Impact of muscle length during stretch-shortening contractions on real-time and temporal muscle performance measures in rats in vivo

Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, West Virginia 26505 Submitted 21 January 2003 ; accepted in final form 25 September 2003 The objective of the present study was to investigate the impact of muscle length during stretch-shortening cycles on static and dynamic muscle performance. Animals were randomly assigned to an isometric (control, Con, n = 12), a short-muscle-length (S-Inj, 1.22-2.09 rad, n = 12), or a long-muscle-length (L-Inj, 1.57-2.44 rad, n = 12) group. The dorsiflexor muscles were exposed in vivo to 7 sets of 10 stretch-shortening contractions (conducted at 8.72 rad/s) or 7 sets of isometric contractions of the same stimulation duration by using a custom-designed dynamometer. Performance was characterized by multipositional isometric exertions and positive, negative, and net work before exposure, 6 h after exposure, and 48 h after exposure to contractions. Real-time muscle performance during the stretch-shortening cycles was characterized by stretch-shortening parameters and negative, positive, and net work. The S-Inj group recovery (force difference) was similar to the Con group force difference at 48 h, whereas the L-Inj group force difference was statistically greater at 1.39, 1.57, and 1.74 rad than the Con group force difference ( P < 0.05). Negative work ( P < 0.05) and net work ( P < 0.05) were statistically lower in the S-Inj and L-Inj groups than in the Con group 48 h after exposure to contractions. Of the real-time parameters, there was a difference in cyclic force with treatment during the stretch-shortening cycles ( P < 0.0001), with the L-Inj group being the most affected. Thus longer ranges of motion result in a more profound isometric force decrement 48 h after exposure to contractions and in real-time changes in eccentric forces. muscle injury; dorsiflexor muscles; cycle work Address for reprint requests and other correspondence: R. G. Cutlip, National Institute for Occupational Safety and Health, Health Effects Laboratory Division, 1095 Don Nehlen Dr., M/S 2027, Morgantown, WV 26505 (E-mail: rgc8{at}cdc.gov ).