- Email: [email protected]
Effects of vibration training in reducing risk of slip-related falls among young adults with obesity
Journal of Biomechanics xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
Effects of vibration training in reducing risk of slip-related falls among young adults with obesity Feng Yang a,⇑, Jose Munoz b, Long-zhu Han c, Fei Yang d a
Department of Kinesiology and Health, Georgia State University, Atlanta, GA 30302, USA Department of Kinesiology, University of Texas at El Paso, El Paso, TX 79968, USA c School of Biological Science and Medical Engineering, Beihang University, Beijing 100191, China d Department of Radiation Oncology, University of Miami School of Medicine, Miami, FL 33136, USA b
a r t i c l e
i n f o
Article history: Accepted 31 March 2017 Available online xxxx Keywords: Fall prevention Dynamic stability Obesity Vibration Randomized controlled trial
a b s t r a c t This study examined the effects of controlled whole-body vibration training on reducing risk of sliprelated falls in people with obesity. Twenty-three young adults with obesity were randomly assigned into either the vibration or placebo group. The vibration and placebo groups respectively received 6-week vibration and placebo training on a side-alternating vibration platform. Before and after the training, the isometric knee extensors strength capacity was measured for the two groups. Both groups were also exposed to a standardized slip induced by a treadmill during gait prior to and following the training. Dynamic stability and fall incidences responding to the slip were also assessed. The results indicated that vibration training significantly increased the muscle strength and improved dynamic stability control at recovery touchdown after the slip occurrence. The improved dynamic stability could be resulted from the enhanced trunk segment movement control, which may be attributable to the strength increment caused by the vibration training. The decline of the fall rates from the pre-training slip to the post-training one was greater among the vibration group than the placebo group (45% vs. 25%). Vibration-based training could be a promising alternative or additional modality to active exercise-based fall prevention programs for people with obesity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Falls present a pressing challenge among elderly (Corso et al., 2006). Slip-related falls frequently lead to hip fractures, limitation of mobility, and fear of falling (Stevens et al., 2006). Obesity has been associated with an increased postural instability (Fjeldstad et al., 2008) and reduced lower limb muscle strength relative to body mass (Lafortuna et al., 2005). Both instability and muscle weakness are related to falls (Ding and Yang, 2016; Rogers et al., 2003). Therefore, obesity heightens the fall risk among adults (Himes and Reynolds, 2012). It has been reported that obesity could increase 19 times the likelihood of falls initiated by a standardized slip among young adults after removing the confounding effects from body height and gender (Yang et al., 2017). Obese individuals also suffer a high fall-related injuries rate (Finkelstein et al., 2007). It is imperative to develop effective fall prevention programs towards this population. ⇑ Corresponding author at: Department of Kinesiology and Health, Georgia State University, PO Box 3975, Atlanta, GA 30302, USA. Fax: +1 404 413 8053. E-mail address: [email protected] (F. Yang).
Recently, controlled whole-body vibration (CWBV) training has emerged to reduce fall risk among elderly (Yang et al., 2015) and individuals with movement disorders (Sanudo et al., 2013; Yang et al., 2016). During CWBV training, trainees stand on a vibration platform that creates sinusoidal vibrations. The transmission of vibrations to the human body stimulates the primary endings of the muscle spindles, which in turn activates a-motor neurons and results in involuntary muscle contractions. CWBV also increases the synchronization of motor units (Cardinale and Bosco, 2003) and the efficiency of agonist/antagonist pairs (Cardinale and Bosco, 2003; Kossev et al., 2001). The increased synchronization of motor units signifies that more muscle fibers are contracted at once and hence more force can be produced. The enhanced muscular performance could improve the risk factors of fall, which has been shown among older adults (Yang et al., 2015). Given the inherent features of CWBV training, such as portability, safety, and effectiveness, CWBV could be an alternative to traditional exercise-based training. Previous studies suggested that CWBV training increases the lower limb muscle strength among people with obesity (Milanese et al., 2013). Given that fall risk factors are not equal
http://dx.doi.org/10.1016/j.jbiomech.2017.03.024 0021-9290/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
2
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
to real-life falls, improved muscle strength (as a fall risk factor) does not necessarily represent a lowered risk of actual falls. Therefore, the effect of CWBV training on reducing real-life falls, particularly slip-related falls, still remains unanswered. An ideal platform to test the effectiveness of CWBV training on reducing slip-related falls in people with obesity would be to expose them to a well-controlled slip perturbation in a laboratory environment. Dynamic gait stability has been identified as a key risk factor of slip-related falls (Yang et al., 2009). Based on a theoretical framework (the feasible stability region theory) (Hof et al., 2005; Yang et al., 2008), dynamic gait stability is characterized by the relationship between the body’s center of mass (COM) motion state and the analytically-derived stability limit (Yang et al., 2009). See Online Supplement for details. The examination of how CWBV training affects dynamic stability control could elucidate the mechanisms through which the training alters the risk of slip-related falls in obese. Lower limb muscle strength, particularly the knee joint strength, is critical to prevent a slip-related fall. Specifically, after the loss balance following a slip, it is paramount to take a quick and successful recovery step to reestablish the base of support (BOS) and to provide sufficient extensor moment to avoid the limb collapse (Cham and Redfern, 2001; Ding and Yang, 2016; Pai et al., 2006). To determine how vibration training modifies knee extensors moment could also provide insights into the mechanisms of vibration training reducing risk of falls in people with obesity. The purposes of this study were (1) to examine if a 6-week CWBV training course could improve the knee extensors muscle strength, and (2) to inspect whether the rate of falls and dynamic gait stability in response to a standardized slip induced on a treadmill during gait would be changed by CWBV training program among young people with obesity. Participants with obesity would be randomly assigned to either the vibration or placebo group. Before and after the training, muscle strength, responses to the slip would be evaluated for both groups. We hypothesized that (1) the training group would demonstrate increased lower-limb muscle strength than the placebo group after the training and (2) that vibration training would lower the rate of slip-related falls and improve dynamic stability responding to the post-training slip.
2. Methods 2.1. Participants Only those who were obese were enrolled. For a male (female) participant, his (her) body mass index must be at least 30 kg/m2 and the body fat percentage should be no less than 25% (35%) (Wu and Madigan, 2014). Participants must also be free of any musculoskeletal disorders, neurological disorders, orthopedic conditions, and cardiovascular conditions. Twenty-eight young adults were initially screened. Twenty-three of them were qualified and randomized into two groups (vibration vs. placebo, Table 1). All participants gave their written consent for participation in the experiment approved by the Institutional Review Board. Six more participants were excluded from the study due to incomplete training (n = 3) or no valid slip trial during post-training test (n = 3) (Fig. 1). Nine from the vibration group and eight from placebo group were included in the final analysis. Of the remaining 17 participants, all successfully completed the intervention with a compliance rate of 100% (number of sessions = 18). None of the participants reported any major discomfort or adverse effect during the training. Itching of legs
(n = 3), which are typical for vibration intervention, were reported in the vibration group. These effects were mild and diminished after approximately 3–5 training sessions.
2.2. Study design This study adopted a two-arm, randomized controlled design (Figs. 1 and 2a). The vibration and placebo groups respectively received a 6-week vibration and placebo training on a vibration platform. Before and after the training, muscle strength, fall rate and dynamic stability during a treadmill-induced slip were assessed. Participants had no knowledge about their group assignment and the investigators who performed the two evaluations were also blinded to participants’ group assignment.
2.3. Evaluation of risk factors of slip-related falls 2.3.1. Muscle strength Strength capacity of the right knee extensors was assessed via an isokinetic dynamometer (Biodex, NY). While they were seated in the dynamometer chair, participants performed maximal voluntary isometric contractions of knee extensors three times with the knee joint flexed at 35°. The contractions lasted seven seconds each and were separated by a 2-min rest interval. The peak torque was recorded during each of the three repetitions. The average value of the three peaks across the three repetitions, normalized to body mass (Nm/kg), was calculated to represent the knee extensors strength capacity.
2.3.2. Slip perturbation Following the muscle performance assessment, all subjects took a 10-min break. After five overground walking trials on a 14-m walkway, all subjects stepped on a regular treadmill over which each participant’s comfortable walking speed was determined. They also walked five minutes to get acquainted with treadmill walking. They were then moved to the ActiveStep treadmill (Simbex, NH) and wore a safety harness attached to an overhead arch through ropes (Fig. 2b). A load cell connecting to the ropes measured the force exerted on it at 600 Hz. Participants were instructed that ‘‘the following trials will be normal walking ones without any perturbation.” After walking 3–5 times at their self-selected gait speed as determined above, they walked three times at the speed of 1.2 m/s. They were then told that ‘‘from the next trial on, you may or may not experience a simulated slip in each trial. If a slip happens, try to recover your balance and to continue walking.” Following the instructions, participants walked two trials at the speed of 1.2 m/ s without perturbation on the treadmill. They were then exposed to the slip perturbation. After 10–12 regular steps in the slip trial, approximately 80–120 ms later than the touchdown of the leading foot, without participants’ knowledge, the belt suddenly accelerated forward 1.2 m/s within 0.2 s, which induced a forward displacement of the subjects’ BOS relative to their COM, creating an unexpected slip perturbation (Yang et al., 2013). The perturbation level was the same for all subjects (Fig. 2c). Full-body kinematics from 26 retro-reflective markers placed on the participants’ body were gathered using an 8-camera motion capture system (Vicon, UK) which was synchronized with the load cell measurement.
2.4. Vibration and placebo training A side-alternating vibration platform (Galileo, German) was used in this study (Fig. 2d). Participants stood over clearly-marked positions on the platform during training (Fig. 2d). They were required to maintain stance on the platform with knees at 20° flexion and the trunk held upright (Mikhael et al., 2010). The vibration frequency and amplitude were 25 Hz and 3.9 mm, respectively. Each training session was comprised of five repetitions of 1-min vibration exposure followed by a 1-min rest (Fig. 2a). The same training was repeated three times a week over six weeks for a total of 18 training sessions (Fig. 2a). The vibration group experienced the vibration training while standing on the platform. The placebo group stood on the same platform which did not vibrate (Fig. 2a). Other than the vibration, participants in both groups followed the identical procedures.
Table 1 Comparisons of the demographic information (in mean ± standard deviation) between vibration and placebo groups. Groups
Age (years)
Gender (female)
Height (m)
Mass (kg)
BMI (kg/m2)
Body fat (%)
Vibration (n = 12) Placebo (n = 11)
26.00 ± 7.29 23.72 ± 2.97
4 4
1.72 ± 0.08 1.69 ± 0.11
102.1 ± 9.4 103.7 ± 24.1
34.44 ± 1.94 35.85 ± 5.36
36.58 ± 6.66 38.16 ± 5.26
p value
0.375
0.879a
0.487
0.828
0.403
0.537
BMI: body mass index. a Fisher’s exact test was used.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
3
Fig. 1. Flowchart of the number of the participants in various stages throughout the study. Out of 28 participants initially recruited, 5 participants did not continue due to not meeting the inclusion criteria. The remaining 23 were randomly assigned to one of the two groups: vibration (n = 12) or placebo training groups (n = 11). After the pretraining evaluation, all participants started the 6-week training program (either the vibration or placebo training). Two from the vibration group and one from the placebo group did not finish the training course. In addition, one participant in the vibration group did not have a valid treadmill-slip during the post-training evaluation because of technical error. Two participants in the placebo group did not take part in the post-training evaluation due to scheduling conflict. Eventually, nine and eight participants were included in the final analysis respectively for the vibration and placebo group.
Fig. 2. Schematics of (a) the experimental protocol of the present study, (b) the treadmill used to produce slip perturbation during gait, (c) the profile of the treadmill slip perturbation, and (d) the side-alternating vibration platform used in the study. After the randomization, both vibration and placebo groups underwent the pre-training evaluation and then started their training. The evaluation consisted of the assessment of muscle strength of the knee extensors and the response to a treadmill (TM)-induced slip during gait. After the completion of the 18 training sessions, both groups were evaluated again for the strength and slip-related falls. The slip trial began with a 2-s ramp up, followed by a steady state with a backward-moving belt speed of 1.2 m/s. After 10–12 regular steps on the slip trial, approximately 80–120 ms later than the touchdown of the leading foot, the top belt was suddenly accelerated to 1.2 m/s forward within 200 ms without the participants’ knowledge. Following the slip perturbation, the belt speed slowly returned to backward direction at 1.2 m/s. Subjects were protected by a full-body safety harness during all trials on the treadmill. Full-body kinematics were collected by a motion capture system from 26 reflective markers affixed to subjects’ body. The perturbation level was characterized by 24-cm slip distance, 2.4-m/s peak slip velocity, and 12-m/s2 slip acceleration. Vibration was delivered in an intermittent way: five repetitions of 1-min vibration followed by a 1-min rest. The same training was repeated three times per week for six weeks, leading to a total of 18 training sessions. There was a minimum 24-h rest period between two consecutive vibration sessions. During training, participants held handlebars for balance and looked directly ahead while standing barefoot on the vibration platform over clearly marked foot positions corresponding to the vibration amplitude. The foot position was closely monitored to avoid any skidding from the desired positions. They were also required to stand on the platform with 20° knee flexion but with an upright trunk to allow the transmission of vibration to the lower limbs but not the spinal cord or brain. They were instructed to try to distribute their body weight evenly over the forefoot and hindfoot bilaterally.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
4
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
2.5. Data reduction The peak load cell force was used to determine the outcome (fall or recovery) of each slip. A slip trial was classified as a fall if the peak load cell force exceeded 30% of body weight (bw) (Yang and Pai, 2011). A recovery occurred when the moving average of the load cell force on the harness did not exceed 4.5% bw over any 1-s period after the slip onset (Yang and Pai, 2011). Marker paths were low-pass filtered at marker-specific cut-off frequencies (ranging from 4.5 to 9 Hz) using fourth-order, zero-lag Butterworth filters (Winter, 2009). The body COM kinematics were computed using genderdependent segmental inertial parameters (de Leva, 1996). The two components of the COM motion state, i.e. its position and velocity, were calculated relative to the rear of the BOS (i.e. the leading heel) and normalized by foot length (lBOS) and pffiffiffiffiffiffiffiffiffiffiffiffiffiffi g bh, respectively, where g is the gravitational acceleration and bh the body height. Dynamic stability was calculated as the shortest distance from the COM motion state to the limit of stability (Yang et al., 2009). Trunk angle was calculated between the trunk segment and a vertical axis. Positive trunk angle represents that the trunk leans backward from the vertical line. The COM motion state, dynamic gait stability and trunk angle were calculated at the instant of recovery foot touchdown after the slip occurrence, which was identified based on foot kinematics.
2.6. Statistical analysis The slip outcome (fall or 1 vs. recovery or 0) was analyzed by using the Generalized Estimating Equation (GEE) with the session (pre-training vs. post-training) being the within-subject factor and group (vibration vs. placebo) being the between-subject factor. Post-hoc McNemar and v2 tests were respectively used to compare the slip outcome within and between groups. Continuous variables, including the knee strength capacity, COM position, COM velocity, dynamic stability, and trunk angle at recovery foot touchdown, were checked for normality first. Variables that were not normally distributed would be log transformed. Then, analyses of variance (ANOVA) with repeated measures were used to analyze these variables with the same within- and between-subject factors. The within and between groups comparisons of these measurements were conducted by post-hoc paired ttest and independent t-test, respectively. For both GEE and ANOVA analyses, the group by session interaction effects were also investigated. All statistical analyses were performed using SPSS 21.0 (IBM, NY) and a significance level of 0.05 was implemented throughout.
3. Results No significant main but group by session interaction effect (p = 0.033, Table 2 and Fig. 3a) was observed for the knee extensors strength. Specifically, the vibration group produced significantly greater strength than the placebo group post training (p = 0.035). The vibration group demonstrated significantly improved knee extensors strength from the pre-training to post-training assessment (p = 0.008). When exposed to an identical slip on the treadmill, the fall rate did not show any significant group-related main or group by session interaction effect (Table 2 and Fig. 3b). It differed significantly between sessions (p = 0.015). Post-hoc test indicated that the fall rate was marginally lower during the post-training than the pretraining session among the vibration group (p = 0.067). The COM position at recovery foot touchdown showed significant main effects of group (p = 0.021) and session (p = 0.004), but not the interaction (Table 2 and Fig. 4a). Post training, the vibration group placed their COM significantly closer to the BOS than did the pla-
Fig. 3. Group mean (column height) and standard deviation (bar) of (a) the maximum voluntary strength (or torque) of the knee extensors, (b) the fall rate in response to the identical slip induced on a treadmill, and (c) the trunk angle at recovery foot touchdown after slip onset between vibration (n = 9) and placebo (n = 8) groups at pre- and post-training evaluations. The strength measurement was normalized to body mass. The fall rate was calculated as the ratio of the number of fallers to the total number of subjects within each group. Trunk angle was calculated between the trunk segment and a vertical axis. Positive trunk angle represents that the trunk leans backward against the vertical line.
cebo group (p = 0.036). In comparison with the pre-training evaluation, the COM in the vibration, but not placebo group, was more anterior during the post-training session (p = 0.003). The COM velocity did not display any significant difference associated with either of the main factors or their interaction (Fig. 4b). Similar to the COM position, dynamic stability exhibited significant main effects (p = 0.030 for group and p = 0.006 for session) but not interaction effects (Table 2 and Fig. 4c). Post-hoc analyses revealed that
Table 2 Summary of p values for all statistical analyses. Analysis of variance with repeated measures and then independent or paired t-tests were employed to analyze the continuous measurements including the muscle strength (the knee extensors), center of mass (COM) position, velocity, and stability, and the trunk angle at the recovery foot touchdown. For binary outcome (i.e., the fall incidences), the Generalized Estimating Equation and subsequently v2 or McNemar tests were used. The p values less than 0.05 are bold. Variable
Knee extensor Falls COM position COM velocity Stability Trunk angle
Group
0.075 0.287 0.021 0.334 0.030 0.056
Session
0.315 0.015 0.004 0.368 0.006 0.063
Interaction
0.033 0.497 0.176 0.320 0.174 0.423
Vibration vs. Placebo
Pre-training vs. Post-training
Pre-training
Post-training
Vibration
Placebo
0.188 0.707 0.221 0.957 0.329 0.588
0.035 0.231 0.036 0.228 0.046 0.020
0.008 0.067 0.003 0.236 0.009 0.067
0.208 0.309 0.264 0.938 0.281 0.454
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
Fig. 4. Comparisons of (a) the center of mass (COM) position, (b) COM velocity, and (c) dynamic gait stability at recovery foot touchdown after the occurrence of the slip for the vibration and placebo groups at pre- and post-training sessions. Both the COM position and velocity were relative to the rear edge of the base of support pffiffiffiffiffiffiffiffiffiffiffiffiffiffi (BOS) and respectively normalized by foot length (lBOS) and g bh, where g represents the gravitational acceleration and bh the body height. Stability is calculated as the shortest distance from the given COM motion state (the combination of its position and velocity) to the computer-predicted boundary against backward balance loss (Yang et al., 2008).
the vibration group was less instable than the placebo group during the post-training slip test (p = 0.046). Further, the stability was significantly improved from pre-training to the post-training test only within the vibration group (p = 0.009). The trunk angle at the recovery foot touchdown showed a marginal main effect related to group (p = 0.056, Table 2, Fig. 3c). The session-associated main effect or the interaction effect was not significant for the trunk angle. During the post-training session, the vibration group showed a significantly smaller backward leaning trunk compared with their placebo counterparts (p = 0.020).
4. Discussion This study sought to investigate the effect of a 6-week CWBV training program in strengthening knee extensors and reducing the risk of slip-related falls among young obese people. The results revealed that the training increased the strength capacity of knee extensors. The training also significantly improved dynamic stability and marginally reduced the rate of falls in response to the unannounced slip. The results supported our first hypothesis. In detail, the isometric strength capacity of the knee extensors significantly increased from the pre-training to post-training test (8.2%) among the vibration group (p < 0.01) while the same strength measurement
5
did not change significantly between the two evaluations in the placebo group (p > 0.05, Fig. 3a). At post-training evaluation, the vibration group could produce greater strength at the knee extensors than the placebo group (p < 0.05) whereas the strength capacity was comparable between groups at pre-training test (p > 0.05, Fig. 3a). Our finding is consistent with previous ones. For example, a 10-week vibration training strengthened the knee extensors by about 14.2% among 28 women with obesity (Milanese et al., 2013). Although the training protocols differ between studies, the similar and positive changes in the leg strength were detected in both studies. A more conservative and less intensive vibration training protocol like the one used in our study still benefits individuals with obesity. This reinforces the notion that CWBV training could be an encouraging modality to improve muscle strength among people with obesity. Although the underlying mechanism(s) involved in strengthening leg muscles are not fully elucidated, it was suggested that CWBV training can possibly cause neurological adaptations or facilitate neural control within muscles (Bogaerts et al., 2007), which contribute to the observed increase in muscle strength (Cardinal and Wakeling, 2005). Individuals with obesity generate lower relative muscle strength to the body mass than their lean counterparts (Maffiuletti et al., 2007). An explanation to the smaller relative muscle strength in people with obesity could be due to the higher rate of fat (Maffiuletti et al., 2007). The fat tissue produces little force but accounts for the total body mass, leading to the small relative muscle strength in obese. Previous studies postulated that a long-term vibration training has the potential to increase the muscle mass (Bogaerts et al., 2007) and reduce the body fat mass in obese individuals (Milanese et al., 2013). In this connection, vibration training could moderate the deficit of the relative muscle strength among people with obesity. A past study uncovered the link between muscle weakness and slip-related falls among older adults (Ding and Yang, 2016). Intuitively, to regain body balance and prevent an actual fall after a slip, one must generate quickly sufficient corrective reactions during the recovery stepping (Cham and Redfern, 2001; Yang et al., 2009). One key reactive response is the extensor moment from the recovery leg providing sufficient anti-gravity support to prevent a limb collapse (Cham and Redfern, 2001; Ding and Yang, 2016). If one can generate great muscle strength, the chance to retard and even reverse the falling after the slip would increase, reducing the probability of falling. It has been reported that a unit decrease in knee extensors strength capacity would increases the odds of slip-related fall by a factor of 3.23 in older adults (Ding and Yang, 2016). In this study, the knee extensors strength in the vibration, but not the placebo group, was significantly increased. Therefore, individuals in the vibration group could produce more knee extensors moment to resist a fall after the slip. Our second hypothesis was partially supported by the results. Dynamic stability, but not the fall rate, in responding to the slip was improved owning to the training. Specifically, dynamic stability was improved from the pre-training to post-training test for the vibration group (Fig. 4c). Such an improvement was not observed among the placebo group. Further, participants in the vibration group were less instable during the slip than the placebo group after the training. In this study, participants in the vibration group primarily adopted the modification of COM position to improve dynamic gait stability. At recovery touchdown, participants in the vibration group placed their COM more anteriorly and closer to the BOS during the post-training test in comparison with the pre-training one. The placebo group did not exhibit such changes in the COM position between evaluations. During the posttraining test, the COM was placed closer to the BOS among the vibration group than in the placebo. A more forward positioned COM requires less forward momentum to enable the COM to catch
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
6
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
the BOS, thus reducing the possibility of backward falling (Yang et al., 2008). Therefore, the vibration group was less instable than the placebo group during the post-training test although the COM velocity relative to the BOS was close between evaluations and groups. A major contributor to the more anteriorly-located COM in the vibration group than in the placebo group during the post-training test could be the less backward-leaning trunk segment after the slip occurrence. At recovery touchdown, the vibration group inclined their trunk backward approximately 3.5° less than the placebo group (Fig. 3c). Less backward leaning of the trunk shifted the COM forward (Espy et al., 2010) given that the HAT (head, arm, and trunk) segment represents about two thirds of the total body mass. The improvement in the HAT segment control could be contributed to the strengthened muscle among the vibration group. It was reported that the knee joint strength can be used to represent the overall muscle strength among adults (Bohannon, 2008). Although the present study did not directly measure the muscle strength around the hip joints, the increase in the knee joint strength could imply the increase in the hip joint muscles, which may play an important role in controlling the trunk segment’s movement. The fall rate demonstrated a significant session-related main effect (p = 0.015). The slip-related fall rate for both groups dropped from the pre-training to the post-training test (67%–22% and 75%– 50% respectively for vibration and placebo group) (Fig. 3b). Such a reduction could also be due to the exposure to the slip perturbation during the pre-training test (Marigold and Patla, 2002). It has been suggested that human central nervous system (CNS) adapts recovery responses to perturbations which are based primarily on prior experience with the perturbation and could rapidly occur within one slip trial (Marigold and Patla, 2002). In current study, participants experienced an identical slip perturbation during both the pre- and post-training sessions. They may acquire some knowledge of the slip perturbation during the pre-training session. The preexisting knowledge might have facilitated the CNS to develop the recovery strategy responding to the post-training slip to a certain degree. This may explain why the control group did not show strength increase but improved control in dynamic stability upon the post-training slip. The rate of falls in response to the identical slip did not show significant group-related main or group by session interaction effects. First, the small sample size may count for this. A power analysis based on the GEE analysis using the effect size derived from the present study indicated that at least 34 participants per group are necessary to reach a significant level of 0.05 with a power of 0.80. Second, the slip outcome is a binary measurement (1 vs. 0). It is not as sensitive as a continuous measurement, like dynamic stability, to detect changes resulting from vibration training. The results about fall rate seem not supporting our second hypothesis. Nevertheless, the fact that the decline in the fall rate within the vibration group (45%, p = 0.067) was greater than the one for the placebo group (25%, p = 0.309) could support the second hypothesis in part (Fig. 3b). This study had limitations. First, only young individuals were enrolled in this study and the falls were produced in a laboratory environment. The external validity of this intervention in terms of its generalizability to other age groups (like older adults with obesity) and other settings (like the everyday-living condition) remains unexplored. Second, only knee extensors strength was considered. It is unknown if the training could induce any meaningful improvement in other lower limb joints, which could also play roles in counteracting a slip-related fall. Third, neither the ground reaction force nor the electromyograph of the knee extensors were recorded due to the technical difficulty in this study. The timing or sequence of knee extensors activation could not be
examined, restricting our ability to further explore the mechanisms of vibration training in improving muscular performance. All issues warrant further investigation. In summary, a 6-week CWBV training could improve knee joint strength capacity and thus enhance dynamic stability control during a slip among young individuals with obesity. The results also showed a trend that the vibration group exhibited a greater decline in the fall rate than the placebo group between tests. Although active exercise is still considered the first option for persons with obesity to reduce their risk of falls, it is conceivable that a future intervention strategy may also include a vibration-based component to boost the resistance to postural disturbances among people with obesity. Conflict of interest The authors declare no conflict of interest. Acknowledgements This work was funded in part by a research grant (2014-070) from The Retirement Research Foundation (FY). The authors thank JaeEun Kim for assistance in data collection and processing and Christina Carrera for editing. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jbiomech.2017. 03.024. References Bogaerts, A., Delecluse, C., Claessens, A.L., Coudyzer, W., Boonen, S., Verschueren, M. P., 2007. Impact of whole-body vibration training versus fitness training on muscle strength and muscle mass in older men: a 1-year randomized controlled trial. J. Gerontol. A Biol. Sci. Med. Sci. 62A, 630–635. Bohannon, R.W., 2008. Is it legitimate to characterize muscle strength using a limited number of measures? J. Strength Cond. Res. 22, 166–173. Cardinal, M., Wakeling, J., 2005. Whole body vibration exercise: are vibrations good for you? Br. J. Sports Med. 39, 585–589. Cardinale, M., Bosco, C., 2003. The use of vibration as an exercise intervention. Exerc. Sport Sci. Rev. 31, 3–7. Cham, R., Redfern, M.S., 2001. Lower extremity corrective reactions to slip events. J. Biomech. 34, 1439–1445. Corso, P.S., Finkelstein, E.A., Miller, T.R., Fiebelkorn, I., Zaloshnja, E., 2006. Incidence and lifetime costs of injuries in the United States. Inj. Prev. 12, 212–218. de Leva, P., 1996. Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters. J. Biomech. 29, 1223–1230. Ding, L., Yang, F., 2016. Muscle weakness is related to slip-initiated falls among community-dwelling older adults. J. Biomech. 49, 238–243. Espy, D.D., Yang, F., Bhatt, T., Pai, Y.-C., 2010. Independent influence of gait speed and step length on stability and fall risk. Gait Posture 32, 378–382. Finkelstein, E.A., Chen, H., Prabhu, M., Trogdon, J.G., Corso, P.S., 2007. The relationship between obesity and injuries among U.S. adults. Am. J. Health Promot. 21, 460–468. Fjeldstad, C., Fjeldstad, A.S., Acree, L.S., Nickel, K.J., Gardner, A.W., 2008. The influence of obesity on falls and quality of life. Dyn. Med. 7, 1–6. Himes, C.L., Reynolds, S.L., 2012. Effect of obesity on falls, injury, and disability. J. Am. Geriatr. Soc. 60, 124–129. Hof, A.L., Gazendam, M.G., Sinke, W.E., 2005. The condition for dynamic stability. J. Biomech. 38, 1–8. Kossev, A., Siggelkow, S., Kapels, H., Dengles, R., Rollnik, J.D., 2001. Crossed effects of muscle vibration on motor-evoked potentials. Clin. Neurophysiol. 112, 453– 456. Lafortuna, C.L., Maffiuletti, N.A., Agosti, F., Sartorio, A., 2005. Gender variations of body composition, muscle strength and power output in morbid obesity. Int. J. Obes. 29, 833–841. Maffiuletti, N.A., Jubeau, M., Munzinger, U., Bizzini, M., Agosti, F., de Col, A., Lafortuna, C.L., Sartorio, A., 2007. Differences in quadriceps muscle strength and fatigue between lean and obese subjects. Eur. J. Appl. Physiol. 101, 51–59. Marigold, D.S., Patla, A.E., 2002. Strategies for dynamic stability during locomotion on a slippery surface: effects of prior experience and knowledge. J. Neurophysiol. 88, 339–353.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx Mikhael, M., Orr, R., Amsen, F., Greene, D., Singh, M.A.F., 2010. Effect of standing posture during whole body vibration training on muscle morphology and function in older adults: a randomised controlled trial. BioMed Central Geriatr. 10, 1–13. Milanese, C., Poiscitelli, F., Zenti, M.G., Moghetti, P., Sandri, M., Zancanaro, C., 2013. Ten-week whole-body vibration training improves body composition and muscle strength in obese women. Int. J. Med. Sci. 10, 307–311. Pai, Y.-C., Yang, F., Wening, J.D., Pavol, M.J., 2006. Mechanisms of limb collapse following a slip among young and older adults. J. Biomech. 39, 2194– 2204. Rogers, M.E., Rogers, N.L., Takeshima, N., Islam, M.M., 2003. Methods to assess and improve the physical parameters associated with fall risk in older adults. Prev. Med. 36, 255–264. Sanudo, B., Carrasco, L., de Hoyo, M., Oliva-Pascual-Vaca, A., Rodriguez-Blanco, C., 2013. Changes in body balance and functional performance following wholebody vibration training in patients with fibromyalgia syndrome: a randomized controlled trial. J. Rehabil. Med. 45, 678–684. Stevens, J.A., Corso, P.S., Finkelstein, E.A., Miller, T.R., 2006. The costs of fatal and non-fatal falls among older adults. Inj. Prev. 12, 290–295. Winter, D.A., 2009. Biomechanics and Motor Control of Human Movement. Wiley, Hoboken, NJ.
7
Wu, X.-F., Madigan, M.L., 2014. Impaired plantar sensitivity among the obese is associated with increased postural sway. Neurosci. Lett. 583, 49–54. Yang, F., Anderson, F.C., Pai, Y.-C., 2008. Predicted threshold against backward balance loss following a slip in gait. J. Biomech. 41, 1823–1831. Yang, F., Bhatt, T., Pai, Y.-C., 2009. Role of stability and limb support in recovery against a fall following a novel slip induced in different daily activities. J. Biomech. 42, 1903–1908. Yang, F., Bhatt, T., Pai, Y.-C., 2013. Generalization of treadmill-slip training to prevent a fall following a sudden (novel) slip in over-ground walking. J. Biomech. 46, 63–69. Yang, F., Estrada, E., Sanchez, M.C., 2016. Vibration training improves disability status in multiple sclerosis: a pretest-posttest pilot study. J. Neurol. Sci. 369, 96–101. Yang, F., Kim, J., Yang, F., 2017. Effects of obesity on dynamic stability control during recovery from a treadmill-induced slip among young adults. J. Biomech. 53, 148–153. Yang, F., King, G.A., Dillon, L., Su, X.-G., 2015. Controlled whole-body vibration training reduces risk of falls among community-dwelling older adults. J. Biomech. 48, 3206–3212. Yang, F., Pai, Y.-C., 2011. Automatic recognition of falls in gait-slip training: harness load cell based criteria. J. Biomech. 44, 2243–2249.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
Effects of vibration training in reducing risk of slip-related falls among young adults with obesity Feng Yang a,⇑, Jose Munoz b, Long-zhu Han c, Fei Yang d a
Department of Kinesiology and Health, Georgia State University, Atlanta, GA 30302, USA Department of Kinesiology, University of Texas at El Paso, El Paso, TX 79968, USA c School of Biological Science and Medical Engineering, Beihang University, Beijing 100191, China d Department of Radiation Oncology, University of Miami School of Medicine, Miami, FL 33136, USA b
a r t i c l e
i n f o
Article history: Accepted 31 March 2017 Available online xxxx Keywords: Fall prevention Dynamic stability Obesity Vibration Randomized controlled trial
a b s t r a c t This study examined the effects of controlled whole-body vibration training on reducing risk of sliprelated falls in people with obesity. Twenty-three young adults with obesity were randomly assigned into either the vibration or placebo group. The vibration and placebo groups respectively received 6-week vibration and placebo training on a side-alternating vibration platform. Before and after the training, the isometric knee extensors strength capacity was measured for the two groups. Both groups were also exposed to a standardized slip induced by a treadmill during gait prior to and following the training. Dynamic stability and fall incidences responding to the slip were also assessed. The results indicated that vibration training significantly increased the muscle strength and improved dynamic stability control at recovery touchdown after the slip occurrence. The improved dynamic stability could be resulted from the enhanced trunk segment movement control, which may be attributable to the strength increment caused by the vibration training. The decline of the fall rates from the pre-training slip to the post-training one was greater among the vibration group than the placebo group (45% vs. 25%). Vibration-based training could be a promising alternative or additional modality to active exercise-based fall prevention programs for people with obesity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Falls present a pressing challenge among elderly (Corso et al., 2006). Slip-related falls frequently lead to hip fractures, limitation of mobility, and fear of falling (Stevens et al., 2006). Obesity has been associated with an increased postural instability (Fjeldstad et al., 2008) and reduced lower limb muscle strength relative to body mass (Lafortuna et al., 2005). Both instability and muscle weakness are related to falls (Ding and Yang, 2016; Rogers et al., 2003). Therefore, obesity heightens the fall risk among adults (Himes and Reynolds, 2012). It has been reported that obesity could increase 19 times the likelihood of falls initiated by a standardized slip among young adults after removing the confounding effects from body height and gender (Yang et al., 2017). Obese individuals also suffer a high fall-related injuries rate (Finkelstein et al., 2007). It is imperative to develop effective fall prevention programs towards this population. ⇑ Corresponding author at: Department of Kinesiology and Health, Georgia State University, PO Box 3975, Atlanta, GA 30302, USA. Fax: +1 404 413 8053. E-mail address: [email protected] (F. Yang).
Recently, controlled whole-body vibration (CWBV) training has emerged to reduce fall risk among elderly (Yang et al., 2015) and individuals with movement disorders (Sanudo et al., 2013; Yang et al., 2016). During CWBV training, trainees stand on a vibration platform that creates sinusoidal vibrations. The transmission of vibrations to the human body stimulates the primary endings of the muscle spindles, which in turn activates a-motor neurons and results in involuntary muscle contractions. CWBV also increases the synchronization of motor units (Cardinale and Bosco, 2003) and the efficiency of agonist/antagonist pairs (Cardinale and Bosco, 2003; Kossev et al., 2001). The increased synchronization of motor units signifies that more muscle fibers are contracted at once and hence more force can be produced. The enhanced muscular performance could improve the risk factors of fall, which has been shown among older adults (Yang et al., 2015). Given the inherent features of CWBV training, such as portability, safety, and effectiveness, CWBV could be an alternative to traditional exercise-based training. Previous studies suggested that CWBV training increases the lower limb muscle strength among people with obesity (Milanese et al., 2013). Given that fall risk factors are not equal
http://dx.doi.org/10.1016/j.jbiomech.2017.03.024 0021-9290/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
2
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
to real-life falls, improved muscle strength (as a fall risk factor) does not necessarily represent a lowered risk of actual falls. Therefore, the effect of CWBV training on reducing real-life falls, particularly slip-related falls, still remains unanswered. An ideal platform to test the effectiveness of CWBV training on reducing slip-related falls in people with obesity would be to expose them to a well-controlled slip perturbation in a laboratory environment. Dynamic gait stability has been identified as a key risk factor of slip-related falls (Yang et al., 2009). Based on a theoretical framework (the feasible stability region theory) (Hof et al., 2005; Yang et al., 2008), dynamic gait stability is characterized by the relationship between the body’s center of mass (COM) motion state and the analytically-derived stability limit (Yang et al., 2009). See Online Supplement for details. The examination of how CWBV training affects dynamic stability control could elucidate the mechanisms through which the training alters the risk of slip-related falls in obese. Lower limb muscle strength, particularly the knee joint strength, is critical to prevent a slip-related fall. Specifically, after the loss balance following a slip, it is paramount to take a quick and successful recovery step to reestablish the base of support (BOS) and to provide sufficient extensor moment to avoid the limb collapse (Cham and Redfern, 2001; Ding and Yang, 2016; Pai et al., 2006). To determine how vibration training modifies knee extensors moment could also provide insights into the mechanisms of vibration training reducing risk of falls in people with obesity. The purposes of this study were (1) to examine if a 6-week CWBV training course could improve the knee extensors muscle strength, and (2) to inspect whether the rate of falls and dynamic gait stability in response to a standardized slip induced on a treadmill during gait would be changed by CWBV training program among young people with obesity. Participants with obesity would be randomly assigned to either the vibration or placebo group. Before and after the training, muscle strength, responses to the slip would be evaluated for both groups. We hypothesized that (1) the training group would demonstrate increased lower-limb muscle strength than the placebo group after the training and (2) that vibration training would lower the rate of slip-related falls and improve dynamic stability responding to the post-training slip.
2. Methods 2.1. Participants Only those who were obese were enrolled. For a male (female) participant, his (her) body mass index must be at least 30 kg/m2 and the body fat percentage should be no less than 25% (35%) (Wu and Madigan, 2014). Participants must also be free of any musculoskeletal disorders, neurological disorders, orthopedic conditions, and cardiovascular conditions. Twenty-eight young adults were initially screened. Twenty-three of them were qualified and randomized into two groups (vibration vs. placebo, Table 1). All participants gave their written consent for participation in the experiment approved by the Institutional Review Board. Six more participants were excluded from the study due to incomplete training (n = 3) or no valid slip trial during post-training test (n = 3) (Fig. 1). Nine from the vibration group and eight from placebo group were included in the final analysis. Of the remaining 17 participants, all successfully completed the intervention with a compliance rate of 100% (number of sessions = 18). None of the participants reported any major discomfort or adverse effect during the training. Itching of legs
(n = 3), which are typical for vibration intervention, were reported in the vibration group. These effects were mild and diminished after approximately 3–5 training sessions.
2.2. Study design This study adopted a two-arm, randomized controlled design (Figs. 1 and 2a). The vibration and placebo groups respectively received a 6-week vibration and placebo training on a vibration platform. Before and after the training, muscle strength, fall rate and dynamic stability during a treadmill-induced slip were assessed. Participants had no knowledge about their group assignment and the investigators who performed the two evaluations were also blinded to participants’ group assignment.
2.3. Evaluation of risk factors of slip-related falls 2.3.1. Muscle strength Strength capacity of the right knee extensors was assessed via an isokinetic dynamometer (Biodex, NY). While they were seated in the dynamometer chair, participants performed maximal voluntary isometric contractions of knee extensors three times with the knee joint flexed at 35°. The contractions lasted seven seconds each and were separated by a 2-min rest interval. The peak torque was recorded during each of the three repetitions. The average value of the three peaks across the three repetitions, normalized to body mass (Nm/kg), was calculated to represent the knee extensors strength capacity.
2.3.2. Slip perturbation Following the muscle performance assessment, all subjects took a 10-min break. After five overground walking trials on a 14-m walkway, all subjects stepped on a regular treadmill over which each participant’s comfortable walking speed was determined. They also walked five minutes to get acquainted with treadmill walking. They were then moved to the ActiveStep treadmill (Simbex, NH) and wore a safety harness attached to an overhead arch through ropes (Fig. 2b). A load cell connecting to the ropes measured the force exerted on it at 600 Hz. Participants were instructed that ‘‘the following trials will be normal walking ones without any perturbation.” After walking 3–5 times at their self-selected gait speed as determined above, they walked three times at the speed of 1.2 m/s. They were then told that ‘‘from the next trial on, you may or may not experience a simulated slip in each trial. If a slip happens, try to recover your balance and to continue walking.” Following the instructions, participants walked two trials at the speed of 1.2 m/ s without perturbation on the treadmill. They were then exposed to the slip perturbation. After 10–12 regular steps in the slip trial, approximately 80–120 ms later than the touchdown of the leading foot, without participants’ knowledge, the belt suddenly accelerated forward 1.2 m/s within 0.2 s, which induced a forward displacement of the subjects’ BOS relative to their COM, creating an unexpected slip perturbation (Yang et al., 2013). The perturbation level was the same for all subjects (Fig. 2c). Full-body kinematics from 26 retro-reflective markers placed on the participants’ body were gathered using an 8-camera motion capture system (Vicon, UK) which was synchronized with the load cell measurement.
2.4. Vibration and placebo training A side-alternating vibration platform (Galileo, German) was used in this study (Fig. 2d). Participants stood over clearly-marked positions on the platform during training (Fig. 2d). They were required to maintain stance on the platform with knees at 20° flexion and the trunk held upright (Mikhael et al., 2010). The vibration frequency and amplitude were 25 Hz and 3.9 mm, respectively. Each training session was comprised of five repetitions of 1-min vibration exposure followed by a 1-min rest (Fig. 2a). The same training was repeated three times a week over six weeks for a total of 18 training sessions (Fig. 2a). The vibration group experienced the vibration training while standing on the platform. The placebo group stood on the same platform which did not vibrate (Fig. 2a). Other than the vibration, participants in both groups followed the identical procedures.
Table 1 Comparisons of the demographic information (in mean ± standard deviation) between vibration and placebo groups. Groups
Age (years)
Gender (female)
Height (m)
Mass (kg)
BMI (kg/m2)
Body fat (%)
Vibration (n = 12) Placebo (n = 11)
26.00 ± 7.29 23.72 ± 2.97
4 4
1.72 ± 0.08 1.69 ± 0.11
102.1 ± 9.4 103.7 ± 24.1
34.44 ± 1.94 35.85 ± 5.36
36.58 ± 6.66 38.16 ± 5.26
p value
0.375
0.879a
0.487
0.828
0.403
0.537
BMI: body mass index. a Fisher’s exact test was used.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
3
Fig. 1. Flowchart of the number of the participants in various stages throughout the study. Out of 28 participants initially recruited, 5 participants did not continue due to not meeting the inclusion criteria. The remaining 23 were randomly assigned to one of the two groups: vibration (n = 12) or placebo training groups (n = 11). After the pretraining evaluation, all participants started the 6-week training program (either the vibration or placebo training). Two from the vibration group and one from the placebo group did not finish the training course. In addition, one participant in the vibration group did not have a valid treadmill-slip during the post-training evaluation because of technical error. Two participants in the placebo group did not take part in the post-training evaluation due to scheduling conflict. Eventually, nine and eight participants were included in the final analysis respectively for the vibration and placebo group.
Fig. 2. Schematics of (a) the experimental protocol of the present study, (b) the treadmill used to produce slip perturbation during gait, (c) the profile of the treadmill slip perturbation, and (d) the side-alternating vibration platform used in the study. After the randomization, both vibration and placebo groups underwent the pre-training evaluation and then started their training. The evaluation consisted of the assessment of muscle strength of the knee extensors and the response to a treadmill (TM)-induced slip during gait. After the completion of the 18 training sessions, both groups were evaluated again for the strength and slip-related falls. The slip trial began with a 2-s ramp up, followed by a steady state with a backward-moving belt speed of 1.2 m/s. After 10–12 regular steps on the slip trial, approximately 80–120 ms later than the touchdown of the leading foot, the top belt was suddenly accelerated to 1.2 m/s forward within 200 ms without the participants’ knowledge. Following the slip perturbation, the belt speed slowly returned to backward direction at 1.2 m/s. Subjects were protected by a full-body safety harness during all trials on the treadmill. Full-body kinematics were collected by a motion capture system from 26 reflective markers affixed to subjects’ body. The perturbation level was characterized by 24-cm slip distance, 2.4-m/s peak slip velocity, and 12-m/s2 slip acceleration. Vibration was delivered in an intermittent way: five repetitions of 1-min vibration followed by a 1-min rest. The same training was repeated three times per week for six weeks, leading to a total of 18 training sessions. There was a minimum 24-h rest period between two consecutive vibration sessions. During training, participants held handlebars for balance and looked directly ahead while standing barefoot on the vibration platform over clearly marked foot positions corresponding to the vibration amplitude. The foot position was closely monitored to avoid any skidding from the desired positions. They were also required to stand on the platform with 20° knee flexion but with an upright trunk to allow the transmission of vibration to the lower limbs but not the spinal cord or brain. They were instructed to try to distribute their body weight evenly over the forefoot and hindfoot bilaterally.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
4
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
2.5. Data reduction The peak load cell force was used to determine the outcome (fall or recovery) of each slip. A slip trial was classified as a fall if the peak load cell force exceeded 30% of body weight (bw) (Yang and Pai, 2011). A recovery occurred when the moving average of the load cell force on the harness did not exceed 4.5% bw over any 1-s period after the slip onset (Yang and Pai, 2011). Marker paths were low-pass filtered at marker-specific cut-off frequencies (ranging from 4.5 to 9 Hz) using fourth-order, zero-lag Butterworth filters (Winter, 2009). The body COM kinematics were computed using genderdependent segmental inertial parameters (de Leva, 1996). The two components of the COM motion state, i.e. its position and velocity, were calculated relative to the rear of the BOS (i.e. the leading heel) and normalized by foot length (lBOS) and pffiffiffiffiffiffiffiffiffiffiffiffiffiffi g bh, respectively, where g is the gravitational acceleration and bh the body height. Dynamic stability was calculated as the shortest distance from the COM motion state to the limit of stability (Yang et al., 2009). Trunk angle was calculated between the trunk segment and a vertical axis. Positive trunk angle represents that the trunk leans backward from the vertical line. The COM motion state, dynamic gait stability and trunk angle were calculated at the instant of recovery foot touchdown after the slip occurrence, which was identified based on foot kinematics.
2.6. Statistical analysis The slip outcome (fall or 1 vs. recovery or 0) was analyzed by using the Generalized Estimating Equation (GEE) with the session (pre-training vs. post-training) being the within-subject factor and group (vibration vs. placebo) being the between-subject factor. Post-hoc McNemar and v2 tests were respectively used to compare the slip outcome within and between groups. Continuous variables, including the knee strength capacity, COM position, COM velocity, dynamic stability, and trunk angle at recovery foot touchdown, were checked for normality first. Variables that were not normally distributed would be log transformed. Then, analyses of variance (ANOVA) with repeated measures were used to analyze these variables with the same within- and between-subject factors. The within and between groups comparisons of these measurements were conducted by post-hoc paired ttest and independent t-test, respectively. For both GEE and ANOVA analyses, the group by session interaction effects were also investigated. All statistical analyses were performed using SPSS 21.0 (IBM, NY) and a significance level of 0.05 was implemented throughout.
3. Results No significant main but group by session interaction effect (p = 0.033, Table 2 and Fig. 3a) was observed for the knee extensors strength. Specifically, the vibration group produced significantly greater strength than the placebo group post training (p = 0.035). The vibration group demonstrated significantly improved knee extensors strength from the pre-training to post-training assessment (p = 0.008). When exposed to an identical slip on the treadmill, the fall rate did not show any significant group-related main or group by session interaction effect (Table 2 and Fig. 3b). It differed significantly between sessions (p = 0.015). Post-hoc test indicated that the fall rate was marginally lower during the post-training than the pretraining session among the vibration group (p = 0.067). The COM position at recovery foot touchdown showed significant main effects of group (p = 0.021) and session (p = 0.004), but not the interaction (Table 2 and Fig. 4a). Post training, the vibration group placed their COM significantly closer to the BOS than did the pla-
Fig. 3. Group mean (column height) and standard deviation (bar) of (a) the maximum voluntary strength (or torque) of the knee extensors, (b) the fall rate in response to the identical slip induced on a treadmill, and (c) the trunk angle at recovery foot touchdown after slip onset between vibration (n = 9) and placebo (n = 8) groups at pre- and post-training evaluations. The strength measurement was normalized to body mass. The fall rate was calculated as the ratio of the number of fallers to the total number of subjects within each group. Trunk angle was calculated between the trunk segment and a vertical axis. Positive trunk angle represents that the trunk leans backward against the vertical line.
cebo group (p = 0.036). In comparison with the pre-training evaluation, the COM in the vibration, but not placebo group, was more anterior during the post-training session (p = 0.003). The COM velocity did not display any significant difference associated with either of the main factors or their interaction (Fig. 4b). Similar to the COM position, dynamic stability exhibited significant main effects (p = 0.030 for group and p = 0.006 for session) but not interaction effects (Table 2 and Fig. 4c). Post-hoc analyses revealed that
Table 2 Summary of p values for all statistical analyses. Analysis of variance with repeated measures and then independent or paired t-tests were employed to analyze the continuous measurements including the muscle strength (the knee extensors), center of mass (COM) position, velocity, and stability, and the trunk angle at the recovery foot touchdown. For binary outcome (i.e., the fall incidences), the Generalized Estimating Equation and subsequently v2 or McNemar tests were used. The p values less than 0.05 are bold. Variable
Knee extensor Falls COM position COM velocity Stability Trunk angle
Group
0.075 0.287 0.021 0.334 0.030 0.056
Session
0.315 0.015 0.004 0.368 0.006 0.063
Interaction
0.033 0.497 0.176 0.320 0.174 0.423
Vibration vs. Placebo
Pre-training vs. Post-training
Pre-training
Post-training
Vibration
Placebo
0.188 0.707 0.221 0.957 0.329 0.588
0.035 0.231 0.036 0.228 0.046 0.020
0.008 0.067 0.003 0.236 0.009 0.067
0.208 0.309 0.264 0.938 0.281 0.454
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
Fig. 4. Comparisons of (a) the center of mass (COM) position, (b) COM velocity, and (c) dynamic gait stability at recovery foot touchdown after the occurrence of the slip for the vibration and placebo groups at pre- and post-training sessions. Both the COM position and velocity were relative to the rear edge of the base of support pffiffiffiffiffiffiffiffiffiffiffiffiffiffi (BOS) and respectively normalized by foot length (lBOS) and g bh, where g represents the gravitational acceleration and bh the body height. Stability is calculated as the shortest distance from the given COM motion state (the combination of its position and velocity) to the computer-predicted boundary against backward balance loss (Yang et al., 2008).
the vibration group was less instable than the placebo group during the post-training slip test (p = 0.046). Further, the stability was significantly improved from pre-training to the post-training test only within the vibration group (p = 0.009). The trunk angle at the recovery foot touchdown showed a marginal main effect related to group (p = 0.056, Table 2, Fig. 3c). The session-associated main effect or the interaction effect was not significant for the trunk angle. During the post-training session, the vibration group showed a significantly smaller backward leaning trunk compared with their placebo counterparts (p = 0.020).
4. Discussion This study sought to investigate the effect of a 6-week CWBV training program in strengthening knee extensors and reducing the risk of slip-related falls among young obese people. The results revealed that the training increased the strength capacity of knee extensors. The training also significantly improved dynamic stability and marginally reduced the rate of falls in response to the unannounced slip. The results supported our first hypothesis. In detail, the isometric strength capacity of the knee extensors significantly increased from the pre-training to post-training test (8.2%) among the vibration group (p < 0.01) while the same strength measurement
5
did not change significantly between the two evaluations in the placebo group (p > 0.05, Fig. 3a). At post-training evaluation, the vibration group could produce greater strength at the knee extensors than the placebo group (p < 0.05) whereas the strength capacity was comparable between groups at pre-training test (p > 0.05, Fig. 3a). Our finding is consistent with previous ones. For example, a 10-week vibration training strengthened the knee extensors by about 14.2% among 28 women with obesity (Milanese et al., 2013). Although the training protocols differ between studies, the similar and positive changes in the leg strength were detected in both studies. A more conservative and less intensive vibration training protocol like the one used in our study still benefits individuals with obesity. This reinforces the notion that CWBV training could be an encouraging modality to improve muscle strength among people with obesity. Although the underlying mechanism(s) involved in strengthening leg muscles are not fully elucidated, it was suggested that CWBV training can possibly cause neurological adaptations or facilitate neural control within muscles (Bogaerts et al., 2007), which contribute to the observed increase in muscle strength (Cardinal and Wakeling, 2005). Individuals with obesity generate lower relative muscle strength to the body mass than their lean counterparts (Maffiuletti et al., 2007). An explanation to the smaller relative muscle strength in people with obesity could be due to the higher rate of fat (Maffiuletti et al., 2007). The fat tissue produces little force but accounts for the total body mass, leading to the small relative muscle strength in obese. Previous studies postulated that a long-term vibration training has the potential to increase the muscle mass (Bogaerts et al., 2007) and reduce the body fat mass in obese individuals (Milanese et al., 2013). In this connection, vibration training could moderate the deficit of the relative muscle strength among people with obesity. A past study uncovered the link between muscle weakness and slip-related falls among older adults (Ding and Yang, 2016). Intuitively, to regain body balance and prevent an actual fall after a slip, one must generate quickly sufficient corrective reactions during the recovery stepping (Cham and Redfern, 2001; Yang et al., 2009). One key reactive response is the extensor moment from the recovery leg providing sufficient anti-gravity support to prevent a limb collapse (Cham and Redfern, 2001; Ding and Yang, 2016). If one can generate great muscle strength, the chance to retard and even reverse the falling after the slip would increase, reducing the probability of falling. It has been reported that a unit decrease in knee extensors strength capacity would increases the odds of slip-related fall by a factor of 3.23 in older adults (Ding and Yang, 2016). In this study, the knee extensors strength in the vibration, but not the placebo group, was significantly increased. Therefore, individuals in the vibration group could produce more knee extensors moment to resist a fall after the slip. Our second hypothesis was partially supported by the results. Dynamic stability, but not the fall rate, in responding to the slip was improved owning to the training. Specifically, dynamic stability was improved from the pre-training to post-training test for the vibration group (Fig. 4c). Such an improvement was not observed among the placebo group. Further, participants in the vibration group were less instable during the slip than the placebo group after the training. In this study, participants in the vibration group primarily adopted the modification of COM position to improve dynamic gait stability. At recovery touchdown, participants in the vibration group placed their COM more anteriorly and closer to the BOS during the post-training test in comparison with the pre-training one. The placebo group did not exhibit such changes in the COM position between evaluations. During the posttraining test, the COM was placed closer to the BOS among the vibration group than in the placebo. A more forward positioned COM requires less forward momentum to enable the COM to catch
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
6
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx
the BOS, thus reducing the possibility of backward falling (Yang et al., 2008). Therefore, the vibration group was less instable than the placebo group during the post-training test although the COM velocity relative to the BOS was close between evaluations and groups. A major contributor to the more anteriorly-located COM in the vibration group than in the placebo group during the post-training test could be the less backward-leaning trunk segment after the slip occurrence. At recovery touchdown, the vibration group inclined their trunk backward approximately 3.5° less than the placebo group (Fig. 3c). Less backward leaning of the trunk shifted the COM forward (Espy et al., 2010) given that the HAT (head, arm, and trunk) segment represents about two thirds of the total body mass. The improvement in the HAT segment control could be contributed to the strengthened muscle among the vibration group. It was reported that the knee joint strength can be used to represent the overall muscle strength among adults (Bohannon, 2008). Although the present study did not directly measure the muscle strength around the hip joints, the increase in the knee joint strength could imply the increase in the hip joint muscles, which may play an important role in controlling the trunk segment’s movement. The fall rate demonstrated a significant session-related main effect (p = 0.015). The slip-related fall rate for both groups dropped from the pre-training to the post-training test (67%–22% and 75%– 50% respectively for vibration and placebo group) (Fig. 3b). Such a reduction could also be due to the exposure to the slip perturbation during the pre-training test (Marigold and Patla, 2002). It has been suggested that human central nervous system (CNS) adapts recovery responses to perturbations which are based primarily on prior experience with the perturbation and could rapidly occur within one slip trial (Marigold and Patla, 2002). In current study, participants experienced an identical slip perturbation during both the pre- and post-training sessions. They may acquire some knowledge of the slip perturbation during the pre-training session. The preexisting knowledge might have facilitated the CNS to develop the recovery strategy responding to the post-training slip to a certain degree. This may explain why the control group did not show strength increase but improved control in dynamic stability upon the post-training slip. The rate of falls in response to the identical slip did not show significant group-related main or group by session interaction effects. First, the small sample size may count for this. A power analysis based on the GEE analysis using the effect size derived from the present study indicated that at least 34 participants per group are necessary to reach a significant level of 0.05 with a power of 0.80. Second, the slip outcome is a binary measurement (1 vs. 0). It is not as sensitive as a continuous measurement, like dynamic stability, to detect changes resulting from vibration training. The results about fall rate seem not supporting our second hypothesis. Nevertheless, the fact that the decline in the fall rate within the vibration group (45%, p = 0.067) was greater than the one for the placebo group (25%, p = 0.309) could support the second hypothesis in part (Fig. 3b). This study had limitations. First, only young individuals were enrolled in this study and the falls were produced in a laboratory environment. The external validity of this intervention in terms of its generalizability to other age groups (like older adults with obesity) and other settings (like the everyday-living condition) remains unexplored. Second, only knee extensors strength was considered. It is unknown if the training could induce any meaningful improvement in other lower limb joints, which could also play roles in counteracting a slip-related fall. Third, neither the ground reaction force nor the electromyograph of the knee extensors were recorded due to the technical difficulty in this study. The timing or sequence of knee extensors activation could not be
examined, restricting our ability to further explore the mechanisms of vibration training in improving muscular performance. All issues warrant further investigation. In summary, a 6-week CWBV training could improve knee joint strength capacity and thus enhance dynamic stability control during a slip among young individuals with obesity. The results also showed a trend that the vibration group exhibited a greater decline in the fall rate than the placebo group between tests. Although active exercise is still considered the first option for persons with obesity to reduce their risk of falls, it is conceivable that a future intervention strategy may also include a vibration-based component to boost the resistance to postural disturbances among people with obesity. Conflict of interest The authors declare no conflict of interest. Acknowledgements This work was funded in part by a research grant (2014-070) from The Retirement Research Foundation (FY). The authors thank JaeEun Kim for assistance in data collection and processing and Christina Carrera for editing. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jbiomech.2017. 03.024. References Bogaerts, A., Delecluse, C., Claessens, A.L., Coudyzer, W., Boonen, S., Verschueren, M. P., 2007. Impact of whole-body vibration training versus fitness training on muscle strength and muscle mass in older men: a 1-year randomized controlled trial. J. Gerontol. A Biol. Sci. Med. Sci. 62A, 630–635. Bohannon, R.W., 2008. Is it legitimate to characterize muscle strength using a limited number of measures? J. Strength Cond. Res. 22, 166–173. Cardinal, M., Wakeling, J., 2005. Whole body vibration exercise: are vibrations good for you? Br. J. Sports Med. 39, 585–589. Cardinale, M., Bosco, C., 2003. The use of vibration as an exercise intervention. Exerc. Sport Sci. Rev. 31, 3–7. Cham, R., Redfern, M.S., 2001. Lower extremity corrective reactions to slip events. J. Biomech. 34, 1439–1445. Corso, P.S., Finkelstein, E.A., Miller, T.R., Fiebelkorn, I., Zaloshnja, E., 2006. Incidence and lifetime costs of injuries in the United States. Inj. Prev. 12, 212–218. de Leva, P., 1996. Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters. J. Biomech. 29, 1223–1230. Ding, L., Yang, F., 2016. Muscle weakness is related to slip-initiated falls among community-dwelling older adults. J. Biomech. 49, 238–243. Espy, D.D., Yang, F., Bhatt, T., Pai, Y.-C., 2010. Independent influence of gait speed and step length on stability and fall risk. Gait Posture 32, 378–382. Finkelstein, E.A., Chen, H., Prabhu, M., Trogdon, J.G., Corso, P.S., 2007. The relationship between obesity and injuries among U.S. adults. Am. J. Health Promot. 21, 460–468. Fjeldstad, C., Fjeldstad, A.S., Acree, L.S., Nickel, K.J., Gardner, A.W., 2008. The influence of obesity on falls and quality of life. Dyn. Med. 7, 1–6. Himes, C.L., Reynolds, S.L., 2012. Effect of obesity on falls, injury, and disability. J. Am. Geriatr. Soc. 60, 124–129. Hof, A.L., Gazendam, M.G., Sinke, W.E., 2005. The condition for dynamic stability. J. Biomech. 38, 1–8. Kossev, A., Siggelkow, S., Kapels, H., Dengles, R., Rollnik, J.D., 2001. Crossed effects of muscle vibration on motor-evoked potentials. Clin. Neurophysiol. 112, 453– 456. Lafortuna, C.L., Maffiuletti, N.A., Agosti, F., Sartorio, A., 2005. Gender variations of body composition, muscle strength and power output in morbid obesity. Int. J. Obes. 29, 833–841. Maffiuletti, N.A., Jubeau, M., Munzinger, U., Bizzini, M., Agosti, F., de Col, A., Lafortuna, C.L., Sartorio, A., 2007. Differences in quadriceps muscle strength and fatigue between lean and obese subjects. Eur. J. Appl. Physiol. 101, 51–59. Marigold, D.S., Patla, A.E., 2002. Strategies for dynamic stability during locomotion on a slippery surface: effects of prior experience and knowledge. J. Neurophysiol. 88, 339–353.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024
F. Yang et al. / Journal of Biomechanics xxx (2017) xxx–xxx Mikhael, M., Orr, R., Amsen, F., Greene, D., Singh, M.A.F., 2010. Effect of standing posture during whole body vibration training on muscle morphology and function in older adults: a randomised controlled trial. BioMed Central Geriatr. 10, 1–13. Milanese, C., Poiscitelli, F., Zenti, M.G., Moghetti, P., Sandri, M., Zancanaro, C., 2013. Ten-week whole-body vibration training improves body composition and muscle strength in obese women. Int. J. Med. Sci. 10, 307–311. Pai, Y.-C., Yang, F., Wening, J.D., Pavol, M.J., 2006. Mechanisms of limb collapse following a slip among young and older adults. J. Biomech. 39, 2194– 2204. Rogers, M.E., Rogers, N.L., Takeshima, N., Islam, M.M., 2003. Methods to assess and improve the physical parameters associated with fall risk in older adults. Prev. Med. 36, 255–264. Sanudo, B., Carrasco, L., de Hoyo, M., Oliva-Pascual-Vaca, A., Rodriguez-Blanco, C., 2013. Changes in body balance and functional performance following wholebody vibration training in patients with fibromyalgia syndrome: a randomized controlled trial. J. Rehabil. Med. 45, 678–684. Stevens, J.A., Corso, P.S., Finkelstein, E.A., Miller, T.R., 2006. The costs of fatal and non-fatal falls among older adults. Inj. Prev. 12, 290–295. Winter, D.A., 2009. Biomechanics and Motor Control of Human Movement. Wiley, Hoboken, NJ.
7
Wu, X.-F., Madigan, M.L., 2014. Impaired plantar sensitivity among the obese is associated with increased postural sway. Neurosci. Lett. 583, 49–54. Yang, F., Anderson, F.C., Pai, Y.-C., 2008. Predicted threshold against backward balance loss following a slip in gait. J. Biomech. 41, 1823–1831. Yang, F., Bhatt, T., Pai, Y.-C., 2009. Role of stability and limb support in recovery against a fall following a novel slip induced in different daily activities. J. Biomech. 42, 1903–1908. Yang, F., Bhatt, T., Pai, Y.-C., 2013. Generalization of treadmill-slip training to prevent a fall following a sudden (novel) slip in over-ground walking. J. Biomech. 46, 63–69. Yang, F., Estrada, E., Sanchez, M.C., 2016. Vibration training improves disability status in multiple sclerosis: a pretest-posttest pilot study. J. Neurol. Sci. 369, 96–101. Yang, F., Kim, J., Yang, F., 2017. Effects of obesity on dynamic stability control during recovery from a treadmill-induced slip among young adults. J. Biomech. 53, 148–153. Yang, F., King, G.A., Dillon, L., Su, X.-G., 2015. Controlled whole-body vibration training reduces risk of falls among community-dwelling older adults. J. Biomech. 48, 3206–3212. Yang, F., Pai, Y.-C., 2011. Automatic recognition of falls in gait-slip training: harness load cell based criteria. J. Biomech. 44, 2243–2249.
Please cite this article in press as: Yang, F., et al. Effects of vibration training in reducing risk of slip-related falls among young adults with obesity. J. Biomech. (2017), http://dx.doi.org/10.1016/j.jbiomech.2017.03.024