Impact of age on the development of fatigue during large and small muscle mass exercise

To examine the impact of aging on neuromuscular fatigue following cycling (CYC; large active muscle mass) and single-leg knee-extension (KE; small active muscle mass) exercise, 8 young (25 +/- 4 years) and older (72 +/- 6 years) participants performed CYC and KE to task failure at a given relative intensity (80% of peak power output). The young also matched CYC and KE workload and duration of the old (iso-work comparison). Peripheral and central fatigue were quantified via pre-/postexercise decreases in quadriceps twitch torque (Delta Q(tw) electrical femoral nerve stimulation) and voluntary activation (Delta VA). Although young performed 77% and 33% more work during CYC and KE, respectively, time to task failure in both modalities was similar to the old (similar to 9.5 min: P > 0.2). The resulting Delta Q(tw) was also similar between groups (CYC similar to 40%, KE -55%; P > 0.3); however, Delta VA was, in both modalities, approximately double in the young (CYC similar to 6%, KE similar to 9%; P < 0.05). While causing substantial peripheral and central fatigue in both exercise modalities in the old, Delta Q(tw) in the iso-work comparison was not significant (CYC; P = 0.2), or similar to 50% lower (KE; P < 0.05) in the young, with no central fatigue in either modality (P > 0.4). Based on iso-work comparisons, healthy aging impairs fatigue resistance during aerobic exercise. Furthermore. comparisons of fatigue following exercise at a given relative intensity mask the age-related difference observed following exercise performed at the same workload. Finally, although active muscle mass has little influence on the age-related difference in the rate of fatigue at a given relative intensity, it substantially impacts the comparison during exercise at a given absolute intensity.