[ARTICLE] Effects of different vibration frequencies on muscle strength, bone turnover and walking endurance in chronic stroke – Full Text

Abstract

This randomized controlled trial aimed to evaluate the effects of different whole body vibration (WBV) frequencies on concentric and eccentric leg muscle strength, bone turnover and walking endurance after stroke. The study involved eighty-four individuals with chronic stroke (mean age = 59.7 years, SD = 6.5) with mild to moderate motor impairment (Fugl-Meyer Assessment lower limb motor score: mean = 24.0, SD = 3.5) randomly assigned to either a 20 Hz or 30 Hz WBV intervention program. Both programs involved 3 training sessions per week for 8 weeks. Isokinetic knee concentric and eccentric extension strength, serum level of cross-linked N-telopeptides of type I collagen (NTx), and walking endurance (6-min walk test; 6MWT) were assessed at baseline and post-intervention. An intention-to-treat analysis revealed a significant time effect for all muscle strength outcomes and NTx, but not for 6MWT. The time-by-group interaction was only significant for the paretic eccentric knee extensor work, with a medium effect size (0.44; 95% CI: 0.01, 0.87). Both WBV protocols were effective in improving leg muscle strength and reducing bone resorption. Comparatively greater improvement in paretic eccentric leg strength was observed for the 30 Hz protocol.

Introduction

Muscle weakness is a major impairment after stroke¹ and is associated with various aspects of physical function² and bone tissue integrity³. According to a recent systematic review⁴, previous studies involving the use of bone imaging techniques such as peripheral quantitative computed tomography (pQCT)^3,5,6 and dual-energy X-ray absorptiometry (DXA)⁷ to investigate the impact of stroke on lower limb bone outcomes reported strong associations of muscle strength and mass with bone mineral density and indices of bone strength. Previous work has also demonstrated an increased rate of bone resorption in people with stroke, which was correlated with lower hip bone density^8,9. Therefore, effective interventions that target muscle strength and bone health are important for stroke rehabilitation.

Whole-body vibration (WBV) augments muscle activation during exercise^10,11. The mechanical vibration induces reflex muscle activation and increases motor cortex excitability^12,13. WBV has also been shown to increase peak muscle torque in lower limb muscles¹⁴, presumably through the recruitment of higher threshold motor units. Improved muscle contractility and force generating capacity have implications for bone health¹⁵ as muscle contractions provide an important source of dynamic mechanical loading for maintaining bone tissue^15,16. There is evidence that WBV can reduce the rate of bone resorption in different populations (e.g., post-menopausal women, children with severe motor disabilities, and people with metabolic acidosis)^17,18,19.

WBV training has been identified as a potentially viable treatment modality in various patient groups with muscle weakness and consequent bone loss^19,20,21,22, such as people after stroke^3,23,24,25. However, research on bone metabolism and muscle strength post-stroke after WBV intervention is scarce and the results are inconclusive^23,24,26. Thus far, only one study has examined the effects of WBV on bone turnover in people with stroke, and found no significant change in both bone formation and resorption markers following an 8-week WBV intervention (9–15 min, 20–30 Hz)²⁴. More research is needed before the use of WBV for modifying bone turnover rate in people with stroke can be considered conclusive. A meta-analyses by Yang et al. demonstrated that the effects of WBV on maximal isometric knee extension strength (5 studies, SMD = 0.23, 95%CI = − 0.27 to 0.74, p = 0.36), and maximal eccentric knee extension strength (2 studies, SMD = 0.09, 95%CI = -0.38 to 0.56, p = 0.71) yielded wide confidence intervals, indicating that the therapeutic value of WBV on improving knee muscle strength post-stroke requires further investigation²⁶.

Many factors may account for the discrepancies in results across previous studies in stroke (e.g., sample characteristics, WBV type, WBV frequency, treatment duration, etc.). As various studies differed on multiple factors, it was not feasible to delineate the effects of each factor by comparing the results of different studies. Nevertheless, among these factors, vibration frequency may be a particularly important parameter, as revealed by both animal and human studies. Animal studies have shown that higher frequency WBV can enhance osteogenesis more effectively than relatively lower frequency WBV²⁷. In people with stroke, a greater level of leg muscle activation, as indicated by electromyography (EMG) findings, was found during exposure to higher WBV frequency (30 Hz) than lower frequency (20 Hz)^11,28. Therefore, repeated exposure to WBV of higher frequencies may lead to a greater strengthening effect of the muscles being stimulated. In a randomized controlled trial, Wei et al. showed that when controlling for the total number of vibrations, a 40 Hz frequency WBV protocol led to the best outcomes in terms of muscle size, strength and physical performance (i.e., 10-m walk test, timed-up-and-go, and sit-to-stand) in patients with sarcopenia^29,30. However, these findings are not necessarily generalizable to individuals with chronic stroke. Stroke-related impairments are heterogeneous in presentation, etiologically complex (compensatory movement patterns, learned disuse, etc.) and are often inconsistent with typical muscle changes and performance deficits associated with atrophy or aging alone^31,32. Only one study has compared the effects of two different WBV protocols in the same sample of people with stroke (20 Hz vs 30 Hz) and found no difference in knee muscle strength after 10 weeks of intervention. However, the number of vibrations was not controlled and bone turnover was not measured³³.

To address these identified gaps in knowledge, we aimed to evaluate the effects of different WBV frequencies in stroke patients. In addition to leg muscle strength and bone turnover, the 6-min walk test (6MWT), an indicator of walking endurance, was also used as an outcome. Leg muscle strength has demonstrated a strong association with 6MWT distance in individuals with stroke^34,35. Therefore, any WBV-induced improvement in leg muscle strength was also thought to result in better walking endurance. We hypothesized that a higher WBV frequency (30 Hz) would induce larger improvements in muscle strength and walking endurance, and greater reduction in the level of bone resorption marker compared with a lower WBV frequency (20 Hz).[…]

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[ARTICLE] Whole-Body Vibration in Horizontal Direction for Stroke Rehabilitation: A Randomized Controlled Trial – Full Text

Abstract

Background

As most of the existing whole-body vibration (WBV) training programs provide vertical or rotatory vibration, studies on the effects of horizontal vibration have rarely been reported. The present study was conducted to investigate the effect of WBV in the horizontal direction on balance and gait ability in chronic stroke survivors.

Material/Methods

This study was designed as a randomized controlled trial. Twenty-one stroke survivors were randomly allocated into 2 groups (whole-body vibration group [n=9] and control group [n=12]). In the WBV group, WBV training in the horizontal direction was conducted for 6 weeks, and a conventional rehabilitation for 30 min, 3 days per week for a 6-week period, was conducted in both the WBV and control groups. Outcome variables included the static balance and gait ability measured before training and after 6 weeks.

Results

On comparing the outcome variables before and after training in the WBV group, significant differences were observed in the cadence and single support time of gait ability. However, there were no significant differences in other variables, including velocity, step length, stride length, and double support time. In addition, after training, no significant differences in all variables were observed between the 2 groups.

Conclusions

The results of this study suggest that WBV training in the horizontal direction has few positive effects on balance and gait function in chronic stroke survivors. However, further investigation is needed to confirm this.

Background

Stroke survivors suffer from central nervous system damage, with sensory and motor system damage, which leads to consequences such as decreased control of muscle tone, delay in muscle contraction, and absence of selective movement [1,2]. In addition, stroke survivors have unstable balance and poor gait ability, which naturally limits their activities of daily living and participation in the community, while losing independence [2,3]. Consequently, the first priority for stroke survivors is recovery of independent activities, and for this, the recovery of balance in a standing posture and gait abilities is essential.

For functional recovery of stroke survivors, various methods have been suggested [4], and whole-body vibration (WBV) is a relatively novel form of exercise intervention that could improve functional recovery [5]. WBV involves the use of a vibrating platform in a static position or while performing dynamic movements. In previous studies, it was suggested that WBV training could improve physical functions. Castrogiovanni et al. [6] reported that a multi-component training, including aerobic activity and other types of training (resistance and/or strength exercises), is the best kind of exercise for improving bone mass and bone metabolism in elderly people and especially in osteopenic and osteoporotic women. With regard to whole-body vibration training, studies have suggested that it could be a valid method. Pichler et al. [7] reported that mechanical stimulation such as treadmill and vibration stimulation training inhibits the activity of RANKL in osteoporosis. In addition, Musumeci et al. [8] suggested that, in certain diseases such as osteoporosis, mechanical stimulation including treadmill and vibration platform training could be a possible therapeutic treatment. Based on their results, they proposed the hypothesis that physical activity could also be used as a therapeutic treatment for cartilage diseases such as osteoarthritis. Van Nes et al. [9] introduced WBV as a means of somatic sensory stimulation for functional recovery of stroke survivors. They also reported that somatosensory stimulation through WBV can significantly improve muscle performance, balance, and daily activities. Balance, defined as the ability to maintain the center of pressure (COP) on the support surface in given circumstances, can be held through adjusted harmony of visual, vestibular, and somatic sensory system [10], and vibration stimulation is reported to cause small changes in the skeletal muscle length of the human body and affect the motor neurons to facilitate activation of the spinal reflexes through short spindle-motor neuron connections [11].

Balance is a major component required for controlling or maintaining the COP in mobility and locomotion in which the support surface changes [12]. The information on changes of the support surface along with the biomechanic information needed for movement control is passed on to the central nervous system by muscle spindles, Golgi tendon organs, and joint receptors in the proprioception sense; thus, they have a very important role in controlling balance [13,14]. In addition, Muller and Redfern [15] performed a comparative analysis of the latency of beginning muscle activity by measuring electromyogram (EMG) activation degree of muscle strength of the lower extremities caused by movement of the COP while the support surface moved back and forth. Consequently, the latency of activation of the tibialis anterior muscle was rapid on the support surface moving forward and that of the soleus muscle was rapid when moving backward. Given these reports, for recovery of balance ability, the horizontal vibration in all directions might be needed more than the vertical or rotatory vibration provided by the original WBV training. Additionally, our bodies maintain standing posture using ankle strategy, hip strategy, or both [16]. The ankle strategy, which is the postural control strategy that starts first in postural sway, enables immediate recovery of standing balance through ankle joint muscle contraction [16]. Horizontal vibration, therefore, may significantly activate not only stimulation of somatosensory, but also ankle strategy or hip strategy.

However, since most of the existing WBV training programs provide only vertical or rotatory vibrations, studies on effects of horizontal vibrations have been rarely reported. Accordingly, the present study examined the effects of horizontal WBV in an antero-posterior or medio-lateral direction on balance and gait abilities of stroke survivors.[…]

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Figure 2
Whole-body vibration in horizontal direction.