Long-term effects of whole-body vibration on motor unit contractile function and myosin heavy chain composition in the rat medial gastrocnemius.

Structural and physiological mechanisms underling functional adaptations of a muscle to chronic whole-body vibration (WBV) are poorly understood. The study aimed at examining the contractile properties of motor units and the myosin heavy chain (MHC) expression in rat medial gastrocnemius muscle in response to 3- or 6-month periods of the WBV. The three-month WBV induced modifications of contractile properties principally in slow (S) and fast resistant to fatigue (FR) motor units. In S units an increase in the maximum tetanus force, a reduction in the twitch force and a decrease in the twitch-to-tetanus force ratio were found. In FR units a shortening in the twitch time parameters, a decrease in the twitch-to-tetanus ratio and an increase in the fatigue resistance were observed. In addition, a decrease in the type I and an increase in the type IIax MHC content were revealed. The six-month WBV caused a decrease in the twitch-to-tetanus force ratio in S and FR units. Other structural and physiological changes in MU properties previously seen were no longer apparent. In conclusion, responses to the long-term WBV stimulus vary between particular types of motor units, what suggests that multiple adaptive processes in muscle tissue are involved.