Age-related differences in corticospinal and reticulospinal adaptations to short-term strength training

High force production depends on effective transmission of motor commands to spinal motoneurones, a capacity that declines with age. Although strength adaptations are often attributed to cortical mechanisms, subcortical pathways providing strong descending drive remain understudied. The reticulospinal tract, with its fast, widespread projections to motoneurones, is well positioned to support force generation, yet its role in human strength adaptation is largely unknown. This study examined age-related differences in corticospinal and reticulospinal adaptations following short-term strength training. Thirteen older adults (67 ± 5 years) and twelve younger adults (26 ± 6 years) completed a two-week unilateral elbow-flexor strength training programme. Neurophysiological and behavioural assessments were performed to evaluate cortical and subcortical activity and their contribution to strength gains. Both groups demonstrated similar increases in strength; however, corticospinal excitability decreased in younger adults and remained unchanged in older adults. Reduced GABAergic inhibition, indexed by shortened cortical silent periods, occurred in both groups. Reaction time and rate of force development in response to startling stimuli improved in both groups, with a more pronounced StartReact effect in older adults, consistent with enhanced reticulospinal responsiveness. Voluntary drive, measured by central activation ratio, increased significantly in older adults (p < 0.001), whereas MMAX remained stable. Early strength gains were accompanied by reduced intracortical inhibition and increased engagement of subcortical descending pathways. The pronounced StartReact responses and enhanced early-force kinetics provide indirect evidence of greater reticulospinal contribution, with older adults appearing to rely more heavily on this pathway when corticospinal plasticity is limited. Short-term strength training improved rate of force development and reaction time in older and younger adults. Corticospinal excitability decreased in younger adults after training, with no change in older adults, indicating age-related differences in corticospinal plasticity. GABAergic inhibition, as indexed by cortical silent period (cSP), was reduced in both groups following training.