Objectives: Several studies have investigated the effects of focal vibration on muscle strength. Non-univocal results have been found. The aim of this study was to evaluate the effect of prolonged focal vibratory stimulation on quadriceps muscle strength at two different frequencies (80 and 300 Hz). The evaluation of muscle strength was performed at different intervals of time after the end of the vibratory stimulation in order to quantify the long-term effects and their trends over time.
Methods: Twenty-seven healthy volunteers were divided into three groups, a control group (no treatment) and two groups treated with vibratory stimulation (80 or 300 Hz) of relaxed quadriceps femoris bilaterally, once a day (30 min) for 5 consecutive days. The quadriceps' strength was measured through an isokinetic dynamometer, before and at three time intervals after the treatment, with a follow-up period of 4 weeks. The outcome measure was the Peak Torque (PT, Nm) of the quadriceps femoris produced by extension movement at three defined angular velocities and during isometric contraction.
Results: No changes in PT were observed in the control group over time, while PT increased in the treated groups. No significant difference in PT behavior was observed between these two groups. PTs recorded before and after the treatment were markedly different, and the increase in the PT persisted until the follow-up at 4 weeks, for all angular velocities tested.
Conclusions: Prolonged vibratory stimulation of the quadriceps femoris, both at 80 and at 300 Hz, leads to an increase in muscle strength. The vibration effect does not appear to fade at the end of treatment, but persists at the follow up, suggesting a likely underlying plastic process. The results of the current study suggest that 30-min per day, 5 day focal vibratory treatment can be helpful during the clinical practice to regain muscular strength. It does not require patient's effort during the treatment, requires a little time, its effects are long-lasting, and there are no known adverse effects.
Keywords: healthy volunteers; muscle strength; quadriceps muscle; rehabilitation; vibration.
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