Force deficits after repeated stretches of activated skeletal muscles in female and male rats

Force deficits after stretches of activated plantar flexor muscles were mea sured in six male Sprague-Dawley rats (285 +/- 10 g, age 62 +/- 4 days, mea n +/- SE) and compared with six age-matched (193 +/- 6 g, age 67 +/- 3 days ) and six weight-matched female rats (273 +/- 7 g, age 141 +/- 9 days). Twe nty stretches, imposed on isometric contractions at 90 degrees (0.2 ms puls e duration, 80 Hz, 5.4 +/- 0.3 V, duty cycle 0.006), were produced by ankle rotation from 90 to 40 degrees. Before the stretch protocol, weight-matche d groups had similar isometric forces at an ankle position of 90 degrees at 5, 10, 20, 40, 60 and 80 Hz but forces were lower for age-matched females. For all groups, normalized force-frequency relationships were similar. Dur ing the stretch protocol, deficits for isometric force at 90 degrees and pe ak stretch force at 400 with stretch number were similar for all groups. On e hour after the stretches, isometric force deficits at 90 degrees at 40, 6 0 and 80 Hz were larger for females in weight-matched groups (e.g. 80 Hz, f emale. 47.8 +/- 1.7%; male: 41.1 +/- 1.7%; P < 0.05), perhaps because of a difference in age (P < 0.05). For age-matched groups, isometric force defic its at 90 degrees were similar at all frequencies. The susceptibility for f orce deficits by stretches of activated skeletal muscles was not gender-dep endent for 2-month-old rats.