The measurement of force/velocity relationships of fresh and fatigued human adductor pollicis muscle

The purpose of the study was to obtain force/velocity relationships for ele ctrically stimulated (80 Hz) human adductor pollicis muscle (n = 6) and to quantify the effects of fatigue. There are two major problems of studying h uman muscle in situ; the first is the contribution of the series elastic co mponent, and the second is a loss of force consequent upon the extent of lo aded shortening. These problems were tackled in two ways. Records obtained from isokinetic releases from maximal isometric tetani showed a late linear phase of force decline, and this was extrapolated back to the time of rele ase to obtain measures of instantaneous force. This method gave usable data up to velocities of shortening equivalent to approximately one-third of ma ximal velocity. An alternative procedure (short activation, SA) allowed the muscle to begin shortening when isometric force reached a value that could be sustained during shortening (essentially an isotonic protocol). At low velocities both protocols gave very similar data (r(2) = 0.96), but for hig h velocities only the SA procedure could be used. Results obtained using th e SA protocol in fresh muscle were compared to those for muscle that had be en fatigued by 25 s of ischaemic isometric contractions, induced by electri cal stimulation at the ulnar nerve. Fatigue resulted in a decrease of isome tric force [to 69 (3)%], an increase in half-relaxation time [to 431 (10)%] , and decreases in maximal shortening velocity [to 77 (8)%] and power [to 4 2 (5)%]. These are the first data for human skeletal muscle to show convinc ingly that during acute fatigue, power is reduced as a consequence of both the loss of force and slowing of the contractile speed.