SHORTENING-INDUCED FORCE DEPRESSION IN HUMAN ADDUCTOR POLLICIS MUSCLE

1. The effects of single isovelocity shortening contractions on force production of the electrically stimulated human adductor pollicis musc le were investigated in seven healthy male subjects. 2. Redeveloped is ometric force immediately following isovelocity shortening was always depressed compared with the isometric force recorded at, the same musc le length but without preceding shortening. The maximal isometric forc e deficit (FD) was (mean +/- S.E.M.) 37 +/- 2 % after 38 deg of shorte ning at 6.1 deg s(-1) 3. The FD was positively correlated with angular displacement (r(2) > 0.98) and decreased with increasing velocity of the shortening step. Stimulation at 20 Hz instead of 50 Hz reduced abs olute force levels during the contractions to about 73% and the FD was decreased to a similar extent. Eighty-nine per cent of tile velocity- related variation in the FD could be explained by the absolute force l es els during shortening. 4. FD was largely abolished by allowing the muscle to relax briefly (approximately 200 ms), a time probably too sh ort for significant metabolic recovery. 5. At all but the highest velo cities there was a linear decline in force during the latter part of t he isovelocity shortening phase, suggesting that the mechanisms underl ying FD were active during shortening. 6. Our results show that shorte ning-induced force deficit is a significant feature of human muscle wo rking in situ and is proportional to the work done by the muscle-tendo n complex. This finding has important implications for experimental st udies of force-velocity relationships in the intact human.