WILLEMS, M. E. T., and W. T. STAUBER. Force deficits by stretches of activated muscles with constant or increasing velocity. Med. Sci. Sports Exerc., Vol. 34, No. 4, pp. 667–672, 2002.
Purpose: Force deficits produced by constant (CV) versus increasing velocity (IV) stretches of rat plantar flexor muscles at low and high levels of nerve activation were studied.
Methods: Twenty repeated stretches were imposed on isometric contractions by ankle rotation from 90° to 40° at 300°·s−1 and at 3000°·s−2 during 80-Hz (CV80 and IV80) and 20-Hz stimulation (CV20 and IV20). Rest periods between contractions were 3 min. Isometric and peak stretch forces during the stretch protocols and force-frequency relationships before and 1 h after the stretch protocols were measured.
Results: Peak stretch forces were similar for IV80-CV80 and for IV20-CV20 rats but were lower for IV20-CV20 than for IV80-CV80 rats throughout the stretch protocol. At the end of the stretch protocol, isometric force deficits were similar for IV80 (49.9 ± 2.1%) and CV80 (54.5 ± 2.5%) and for IV20 (16.4 ± 2.8%) and CV20 (15.8 ± 1.9%) but lower for IV20-CV20 rats. In contrast, for all groups, deficits in peak stretch force were similar at the end of the stretch protocol (IV80: 35.0 ± 1.8%, CV80: 32.3 ± 2.2%, IV20: 26.8 ± 3.6%, CV20: 28.0 ± 2.0%). After 1 h, isometric force deficits were similar for either IV80-CV80 or IV20-CV20 at 5, 10, 20, 40, 60, and 80 Hz stimulation but were lower for IV20-CV20.
Conclusions: Variation in velocity of ankle rotation with similar peak stretch forces did not influence the amount of stretch-induced force deficits. High peak stretch forces produced greater isometric force deficits than low peak stretch forces, but the relative loss in peak stretch force was not force dependent. Different mechanisms may account for isometric force deficits and peak stretch force deficits caused by repeated stretches of activated skeletal muscles.