Changes in force by repeated stretches of skeletal muscle in young and oldfemale Sprague Dawley rats

We examined whether the decline in isometric force and peak stretch force d uring repeated stretches of activated plantar flexor muscles was larger in old (24 months, N=5) compared with young (4 months, N=5) female Sprague Daw ley rats. Thirty stretches were imposed on isometric contractions (stimulat ion time 1.9 s, rest periods 180 s) by ankle rotation from 90 degrees to 40 degrees at 50 degrees s(-1) Even though muscle weights were similar [2094/-54 (young) vs 2033+/-73 mg (old) (mean+/-SE)], the isometric force (IF) a t 90 degrees before stretch 1 and peak stretch force (PSF) at 40 degrees fo r stretch 1 were lower in old rats [18.9+/-0.5 vs 15.5+/-1.4 N (p=0.024) an d 25.8+/-1.3 vs 20.5+/-0.9 N (p=0.017)]. After the stretches, the IF defici t teas similar [38.5+/-3.0% (young) vs 39.8+/-3.4% (old)] and did not recov er after 1 hour of rest [35.1+/-4.1% (young) vs 36.9+/-4.3% (old)]. The dec line in PSF was also similar (36.0+/-5.2% (young) vs 26.6+/-1.8% (old)]. Sk eletal muscles of 24-month-old female Sprague Dawley rats were weaker than 4-month-old rats but had similar susceptibility to develop IF deficits afte r stretches of activated muscles.