ADAPTIVE RESPONSES TO MUSCLE LENGTHENING AND SHORTENING IN HUMANS

We tested the hypothesis that exercise training with maximal eccentric (lengthening) muscle actions results in greater gains in muscle stren gth and size than training with concentric (shortening) actions. Chang es in muscle strength, muscle fiber size, and surface electromyographi c (EMG) activity of the quadriceps muscle were compared after 36 sessi ons of isokinetic concentric (n = 8) or eccentric (n = 7) exercise tra ining over 12 wb with use of a one-leg model. Eccentric training incre ased eccentric strength 3.5 times more (pre/post 46%, P < 0.05) than c oncentric training increased concentric strength (pre/post 13%). Eccen tric training increased concentric strength and concentric training in creased eccentric strength by about the same magnitude (5 and 10%, res pectively, P > 0.05). Eccentric training increased EMG activity seven times more during eccentric testing (pre/post 86%, P < 0.05) than conc entric training increased EMG activity during concentric testing (pre/ post 12%). Eccentric training increased the EMC activity measured duri ng concentric tests and concentric training increased the EMG activity measured during eccentric tests by about the same magnitude (8 and 11 %, respectively, P > 0.05). Type I muscle fiber percentages did not ch ange significantly, but type IIa fibers increased and type IIb fibers decreased significantly (P < 0.05) in both training groups. Type I fib er areas did not change significantly (P > 0.05), but type II fiber ar ea increased similar to 10 times more (P < 0.05) in the eccentric than in the concentric group. It is concluded that adaptations to training with maximal eccentric contractions are specific to eccentric muscle actions that are associated with greater neural adaptation and muscle hypertrophy than concentric exercise.