Sv. Brooks et Ja. Faulkner, ISOMETRIC, SHORTENING, AND LENGTHENING CONTRACTIONS OF MUSCLE-FIBER SEGMENTS FROM ADULT AND OLD MICE, The American journal of physiology, 267(2), 1994, pp. 30000507-30000513
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.