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Whole-Body Vibration Has No Effect on Neuromotor Function and Falls in Chronic Stroke

LAU, RICKY W. K.1; YIP, SHEA PING2; PANG, MARCO Y. C.1

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Medicine & Science in Sports & Exercise: August 2012 - Volume 44 - Issue 8 - p 1409-1418
doi: 10.1249/MSS.0b013e31824e4f8c
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Abstract

Many individuals with chronic stroke continue to experience deficits in various aspects of neuromotor performance (e.g., reduced postural control, muscle power and mobility), which may contribute to an increased risk of falling (11). Much research has therefore been directed at investigating effective strategies to optimize neuromotor performance and reduce the risk of falls in this vulnerable population.

Whole-body vibration (WBV) therapy has gained increasing popularity in clinical practice in improving neuromotor performance in different patient populations. In this form of treatment, the individual is required to perform various static (i.e., sustaining a particular posture) or dynamic (i.e., performing certain body movements) exercises while standing on an oscillating platform that generates mechanical vibrations. The vibration signals activate the muscle spindles in the leg musculature, which in turn induces reflexive activation of the motor units (37). There is also some evidence that WBV can modulate the excitability of the spinal motor neuronal pool (25) and improve proprioception (14,50). Several studies have reported the positive effects of WBV therapy on balance performance (6,8), functional mobility (8,41), muscle strength (41,53,54), and the likelihood of falls (5,55) in older adults. Some preliminary research has also suggested the potential benefit of WBV in populations with different chronic diseases (36,44,50,51).

The promising results reported in older adult populations raised the possibility that stroke patients may also benefit from WBV therapy. To date, only two randomized controlled trials have investigated the effect of WBV after stroke. In a study on patients with subacute stroke (48), it demonstrated that the addition of WBV exercise training to conventional physiotherapy induced significantly greater improvement in paretic knee extensor muscle strength when compared with conventional physiotherapy alone after 4 wk of treatment. In contrast, van Nes et al. (52) showed that a WBV program and control treatment (exercises on music) both resulted in similar significant improvement in balance and mobility outcomes after 6 wk of training in patients with subacute stroke. In summary, research on WBV therapy in people with stroke is scarce, and the results are inconclusive. To date, no study has examined the effect of WBV exercise training on different aspects of neuromotor performance and the likelihood of falls in patients with chronic stroke, who are particularly susceptible to physical deconditioning and functional decline (33,56).

The aim of this randomized controlled study was to compare the effects of a combined WBV and dynamic exercise program with those of dynamic exercise alone on balance, mobility, leg muscle strength, fall-related self-efficacy, and the likelihood of falls in individuals with chronic stroke. The research hypotheses of this study were as follows: 1) the experimental group should have significantly better balance, mobility, leg muscle strength, and fall-related efficacy outcomes than the control group immediately after treatment and also at the 1-month follow-up; and 2) the experimental group should have a significantly lower rate of falls than the control group during the 6-month follow-up period.

METHODS

Study design

This was a single-blinded randomized controlled intervention trial, in which the assessors were blinded to the group allocation of the subjects. The study was registered at ClinicalTrials.gov (NCT00937339). The reporting of this trial followed the recommendations of the International Society of Musculoskeletal and Neuronal Interactions (40).

Subjects and calculation of sample size

Subjects were recruited from the stroke self-help groups in the community. The inclusion criteria were as follows: 1) has a diagnosis of hemispheric stroke, 2) the stroke onset was more than 6 months previously, 3) is medically stable, 4) scored 6 or higher in the Abbreviated Mental Test, 5) is able to stand independently with or without aids for at least 1.5 min, and 6) age ≥18 yr. The exclusion criteria were as follows: 1) has other neurological conditions, 2) has significant musculoskeletal conditions (e.g., amputation), 3) experiences pain that affected the performance of physical activities, 4) has metal implants or recent fractures in the lower extremity, 5) has vestibular disorders, 6) has peripheral vascular disease, 7) has other serious illnesses that precluded participation in the study (e.g., malignancy), and 8) is pregnant. All eligible subjects gave written informed consent before participating in the study. All the experimental procedures were conducted in accordance with the Declaration of Helsinki for human experiments. Ethical approval was obtained from the Human Subjects Ethics Subcommittee of the Hong Kong Polytechnic University.

A few previous studies of older adults have shown that WBV therapy can result in significant beneficial effects on balance and mobility measures (2,8,17), with reported effect sizes ranging from 0.4 to 1.4. Moreover, based on a sample of 20 subjects with stroke, Tihanyi et al. (48) showed that a 4-wk WBV program resulted in a significant increase in isometric knee muscle strength, with an effect size of 0.4 (small to medium effect size). In light of the overall available scientific evidence from the literature, a medium to large effect size of 0.7 was expected for this study. Based on an α value of 5%, power of 80%, and an attrition rate of 20%, the minimum sample size required for each treatment arm was 41.

Randomization

Eligible subjects were randomly allocated to either the experimental or control group in blocks of 20 using sealed opaque envelopes. To ensure concealed allocation, the procedures were performed by an “off-site” researcher who was not involved in other parts of the study.

Intervention

The subjects in the experimental group received WBV training three times a week for 8 wk. This training duration was chosen as previous studies in stroke patients have shown that it is possible to achieve gain in neuromotor performance (e.g., muscle strength, mobility) with WBV or other forms of exercise training within 4–10 wk (47,48). Additional sessions were arranged if subjects were unable to attend a particular session to ensure that all received 24 training treatments. Each session included 15 min of warm-up exercises (general mobilization and stretching) in the sitting position. Vertical (synchronous) WBV signals were delivered by the Jet-Vibe System (Danil SMC Co. Ltd., Seoul, South Korea). This device is capable of providing WBV with frequency ranging between 20 and 50 Hz. Each frequency is associated with a specific, unadjustable amplitude value. The vibration parameters were verified by using a triaxial accelerometer (40).

The WBV protocol is shown in Table 1. The training volume was systematically increased over the course of the training program (53). This was achieved by increasing the duration of each exercise (Table 1). The training intensity, indicated by the g force (g represents the Earth’s gravitational acceleration at 9.81 m·s−2), was also increased over time by increasing the frequency of the WBV signals, as the subjects’ strength and endurance improved (16,53).

TABLE 1
TABLE 1:
WBV and exercise protocol.

A frequency range of 20–30 Hz (with associated amplitude of 0.44–0.60 mm and peak acceleration of 9.5–15.8 m·s−2 and g force of 0.97g–1.61g) was used in this study after careful consideration. Vertical vibrations below 20 Hz were not adopted because substantial augmentation of the signals may occur owing to body resonances, which may in turn cause adverse effects such as damage to internal organ or dizziness (20,34). On the other hand, frequencies higher than 30 Hz were not used in this study for several reasons. First, a substantial proportion of subjects complained of discomfort in our pilot testing when a frequency range of 35–40 Hz was used. Second, higher frequencies would also lead to higher peak acceleration. Chronic stroke patients often have considerably compromised bone mass (19). A protocol with high peak acceleration may not be desirable for patients with a fragile skeleton. Third, there is some evidence that WBV at 20–30 Hz can produce a therapeutic effect in older adults (8,9,16,42) and subacute stroke patients (48) without significant adverse effects (28,30). Although Kiiski et al. (24) reported that substantial amplification of peak acceleration could occur in the ankle and knee at 20–30 Hz, their results could not be directly applied to our study because their data were collected when the subjects were in an erect standing position with knees only slightly bent, whereas our subjects engaged in dynamic activities while receiving WBV stimulation and the knees were in more flexion (Table 1). The different body postures assumed and movements performed would influence the transmission of vibration and hence potential amplification of peak acceleration (1,46). In summary, the current intervention protocol was adopted after considering the general health status and physical ability of our stroke patients and carefully weighing the potential risks and benefits.

The subjects were required to wear the same sports footwear for each session and were instructed to perform six different sets of exercises while standing on the vibration platform (Table 2). The exercises were dynamic in nature and were modified from the ones used in previous WBV trials in other populations (6,49). They were aimed to not only strengthen the major leg muscle groups (12,22) (especially exercise 2, 5, and 6 in Table 1), but also enhance the ability to weight-shift in different directions (e.g., exercise 1, 3, 4, and 6), which is essential in successful execution of various functional tasks such as walking and transfer. The movements involved in semisquat (exercise 2) and deep squat (exercise 5) are also key components of the sit-to-stand task, which is another important daily functional activity for stroke patients. Adding WBV to dynamic squatting exercises has been shown to increase the leg muscle activity (22). Verbal instructions and manual assistance were provided as necessary to ensure correct performance of the exercises.

TABLE 2
TABLE 2:
Subjects’ characteristics.

The subjects in the control group performed the same exercises while standing on the same platform but with no vibration. The exercise training for the WBV and control groups took place in the same exercise room in the university, but at different times of the day, so that the subjects in one group were unable to observe the training procedures of the other group. For each of the experimental and control groups, three to four subjects would participate in the same training session (1 h in duration). Each subject would take turns to perform the exercises on the platform one by one under the supervision of the therapist. Such an arrangement would also allow the opportunities for rest between each set of exercise and would also allow social interaction with other subjects. Based on our clinical observation, individuals with stroke enjoy the social interaction involved in this type of group exercise training, which is often an important motivating factor for them to participate in physical activity (37). The exercise instructions and progression pattern were exactly the same for both groups.

Outcome measurements

Demographic data and other relevant information (e.g., medical history) were obtained at baseline. The upper and lower limb impairment levels were assessed by Chedoke–McMaster stroke assessment (18). Subjects were also asked whether they had sustained any falls in the past 3 months. The following outcomes were measured at baseline, immediately after the 24 training sessions, and 1 month after the termination of training.

The 14-item Berg Balance Scale (BBS) was used to evaluate balance performance in functional activities that the subjects frequently encountered in everyday life (3). A systematic review demonstrated that BBS has good psychometric properties when used in the stroke population, with excellent internal consistency (Cronbach α = 0.92–0.98), interrater reliability (intraclass correlation coefficient (ICC) = 0.95–0.98), and intrarater reliability (ICC = 0.97) (4).

Dynamic postural control was assessed by the limit-of-stability (LOS) test using the SMART balance system, which is composed of moveable dual force plates that are connected to a computer system and a screen display located in front of the subject (NeuroCom, SMART EquiTest; NeuroCom International, Inc., Clackamas, OR) (10). The LOS test is designed to quantify an individual’s ability to move his or her center of pressure (COP) voluntarily by leaning in different directions without losing stability or stepping. Each subject was asked to stand on the force plates barefeet with the feet placed at specified positions. At the beginning of the test, the subjects were allowed to look at the screen in front. The screen display contained eight target boxes placed at the periphery (forward, backward, left, right, forward right, forward left, backward right, and backward left) and a center box. The cursor on the screen represented the COP, and the subject was required to maintain the COP cursor within the center box as the starting position. During the test, the subjects were instructed to move the COP cursor toward a highlighted target box as quickly and accurately as possible and then maintain the position as close to the target as possible. The subjects were given 8 s to complete each trial. The measured outcomes included the following: movement velocity (MVL; average speed of COP movement in degrees per second), end point excursions (EPE; distance traveled of the COP on the first attempt toward the target expressed as a percentage of maximum LOS distance), maximum excursion (MXE; maximum distance traveled of the COP during all attempts), and directional control (DCL; comparison of the amount of movement toward the target with that away from it) (10). The LOS has a high test–retest reliability (ICC = 0.78–0.91) and acceptable predictive validity for comprehensive activities of daily living function (r2 = 0.15–0.17) in stroke patients (10).

The 10-m walk test (10 MWT) was used to evaluate mobility (45). The subjects were instructed to walk using their self-selected walking speed with their usual ambulatory devices or orthoses along a 15-m walkway. The time required (s) to walk the middle 10 m of the 15-m walkway was recorded using a stopwatch. The 10 MWT has good reliability (ICC = 0.93) in stroke patients (13). The 10 MWT also has significant correlations with the Rivermead Mobility Index (r = −0.57), thus demonstrating good concurrent validity in stroke patients (45).

The 6-min walk test (6 MWT) was used to evaluate walking endurance (13,15). The subjects were asked to walk along a 15-m walkway and to cover as much distance as possible in 6 min, using their usual ambulatory devices or orthoses. The devices or orthoses used during the test were documented. The subjects were allowed to rest if absolutely necessary. The total distance (m) covered within the 6-min period was recorded. The test–retest reliability of the 6 MWT was high (ICC = 0.97–0.99) in stroke patients (13,15).

Isometric knee flexion and extension strength on the paretic side was measured at 70° knee flexion, using a Cybex dynamometer (Computer Sports Medicine, Inc., Stoughton, MA). The upper body was stabilized by placing a strap across the chest. The tested leg was stabilized by strapping the thigh to the seat. The lateral femoral condyle was aligned with the rotation axis of the dynamometer. The distal part of the tested lower leg (at about 5 cm proximal to the lateral malleoli) was strapped to the ankle cuff of the level arm. The other leg was comfortably fixed and stabilized on the chair. Each subject’s tested lower limb was weighted during the gravity correction procedures to correct for the effect of gravity on torque. To familiarize the subjects with the testing procedures, a practice trial was given before the actual recording. Three trials were performed, and the mean peak torque value (N·m·kg−1) obtained in each type of contraction was used as the outcome measure. Based on the data from our subjects’ baseline measurements, the test–retest reliability was found to be high for the various muscle strength outcomes (ICC3,1 = 0.923–0.926).

Fall-related self-efficacy was evaluated using the activities-specific balance confidence (ABC) scale (30). The subjects were asked to rate their level of confidence in performing each activity without losing their balance using a numerical rating scale from 0 to100, with a higher score indicating better balance confidence. The scores for each item were summed and then averaged to obtain the ABC score for each subject for subsequent analysis. The ABC scale has been found to be a reliable and valid tool for measuring self-perceived balance confidence in patients with stroke (7). The validated Chinese version was used for this study (30).

Subjects were given a logbook to record any occurrence and circumstances of falls, and the data were collected during a monthly telephone interview until 6 months after the termination of training.

Statistical analysis

Independent t-tests (for continuous variables), Mann–Whitney U tests (for ordinal variables), and the χ2 test (for nominal variables) were used to compare the baseline characteristics of the experimental and control groups. In our primary analysis, intention-to-treat analysis was performed to assess the effect of the experimental treatment. For subjects who withdrew from the study, the baseline assessment results were carried over to the follow-up assessments. For balance, mobility, strength, and fall-related self-efficacy measures, ANOVA with mixed design (within-subject factor = time, between-subject factor = group) was used to compare each outcome variable across the three time points (i.e., at baseline, immediately after termination of training, at the 1-month follow-up). Post hoc contrast analysis was then performed when appropriate. Effect sizes (ηp2) were reported for each outcome. ηp2 values of 0.14, 0.06, and 0.01 were considered as large, medium, and small effect sizes, respectively (35). The between-group difference in the proportion of fallers at the end of the 6-month follow-up period was compared using the χ2 test. The McNemar test was used to determine whether there was a significant change in the proportion of fallers before and after treatment in each group. To determine whether there was a gender difference in treatment effect, the above analyses were repeated after separating the data of the two gender groups.

Secondary data analysis was carried out to further explore whether the treatment effects were related to any subject characteristics or the baseline values of the outcomes. For each outcome variable, the change score was obtained by subtracting the baseline score from the posttest score. A positive change score indicates that a posttest score had a greater value than the corresponding pretest score. The associations between the change score for each outcome measure and baseline characteristics was determined using Pearson r or Spearman ρ, depending on the level of measurement. All statistical analyses were performed using SPSS 17.0 (SPSS, Inc., Chicago, IL) with a significance level of 0.05 (two-tailed).

RESULTS

The subjects were enrolled during the period of June 2009 and July 2010. The last 6-month follow-up assessment of falls was conducted in March 2011. A total of 102 people volunteered to participate in the study, 82 of whom fulfilled the inclusion and exclusion criteria (see the CONSORT flow diagram in Fig. 1). After randomization, 41 subjects were allocated to each of the experimental and control groups, respectively. No between-group difference was identified in any of the demographic (Table 2) or outcome (Table 3) variables at baseline. During the study, six subjects (three from each treatment arm) dropped out of the study for reasons that were not related to the training program (Fig. 1). A total of 76 subjects completed the training program and the follow-up assessments. No significant differences in baseline characteristics or outcome measures were found between the dropouts and those who completed the study. Among those who completed all follow-up assessments, no significant difference was observed in the mean time taken to complete the 24-session training program between the experimental group and control group (experimental group = 64.7 ± 3.5 d, control group = 62.2 ± 6.7 d, P = 0.177). The maximum time lapse between two exercise sessions was also similar between the two groups (experimental group = 7.7 ± 3.4 d, control group = 7.1 ± 2.7 d, P = 0.460).

TABLE 3
TABLE 3:
Outcome measures.
FIGURE 1
FIGURE 1:
CONSORT flow diagram.

In the intention-to-treat analysis, ANOVA revealed a significant time effect for the BBS score, 10 MWT, 6 MWT, isometric knee flexion and extension muscle strength, and ABC score (P < 0.05; Table 4). However, the interaction effects of time × group and group effects were not significant for these variables (P > 0.05). For the LOS test, a significant time effect was identified for MVL, EPE, and MXE (P < 0.05) but no time effect for DCL (P > 0.05). No significant time × group interaction or group effects were detected for these variables related to the LOS test, however. For those outcomes that showed a significant time effect, a post hoc contrast analysis revealed a significant improvement immediately after the training period in both groups (P < 0.05) and the values remained stable during the 1-month follow-up period (P > 0.05; Table 4). No significant between-group difference in the incidence of falls was found, with three subjects in each of group reporting a fall during the 6-month follow-up period (i.e., six falls in total; χ2 = 0.000, P = 1.000), nor was there any significant change found in the proportion of fallers before and after the treatment period in either group (McNemar test, P = 1.000; Table 5). None of the falls resulted in any injuries that required medical attention. Gender-specific analysis yielded results similar to the primary analysis, with no significant time × group interaction effect for all outcome variables for both gender groups (not shown). A separate analysis was also performed after removal of the dropouts (i.e., on-protocol analysis), and similar results were obtained (not shown).

TABLE 4
TABLE 4:
Post hoc contrast analysis on time effect.
TABLE 5
TABLE 5:
Fall incidences before and after training.

In the secondary analysis, the change score of each outcome was correlated with the respective baseline value and other relevant factors (e.g., compliance). It was found that the change scores for 10 MWT (r = −0.575, P < 0.001), BBS (r = −0.663, P < 0.001), and ABC score (r = −0.597, P < 0.001) showed significant negative correlations with their respective baseline scores, indicating that the subjects with poorer neuromuscular function and fall-related self-efficacy tended to have greater improvement. None of the change scores was significantly correlated with measures of compliance (e.g., mean time taken to complete the 24 sessions of training or maximum time lapse between two exercise sessions).

No severe adverse effects were reported by the subjects, although three reported mild dizziness during WBV therapy and four had lower limb soreness and fatigue (two from the WBV group). The symptoms gradually subsided after the first few sessions of training.

DISCUSSION

This is the first study to evaluate the efficacy of WBV therapy on neuromotor performance and the likelihood of falls in people who have had a chronic stroke. The results showed that, after the treatment period, both the WBV and control groups had similar significant improvement in most of the neuromotor outcomes measured. The study also provides no evidence that the present WBV protocol is effective in reducing the incidence of falls in patients with chronic stroke.

WBV has been proposed to improve neuromotor performance through several mechanisms, including activation of motor units (38), modulation of excitability of the spinal motoneuronal pool (25), and improved proprioception (14,50). The efficacy of WBV training on neuromotor function in older adults remains controversial. Although some trials showed positive effects of WBV, a good number of trials reported no significant effects (5,6,8,17,26,41,42,52–55). A major reason for the discrepancy in these results could be the difference in study design. Many previous WBV trials in older adults or stroke patients did not incorporate a control group that performed the same activities without the stimulus of WBV. For example, the randomized control trial in subacute stroke patients conducted by van Nes et al. (52) showed that the 6-wk WBV program induced similar gains in functional balance performance and walking status to those in the comparison group that underwent exercise therapy on music. In another study on patients with subacute stroke, Tihanyi et al. (48) reported better leg muscle strength outcomes in the experimental group that underwent 4 wk of conventional rehabilitation therapy in combination with WBV stimulation and leg exercise when compared with the control group that received conventional rehabilitation therapy only. Whether the reported effects can be attributed to WBV stimulation itself or the leg exercises performed while standing on the vibration platform remains a question. In contrast, our control group performed exactly the same leg exercises as the experimental group, thereby allowing us to delineate clearly the effects of WBV. In addition, the sample size in previous stroke trials was relatively small and the interpretation of their results thus warrants caution. In contrast, our study used a much larger sample size (82 subjects).

Another possible reason that accounts for the discrepancy in results is the difference in the WBV protocol used. Both Bautmans et al. (2) and Rees et al. (42) adopted a study design similar to ours (i.e., comparing between exercise group and WBV + exercise group). They documented significantly more improvement in balance and mobility among older adults when WBV was added. Interestingly, their WBV protocols were of higher intensities (30–54 Hz, 71.1–575.6 m·s−2 for Bautmans et al.; 26 Hz, 66.7–106.7 m·s−2 for Rees et al.) than ours (20–30 Hz, 9.5–15.8 m·s−2). It is thus possible that our training protocol may not be of sufficient intensity to induce a positive training effect in stroke patients. However, in the same trials by Bautmans et al. (2) and Rees et al. (42), no significant treatment effect on leg muscle strength was reported. It seems that the most optimal WBV parameter may differ depending on the outcome measured.

All the six exercises used were primarily dynamic in nature. The dynamic exercises were emphasized because the main outcomes in this study (i.e., balance, mobility, and falls) were related to dynamic activities. In particular, most falls in stroke patients occur during dynamic tasks such as transfers, turning, rising from a chair, and walking (21,23,29). It has also been demonstrated that leg muscle activity can be augmented by WBV during the performance of dynamic squatting exercises (22). Our pilot testing also showed that the stroke patients have great difficulty in sustaining certain postures (especially partial squat or deep squat, forward lunge) for prolonged periods. Are the negative results of this study related to the lack of a static exercise component? Available literature on older adults provides no clear evidence that static exercises or combined static/dynamic exercises are superior to dynamic exercises or vice versa. For example, positive treatment effect on balance or mobility was reported in WBV trials using static (2,17), dynamic (54), and combined static/dynamic exercise protocols (5,28,42,53). In contrast, both Cheung et al. (9) and Raimundo et al. (39) reported mixed results in their respective trials using a static exercise protocol. Marin et al. (31) and Bogaerts et al. (6) used a combined static/dynamic exercise paradigm and reported no significant effect on balance.

Our subjects underwent three exercise sessions per week for 8 wk (i.e., 24 sessions). Rees et al. (42) used the same program duration as ours (24 sessions in 8 wk), and the training program in Bautmans et al. (2) was even shorter (18 sessions in 6 wk). Despite this, both of these studies in older adults reported significant treatment effect on neuromotor outcomes. Perhaps a longer training program duration (>24 sessions) may be required before treatment effects can be obtained in stroke patients, who have varying degrees of physical disability. Indeed, it is likely that individuals with different levels of impairment may probably require different training protocols to obtain optimal therapeutic effects. The available evidence suggests that the effect of WBV treatment on neuromuscular function is more apparent in more heavily compromised populations (e.g., older adults and postmenopausal women) than in younger adults (17,26,32,43,49,53,55). The application of WBV in populations with chronic diseases, including stroke, is fairly recent. It is currently unknown what WBV protocol is the most optimal for improving different outcomes in stroke patients. Our secondary analysis indeed showed that those with poorer neuromotor performance tended to have better outcomes. Perhaps a more homogeneous group of stroke survivors with more severe neuromotor impairment can benefit more from the WBV training and should be considered in future trials. Further research should also address the efficacy of various WBV training protocols in different stroke subpopulations and the related physiological mechanisms. For example, it would be interesting to compare the effects of protocols with different WBV frequencies (e.g., 20, 30, 40 Hz), intensities (e.g., lower vs higher peak accelerations), number of training sessions per week (e.g., three vs seven), and exercises used (e.g., dynamic vs static exercises).

Because leg muscle weakness, balance deficits, and mobility problems are major risk factors of falls, it would be interesting to determine whether WBV treatment would reduce the fall rate. In our study, no significant difference in fall rate was found between the experimental and control groups. To date, only one previous study has examined the influence of WBV on falls. In a recent study on postmenopausal women (55), the total number of fall incidences in the combined WBV and conventional exercise group was significantly lower than the no-intervention control group during the training period. However, the fall incidence in the conventional exercise group was not significantly different from the control group. Based on this finding, the authors concluded that the addition of WBV to a conventional exercise program is beneficial in reducing falls. However, it should be noted that the fall incidence in the combined WBV and conventional exercise group was not significantly different from that in the conventional exercise group. Therefore, the added value of WBV on reducing falls is still questionable. In another study of elderly women (5), the influence of WBV on fall risk, rather than actual fall rate, was evaluated. Using the Physiological Profile Assessment (27), which incorporates evaluation of visual function, sensation, reaction time, muscle strength, and balance ability, a fall risk score was computed before and after the 6-month WBV training. Their results showed that the fall risk was not significantly different between the WBV group and the no-intervention control group after the training period. However, the proportion of people in the high-fall-risk group was significantly reduced after WBV training compared with controls (5). It concurs with previous research that WBV may have more apparent therapeutic effects in more compromised patient populations. Future WBV trials may consider targeting those patients with stroke who have a positive history of recurrent falls.

When examining the time effect in more detail (Table 4), it was found that for those outcomes that showed a significant time effect, the effect sizes were mainly small to medium in the pretest versus posttest comparison, except BBS and ABC scores, which demonstrated a large effect size (ηp2 > 0.14). The training effect was also well maintained at the 1-month follow-up (pretest vs follow-up comparison). The results thus indicate that leg exercise training, regardless of whether WBV is added or not, has beneficial effects on some aspects of neuromotor function and fall-related self-efficacy among individuals with chronic stroke. It is well known that stroke patients are prone to functional deterioration in the chronic stage, when they no longer receive intensive rehabilitation services in the hospital. Our results have demonstrated that through intensive exercise training, improvement in certain neuromotor functions is still possible in the chronic stage of stroke and can be achieved in a period as short as 8 wk. Implementing WBV training program in the patients’ local communities may be a viable way to encourage participation in physical exercise and social activity on a regular basis and is worth exploring.

There are several limitations in this study. First, the total duration of training and frequency of treatment sessions varied among the subjects, many of whom could not follow the intended schedule of treatment (three sessions per week for eight consecutive weeks) due to various personal reasons (e.g., time clash with other activities, bad weather, no caregiver to accompany them to the training). Second, most subjects were mobile. The results can therefore only be generalized to a subpopulation of individuals who have regained some ambulatory function, with mild to moderate impairment from the stroke. The effect of WBV training on those who are nonambulatory requires further investigation. It would be particularly interesting to determine whether WBV has any beneficial effects in stroke patients who are unable to perform conventional exercises. Third, the subjects were not blinded to the group allocation, which is often difficult to achieve in exercise trials. The use of sham vibration would have been a good method to blind the subjects, but this could not be done because of the limitation of our WBV device.

In summary, this clinical trial indicated that the addition of the present WBV protocol to the dynamic leg exercise program confers no supplementary benefits for improving neuromotor performance and reducing falls when compared with leg exercise alone in chronic stroke patients with mild to moderate motor impairments. Further research is required to determine the clinical efficacy of different WBV protocols in various subgroups of patients after chronic stroke.

This study is supported by Hang Seng Bank Golden Jubilee Research Endowment Fund.

The authors declare that they have no conflict of interest.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

CEREBROVASCULAR ACCIDENT; BALANCE; MOBILITY; MUSCLE STRENGTH; EXERCISE; REHABILITATION

©2012The American College of Sports Medicine