Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad(.)s(-1)) or slow (0.52 rad(.)s(-1)) velocity eccentric musc le contractions. A group of 20 non-resistance trained subjects were assigne d to a FAST (n = 7), SLOW (n = 6) or non-training CONTROL (n = 7) group. A unilateral training protocol targeted the elbow flexor muscle group and con sisted of 24 maximal eccentric isokinetic contractions (four sets of six re petitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and c oncentric and eccentric torque at 0.52 and 3.14 rad(.)s(-1) were examined a t 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significan t [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [ 27.4 (7.3) %] at 3.14 rad(.)s(-1), isometric torque [21.3 (4.3)%] and eccen tric torque [25.2 (7.2) %] at 0.52 rad(.)s(-1). The percentage of type I fi bres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while ty pe lib fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P < 0.05 ]. In contrast. the SLOW group did not experience significant changes in mu scle fibre type or muscle torque. We conclude that neuromuscular adaptation s to eccentric training stimuli may be influenced by differences in the abi lity to cope with chronic exposure to relatively fast and slow eccentric co ntraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contract ions.