Effects of isometric training on skeletal myosin heavy chain expression

This studytested the hypothesis that an isometric resistance-training programinduces upregulation of slow myosin heavy chain (MHC) expression in afast-twitch skeletal muscle. Thus we studied the effects of tworesistance-training programs on rodent medial gastrocnemius (MG) musclethat were designed to elicit repetitive isometric contractions(10-12 per set; 4 sets per session) of different duration (8 vs. 5 s) and activation frequency (100 vs. 60 Hz) per contraction during eachtraining session (total of 6 and 12 sessions). Results showed that bothtraining paradigms produced significant increases in muscle weight(~11-13%) after completion of training(P < 0.05). Significanttransformations in MHC expression occurred and involved specifically adecrease in the relative expression of the fast type IIb MHC andconcomitant increased expression of the fast type IIx MHC.These adaptations were observed in both the "white" and"red" regions of the MG, and they occurred at both the mRNA andprotein levels. These adaptations were detected after onlysix training sessions. Neither of the training programs produced anychange in the relative expression of either the slow type I MHC or themoderately fast type IIa MHC, which can be upregulated in the red MG bychronic functional overload. These findings show that theisometric protocols used in this investigation were not sufficient toinduce the hypothesized changes in the myosin heavy chain isoformexpression in rodent skeletal muscle.