Sympathetic Stress and Whole Body Vibration Reflex

Plain-language summary

Whole body vibration (WBV) is a therapeutic modality in the form of exercise on a vibrating platform with an amplitude of 2-4 mm at a frequency of 25-50 Hz, which is used with increased popularity in sports medicine and rehabilitation due to its beneficial effects on muscle strength, balance, postural control, bone formation, and circulation. Beneficial effects on muscle strength and athletic performance have been reported. Spinal reflexes explain these beneficial neuromuscular effects. However, the neuronal circuit and receptors of the reflex response have not been defined precisely. A group of researchers propose that the reflex system underlying the neuromuscular effects of WBV is the Tonic vibration reflex (TVR), whose receptor is the muscle spindle; other researchers claim that the reflex latency induced by WBV is 4-5 ms longer than the TVR latency, so it is a bone myoregulation reflex whose receptor is osteocytes. The muscle spindle has sympathetic innervation. It has been reported that in case of increased sympathetic activity, muscle spindle sensitivity may increase and short-latency stretch reflex may be facilitated. The variation of muscle spindle activity with sympathetic activity may provide an opportunity to define the nature of the reflex response during WBV. Muscle spindles are more sensitive to vibrations around 100 Hz. This study has two hypotheses: According to the first hypothesis, WBV activates muscle spindles and the reflex latency induced by WBV is the same as TVR latency, and the latency does not change with increased sympathetic activity. According to the alternative hypothesis, WBV activates osteocytes, and WBV-induced reflex latency is longer than TVR latency. With increased sympathetic activity, the WBV reflex becomes dominant and the WBV-induced reflex latency becomes shorter. The aim of this research is to determine which of these two hypotheses is valid.

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