EFFECT OF EXERCISE TRAINING ON PASSIVE STIFFNESS IN LOCOMOTOR SKELETAL-MUSCLE - ROLE OF EXTRACELLULAR-MATRIX

The purpose of this study was to evaluate the effect of endurance exer cise training on both locomotor skeletal muscle collagen characteristi cs and passive stiffness properties in the young adult and old rat. Yo ung (3-mo-old) and senescent (23-mo-old) male Fischer 344 rats were ra ndomly assigned to either a control or exercise training group [young control (YC), old control (OC), young trained (YT), old trained (OT)]. Exercise training consisted of treadmill running at similar to 70% of maximal oxygen consumption (45 min/day, 5 days/wk, for 10 wk). Passiv e stiffness (stress/strain) of the soleus (Sol) muscle from all four g roups was subsequently measured in vitro at 26 degrees C. Stiffness wa s significantly greater for Sol muscles in OC rats compared with YC ra ts, but in OT rats exercise training resulted in muscles with stiffnes s characteristics not different from those in YC rats. Sol muscle coll agen concentration and the level of the nonreducible collagen cross-li nk hydroxylysylpyridinoline (HP) significantly increased from young ad ulthood to senescence. Although training had no effect on Sol muscle c ollagen concentration in either age group, it resulted in a significan t reduction in the level of Sol muscle HP in OT rats. In contrast, exe rcise had no effect on HP in the YT animals. These findings indicate t hat 10 wk of endurance exercise significantly alter the passive viscoe lastic properties of Sol muscle in old but not in young adult rats. Th e coincidental reduction in the principal collagen cross-link HP also observed in response to training in OT muscle highlights the potential role of collagen in influencing passive muscle viscoelastic propertie s.