The aim of this study was to examine changes in corticomotor excitability and plasticity after a thumb abduction-training task in young and old adults. Electromyographic (EMG) recordings were obtained from the right abductor pollicis brevis (APB, target muscle) and abductor digiti minimi (ADM, control muscle) in 14 young (18-24 years) and 14 old (61-82 years) adults. The training task consisted of 300 ballistic abductions of the right thumb to maximize peak thumb abduction acceleration (TAAcc). Transcranial magnetic stimulation of the left primary motor cortex was used to assess changes in APB and ADM motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before, immediately after, and 30 minutes after training. No differences in corticomotor excitability (resting and active TMS thresholds, MEP input-output curves) or SICI were observed in young and old adults before training. Motor training resulted in improvements in peak TAAcc in young (177% improvement, P<0.001) and old (124%, P=0.005) subjects, with greater improvements in young subjects (P=0.002). Different thumb kinematics were observed during task performance, with increases in APB EMG related to improvements in peak TAAcc in young (r²=0.46, P = 0.008) but not old adults (r²=0.09, P=0.3). After training, APB MEPs were 50% larger (P<0.001 compared with before) in young subjects, with no change after training in old subjects (P=0.49), suggesting reduced use-dependent corticomotor plasticity with advancing age. These changes were specific to the APB muscle, as no training-related change in MEP amplitude was observed in ADM. No significant association was observed between the change in APB MEP and the improvement in TAAcc with training in individual young and old subjects. SICI remained unchanged following training in both groups, suggesting that it was not responsible for the diminished use-dependent corticomotor plasticity for this task in older adults.