The effect of whole-body vibration training with different amplitudes on bone mineral density in elderly women

Abstract

BACKGROUND:

The effects of whole-body vibration training (WBVT) with same frequency and different amplitudes on bone mineral density (BMD) in the elderly is not reported.

OBJECTIVE:

To compare the effect of 45-Hz WBVT with different amplitudes on the BMD in elderly women.

METHODS:

Age-, height-, and weight-matched patients were assigned to a low-amplitude group ( $n=$ 19, amplitude of 2 mm), medium-amplitude group ( $n=$ 18, amplitude of 3 mm), high-amplitude group ( $n=$ 19, amplitude 4 mm), and control group ( $n=$ 20). The WBVT was conducted for 24 weeks in the three amplitude groups. The BMD at lumbar vertebrae L ${}_{2-4}$ and the proximal femur was measured at 0 and 24 weeks.

RESULTS:

The BMD at lumbar vertebrae L ${}_{2-4}$ was higher in the high-amplitude group than in the low-amplitude and middle-amplitude groups, and the BMD of the greater trochanter was significantly higher than that in the low-amplitude group ( $p<$ 0.05). The BMD of the greater trochanter was significantly higher in the middle- than low-amplitude group ( $p<$ 0.05).

CONCLUSION:

A higher amplitude should be considered when WBVT is performed in elderly patients to increase bone density and prevent osteoporosis.

1. Introduction

Table 1

Participants’ demographic characteristics at baseline

Group	Age (y)	Height (cm)	Weight (kg)	BMI (kg/m ${}^2$ )	Vibration time (w)
Low amplitude group ( $n=$ 19)	63.9 $\pm$ 2.1	158.8 $\pm$ 4.6	57.3 $\pm$ 3.3	22.7 $\pm$ 3.9	24
Middle amplitude group ( $n=$ 18)	64.1 $\pm$ 1.7	158.5 $\pm$ 5.2	57.2 $\pm$ 4.6	22.8 $\pm$ 4.8	24
High amplitude group ( $n=$ 19)	64.2 $\pm$ 1.8	159.1 $\pm$ 6.6	56.9 $\pm$ 4.1	22.5 $\pm$ 5.5	24
Control group ( $n=$ 20)	64.0 $\pm$ 2.0	159.0 $\pm$ 5.1	57.0 $\pm$ 5.0	22.5 $\pm$ 5.0	0

BMI, body mass index.

The incidence of osteoporosis has been increasing along with the aging of the Chinese population, and how to improve the bone mineral density (BMD) in the elderly population to prevent osteoporosis has become a research topic [1, 2]. Previous studies have confirmed that whole-body vibration training (WBVT) could affect the BMD of elderly patients [3, 4, 5, 6]. WBVT is reportedly suitable for people with motor loss, cognitive impairment, or disaccustomed exercise [7, 8]. Meta-analyses have shown that WBVT with an intervention time of $>$ 24 weeks, vibration frequency of 20 to 50 Hz, and amplitude of 2 to 5 mm has a positive effect on the BMD of elderly patients [9, 10]. Some scholars have also studied the effects of WBVT with different frequencies at the same amplitude on the BMD of elderly patients. Lu [7] found that high-frequency WBVT (40–45 Hz) more effectively improved the BMD than low- to intermediate-frequency WBVT (10–30 Hz). Ba and Cheng [11] and Cheng [12] reported the same findings. However, differences in the effects of different amplitudes on BMD have not been reported. One study showed that when the participants underwent WBVT on a vibrating platform, they were subjected to a combination of amplitude (centrifugal force on the muscle), vibration frequency (acceleration on the muscle), and load (muscle tension) on the limb [13, 14, 15]. To this end, the various effects of different amplitudes are worth exploring.

On the basis of previous research, we selected a vibration frequency (uniform frequency) that has a positive effect on the BMD of elderly patients [6, 11, 12], and used the weight of the human body as the load (i.e., no additional load) to design WBVT of different amplitudes for elderly women. We then tested the resultant changes in BMD. Our hypothesis was that WBVT of different amplitudes at the same frequency had different effects on the BMD of elderly women and that a high amplitude was more effective than a low to medium amplitude.

2. Materials and methods

2.1 Participants

This study was approved by the ethics committee of our institution (approval no. 2019307). The inclusion criteria were as follows: (1) 60 to 70 years of age and menopausal; (2) completion of the Physical Activity Readiness Questionnaire and a questionnaire on exercise habits; (3) completion of a medical examination; (4) no treatment with estrogen, calcium, or calcitonin in the last 2 years; and (5) consistent with the Declaration of Helsinki, all participants provided signed written informed consent. The exclusion criteria were as follows: (1) diagnosis of osteoporosis in accordance with the World Health Organization diagnostic criteria for osteoporosis in postmenopausal women (T-score of less than $-$ 2.5 SD), (2) a history of injury or surgery involving the trunk and lower limb joints, (3) epilepsy or cardiovascular disease, and (4) presence of a pacemaker.

Our study design was based on previous studies of the effects of WBVT on the BMD of elderly patients [9, 10]. We used a 4 $\times$ 2 experimental design and planned for a sample drop out rate of about 15%. We calculated the need for at least 90 subjects using G-power software (effect size of 0.3, power of 0.8, and $\alpha$ level of 0.05). At the beginning of the experiment, 90 subjects were recruited. Six withdrew from the study for personal or family reasons, three did not participate in the study, and four did not meet the experimental requirements. Finally, 76 participants completed the entire study (a loss rate of 15.6%).

The participants were divided into a low-amplitude group ( $n=$ 19), middle-amplitude group ( $n=$ 18), high-amplitude group ( $n=$ 19), and control group ( $n=$ 20) according to a random distribution of numbers (18 ones, 19 twos, 19 threes, 20 fours). There were no significant differences in age, height, or weight among the groups ( $p>$ 0.05). The demographic characteristics of the study population are shown in Table 1. In accordance with the study plan, the four groups of participants maintained their original living habits. Every 2 weeks, they were visited by WeChat or an interview to record their living conditions [16], including their daily activities, diet, sunlight exposure, and whether they exercised regularly in any other ways, performed resistance training, or took drugs that may affect BMD [1].

Table 2

Bone mineral density at L ${}_{2-4}$ and proximal femur (g/cm ${}^2$ )

Group	Lumbar vertebrae L ${}_{2-4}$		Femoral neck		Greater trochanter		Ward’s triangle
	0 weeks	24 weeks	0 weeks	24 weeks	0 weeks	24 weeks	0 weeks	24 weeks
Low amplitude group	0.97 $\pm$ 0.08	0.99 $\pm$ 0.11 ${}^{\text{c}}$	0.80 $\pm$ 0.07	0.80 $\pm$ 0.05	0.66 $\pm$ 0.03	0.66 $\pm$ 0.05 ${}^{\text{cd}}$	0.60 $\pm$ 0.03	0.62 $\pm$ 0.06
( $n=$ 19)
Middle amplitude group	0.96 $\pm$ 0.09	0.96 $\pm$ 0.10 ${}^{\text{c}}$	0.79 $\pm$ 0.04	0.80 $\pm$ 0.03	0.67 $\pm$ 0.11	0.74 $\pm$ 0.06 ${}^{\text{ab}}$	0.61 $\pm$ 0.05	0.66 $\pm$ 0.07
( $n=$ 18)
High amplitude group	0.97 $\pm$ 0.12	1.04 $\pm$ 0.08 ${}^{\text{ab}}$	0.79 $\pm$ 0.03	0.82 $\pm$ 0.06	0.66 $\pm$ 0.09	0.73 $\pm$ 0.11 ${}^{\text{ab}}$	0.60 $\pm$ 0.08	0.68 $\pm$ 0.05
( $n=$ 19)
Control group ( $n=$ 20)	0.96 $\pm$ 0.11	0.97 $\pm$ 0.07	0.80 $\pm$ 0.06	0.81 $\pm$ 0.09	0.68 $\pm$ 0.06	0.70 $\pm$ 0.04	0.61 $\pm$ 0.03	0.62 $\pm$ 0.04

${}^{\text{a}}P<$ 0.05 in comparisons between week 24 and week 0 in the same group. ${}^{\text{b}}P<$ 0.05 in comparisons between the low-, middle-, and high-amplitude groups and the control group at 24 weeks. ${}^{\text{c}}P<$ 0.05 in comparisons between the low- and middle-amplitude groups and the high-amplitude group at 24 weeks. ${}^{\text{d}}P<$ 0.05 in comparisons between the low- and middle-amplitude groups at 24 weeks.

2.2 Whole-body vibration training

Previous studies have suggested that high-frequency WBVT (40–50 Hz) has a positive effect on the BMD of elderly patients [6, 11, 12]. For this reason, the vibration frequency of the low-, middle-, and high-amplitude groups (intervention groups) was set at a fixed 45 Hz and the amplitude was set at 2 mm (low-amplitude group), 3 mm (middle-amplitude group), and 4 mm (high-amplitude group). Under the guidance of a specialist experimenter, the participants underwent WBVT for 24 weeks using a Power Plate vibrator system (Performance Health Systems, Northbrook, IL, USA). With reference to previous reports [7, 11], the experimental cycle was 24 weeks with a frequency of three times per week for 20 minutes per session. Half-squats (about 90 ${}^{\circ }$ knee angle), full squats, heel lifts, and left-and-right alternating single-leg squats (5 groups per movement, 10 per group, 30-second rest between groups) were performed in the intervention group under the condition of the vibrator. The control group completed the same action as the intervention group with the vibrator system turned off.

2.3 BMD testing

A dual-energy X-ray BMD system (XR46; Norland at Swissray, Fort Atkinson, WI, USA) was used to measure the BMD of the femoral neck, Ward’s triangle, greater trochanter, and lumbar vertebrae L ${}_{2-4}$ . The error coefficient ranged from 1% to 2% for lumbar vertebrae L ${}_{2-4}$ , the femoral neck, and the greater trochanter and from 2.5% to 5.0% for Ward’s triangle [11].

2.4 Statistical analysis

The mean $\pm$ standard deviation of the BMD at 0 and 24 weeks in the four groups was calculated with SPSS 20.0 (IBM Corp., Armonk, NY, USA). The Shapiro-Wilk test was used to determine whether the data were normally distributed; if the data were non-normally distributed, they were converted. According to the distribution shape of the variable, if it is moderate skewness (SKewness 2–3 times of the standard error), the root sign value is used to convert. If it is high skewness (SKewness more than 3 times the standard error), logarithmic conversion is used. Single-factor (one-way) analysis of variance was performed to compare the same time points between groups, and a paired-samples $t$ test was used to compare the different time points within a group. The significance level was set at $\alpha =$ 0.05.

3. Results

The BMD at 0 and 24 weeks in the four study groups is shown in Table 2. The Shapiro-Wilk test showed that all test data conformed to a normal distribution. Comparison of lumbar vertebrae L ${}_{2-4}$ at 0 weeks among the groups showed no significant difference in the BMD of the femoral neck, greater trochanter, or Ward’s triangle ( $p>$ 0.05), indicating that the four groups were homogeneous.

The paired-samples $t$ test of 24 weeks and 0 weeks in the low-amplitude group showed that there was no significant change in the BMD of lumbar vertebrae L ${}_{2-4}$ , the femoral neck, the greater trochanter, or Ward’s triangle ( $p>$ 0.05). The BMD on the dominant side in the middle-amplitude group significantly increased by 10.4% ( $p=$ 0.036); however, there was no significant change in the other indexes ( $p>$ 0.05). In the high-amplitude group, the BMD at lumbar vertebrae L ${}_{2-4}$ and the greater trochanter increased by 7.2% and 10.6%, respectively ( $p=$ 0.042, $p=$ 0.034).

The groups were also compared at 24 weeks. This comparison showed that there was no significant difference in the BMD between the control group and the low-amplitude group ( $p>$ 0.05). However, there was a significant difference in the BMD of the greater trochanter between the control group and middle-amplitude group ( $p=$ 0.035) and in the BMD of lumbar vertebrae L ${}_{2-4}$ and the greater trochanter between the control group and high-amplitude group ( $p=$ 0.040 and $p=$ 0.030, respectively). The BMD of lumbar vertebrae L ${}_{2-4}$ in the high-amplitude group was significantly higher than that in the low-amplitude group ( $p=$ 0.040) and middle-amplitude group ( $p=$ 0.039), and the BMD of the greater trochanter in the high-amplitude group was significantly higher than that in the low-amplitude group ( $p=$ 0.029). The BMD of the greater trochanter in the middle-amplitude group was significantly higher than that in the low-amplitude group ( $p=$ 0.039).

4. Discussion

In this study, we examined the effects of WBVT at 45 Hz with different amplitudes (2, 3, and 4 mm) on the BMD of the lumbar region and dominant lateral proximal femur in elderly women, expanding the theory that WBVT improves the BMD and may help in preventing osteoporosis in elderly women. It was indicated that the BMD of the greater trochanter in the middle-amplitude group increased significantly by 10.4%. In the high-amplitude group, the BMD at L ${}_{2-4}$ and the greater trochanter increased significantly by 7.2% and 10.6%, respectively, but the change in the BMD in the low-amplitude group was not significant.

To the best of our knowledge, these findings are original. of elderly women. Russo et al. [17] confirmed the positive effect of 24 weeks of WBVT on the BMD of elderly people. WBVT with a 2-mm amplitude (frequency of 28 Hz) increased the BMD at L ${}_{2-4}$ and the greater trochanter but not at the femoral neck in elderly women. Lu [7] reported that WBVT with a 3-mm amplitude (frequency of 25–45 Hz) improved the BMD at the proximal femur of elderly women but had no significant effect on the femoral neck. Ba and Cheng [3] reached the same conclusion, reporting that WBVT with a 3-mm amplitude (frequency of 20–35 Hz) increased the BMD at L ${}_{2-4}$ , the greater trochanter, and Ward’s triangle but had no significant effect on the femoral neck in elderly women. The vibration frequency in the present study was 45 Hz, in which the high-amplitude group, consistent with the studies by Russo et al. [17], Lu [7], and Ba and Cheng [3]. However, some studies have produced concluded differently. Ruan et al. [18] reported that WBVT with a 5-mm amplitude (frequency of 30 Hz) significantly increased the BMD of the femoral neck in elderly women. A meta-analysis suggested that WBVT did not improve the lumbar BMD in elderly patients with osteoporosis [9]. This study suggests that individual differences and different actions on the vibration platform may be the causes of different conclusions.

There are two main theories to explain the mechanism underlying how WBVT influences human BMD. The first theory involves muscle dynamics. The theory of “muscle dynamics” [15, 19] suggests that when the whole body vibrates, muscles can be triggered to release “low-value, high-frequency” mechanical stimulation and then stimulate the bone tissue. As people age, this mechanical stimulation with “low value and high frequency” is gradually weakened due to muscle atrophy, causing changes in bone structure. However, WBVT stimulates the muscle by vibration to increase the low value high frequency force, and the related muscle group continues to perform involuntary contraction to maintain the pressure load to the bone, thus promoting osteogenesis growth [19]. The second theory relates to an increased blood perfusion to the bone. Research has shown that WBVT promotes contraction of type IIa muscle fibers in the vibrating part of the human body, increases the blood flow in bone tissue, produces high pressure, increases peripheral lymph and venous drainage, promotes bone growth, and inhibits bone loss [20, 21]. All of these changes have a positive impact on the BMD. At the same frequency, amplitude increases while centrifugal contractility in the muscles of the subject’s vibrated parts also increase. This could increase bone surface stress. Different amplitudes could possibly lead to different conclusions. However, this study found that irrespective of the amplitude, the effect on the femoral neck was not significant, which might have been related to insufficient intervention time. Two studies seem to confirm our conjecture, Gusi et al. [22] found that 32 weeks of WBVT for elderly women (amplitude of 3 mm), significantly increased bone mineral density in the femoral neck. Clinton et al. [23] found that 48 weeks of WBVT (amplitude of 2 mm) significantly inhibited the loss of femoral neck volume in elderly women.

The main limitation of this study is that the muscle strength and the indices affecting bone metabolism were not tested. Also, the sample was limited to the female gender. These measurements represent an important future research direction.

5. Conclusions

High-amplitude WBVT can significantly improve the BMD at lumbar vertebrae L ${}_{2-4}$ and the greater trochanter of elderly women. Medium-amplitude WBVT only improves the BMD of the greater trochanter, and low-amplitude WBVT is inadequate to cause beneficial changes in the BMD of elderly women. These findings suggest that a higher amplitude should be considered when WBVT is performed in elderly women to increase bone density and prevent osteoporosis.

Author contributions

CONCEPTION: Wenliang Song and Yilin Yang.

PERFORMANCE OF WORK: Wenliang Song and Yilin Yang.

INTERPRETATION OR ANALYSIS OF DATA: Wenliang Song and Yilin Yang.

PREPARATION OF THE MANUSCRIPT: Wenliang Song and Yilin Yang.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Wenliang Song and Yilin Yang.

SUPERVISION: Wenliang Song and Yilin Yang.

Ethical considerations

This study was approved by the ethics committee of our institution (approval no. 2019307, Ethics committee of the Sun Yat-Sen University, Guangzhou, China). Before the assessment, every participant received the same detailed information about the testing procedure. Every participant signed the informed consent.

Funding

The authors report no funding.