There is discrepancy in the literature regarding the degree to which old age affects muscle bioenergetics. These discrepancies are likely influenced by several factors, including variations in physical activity (PA) and differences in the muscle group investigated. To test the hypothesis that age may affect muscles differently, we quantified oxidative capacity of tibialis anterior (TA) and vastus lateralis (VL) muscles in healthy, relatively sedentary younger (8 YW, 8 YM; 21–35 years) and older (8 OW, 8 OM; 65–80 years) adults. To investigate the effect of physical activity on muscle oxidative capacity in older adults, we compared older sedentary women to older women with mild-to-moderate mobility impairment and lower physical activity (OIW, n = 7), and older sedentary men with older active male runners (OAM, n = 6). Oxidative capacity was measured in vivo as the rate constant, kPCr, of postcontraction phosphocreatine recovery, obtained by31P magnetic resonance spectroscopy following maximal isometric contractions. While kPCrwas higher in TA of older than activity-matched younger adults (28%; p = 0.03), older adults had lower kPCrin VL (23%; p = 0.04). In OIW compared with OW, kPCrwas lower in VL (∼45%; p = 0.01), but not different in TA. In contrast, OAM had higher kPCrthan OM (p = 0.03) in both TA (41%) and VL (54%). In older adults, moderate-to-vigorous PA was positively associated with kPCrin VL (r = 0.65, p < 0.001) and TA (r = 0.41, p = 0.03). Collectively, these results indicate that age-related changes in oxidative capacity vary markedly between locomotory muscles, and that altered PA behavior may play a role in these changes.

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, n = 8M, 8F) adults who were matched to young adults (21–35; 8M, 8F) 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 deg·s−1. Fatigue was induced with 4 min 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 MVC torque than young subjects during isometric contractions (mean±SE of 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.

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It is well established that exercise training results in increased muscle oxidative capacity. Less is known about how oxidative capacities in distinct muscles, in the same individual, are affected by different levels of physical activity. We hypothesized that 1) trained individuals would have higher oxidative capacity than untrained individuals in both tibialis anterior (TA) and vastus lateralis (VL) and 2) oxidative capacity would be higher in TA than VL in untrained, but not in trained, individuals. Phosphorus magnetic resonance spectroscopy was used to measure the rate of phosphocreatine recovery (k(PCr)), which reflects the rate of oxidative phosphorylation, following a maximal voluntary isometric contraction of the TA and VL in healthy untrained (7 women, 7 men, 25.7 +/- 3.6 yr; mean +/- SD) and trained (5 women, 7 men, 27.5 +/- 3.4 yr) adults. Daily physical activity levels were measured using accelerometry. The trained group spent threefold more time ( approximately 90 vs. approximately 30 min/day; P < 0.001) in moderate to vigorous physical activity (MVPA). Overall, k(PCr) was higher in VL than in TA (P = 0.01) and higher in trained than in untrained participants (P < 0.001). The relationship between k(PCr) and MVPA was more robust in VL (r = 0.64, P = 0.001, n = 25) than in TA (r = 0.38, P = 0.06, n = 25). These results indicate greater oxidative capacity in vivo in trained compared with untrained individuals in two distinct muscles of the lower limb and provide novel evidence of higher oxidative capacity in VL compared with TA in young humans, irrespective of training status. The basis for this difference is not known at this time but likely reflects a difference in usage patterns between the muscles.