ISOMETRIC, SHORTENING, AND LENGTHENING CONTRACTIONS OF MUSCLE-FIBER SEGMENTS FROM ADULT AND OLD MICE

In old animals, skeletal muscle force decreases during both isometric and shortening contractions. In contrast, force during lengthening app ears to be unaffected by aging. We hypothesized that with aging single permeabilized muscle fibers would demonstrate the same impairments in force as are observed for whole muscles. For single permeabilized fib ers from extensor digitorum longus muscles of adult and old mice, forc es were measured during isometric, shortening, and lengthening contrac tions performed at 15 degrees C. Maximum isometric forces normalized f or fiber area were not different for fibers from adult and old mice. D uring submaximal isometric contractions a decreased calcium sensitivit y resulted in lower forces for fibers from old compared with adult mic e. In contrast to a lack of difference in forces developed by fibers f rom old and adult mice during shortening contractions, during lengthen ing contractions fibers from old mice developed forces similar to 30% higher than those of adult mice. We conclude that the impairments in f orce of whole muscles with aging are not the result of impairments in intrinsic force-generating capacity of cross bridges, but changes do o ccur in single permeabilized muscle fibers of old mice that result in higher forces during stretch.