Tissue displacement is a causative factor in vibration-induced muscle injury

The early skeletal muscle response to vibration stimulus at two different displacement levels was examined. Twelve rats were anaesthetized and the hind limb was exposed to vibration, 80 Hz, 63 micronsrms (root mean square) (group 1) and 40 Hz; 130 micronsrms (group 2) for 5 hours/day for 2 days. Cross-sectional areas of vibrated muscle fibres were significantly larger in group 2. Sizes of different fibre types were differently affected: the slow-twitch type 1 fibres were significantly enlarged in both groups, while the fast-twitch type 2 fibres demonstrated a mixed response pattern. Centrally positioned muscle fibre nuclei increased significantly after vibration in group 2. It is concluded that the level of tissue displacement is a crucial factor for development of vibration-induced muscle injury.