A Pilot Mixed-Design Study of Cross-Education-Related Strength Gains and Reticulospinal Involvement in Young and Older Adults.

Cross-education refers to strength gains in the untrained limb following unilateral training, typically attributed to corticospinal adaptations. These changes are modest and may not fully explain interlimb transfer in older adults. This study assessed cortico-reticular and reticulospinal tract excitability in young and older adults using a multimodal neurophysiological approach to determine whether subcortical pathways contribute when corticospinal plasticity is age limited. Thirteen older (67 ± 5 years) and 12 young adults (26 ± 6 years) completed two weeks of unilateral elbow flexor strength training. Strength, cortical excitability, and reticulospinal function were evaluated in the untrained limb using single- and paired-pulse TMS, StartReact, and behavioral measures. Both younger and older adults showed significant strength gains in the untrained arm (young: +22%, older: +20%), with no group × time interaction (p = 0.556). Corticospinal excitability, assessed via stimulus-response curves (100%-170% aMT), remained unchanged (p = 0.624). Intracortical facilitation increased at Week 2 (p = 0.011), while short-interval intracortical inhibition did not change (p = 0.269). Reaction times were significantly reduced across groups (p < 0.001), and the rate of force development increased at retention (p = 0.019), consistent with enhanced reticulospinal excitability. Cross-activation of the ipsilateral motor cortex increased at all time points in young adults (p < 0.001), but only at retention in older adults (p < 0.001). Voluntary activation improved exclusively in older adults (p < 0.001). Cross-education was mediated by subcortical mechanisms, notably increased reticulospinal excitability. Cortical adaptations are modest and age-independent. The bilateral organization of the reticulospinal system positions it as a promising target for interventions to enhance contralateral motor output through unilateral training.