Adaptive responses to muscle lengthening and shortening in man. Hortobagyi, Tibor, Jeff P. Hill, Joseph A. Houmard, David D. Fraser, Nancy J. Lambert, Richard G. Israel. Biomechanics and Human Performance, Laboratories and the Departments of Medicine and Physical Therapy, East Carolina University, Greenville, NC 27858, U.S.A.
APStracts 2:0464A, 1995.
The hypothesis was tested that exercise training with maximal eccentric (lengthening) muscle actions results in greater gains in muscle strength and size compared to training with concentric (shortening) actions. Changes in muscle strength, muscle fiber size, and surface electromyographic (EMG) activity of the quadriceps muscle have been compared following 36 sessions/12 weeks of either isokinetic concentric (n = 8) or eccentric (n = 7) exercise training using a one-leg model. Eccentric training increased eccentric strength 3.5 times more (pre/post 46%, P &LT 0.05) than concentric training increased concentric strength (pre/post 13%). Eccentric training increased concentric strength and concentric training increased eccentric strength by about the same magnitude (5% and 10%, respectively P&GT 0.05). Eccentric training increased 7 times more the EMG activity during eccentric testing (pre/post 86%, P&LT 0.05) than concentric training increased EMG activity during concentric testing (pre/post 12%). Eccentric training increased the EMG activity measured during concentric tests and concentric training increased the EMG activity measured during eccentric tests by about the same magnitude (8% and 11%, respectively P &GT 0.05). No significant changes occurred in Type I muscle fiber percentages, but Type IIa increased and Type IIb decreased significantly (P &LT 0.05) in both training groups. No significant changes occurred in Type I fiber areas (P &GT 0.05), but Type II fiber area increased about 10 times more (P &LT 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 compared to concentric exercise. 

Received 23 November 1994; accepted in final form 10 October
1995.
APS Manuscript Number A1197-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95