Modulation of primary motor cortex after experimentally induced and use-dependent plasticity in young and older adults.

The ability to induce plasticity in human primary motor cortex (M1) may be diminished with advancing age. Intracortical inhibition is critical for M1 plasticity and regular participation in physical activity can promote M1 plasticity. This study assessed modulation of M1 excitability and inhibition after paired associative stimulation (PAS) and motor skill acquisition in young and older adults, which also considered the cardiorespiratory fitness of each participant. Thirty-one older (60-88 years) and 20 young (20-33 years) adults were recruited. Electromyographic recordings were obtained from the dominant hand first dorsal interosseous muscle. A sequential visual isometric force task that required index finger abduction was used to investigate motor skill acquisition. Transcranial magnetic stimulation protocols were used to examine corticomotor excitability and short- and long-interval intracortical inhibition (SICI and LICI) before and after each intervention. Corticomotor excitability, normalized to baseline, increased after PAS in young but not older adults, while no age-related differences were observed after skill acquisition. Facilitation of corticomotor excitability after PAS, but not skill acquisition, was positively correlated with cardiorespiratory fitness. SICI decreased after PAS and increased after skill acquisition, with no differences between age groups. LICI increased for young adults and decreased for older adults after PAS, but did not change after skill acquisition. Overall, there was intervention-specific modulation of inhibition, an age-related difference in LICI after PAS, and higher cardiorespiratory fitness was associated with greater corticomotor excitability facilitation after PAS. These findings may help inform future endeavors focused on attenuating age-related declines in brain and motor-cognitive function.