SKELETAL-MUSCLE FATIGUE AND PHYSICAL ENDURANCE OF YOUNG AND OLD MICE

In order to evaluate the role played by muscular and extramuscular fac tors in the development of fatigue in old age, the time course of fati gue in isolated skeletal muscles and spontaneous motor activity and en durance of whole animals were monitored using young (3-6 months) and o ld (34-36 months) CF57BL/6J mice. The isolated extensor digitorum long us (EDL) and soleus muscles from old mice had smaller (P < 0.05) mass and developed lower (P < 0.02) maximal tetanic tension at 100-Hz stimu lation than the muscles of young mice. During stimulation at 30 Hz eve ry 2.5 s, a 50% decline in original tetanic tension occurred by 109 s in young EDL and 129 s in old EDL, but by 482 s in young soleus and 11 34 s (projected) in old soleus, indicating more (P < 0.05) resistance to fatigue in old than young soleus. However, the old mice showed sign ificantly fewer (P < 0.002) spontaneous ambulatory movements than the young mice. On a treadmill with a belt speed of 10 m/min at an inclina tion of 0 degrees, the old mice could only run for 22 min compared to 39 min ran by young mice (P < 0.02). They took more rest periods (P < 0.02) than the young mice. In a quantitative swimming monitor, the old mice swam for a shorter (P < 0.05) time than young mice (20.4 min com pared to 28.6 min). Integrated swimming activity at 20 min was smaller (P < 0.05) in old mice than in young mice (413 g/s compared to 628 g/ s). Hence increased fatigue in old age is not caused by impairment of processes within the muscles, but by impairment of central or extramus cular processes. (C) 1998 John Wiley & Sons, Inc.