This study compared the contribution of supraspinal fatigue to muscle fatigue in old and young adults. Transcranial magnetic stimulation (TMS) of motor cortex was used to assess voluntary activation during maximal voluntary contractions (MVCs) of elbow flexor muscles in 17 young adults (25.5 ± 3.6 yr; mean±SD) and 7 old adults (73.0 ± 3.3 yr). Subjects performed a fatigue task involving 6 sustained MVCs (22-s duration, separated by 10 s). Young adults exhibited greater reductions in maximal voluntary torque (67 ± 15% of baseline) than the old (37 ± 6%, P<0.001). Increments in torque (superimposed twitch) generated by TMS during sustained MVCs increased for the young and old (P<0.001) but were larger for the old adults at the start of the sustained contractions and during recovery (P<0.05). Voluntary activation was less for the old adults at the start of some sustained contractions and during recovery (P=0.02). Motor evoked potential area increased similarly with age during the fatiguing task but was greater for the old adults than young during recovery. Silent period duration lengthened less for the old adults during the fatigue task. At the end of the fatiguing task, peak relaxation rate of muscle fibres had declined more in the young than the old adults. The greater endurance with age is largely due to a difference in mechanisms located within the muscle. However, recovery from the fatiguing exercise is impaired for old adults because of greater supraspinal fatigue than in the young.