forces in 0G were less than those reported by Munro et al. (1986). This finding was not unexpected, and agrees with the findings during past evaluations of running in microgravity (Cavanagh et al., 2010; De Witt et al., 2010; Genc et al., 2010, Gosseye et al., 2010). A major difference, however, between past research and our study was that past data were collected during short durations of locomotion, or during running at lower speeds (i.e., less than 11.2 kph). These studies were limited in determining if the generation of lower GRF was due to lack of subject adaptation, or in quantifying the GRF developed during faster running speeds. Our study has been able to demonstrate that in absolute terms, the vertical GRF at a given speed will be less than in 1G, and that the effect is relatively consistent. For both peak impact and peak propulsive forces, the loss of mechanical loading in 0G compared to 1G can be compensated for by exercising at higher speeds. Nilsson and Thorstensson (1989) demonstrated that in 1G, GRF are directly related to running speed. Our data indicate a similar response in GRF relative to speed in 0G. While this result is not surprising, the implications, especially when taken together with the GRF
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response at different BF levels are important. Even at BF levels less than 1 BW, the mechanical loading during treadmill running at lower speeds in 1G can be attained in 0G. Cavanagh et al., (2010) have suggested that increasing mechanical loading experienced by astronauts is vital if goals are to reduce the loss in bone health during spaceflight. Although our data do not include pre or post-flight health measures so we cannot comment on if the higher GRF attained by those astronauts who ran at higher speeds was beneficial, we can speculate that due to increased GRF and increased stride frequency at higher speeds, mechanical loading levels, and opportunities were greater. Therefore the use of high speed running as an exercise prescription should be highly considered when creating programs designed to protect astronaut health. Interestingly, although peak impact force and peak propulsive force magnitudes in 1G could be attained in 0G by running faster, loading rates in 0G were always less than those occurring in 1G regardless of BF level or running speed. Gosseye et al. (2010) found loading rates in 0G during parabolic flight to be similar to those occurring in 1G. Although we cannot confirm, we speculate that the decrease in 0G loading rate occurs due to the VIS upon which the treadmill resides. The treadmill on which Gosseye et al. (2010) collected their data from was hard-mounted to the deck of the parabolic aircraft. The purpose of the VIS is to reduce the force transmission from the treadmill to the ISS structure, and is a common feature of all exercise equipment. Our data suggest that in addition to reducing the vibrations transmitted to the ISS, the VIS also reduces the rate of force application to the musculoskeletal system, and potentially adversely affects the mechanical loading experienced by the astronauts. The loss in loading rate during 0G treadmill exercise may be an important factor when explaining how effective a
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countermeasure treadmill running may be for bone health, as rate of force application (Turner, 1998; Turner, Owan & Takano, 1995). Researchers who are attempting to quantify mechanical loading in 0G and comparing their data to 1G should consider accounting for the loss in loading rate in their approaches. Loading rate did increase with increasing speeds in 0G. While the rates were less than those attained in 1G at a given speed, loading rates in 0G at 20.1 kph approached those attained in 1G at 8 kph, giving further support to exercising at higher speeds to better attain beneficial bone loading stimuli. Increased BF levels were also beneficial to increasing GRF parameters at all speeds. The current harness-bungee system is limited by bungee force-length characteristics. Because BF is related to length, and bungee length will change during the gait cycle due to vertical oscillation of the subject, the applied BF during the gait cycle is not constant, which may in turn influence the resulting GRF magnitude. Although higher gravity-replacement forces are possible by placing multiple bungees in parallel on either side of the astronaut, this approach greatly increases the bungee stiffness and could cause comfort issues (De Witt et al., 2014). A subject loading system (SLS) designed by the European Space Agency is currently being assessed and is planned for installation on the ISS. When the SLS becomes operational, it will be possible for crewmembers to exercise with gravity replacement forces that are much higher than currently possible with the bungee system. It is possible that 1G like mechanical loading at a given speed could be replicated in 0G with greater gravity replacement forces (Gosseye et al., 2010). However, it is also possible that because the weight of the entire body is being replaced by bilateral force vectors attached to the hip and shoulders, that running motions and comfort could
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be influenced. It is important that we understand how running with higher gravity replacement force could influence exercise performance, and not extrapolate the results found in this experiment with the limited BF levels currently available. It is highly possible, based on experience during parabolic flight, that subject comfort, biomechanics, and exercise efficacy is influenced by higher EL. These influences should be examined given the opportunity. In summary, GRF attained during steady state exercise in 0G are similar in profile but less in magnitude to those attained in 1G at similar speeds. However, increasing exercise speed in 0G can compensate for the loss of peak impact and peak propulsive force magnitudes. Loading rates, on the other hand, cannot be compensated for with the increase in speed. When attempting to maximize mechanical loading, high speed running should be considered when creating training programs in 0G, with the knowledge that on the current treadmill on the ISS, loading rates will always be less than those occurring in 1G.
Conflict of interest statement
None of the authors had any financial or personal conflict of interest with regard to this study.
Acknowledgements We thank Roxanne Buxton, Melissa Scott-Pandorf, and Renita Fincke for assistance in collecting and processing data and creating analysis software. We also thank Peter Cavanagh for his advice in the processing of the GRF data. We would like to thank NASA’s ISS Medical Projects Group (ISSMP) for their assistance in implementing this
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project. Thanks specifically to Mr. Steve Hing of ISSMP as the primary contact. This study was funded by NASA’s Human Research Program.
References
Cavanagh, P.R., Genc, K.O., Gopalakrishnan, R., Kuklis, M.M., Maender, C.C., Rice, A.J., 2010. Foot forces during typical days on the international space station. J. Biomechanics. 43 (11), 2182-2188. Cavanagh, P.R., LaFortune, M.A., 1980. Ground reaction forces in distance running, Journal of Biomechanics, 13, 397-406. Chang, Y.H., Huang, H.W., Hamerski, C.M., Kram, R., 2000. The independent effects of gravity and inertia on running mechanics. Journal of Experimental Biology. 203 (2), 229238. Cohen J., 1992. A power primer. Psychological Bulletin. 12 (1), 155-159. De Witt, J.K., Perusek, G.P., Lewandowski, B.E., Gilkey, K.M., Savina, M.C., Samorezov, S., Edwards, W.B., 2010. Locomotion in simulated and real microgravity: Horizontal suspension vs. parabolic flight. Aviation, Space, and Environmental Medicine. 81 (12), 1092-1099. De Witt, J.K., Schaffner, G., Ploutz-Snyder, L.L., 2014, Bungee force level, stiffness, and variation during treadmill locomotion in simulated microgravity. Aviation, Space, and Environmental Medicine, 85, 1-7. Genc, K.O., Gopalakrishnan, R., Kuklis, M.M., Maender, C.C., Rice, A.J., Bowersox, K.D., Cavanagh, P.R., 2010. Foot forces during exercise on the International Space Station. Journal of Biomechanics. 43 (15), 3020-3027.
17
Genc, K.O., Mandes, V.E., Cavanagh, P.R., 2006. Gravity replacement during running in simulated microgravity. Aviation, Space, and Environmental Medicine. 77 (11), 11171124. Gibala, M.J., McGee, S.L., 2008. Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain?. Exercise and Sport Sciences Reviews. 36 (2), 58-63. Gopalakrishnan, R., Genc, K.O., Rice, A.J., Lee, S., Evans, H.J., Maender, C.C., Cavanagh, P.R., 2010. Muscle volume, strength, endurance, and exercise loads during 6month missions in space. Aviation, Space, and Environmental Medicine. 81 (2), 91-104. Gosseye, T.P., Willems, P.A., Heglund, N.C., 2010. Biomechanical analysis of running in weightlessness on a treadmill equipped with a subject loading system. European Journal of Applied Physiology. 110 (4), 709-728. Grigoriev A.I., Oganov V.S., Bakulin A.V., Poliakov V.V., Voronin L.I., Morgun V.V., Shnaider V.S., Murashko L.V., Novikov V.E., LeBlanc A., Shakleford L., 1998. [Clinical and physiological evaluation of bone changes among astronauts after long-term space flights]. Aviakosm Ekolog Med. 32, 21-25. Lang T, LeBlanc A, Evans H, Lu Y, Genant H, Yu A., 2004. Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight. Journal of Bone and Mineral Research. 19 (6), 1006–12. Munro, C.F., Miller, D.I., Fuglevand, A.J., 1987. Ground reaction forces in running: a reexamination. Journal of Biomechanics. 20 (2), 147-155. Nilsson, J., & Thorstensson, A., 1989. Ground reaction forces at different speeds of human walking and running. Acta Physiologica Scandinavica. 136 (2), 217-227.
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Novotny, S.C., Perusek, G.P., Rice, A.J., Comstock, B.A., Bansal, A., Cavanagh, P.R., 2013. A Harness for Enhanced Comfort and Loading during Treadmill Exercise in Space. Acta Astronautica. 89, 205-214. Rubin, C.T., Lanyon, L.E., 1984. Regulation of bone formation by applied dynamic loads. The Journal of Bone & Joint Surgery, 66 (3), 397-402. Turner C.H., 1998. Three rules for bone adaptation to mechanical stimuli. Bone. 23, 399407. Schaffner, G., DeWitt, J., Bentley, J., Yarmanova, E., Kozlovskaya, I., Hagan, D., 2005. Effect of load levels of subject loading device on gait, ground reaction force, and kinematics during human treadmill locomotion in a weightless environment. National Aeronautics and Space Administration NASA/TP-2005-213169. Sibonga, J.D., Cavanagh, P.R., Lang, T.F., LeBlanc, A.D., Schneider, V.S., Shackelford, L.C., Smith, S.M., Vico, L., 2008. Adaptation of the skeletal system during long-duration spaceflight. Clinical Reviews in Bone and Mineral Metabolism. 5 (4), 249–261. Smith, S.M., Wastney, M.E., O'Brien, K.O., Morukov, B.V., Larina, I.M., Abrams, S.A., Davis-Street, J.E., Oganov, V., Shackelford, L.C., 2005. Bone markers, calcium metabolism, and calcium kinetics during extended-duration space flight on the Mir space station. Journal of Bone and Mineral Research. 20 (2): 208–218. Smith, S.M., Heer, M.A., Shackelford, L.C., Sibonga, J.D., PloutzǦSnyder, L., Zwart, S.R., 2012. Benefits for bone from resistance exercise and nutrition in longǦduration spaceflight: Evidence from biochemistry and densitometry. Journal of Bone and Mineral Research. 27 (9), 1896-1906.
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Stein, T.P., 2013. Weight, muscle and bone loss during space flight: another perspective. European Journal of Applied Physiology. 113 (9), 2171-2181. Turner, C.H., Owan, I., Takano, Y., 1995. Mechanotransduction in bone: Role of strain rate. American Journal of Physiology-Endocrinology and Metabolism. 269 (3), E438E442.
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Table 1. Number of footfalls analyzed at each speed in 1G and each speed and BF level in 0G across all subjects. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
144 125 146 170 177 178 175 146 147 113
0G 40-49% BW
50-59% BW
60-69% BW
678
1073 1108 4236 1926 5061 2655 4791 822 564 407 435 95 104 47 70 62
353 1429 6932 5224 3313 2159 1250 2626 1112 388 257 221 88 131 81 110
628 666 566 511
70-79% BW
80-89% BW
534 1884 7387 3074 1466 1328 983 2167 288 534 119 150
360 777 2316 1206 257 578 64
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Table 2. Absolute and relative bungee forces used during exercise sessions for all astronauts expressed in newtons (N) and relative to 1G BW (%BW).
Sessi on
1 N
1 2 3 4 5 6
Mea n± SD
530 .5 508 .2 552 .8 552 .8 499 .3 548 .3 532 .0 ± 23. 5
2 %B W 56 % 54 % 58 % 58 % 53 % 58 % 56 ±2
N 481 .4 512 .6 517 .1 534 .9 543 .8 557 .2 524 .5 ± 26. 8
Subject 4
3 %B W 69 % 74 % 74 % 77 % 78 % 80 % 75 ±4
N 490 .4 601 .8 606 .3 641 .9 637 .5 641 .9 603 .3 ± 58. 2
%B W 52 % 64 % 64 % 68 % 68 % 68 % 64 ±6
N 378 .9 289 .8 387 .8 387 .8 405 .7 405 .7 375 .9 ± 43. 6
%B W 52 % 40 % 53 % 53 % 56 % 56 % 52 ±6
5 N 481. 4 490. 4 477. 0 539. 4 DN C* DN C 497. 0± 28.8
6 %B W 77 % 79 % 77 % 87 % DN C DN C 80 ±5
N 468 .1 548 .3 548 .3 624 .1 615 .2 DN C 560 .8 ± 63. 0
7 %B W 53 % 62 % 62 % 70 % 69 % DN C 63 ±7
N 441 .3 459 .1 481 .4 477 .0 526 .0 552 .8 489 .6 ± 42. 0
%B W 52 % 54 % 56 % 56 % 62 % 65 % 58 ±5
*DNC = did not complete session due to scheduling issues
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Table 3. Peak impact force relative to 1G BW at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
1.36 ± 0.17 1.38 ± 0.17 1.44 ± 0.18 1.61 ± 0.24 1.64 ± 0.20 1.76 ± 0.25 1.85 ± 0.27 1.82 ± 0.25 1.91 ± 0.27 2.09 ± 0.29
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
0.71 ± 0.07
0.78 ± 0.12 0.81 ± 0.15 0.88 ± 0.18 1.01 ± 0.24 0.94 ± 0.21 0.95 ± 0.14 0.95 ± 0.15 1.18 ± 0.26 1.35 ± 0.26 1.34 ± 0.27 1.26 ± 0.27 1.34 ± 0.28 1.74 ± 0.10 1.80 ± 0.09 1.77 ± 0.09 1.78 ± 0.10
1.00 ± 0.11 1.10 ± 0.10 1.06 ± 0.12 1.13 ± 0.10 1.20 ± 0.12 1.24 ± 0.15 1.20 ± 0.12 1.23 ± 0.13 1.29 ± 0.12 1.50 ± 0.18 1.55 ± 0.19 1.73 ± 0.18 1.86 ± 0.17 1.83 ± 0.12 1.86 ± 0.15 1.88 ± 0.13
1.09 ± 0.08 1.17 ± 0.10 1.21 ± 0.12 1.33 ± 0.14 1.43 ± 0.12 1.45 ± 0.11 1.49 ± 0.10 1.46 ± 0.10 1.52 ± 0.11 1.60 ± 0.10 1.65 ± 0.10 1.67 ± 0.09
1.24 ± 0.08 1.27 ± 0.07 1.32 ± 0.08 1.38 ± 0.09 1.48 ± 0.07 1.50 ± 0.07 1.47 ± 0.08
0.71 ± 0.06 0.74 ± 0.07 0.80 ± 0.10 0.75 ± 0.06 0.76 ± 0.06
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Table 4. LR expressed relative to 1G BW in BW/s at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. Speed
1G
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
47.70 ± 13.81 50.20 ± 16.43 52.19 ± 17.88 69.19 ± 23.14 72.00 ± 18.50 81.24 ± 20.24 88.98 ± 25.45 84.87 ± 23.57 94.39 ± 27.63 114.62 ± 27.25
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
14.30 ± 3.31
17.17 ± 3.79 17.67 ± 2.97 19.15 ± 3.69 21.29 ± 3.85 21.32 ± 3.55 21.47 ± 3.15 22.23 ± 2.98 23.51 ± 4.10 25.79 ± 4.49 26.41 ± 3.37 25.81 ± 4.31 27.25 ± 3.87 29.99 ± 2.98 31.72 ± 2.61 30.89 ± 2.47 31.30 ± 2.95
19.20 ± 3.45 22.83 ± 3.64 22.09 ± 3.91 23.66 ± 3.39 24.94 ± 3.69 26.59 ± 3.69 26.99 ± 3.00 27.16 ± 3.31 27.89 ± 3.50 29.31 ± 4.17 31.32 ± 3.03 30.58 ± 4.32 33.12 ± 4.43 32.14 ± 3.39 32.89 ± 3.94 33.37 ± 3.65
26.43 ± 3.91 27.14 ± 3.89 27.91 ± 4.74 32.12 ± 3.46 33.00 ± 4.57 33.50 ± 3.81 34.57 ± 3.85 34.37 ± 3.40 33.86 ± 3.91 35.48 ± 3.50 36.92 ± 3.01 31.46 ± 4.58
28.48 ± 2.80 29.83 ± 4.18 31.69 ± 3.34 32.89 ± 4.01 35.12 ± 3.29 36.02 ± 3.09 34.42 ± 2.14
16.38 ± 2.10 18.07 ± 2.39 19.22 ± 3.57 18.18 ± 2.03 18.76 ± 2.42
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Table 5. Peak propulsive force expressed relative to 1G BW at each speed in 1G and at each BF level in 0G (mean ± SD). Blank cells occur for conditions where no data were collected. *1G data from Munro et al., 1987. Speed
1G*
kph 8 8.8 9.7 10.5 11.3 12.1 12.9 13.7 14.5 15.3 16.1 16.9 17.7 18.5 19.3 20.1
2.27 2.35 2.42 2.49 2.55 2.60 2.65 2.69 2.73
0G 40-49% BW
50-59% BW
60-69% BW
70-79% BW
80-89% BW
0.82 ± 0.04
0.96 ± 0.10 1.02 ± 0.10 1.10 ± 0.12 1.18 ± 0.15 1.13 ± 0.13 1.18 ± 0.13 1.13 ± 0.12 1.30 ± 0.14 1.38 ± 0.13 1.39 ± 0.11 1.35 ± 0.08 1.36 ± 0.11 1.50 ± 0.05 1.52 ± 0.06 1.49 ± 0.05 1.52 ± 0.07
1.32 ± 0.12 1.34 ± 0.12 1.35 ± 0.10 1.36 ± 0.11 1.49 ± 0.12 1.44 ± 0.13 1.38 ± 0.08 1.40 ± 0.09 1.45 ± 0.08 1.59 ± 0.15 1.56 ± 0.15 1.72 ± 0.11 1.78 ± 0.11 1.76 ± 0.11 1.78 ± 0.11 1.78 ± 0.11
1.39 ± 0.05 1.48 ± 0.10 1.51 ± 0.10 1.58 ± 0.12 1.68 ± 0.11 1.68 ± 0.11 1.73 ± 0.09 1.67 ± 0.05 1.74 ± 0.05 1.73 ± 0.04 1.72 ± 0.04 1.76 ± 0.04
1.48 ± 0.03 1.53 ± 0.05 1.58 ± 0.08 1.67 ± 0.10 1.76 ± 0.10 1.82 ± 0.09 1.71 ± 0.04
0.86 ± 0.03 0.89 ± 0.04 0.92 ± 0.01 0.92 ± 0.03 0.93 ± 0.04
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Figure 1. Typical vertical GRF trajectories for multiple steps as a subject ran at 14.5 kph. Each trajectory is processed data from a single footfall. Multiple footfalls from a single trial are plotted indicating consistency in GRF shape.
Figure 2. Relative peak impact force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between peak impact force and running speed.
Figure 3. Relative loading rate for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed.
Figure 4. Relative peak propulsive force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed. Mean peak propulsive forces in 1G are estimated from Munro et al. (1986).
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Figure 5. Effect size (Cohen’s d) for mean peak impact force between 0G and 1G. Each panel is a scatterplot of ES versus treadmill speed in 1G for a specific speed in 0G. Speeds in 0G increase for each panel. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean peak impact forces in each gravitational condition. Positive effect size indicates that mean 1G peak impact forces are greater than 0G mean peak impact force for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating similarities in peak impact forces developed. Effect size comparisons within BF level indicate smaller effect sizes for a given speed in 0G at greater loading levels. These data indicate that running faster, if matching peak impact forces in 0G to 1G is the desired outcome, can compensate for lack of gravity replacement force. In addition, these data indicate that there are situations at higher speeds where peak impact forces in 0G are greater than those in 1G, as demonstrated by negative effect sizes.
Figure 6. Effect size (Cohen’s d) for mean loading rate between 0G and 1G. Speeds in 0G are depicted in each facet, and speeds in 1G are listed on the horizontal axis for each facet. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean loading rate in each gravitational condition. Positive effect size indicates that mean 1G loading rates are greater than 0G loading rates for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating the relative separation between 1G and 0G loading rates also decreases. However, because the effect size never becomes
27
approximately 0, loading rates obtained in 1G are unattainable in 0G at the speeds and BF levels tested.
28
Figure
Figure 1. Typical vertical GRF trajectories for multiple steps as a subject ran at 16.9 ± 0.8 kph. Each trajectory is processed data from a single footfall. Multiple footfalls from a single trial are plotted indicating consistency in GRF shape.
Figure 2. Relative peak impact force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between peak impact force and running speed.
Figure 3. Relative loading rate for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed.
Figure 4. Relative peak propulsive force for all recorded footfalls versus running speed. Each point is a specific footfall, and points are slightly adjusted horizontally to allow for identification of point density at each speed. Lines of best fit for each condition as found using standard linear regression depict mean relationship between loading rate and running speed. *Mean peak propulsive forces in 1G are estimated from Munro et al. (1986).
Figure 5. Effect size (Cohen’s d) for mean peak impact force between 0G and 1G. Each panel is a scatterplot of ES versus treadmill speed in 1G for a specific speed in 0G. Speeds in 0G increase for each panel. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean peak impact forces in each gravitational condition. Positive effect size indicates that mean 1G peak impact forces are greater than 0G mean peak impact force for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating similarities in peak impact forces developed. Effect size comparisons within BF level indicate smaller effect sizes for a given speed in 0G at greater loading levels. These data
indicate that running faster, if matching peak impact forces in 0G to 1G is the desired outcome, can compensate for lack of gravity replacement force. In addition, these data indicate that there are situations at higher speeds where peak impact forces in 0G are greater than those in 1G, as demonstrated by negative effect sizes.
Figure 6. Effect size (Cohen’s d) for mean loading rate between 0G and 1G. Speeds in 0G are depicted in each facet, and speeds in 1G are listed on the horizontal axis for each facet. Each color represents a different BF level. Horizontal lines delineate the zone where effect size is between -0.35 and 0.35, indicating no difference between mean loading rate in each gravitational condition. Positive effect size indicates that mean 1G loading rates are greater than 0G loading rates for the given speed comparison. The downward right trend indicates that as speed increases, the effect size between gravitational conditions decreases indicating the relative separation between 1G and 0G loading rates also decreases. However, because the effect size never becomes approximately 0, loading rates obtained in 1G are unattainable in 0G at the speeds and BF levels tested.