[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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