Age-related fatigue resistance in the knee extensor muscles is specific to contraction mode

The question of whether skeletal muscle fatigue is preserved or enhanced in older adults is a point of controversy. Disparate findings may be attributed to differences in subject population and study protocols, including contraction mode. The purpose of this study was to test the hypotheses that healthy older (65-80 years of age, 8 males and 8 females) adults who were matched to young adults (21-35 years of age; 8 males and 8 females) with similar physical activity levels would: (1) fatigue less during isometric knee extensor (KE) contractions, but (2) would show similar fatigue during dynamic KE contractions performed at 120 degrees s(-1). Fatigue was induced with 4 minutes of intermittent, isometric, or dynamic maximal voluntary contractions, performed on separate days. Electrically stimulated contractions were used to evaluate central activation during both fatigue protocols. Older subjects maintained a higher percentage of baseline maximum voluntary contraction (MVC) torque than young subjects during isometric contractions (mean +/- SE: 71 +/- 3% and 57 +/- 3%, respectively, P < 0.01). In contrast, there was no difference between age groups in torque maintenance during dynamic contractions (43 +/- 3% and 44 +/- 3%, respectively, P = 0.86). For both groups, changes in electrically stimulated and voluntary contractions followed similar trends, suggesting that central activation did not play a role in the age-related differences in fatigue. Fatigue during the isometric protocol was associated with fatigue during the dynamic protocol in the young group only (r = 0.62, P = 0.01), suggesting that distinct mechanisms influence fatigue during isometric and dynamic contractions in older adults. Muscle Nerve 39: 692-702, 2009.