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Physiological benefits of exercise in artificial gravity: A broadband countermeasure to space flight related deconditioning
Acta Astronautica 63 (2008) 2 – 7 www.elsevier.com/locate/actaastro
Physiological benefits of exercise in artificial gravity: A broadband countermeasure to space flight related deconditioning Jessica L. Edmonds∗ , Thomas Jarchow, Laurence R. Young Massachusetts Institute of Technology, USA Available online 18 April 2008
Abstract Current countermeasures to space flight related physiological deconditioning have not been sufficiently effective. We believe that a comprehensive countermeasure is the combination of intermittent centrifugation (artificial gravity) and exercise. We aim to test the long-term effectiveness of this combination in terms of fitness benefits. As a first-order determination of effectiveness, subjects participated in an eight-week exercise program. Three times per week, they exercised using a stair-stepper on a shortradius (2 m) centrifuge spinning at 30 RPM, maintaining a target heart rate that was systematically increased over the exercise period. During the sessions, foot forces and stepping cadence, heart rate, and perceived exertion were measured. Before and after the eight-week exercise program, measurements included: body fat percentage, bone mineral content, quadriceps extension strength, push-ups endurance, stepping cadence for a given heart rate, and maximum stepping endurance. We find that stairstepping on a centrifuge is safe and comfortable. Preliminary fitness results indicate that stair-stepping on a centrifuge may be effective in improving aerobic fitness, body composition, and strength. These results indicate that such a combination may also be effective as a countermeasure to space flight deconditioning. © 2007 Elsevier Ltd. All rights reserved.
1. Introduction The future of space exploration lies in the capability of astronauts to travel safely to the moon, Mars, and beyond. Without any countermeasures, long-duration space flight can result in a loss of 1.5% bone mineral density per month [1], up to 17% of muscle volume (seven-month period) [2], intolerance to orthostatic cardiovascular stress [3], and balance abnormalities [4]. Current countermeasures have not been sufficiently effective. Rather than simply treating the symptoms, it is possible to remove the cause of the deconditioning by placing the astronaut in a gravity-like environment. ∗ Corresponding author.
E-mail addresses: [email protected] (J.L. Edmonds), [email protected] (T. Jarchow), [email protected] (L.R. Young). 0094-5765/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2007.12.026
We believe that a comprehensive countermeasure is the combination of intermittent centrifugation (artificial gravity) and exercise. We aim to test the long-term effectiveness of this combination in terms of fitness benefits. Previous experiments have investigated the use of an exercise device on a centrifuge for use as a possible space flight countermeasure. Experimenters have implemented cycle ergometry [5–10] and squats [11,12], on short-radius centrifuges. These experiments have investigated cardiovascular and aerobic parameters during exercise [5–8,10,13,14], musculoskeletal challenges [11,12,15], and biomechanical properties of exercise, especially with respect to the Coriolis environment [11]. In bedrest studies, cycle ergometry on a centrifuge maintains some cardiovascular and muscular parameters, versus no-countermeasure groups
J.L. Edmonds et al. / Acta Astronautica 63 (2008) 2 – 7
3
[8,13–15]. To complement these findings, this study aims to demonstrate general effectiveness of stairstepping on a centrifuge for healthy subjects. Subjects participated in an eight-week exercise program on a short-radius (2 m) centrifuge spinning at 30 RPM. A suite of measurements before and after this exercise period indicates the fitness benefits of stairstepping on a centrifuge. 2. Methods Nine male subjects took part in an exercise program on a short-radius centrifuge, exercising three times per week. The centrifuge spins at rates of up to 30 RPM, corresponding to approximately 1.6–1.8-Gz at the feet, depending on the height of the subject. During centrifugation, the subject lay supine on the centrifuge, with his head placed at the center of rotation, and his feet on the exercise device in the radial direction. Adjustments were made to facilitate comfortable exercise: the footplate could be moved toward or away from the subject’s body, and shoulder pads could also be adjusted in this way (subjects pushed against these shoulder pads while exercising). The exercise equipment included a Kettler Vario䉸 mini-stepper (Redditch, Worcs, UK), which was mounted vertically on a surface at the subject’s feet [16]. The stepper uses hydraulic dampers as resistance to stepping. When the subject steps, he has a limited range of motion, and usually bottoms out at the stoppers. For this reason the subject experiences an impact load on his foot during each step. The stepper was instrumented with force plates (modified digital bathroom scales). Additionally, resistive arm bands (PowerSystems Premium Versa-Tube䉸 , Knoxville, TN) were mounted to the surface of the centrifuge, allowing for arm exercise. Subjects could choose among four resistance levels of these bands, and used them in varying motions. The centrifuge with all exercise equipment is shown in Fig. 1. Volunteers with low aerobic fitness were chosen as test subjects because they were more likely to improve in a short period of time. Therefore, they were selected if they had not been part of a regular exercise program for at least 1 year, and if their estimated V˙O2 -max did not exceed 42.4 mL/min/kg [17]. Estimated V˙O2 -max was determined by a graded treadmill test, during which the subject wore a VO2000䉸 face mask unit (MedGraphics, St. Paul, MN), which measured respiratory parameters. The test was terminated when subjects reached a heart rate of 200-Age [18]. The estimated V˙O2 -max was extrapolated from the data that were obtained,
Fig. 1. Exercise equipment on the short-radius centrifuge.
using a nomogram that allows for estimation of maximal oxygen uptake from submaximal oxygen uptake values [19]. All selected subjects reported that they had no general health or orthopedic problems, and were tolerant to motion sickness. Before participating, they read and signed an informed consent that was approved by the MIT Committee on the Use of Humans as Experimental Subjects. After the determination of eligibility, subjects underwent the following evaluations, which were then repeated after four and eight weeks of the exercise program: 1. An upright stepping test included a warm-up, 2 min of constant cadence exercise, 2 min of constant heart rate exercise, 2 min of maximal stair-stepping, and 4 min of rest. Measures included heart rate (Acumen TZ-max 100䉸 , Sterling, VA), beat-tobeat blood pressure (PortaPres䉸 model 2.0 unit, Finapres Medical Systems, Amsterdam, The Netherlands), foot forces (modified digital bathroom scales mounted to each stepper foothold), and respiratory parameters including oxygen uptake, ventilation, and breathing rate (VO2000䉸 face mask unit). 2. A quadriceps extension test required the subject to sit in a chair that was fixed to a frame, while facing a wall. The stair-stepper with footholds was held vertically against the wall. The subject sat in the chair, which was covered with a rubber mat to prevent slippage. The subject placed the ball of his right foot against one force plate of the stairstepper, such that the angle of his knee was slightly greater than 90◦ . Without pushing against the back of the chair, he pushed his leg against the stepper as hard as possible, slowly extending to maximum
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contraction over a 5 s period. The test was repeated two more times after brief rest periods, always with the right leg. The force measured by the force plate of the stepper was taken as a measure of leg strength. 3. A push-ups test to failure. The subject was instructed to perform as many full-body push-ups as he could without resting, and the number of push-ups was counted. 4. A dual energy X-ray absorptiometry (DEXA) scan (QDR 4500䉸 , Hologic Inc., Bedford, MA) was used to measure body composition: fat, lean, and bone mineral content of the whole body, as well as selected individual regions. 5. A Sharpened Romberg test [20] was used to measure balance. Subjects stood heel-to-toe with their hands crossed above their chest, closed their eyes, and attempted to hold that position without faltering for 60 s. If the first attempt was not successful, the subject repeated this test two more times. Following the initial test battery, subjects attended exercise sessions three times per week. Before all sessions, subjects’ sitting heart rate and blood pressure were taken, as well as body weight. Subjects were helped onto the centrifuge and necessary adjustments were made to the shoulder pads and footplate distance. A portable DVD player was mounted above the subject’s face with his choice of a movie, and he was allowed to wear earphones to hear the movie (verbal communication was still possible with the operator, who was in the same room during the sessions). Additionally, subjects were instrumented with an Acumen heart rate monitor, and were able to monitor their heart rate using the wristwatch receiver mounted above their face; the average heart rate during the exercise session was also recorded. For the subject’s comfort, the room was completely dark during the exercise session, apart from the movie screen and the heart rate display. The centrifuge was spun at 30 RPM, creating 1–Gz per meter radius. The direction of centrifugation (clockwise or counterclockwise) was alternated for each session to balance the lateral displacement effect of Coriolis accelerations on the knees and hips. Subjects warmed up for 3 min by stepping at a speed of their choice. They were then instructed to increase their heart rate to the target level, and maintain that level. Exercise sessions lasted 20, 30, or 40 min, increasing as the subject gained experience. The target heart rate was determined as 40–55% heart rate reserve (HRR1 [21]), depending on the exercise session. The subject 1 Target %×(maximum heart rate−resting heart rate)+resting heart rate.
could modulate his heart rate by stepping faster or slower, or using higher resistance arm bands. Target heart rate assignment were based on the subjects’ rating of perceived exertion in each session [22]; the experimenter attempted to assign a heart rate corresponding to a Borg rating of “somewhat hard” to “hard”. Over the eight weeks, the target heart rate was increased while maintaining a desirable Borg rating. Aside from heart rate and Borg rating, the peak foot force attained during each step was extracted from foot force data over the whole session. A value for the average of these peak foot forces was obtained per session. All statistical analysis used repeated measures analysis under a general linear model; thus, each subject acted as his own control. One of the nine subjects completed only four weeks, and is excluded from statistical tests but shown in Figs. 4–7 for comparison. 3. Results Subjects reported no discomfort, other than exercise fatigue, when exercising on the spinning centrifuge. One subject terminated the exercise program after four weeks due to headaches between sessions. None of the other subjects reported any motion sickness. There was no distinguishable change in resting (sitting) heart rate or blood pressure, or body weight, over the test sessions. During the exercise sessions, subjects were able to maintain their heart rate within 4% of the target on average, while maintaining an average rating of perceived exertion of 12.9 (between 11=“fairly light” and 13=“somewhat hard”). See Fig. 2. Peak foot forces during exercise, averaged over all sessions, were 45–124% of body weight, depending on the subject; the overall average among subjects was 83% body weight (Fig. 3). During the upright stepping test, when stepping at a constant heart rate (the same heart rate, 50% HRR, at zero, four, and eight weeks), cadence increased significantly between weeks four and eight (Fig. 4). In a maximal upright stepping test, in which the subject attempted to step as many times as possible in 2 min, all subjects increased the number of steps they did on average by 20% after four weeks and 43% after eight weeks. The maximum heart rate and maximum minute ventilation achieved during this endurance test significantly increased over eight weeks (17% (p = 0.01) and 23% (p = 0.024), respectively) (Fig. 5). Overall percent body fat (as determined by a DEXA scan) decreased by an average of 0.7% after eight weeks, but these changes were not significant; changes in the
J.L. Edmonds et al. / Acta Astronautica 63 (2008) 2 – 7
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Fig. 4. Stepping cadence increased significantly while holding a constant heart rate, in the last four weeks of training.
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Fig. 3. Peak stepping forces for each subject (average over all exercise sessions), expressed as a percentage of body weight. Inset. Typical force profile when stepping (one foot shown).
legs (1.0% and 1.1% in the left and right legs, respectively), approached or reached significance after four or eight weeks (Fig. 6). Additionally, pelvic bone mineral content increased significantly by 2.4% on average. In a test of maximal quadriceps extension strength, the average force exerted by the subjects increased by
Fig. 5. Maximum number of steps the subject was able to do in an upright two-minute endurance test.
15.2% after eight weeks of training, but these results were not significant. In a timed test to measure the number of full-body push-ups, subjects increased the number of push-ups they were able to do by an average of 18.2% after eight weeks (Fig. 7). A Sharpened Romberg balance test revealed no changes in balance after four or eight weeks of exercise.
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50 p = 0.094, right leg (solid lines) p = 0.013, left leg (dashed lines)
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4. Discussion We find minor fitness improvements in eight out of nine subjects after an eight week exercise program on the centrifuge. There is evidence in these results of exercise specificity [18]: the upright stair-stepping test generated the most obvious fitness changes after four weeks, since subjects were tested using the same type of exercise as they used to train. Specifically, when stepping at the same heart rate, subjects were apparently
able to perform the exercise with considerably less effort after eight weeks (significantly higher cadence for the same heart rate). The subject who terminated the exercise program after four weeks due to headaches between sessions was probably not sufficiently evaluated prior to his selection for the study. Unlike the other subjects, he did not enjoy being on the centrifuge, and strongly disliked exercising beyond 20 min. Nonetheless, motion sickness symptoms will be carefully monitored for future subjects. Some unusual artifacts of artificial gravity exercise should be noted. Subjects reported that exercising in the dark felt somewhat unnatural; however, they became accustomed to it after 3–4 sessions in general. Some subjects also subjectively reported that increasing their heart rate to the target level was more difficult on the centrifuge than it was when stepping upright. This is probably due to the fact that, although they were in an artificial gravity environment, they were still lying down, resulting in a higher stroke volume [23] but, presumably, a lower base heart rate. This does not necessarily confound our study, since in general heart rate is lower in microgravity than it is on Earth [24]. Although it was not explicitly recorded here, a simple experiment could be used to test this subjective report: compare heart rate levels at certain Borg ratings of perceived exertion, both upright and when exercising on the centrifuge. Practically, using the resistive arm bands helped subjects to modulate their heart rate when on the centrifuge. Target heart rate can only be used loosely in exercise prescription [21]; the trainer must be attentive to individual differences. For this reason, we deviated significantly from the generalized heart rate assignments in order to maintain the desired rating of perceived exertion. For example, sessions 4, 5, and 6 were loosely designated 45% HRR sessions. One subject reported a rating of “somewhat hard” during these sessions, and so the experimenter asked him to maintain this heart rate value. Another subject, by session 4, had his target heart rate increased to 55% HRR, due to earlier ratings of “fairly light” at lower target heart rates. The absence of some effects should also be discussed. During the sessions and throughout all sessions, subjects reported no knee or hip pain due to Coriolis accelerations. This may or may not be due to spinning the centrifuge in alternate directions on alternate days, and this negative finding is in agreement with previous studies [11]. Subjects did not experience diminished balance as measured by the Sharpened Romberg test. Although a gross measure of balance, we may be reasonably assured that stair-stepping on a centrifuge three times per week does not seriously impair the subject’s balance in
J.L. Edmonds et al. / Acta Astronautica 63 (2008) 2 – 7
a non-spinning Earth-gravity environment, nor does it impair his normal 1-g locomotion. As a space flight countermeasure, stair-stepping on a centrifuge has the desirable quality of producing relatively high foot forces. Peak impact foot forces during the exercise sessions (up to 124% body weight) are on the order of weightless treadmill running using bungee tie-downs (∼ 130% body weight peak impact forces, tested in parabolic flight [25]). These forces could be increased by using higher centrifuge velocities, or loading the subject further using bungees. The motion of stair-stepping also more closely mimics that of walking than, for example, cycling does. Locomotion training may be important during a long-duration space flight, particularly en route to a planetary surface where walking will be necessary during EVA. Ideally, the exercise device would also include a forward stroke; perhaps it would be possible to implement some type of treadmill device on the centrifuge. Our results indicate that stair-stepping on a centrifuge is likely to improve aerobic fitness, body composition, and strength in a male population of healthy subjects. As such, stair-stepping on a centrifuge may also serve to prevent deconditioning of the same parameters during long-duration space flight. Acknowledgments This work was supported by a NASA GSRP fellowship (NNX06AH21H), as well as the NASA International Multidisciplinary Artificial Gravity project (NNJ04HD64G). The authors are grateful to Dr. William Paloski for his support of this project, and to Esther Hu for her line drawing of the centrifuge. References [1] T. Lang, et al., Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight, Journal of Bone and Mineral Research 19 (6) (2004) 1006–1012. [2] A. LeBlanc, et al., Muscle volume, MRI relaxation times (T2), and body composition after spaceflight, Journal of Applied Physiology 89 (6) (2000) 2158–2164. [3] J.C. Buckey Jr., et al., Orthostatic intolerance after spaceflight, Journal of Applied Physiology 81 (1) (1996) 7–18. [4] J.C.J. Buckey, Space Physiology, Oxford University Press, New York, 2006. [5] V.J. Caiozzo, et al., Hemodynamic and metabolic responses to hypergravity on a human-powered centrifuge, Aviation Space and Environmental Medicine 75 (2) (2004) 101–108. [6] J.E. Greenleaf, et al., Short-arm (1.9 m) + 2.2 Gz acceleration: isotonic exercise load-O2 uptake relationship, Aviation Space and Environmental Medicine 70 (12) (1999) 1173–1182. [7] S. Iwase, et al., Effects of simultaneous load of centrifuge-induced artificial gravity and ergometer exercise
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as the countermeasures for space deconditioning on human cardiovascular function, Journal of Gravitational Physiology 10 (1) (2003) P-101–P-105. K. Katayama, et al., Acceleration with exercise during headdown bed rest preserves upright exercise responses, Aviation Space and Environmental Medicine 75 (12) (2004) 1029–1035. A. Kreitenberg, et al., The “Space Cycle” Self Powered Human Centrifuge: a proposed countermeasure for prolonged human spaceflight, Aviation Space and Environmental Medicine 69 (1) (1998) 66–72. I.F. Vil-Viliams, Y.B. Shul’zhenko, Functional state of the cardiovascular system under the combined effect of 28day immersion, rotation of a short-arm centrifuge, and exercise on a bicycle ergometer, Kosmicheskaya Biologiya I Aviakosmicheskaya Meditsina 2 (1980) 42–45. K.R. Duda, Squat Exercise Biomechanics During Short-Radius Centrifugation, Ph.D. Thesis, MIT, Cambridge, MA, 2006. Y. Yang, et al., Space cycle: a human-powered centrifuge that can be used for hypergravity resistance training, Aviation Space and Environmental Medicine 78 (1) (2007) 2–9. S. Iwase, Effectiveness of centrifuge-induced artificial gravity with ergometric exercise as a countermeasure during simulated microgravity exposure in humans, Acta Astronautica 57 (2–8) (2005) 75–80. S. Iwase, et al., Effect of centrifuge-induced artificial gravity and ergometric exercise on cardiovascular deconditioning myatrophy, and osteoporosis induced by a −6 degrees headdown bedrest, Journal of Gravitational Physiology 11 (2) (2004) P243–P244. H. Akima, et al., Intensive cycle training with artificial gravity maintains muscle size during bed rest, Aviation Space and Environmental Medicine 76 (10) (2005) 923–929. J.L. Edmonds, T. Jarchow, L.R. Young, A stair-stepper for exercising on a short-radius centrifuge, Aviation Space and Environmental Medicine 78 (2) (2007) 129–134. V.H. Heyward, Advance Fitness Assessment and Exercise Prescription, third ed., Human Kinetics, Champaign, IL, 1998. P.-O. Astrand, et al., Textbook of Work Physiology: Physiological Bases of Exercise, fourth Ed., Human Kinetics, Champaign, IL, 2003. I. Astrand, Aerobic work capacity in men and women with special reference to age, Acta Physiologica Scandinavica Supplementary 49 (169) (1960) 1–92. R.V. Kenyon, L.R. Young, M.I.T./Canadian vestibular experiments on Spacelab-1 mission: 5. Postural responses following exposure to weightlessness, Experimental Brain Research 64 (1986) 335–346. M.H. Whaley (Ed.), ACSM’s Guidelines for Exercise Testing and Prescription, seventh ed., Lippincott Williams & Wilkins, Philadelphia, 2006. G.A. Borg, Perceived exertion, Exercise and Sport Sciences Reviews 2 (1974) 131–153. S. Bevegard, A. Holmgren, B. Jonsson, Circulatory studies in well trained athletes at rest and during heavy exercise. With special reference to stroke volume and the influence of body position, Acta Physiologica Scandinavica 57 (1963) 26–50. J.M. Fritsch-Yelle, et al., Microgravity decreases heart rate and arterial pressure in humans, Journal of Applied Physiology 80 (3) (1996) 910–914. G. Schaffner, et al., Effect of load levels of subject loading device on gait, ground reaction force, and kinematics during human treadmill locomotion in a weightless environment, TP2005-213169, NASA, 2005.
Physiological benefits of exercise in artificial gravity: A broadband countermeasure to space flight related deconditioning Jessica L. Edmonds∗ , Thomas Jarchow, Laurence R. Young Massachusetts Institute of Technology, USA Available online 18 April 2008
Abstract Current countermeasures to space flight related physiological deconditioning have not been sufficiently effective. We believe that a comprehensive countermeasure is the combination of intermittent centrifugation (artificial gravity) and exercise. We aim to test the long-term effectiveness of this combination in terms of fitness benefits. As a first-order determination of effectiveness, subjects participated in an eight-week exercise program. Three times per week, they exercised using a stair-stepper on a shortradius (2 m) centrifuge spinning at 30 RPM, maintaining a target heart rate that was systematically increased over the exercise period. During the sessions, foot forces and stepping cadence, heart rate, and perceived exertion were measured. Before and after the eight-week exercise program, measurements included: body fat percentage, bone mineral content, quadriceps extension strength, push-ups endurance, stepping cadence for a given heart rate, and maximum stepping endurance. We find that stairstepping on a centrifuge is safe and comfortable. Preliminary fitness results indicate that stair-stepping on a centrifuge may be effective in improving aerobic fitness, body composition, and strength. These results indicate that such a combination may also be effective as a countermeasure to space flight deconditioning. © 2007 Elsevier Ltd. All rights reserved.
1. Introduction The future of space exploration lies in the capability of astronauts to travel safely to the moon, Mars, and beyond. Without any countermeasures, long-duration space flight can result in a loss of 1.5% bone mineral density per month [1], up to 17% of muscle volume (seven-month period) [2], intolerance to orthostatic cardiovascular stress [3], and balance abnormalities [4]. Current countermeasures have not been sufficiently effective. Rather than simply treating the symptoms, it is possible to remove the cause of the deconditioning by placing the astronaut in a gravity-like environment. ∗ Corresponding author.
E-mail addresses: [email protected] (J.L. Edmonds), [email protected] (T. Jarchow), [email protected] (L.R. Young). 0094-5765/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2007.12.026
We believe that a comprehensive countermeasure is the combination of intermittent centrifugation (artificial gravity) and exercise. We aim to test the long-term effectiveness of this combination in terms of fitness benefits. Previous experiments have investigated the use of an exercise device on a centrifuge for use as a possible space flight countermeasure. Experimenters have implemented cycle ergometry [5–10] and squats [11,12], on short-radius centrifuges. These experiments have investigated cardiovascular and aerobic parameters during exercise [5–8,10,13,14], musculoskeletal challenges [11,12,15], and biomechanical properties of exercise, especially with respect to the Coriolis environment [11]. In bedrest studies, cycle ergometry on a centrifuge maintains some cardiovascular and muscular parameters, versus no-countermeasure groups
J.L. Edmonds et al. / Acta Astronautica 63 (2008) 2 – 7
3
[8,13–15]. To complement these findings, this study aims to demonstrate general effectiveness of stairstepping on a centrifuge for healthy subjects. Subjects participated in an eight-week exercise program on a short-radius (2 m) centrifuge spinning at 30 RPM. A suite of measurements before and after this exercise period indicates the fitness benefits of stairstepping on a centrifuge. 2. Methods Nine male subjects took part in an exercise program on a short-radius centrifuge, exercising three times per week. The centrifuge spins at rates of up to 30 RPM, corresponding to approximately 1.6–1.8-Gz at the feet, depending on the height of the subject. During centrifugation, the subject lay supine on the centrifuge, with his head placed at the center of rotation, and his feet on the exercise device in the radial direction. Adjustments were made to facilitate comfortable exercise: the footplate could be moved toward or away from the subject’s body, and shoulder pads could also be adjusted in this way (subjects pushed against these shoulder pads while exercising). The exercise equipment included a Kettler Vario䉸 mini-stepper (Redditch, Worcs, UK), which was mounted vertically on a surface at the subject’s feet [16]. The stepper uses hydraulic dampers as resistance to stepping. When the subject steps, he has a limited range of motion, and usually bottoms out at the stoppers. For this reason the subject experiences an impact load on his foot during each step. The stepper was instrumented with force plates (modified digital bathroom scales). Additionally, resistive arm bands (PowerSystems Premium Versa-Tube䉸 , Knoxville, TN) were mounted to the surface of the centrifuge, allowing for arm exercise. Subjects could choose among four resistance levels of these bands, and used them in varying motions. The centrifuge with all exercise equipment is shown in Fig. 1. Volunteers with low aerobic fitness were chosen as test subjects because they were more likely to improve in a short period of time. Therefore, they were selected if they had not been part of a regular exercise program for at least 1 year, and if their estimated V˙O2 -max did not exceed 42.4 mL/min/kg [17]. Estimated V˙O2 -max was determined by a graded treadmill test, during which the subject wore a VO2000䉸 face mask unit (MedGraphics, St. Paul, MN), which measured respiratory parameters. The test was terminated when subjects reached a heart rate of 200-Age [18]. The estimated V˙O2 -max was extrapolated from the data that were obtained,
Fig. 1. Exercise equipment on the short-radius centrifuge.
using a nomogram that allows for estimation of maximal oxygen uptake from submaximal oxygen uptake values [19]. All selected subjects reported that they had no general health or orthopedic problems, and were tolerant to motion sickness. Before participating, they read and signed an informed consent that was approved by the MIT Committee on the Use of Humans as Experimental Subjects. After the determination of eligibility, subjects underwent the following evaluations, which were then repeated after four and eight weeks of the exercise program: 1. An upright stepping test included a warm-up, 2 min of constant cadence exercise, 2 min of constant heart rate exercise, 2 min of maximal stair-stepping, and 4 min of rest. Measures included heart rate (Acumen TZ-max 100䉸 , Sterling, VA), beat-tobeat blood pressure (PortaPres䉸 model 2.0 unit, Finapres Medical Systems, Amsterdam, The Netherlands), foot forces (modified digital bathroom scales mounted to each stepper foothold), and respiratory parameters including oxygen uptake, ventilation, and breathing rate (VO2000䉸 face mask unit). 2. A quadriceps extension test required the subject to sit in a chair that was fixed to a frame, while facing a wall. The stair-stepper with footholds was held vertically against the wall. The subject sat in the chair, which was covered with a rubber mat to prevent slippage. The subject placed the ball of his right foot against one force plate of the stairstepper, such that the angle of his knee was slightly greater than 90◦ . Without pushing against the back of the chair, he pushed his leg against the stepper as hard as possible, slowly extending to maximum
4
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contraction over a 5 s period. The test was repeated two more times after brief rest periods, always with the right leg. The force measured by the force plate of the stepper was taken as a measure of leg strength. 3. A push-ups test to failure. The subject was instructed to perform as many full-body push-ups as he could without resting, and the number of push-ups was counted. 4. A dual energy X-ray absorptiometry (DEXA) scan (QDR 4500䉸 , Hologic Inc., Bedford, MA) was used to measure body composition: fat, lean, and bone mineral content of the whole body, as well as selected individual regions. 5. A Sharpened Romberg test [20] was used to measure balance. Subjects stood heel-to-toe with their hands crossed above their chest, closed their eyes, and attempted to hold that position without faltering for 60 s. If the first attempt was not successful, the subject repeated this test two more times. Following the initial test battery, subjects attended exercise sessions three times per week. Before all sessions, subjects’ sitting heart rate and blood pressure were taken, as well as body weight. Subjects were helped onto the centrifuge and necessary adjustments were made to the shoulder pads and footplate distance. A portable DVD player was mounted above the subject’s face with his choice of a movie, and he was allowed to wear earphones to hear the movie (verbal communication was still possible with the operator, who was in the same room during the sessions). Additionally, subjects were instrumented with an Acumen heart rate monitor, and were able to monitor their heart rate using the wristwatch receiver mounted above their face; the average heart rate during the exercise session was also recorded. For the subject’s comfort, the room was completely dark during the exercise session, apart from the movie screen and the heart rate display. The centrifuge was spun at 30 RPM, creating 1–Gz per meter radius. The direction of centrifugation (clockwise or counterclockwise) was alternated for each session to balance the lateral displacement effect of Coriolis accelerations on the knees and hips. Subjects warmed up for 3 min by stepping at a speed of their choice. They were then instructed to increase their heart rate to the target level, and maintain that level. Exercise sessions lasted 20, 30, or 40 min, increasing as the subject gained experience. The target heart rate was determined as 40–55% heart rate reserve (HRR1 [21]), depending on the exercise session. The subject 1 Target %×(maximum heart rate−resting heart rate)+resting heart rate.
could modulate his heart rate by stepping faster or slower, or using higher resistance arm bands. Target heart rate assignment were based on the subjects’ rating of perceived exertion in each session [22]; the experimenter attempted to assign a heart rate corresponding to a Borg rating of “somewhat hard” to “hard”. Over the eight weeks, the target heart rate was increased while maintaining a desirable Borg rating. Aside from heart rate and Borg rating, the peak foot force attained during each step was extracted from foot force data over the whole session. A value for the average of these peak foot forces was obtained per session. All statistical analysis used repeated measures analysis under a general linear model; thus, each subject acted as his own control. One of the nine subjects completed only four weeks, and is excluded from statistical tests but shown in Figs. 4–7 for comparison. 3. Results Subjects reported no discomfort, other than exercise fatigue, when exercising on the spinning centrifuge. One subject terminated the exercise program after four weeks due to headaches between sessions. None of the other subjects reported any motion sickness. There was no distinguishable change in resting (sitting) heart rate or blood pressure, or body weight, over the test sessions. During the exercise sessions, subjects were able to maintain their heart rate within 4% of the target on average, while maintaining an average rating of perceived exertion of 12.9 (between 11=“fairly light” and 13=“somewhat hard”). See Fig. 2. Peak foot forces during exercise, averaged over all sessions, were 45–124% of body weight, depending on the subject; the overall average among subjects was 83% body weight (Fig. 3). During the upright stepping test, when stepping at a constant heart rate (the same heart rate, 50% HRR, at zero, four, and eight weeks), cadence increased significantly between weeks four and eight (Fig. 4). In a maximal upright stepping test, in which the subject attempted to step as many times as possible in 2 min, all subjects increased the number of steps they did on average by 20% after four weeks and 43% after eight weeks. The maximum heart rate and maximum minute ventilation achieved during this endurance test significantly increased over eight weeks (17% (p = 0.01) and 23% (p = 0.024), respectively) (Fig. 5). Overall percent body fat (as determined by a DEXA scan) decreased by an average of 0.7% after eight weeks, but these changes were not significant; changes in the
J.L. Edmonds et al. / Acta Astronautica 63 (2008) 2 – 7
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Fig. 4. Stepping cadence increased significantly while holding a constant heart rate, in the last four weeks of training.
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Fig. 3. Peak stepping forces for each subject (average over all exercise sessions), expressed as a percentage of body weight. Inset. Typical force profile when stepping (one foot shown).
legs (1.0% and 1.1% in the left and right legs, respectively), approached or reached significance after four or eight weeks (Fig. 6). Additionally, pelvic bone mineral content increased significantly by 2.4% on average. In a test of maximal quadriceps extension strength, the average force exerted by the subjects increased by
Fig. 5. Maximum number of steps the subject was able to do in an upright two-minute endurance test.
15.2% after eight weeks of training, but these results were not significant. In a timed test to measure the number of full-body push-ups, subjects increased the number of push-ups they were able to do by an average of 18.2% after eight weeks (Fig. 7). A Sharpened Romberg balance test revealed no changes in balance after four or eight weeks of exercise.
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50 p = 0.094, right leg (solid lines) p = 0.013, left leg (dashed lines)
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4. Discussion We find minor fitness improvements in eight out of nine subjects after an eight week exercise program on the centrifuge. There is evidence in these results of exercise specificity [18]: the upright stair-stepping test generated the most obvious fitness changes after four weeks, since subjects were tested using the same type of exercise as they used to train. Specifically, when stepping at the same heart rate, subjects were apparently
able to perform the exercise with considerably less effort after eight weeks (significantly higher cadence for the same heart rate). The subject who terminated the exercise program after four weeks due to headaches between sessions was probably not sufficiently evaluated prior to his selection for the study. Unlike the other subjects, he did not enjoy being on the centrifuge, and strongly disliked exercising beyond 20 min. Nonetheless, motion sickness symptoms will be carefully monitored for future subjects. Some unusual artifacts of artificial gravity exercise should be noted. Subjects reported that exercising in the dark felt somewhat unnatural; however, they became accustomed to it after 3–4 sessions in general. Some subjects also subjectively reported that increasing their heart rate to the target level was more difficult on the centrifuge than it was when stepping upright. This is probably due to the fact that, although they were in an artificial gravity environment, they were still lying down, resulting in a higher stroke volume [23] but, presumably, a lower base heart rate. This does not necessarily confound our study, since in general heart rate is lower in microgravity than it is on Earth [24]. Although it was not explicitly recorded here, a simple experiment could be used to test this subjective report: compare heart rate levels at certain Borg ratings of perceived exertion, both upright and when exercising on the centrifuge. Practically, using the resistive arm bands helped subjects to modulate their heart rate when on the centrifuge. Target heart rate can only be used loosely in exercise prescription [21]; the trainer must be attentive to individual differences. For this reason, we deviated significantly from the generalized heart rate assignments in order to maintain the desired rating of perceived exertion. For example, sessions 4, 5, and 6 were loosely designated 45% HRR sessions. One subject reported a rating of “somewhat hard” during these sessions, and so the experimenter asked him to maintain this heart rate value. Another subject, by session 4, had his target heart rate increased to 55% HRR, due to earlier ratings of “fairly light” at lower target heart rates. The absence of some effects should also be discussed. During the sessions and throughout all sessions, subjects reported no knee or hip pain due to Coriolis accelerations. This may or may not be due to spinning the centrifuge in alternate directions on alternate days, and this negative finding is in agreement with previous studies [11]. Subjects did not experience diminished balance as measured by the Sharpened Romberg test. Although a gross measure of balance, we may be reasonably assured that stair-stepping on a centrifuge three times per week does not seriously impair the subject’s balance in
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a non-spinning Earth-gravity environment, nor does it impair his normal 1-g locomotion. As a space flight countermeasure, stair-stepping on a centrifuge has the desirable quality of producing relatively high foot forces. Peak impact foot forces during the exercise sessions (up to 124% body weight) are on the order of weightless treadmill running using bungee tie-downs (∼ 130% body weight peak impact forces, tested in parabolic flight [25]). These forces could be increased by using higher centrifuge velocities, or loading the subject further using bungees. The motion of stair-stepping also more closely mimics that of walking than, for example, cycling does. Locomotion training may be important during a long-duration space flight, particularly en route to a planetary surface where walking will be necessary during EVA. Ideally, the exercise device would also include a forward stroke; perhaps it would be possible to implement some type of treadmill device on the centrifuge. Our results indicate that stair-stepping on a centrifuge is likely to improve aerobic fitness, body composition, and strength in a male population of healthy subjects. As such, stair-stepping on a centrifuge may also serve to prevent deconditioning of the same parameters during long-duration space flight. Acknowledgments This work was supported by a NASA GSRP fellowship (NNX06AH21H), as well as the NASA International Multidisciplinary Artificial Gravity project (NNJ04HD64G). The authors are grateful to Dr. William Paloski for his support of this project, and to Esther Hu for her line drawing of the centrifuge. References [1] T. Lang, et al., Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight, Journal of Bone and Mineral Research 19 (6) (2004) 1006–1012. [2] A. LeBlanc, et al., Muscle volume, MRI relaxation times (T2), and body composition after spaceflight, Journal of Applied Physiology 89 (6) (2000) 2158–2164. [3] J.C. Buckey Jr., et al., Orthostatic intolerance after spaceflight, Journal of Applied Physiology 81 (1) (1996) 7–18. [4] J.C.J. Buckey, Space Physiology, Oxford University Press, New York, 2006. [5] V.J. Caiozzo, et al., Hemodynamic and metabolic responses to hypergravity on a human-powered centrifuge, Aviation Space and Environmental Medicine 75 (2) (2004) 101–108. [6] J.E. Greenleaf, et al., Short-arm (1.9 m) + 2.2 Gz acceleration: isotonic exercise load-O2 uptake relationship, Aviation Space and Environmental Medicine 70 (12) (1999) 1173–1182. [7] S. Iwase, et al., Effects of simultaneous load of centrifuge-induced artificial gravity and ergometer exercise
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