High-Frequency Whole-Body Vibration Improves Balancing Ability in Elderly Women

Methods 

Statistical Analysis 

The primary endpoint of this study was movement velocity because other studies indicated an improvement of mobility or movement after vibration treatments.8, 12 The null hypothesis of this study was that WBV provided no effect on the balancing ability, whereas the alternative was that WBV enhanced the outcomes of the treated subjects. A sample size of 50 for the study was estimated to achieve a statistical power 0.8. This calculation was based on a previous similar study,10 with an expectation of 16% increase of dynamic muscle strength in postmenopausal women after 6 months of WBV training. Only the subjects achieving 70% or higher compliance2 in the WBV intervention group would be included in the analysis, and it was estimated that only half of the subjects in this group would reach 70% or higher compliance; therefore, 50 subjects in the intervention group and 25 in the control group were needed.

The data were expressed in mean ± standard deviation (SD) and analyzed with SPSS.c An independent t test was used to compare the demographic data of both groups. Two-way analysis of variance was used to analyze the parameters for the limits of stability and the FRT between 2 groups. P values less than .05 were considered to be significant.

Results 

Demographics 

Seventy-five of the 85 elderly women recruited were eligible and willing to participate in the study. Twenty-five were randomized to the control group and 50 to the intervention group. Only 24 and 45 subjects completed the study in the control and intervention group, respectively (average age, 72.36±4.93y). There were 18 (26.1%) subjects who had a history of fall in the previous year. The mean number of diseases and prescriptions of the subjects was 4.33±2.43 and 1.71±1.58, respectively, during the study period. There was no significant difference in the demographic and baseline data, as shown in table 1.

Table 1. Demographic and Baseline Data of Subjects in Both Control and WBV Groups

	Parameter	Control (n=24)	WBV Treatment (n=45)	P
	Age (y)	72.0±6.01	72.51±4.45	.689
	Height (cm)	150.88±5.33	151.51±5.62	.650
	Weight (kg)	58.50±8.95	57.63±9.42	.711
	No. of comorbidities	4.79±2.64	4.08±2.17	.223
	No. of medication	1.96±1.60	1.58±1.57	.345
	History of fall, n (%)	5(20.8)	13(28.9)	.468
	Reaction time (s)	1.17±0.36	1.22±0.42	.694
	Movement velocity (deg/s)	1.96±0.61	1.89±0.46	.635
	Endpoint excursion (% of limits of stability)	39.39±9.25	43.27±7.67	.092
	Maximum point excursion (% of limits of stability)	57.26±12.00	56.54±9.55	.791
	Directional control (% of accuracy)	63.57±10.54	62.43±10.53	.688
	Functional reach (cm)	22.00±11.09	21.39±7.13	.782

NOTE. Values are mean ± SD or as otherwise indicated.

Dropout and Compliance 

Six subjects dropped out (5 from the intervention group, 1 from the control group) during the study period and refused to come back for reassessment for health or other personal reasons. Of the remaining subjects (n=45) in the treatment group, all had completed the 3 months of intervention, with a mean compliance of 93.3% (range, 75%–100%) (fig 1). All subjects in both groups did not change their activity habits throughout the 3-month period.

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Fig 1. CONSORT flowchart of the study.

Limits of Stability and Functional Reach 

Comparing the findings between the control and treatment groups (table 2), significant improvement was shown in movement velocity (P<.01) and maximum point excursion (P<.01). Directional control also reached marginal significant level (P<.05). Other parameters, including reaction time and endpoint excursion, however, were not significant (P>.05). For the FRT, the intervention group showed more improvement than the control group, although it was not statistically significant (P=.22).

Table 2. Limits of Stability and Function Reach Compared Between Control and WBV Groups

	Parameter (% change)	Control (n=24)	WBV Treatment (n=45)	P
	Reaction time (s)	−25.59±24.37	−34.47±26.03	.210
	Movement velocity (deg/s)	14.96±31.72	53.49±54.38	.003†
	Endpoint excursion (% of limits of stability)	11.12±33.46	20.36±30.07	.261
	Maximum point excursion (% of limits of stability)	3.36±20.91	18.84±18.26	.003†
	Directional control (% of accuracy)	−6.61±19.72	4.32±19.64	.049⁎
	Functional reach (cm)	6.59±34.56	23.77±63.01	.221

NOTE. Values are mean percentage change ± SD.

⁎ P<.05. † P<.01.

Discussion 

The present study questioned the efficacy of WBV on balancing ability with regard to the limits of stability and the FRT in elderly women. To our knowledge, this was the first investigation to evaluate the effect of side-alternating vibration machine on balancing ability of elderly women in terms of the limits of stability. The findings showed significant enhancement in stability with respect to movement velocity, maximum point excursion, and directional control in elderly women after the WBV interventions as compared with the controls.

The present prospective RCT showed that 3 months of WBV could significantly improve limits of stability parameters to different extents. This finding was substantiated by others who have reported significant improvement after 6 months of WBV on speed of movement and knee-extension strength in older women,8 whereas the 6-week WBV intervention for institutionalized elderly also indicated a positive impact on motor control in the subjects.12 Enhancement in postural control and mobility was detected in MS patients13 as well as unilateral chronic stroke patients.14 All the enhancements in muscle function related outcomes shown in these studies support the effectiveness and feasibility of using WBV on enhancing the muscle performance and hence the balancing ability. Clinically, poor balancing ability is closely related to the risk of falling in the elderly, which may lead to serious medical consequences including hip fractures and head injuries.31 The findings of the present study will open up a new area to apply WBV in the prevention of falls and its related injuries among the elderly in the future.

In the present study, movement velocity, maximum point excursion, and directional control were improved after WBV interventions. Because these functional parameters involve the ability to recruit muscle fibers, muscular adaptation, and neuromuscular coordination, these improvements indicate that WBV would be effective in enhancing neuromuscular rehabilitation. This could be particularly well reflected by the beneficial effects of WBV on stroke14 and MS13 patients. These outcomes also agreed with other mechanistic studies that WBV could induce several neural and muscular changes such as the stimulation of human spindle endings20, 21 and changes in biogenic amines.22 Among these improvements in the limits of stability parameters, movement velocity and maximum point excursion were the most sensitive in response to the WBV treatment, as reflected from their high significance levels. Besides, the treatment protocol used in this study was 3 minutes a day and 3 days a week, which was a mild and fast regimen for elderly women. Other studies applied complicated programs for the elderly, involving different exercises and durations in different weeks8, 12 or systematic increase of training volume.10 Therefore, the protocol applied in this study is relatively user friendly for the elderly, and the application of WBV can be used to prevent fall incidences caused by poor balancing ability, thus potentially reducing the rate of osteoporotic fractures.

Measuring limits of stability for the assessment of balancing ability was used in this study because limits of stability is a good and reliable indicator of dynamic balance.28, 29 The ability to control the movement of the COG over the base of support is critical to normal balance.27 Therefore, increased fall risk can be reflected from delayed reaction time, slow movement velocity, constricted limits of stability boundary, or uncontrolled COG movement. The key advantage of this computerized assessment of limits of stability is the ability to obtain objective, quantitative measurements quickly and reliably while the subjects are providing biofeedback. In this study, functional reach was assessed in addition to the limits of stability because the FRT is a single-item measure of dynamic balance clinically used by physiotherapists. Despite the similar nature of both tests, the FRT was still necessary because it had been reported that the FRT and the limits of stability tests should not be used interchangeably.32

On the other hand, Torvinen et al’s studies showed that the WBV intervention did not affect the dynamic or static balance of the subjects in short-term (4-mo)6 or long-term (8-mo) treatment.11 This discrepancy from our findings could be explained by the difference in target subjects between the 2 studies. In most of the studies with positive effects on muscle performance, including the present one, target subjects were mostly elderly or postmenopausal women8, 10, 12 or physically weak people like MS13 and unilateral chronic stroke patients.14 However, the target subjects in Torvinen’s studies6, 11 were young healthy nonathletic volunteers aged 19 to 38 years. This implies that young healthy people with good muscle quality may not readily gain additional benefits from WBV. But this does not completely exclude the efficacy of WBV on young subjects because Cochrane and Stannard5 have reported that acute WBV training increased flexibility performance in elite female field hockey players. This might be attributed to the intensive training in Cochrane’s study, in which WBV at 26Hz accompanied with 6 different exercises was performed, whereas the oscillation was ascended from 25 to 40Hz for 4 minutes only in Torvinen’s study.11

Variations in vibration machines and training protocols may also explain the different outcomes in these clinical studies. In most studies, vibration platforms with a vertical mode of oscillation were used including Power Plate,8, 10, 12,d Zeptor-Med system,13,e and Kuntotäry.6, 11, 33,f Other studies,5, 14 including the present one, used the Galileo platform, which is of a side-alternating vibration mode. Although no study has compared the effects or functions before, the 2 vibration modes differ in terms of the mechanism and stimulated muscles, which need further investigations to depict their differences.

Study Limitations 

The limitation of this study was the lack of dummy machines for placebo control. Subjects in the control group had to keep sedentary with normal daily life throughout the whole study. This limitation was minimized by the monitoring of their involvement in habitual exercise or supervised exercise. Those exceeding the amount of exercise were not recruited in this study. Also, the imbalanced number of subjects in 2 arms was because of our overestimation of the dropout rate for the intervention group.

Conclusions 

Three-month high-frequency vibration therapy with 3 minutes a day and 3 days a week significantly enhanced the balancing ability of elderly women in terms of movement velocity, maximum point excursion, and directional control. Of these parameters, movement velocity and maximum point excursion were the parameters with the most significant improvement. These findings provide good evidence to support the efficacy of WBV on enhancing muscular performance in elderly women and hence balancing ability. The long-term impact of WBV on balancing ability and its potential application in prevention of falls or related injuries are also worthwhile for further studies.