NUMBER OF SARCOMERES IN SERIES IS UNAFFECTED BY SIX WEEKS OF CONDITIONING WITH LENGTHENING CONTRACTIONS
Susan V. Brooks, S A. Rashes, John A. Faulkner · Medicine & Science in Sports & Exercise · 2002
Despite greater susceptibility to contraction-induced injury of muscles in old compared with younger animals, muscles in adult and old animals can be conditioned for protection from injury. The adaptations underlying the protection are not known, but changes that prevent excessive stretch of sarcomeres during contraction could protect muscles from injury. For a given muscle stretch, sarcomere stretch is less in muscles with longer fibers and consequently more sarcomeres in series. PURPOSE: Test the hypothesis that the number of sarcomeres in series is greater for fibers in muscles conditioned for protection from injury compared with unconditioned muscles in adult (10 mos.) and old (22 mos.) rats. METHODS: Dorsiflexor muscles were conditioned in vivo using 180 lengthening contractions performed once a week for 6 weeks. After conditioning, extensor digitorum longus (EDL) muscles were administered an in situ lengthening contraction protocol that produces a 60% isometric force deficit in uncondtioned muscles. In both adult and old rats, the force deficit for conditioned muscles was 15%. Conditioned and contralateral unconditioned EDL muscles were harvested, fixed in 10% formalin, digested in 20% nitric acid, and individual fibers were isolated. Average length of 10 sarcomeres was measured for five regions along each fiber using an image processing system. No regional differences were found in sarcomere length so sarcomere number was calculated by dividing fiber length by mean sarcomere length. RESULTS: A total of 173 fibers from 6 (3 adult, 3 old) pairs of conditioned and unconditioned muscles were analyzed. A two-way analysis of variance showed no differences in the number of sarcomeres in series with age or conditioning, and a paired t-test confirmed no effect of conditioning on sarcomere number (conditioned: 4709 ± 131; unconditioned: 4448 ± 93). CONCLUSION: A 6-week conditioning program that induces protection from subsequent contraction-induced injury does not increase the number of sarcomeres in series in muscles of adult or old rats. Supported by AG-06157.