Effects of lengthened immobilization on functional and histochemical properties of rabbit tibialis anterior muscle.

The imposition of long-term lengthened immobilization on rabbit tibialis anterior (TA) muscles resulted in rapid increases in slow oxidative (SO) fibre number. After 2 weeks, SO number had increased 2-fold and was 5 times greater after 6 weeks immobilization. There were also fibre-type-specific effects on SO, fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibre areas. Twitch strength was unchanged throughout immobilization whilst production of tetanic tension was impaired during the initial period but had returned to control levels by 6 weeks. Twitch contraction times and isometrically determined rates of rise and relaxation were largely unaffected by immobilization despite the marked increase in expression of slow myosin. The change in phenotypic expression of the lengthened TA was not attributable to chronically increased levels of activation since integrated EMG activity was unchanged from control values throughout the immobilization period. Thus it is suggested that a chronic increase in tension consequent on the lengthening procedure is a potent stimulus for fast-to-slow myosin transformation.