The force‐velocity relationship of human adductor pollicis muscle during stretch and the effects of fatigue

1 We have examined the force‐velocity characteristics of tetanically activated human adductor pollicis working in vivo , in the fresh and fatigued states. 2 The increase in force in response to stretch was divided into two major components. The first, steady , component persisted after the stretch and is concluded not to be a function of active cycling cross‐bridges because it was not affected by either the velocity of the stretch or the level of muscle activation. 3 The origin of the second, transient , component of the increased force seen during stretch is consistent with cross‐bridge activity since it increased with increasing velocity of stretch and was proportional to the level of activation. 4 It is likely that both components of the stretch response make a significant contribution to muscle performance when acting to resist a force. For the fastest stretch used, the contributions of cross‐bridge and non‐cross‐bridge mechanisms were equal. For the slowest stretch, lasting 10 s and over the same distance, the force response was attributed almost entirely to non‐cross‐bridge mechanisms. 5 As a result of acute fatigue (50 % isometric force loss) there were only small reductions in the non‐cross‐bridge component of the force response to stretch, while the cross‐bridge component decreased in absolute terms. 6 The transient component of the stretch response increased as a result of fatigue, relative to the isometric force, while the force during shortening decreased. The results are consistent with a decrease in cross‐bridge turnover in fatigued muscle.