Cross-Education of Strength Depends on Limb Dominance

Interhemispheric Plasticity in Humans

“…The value of 21.8% cross education observed here agrees with the magnitude (27%, adjusted for the changes in the control group) reported previously for a similarly unfamiliar movement, ulnar deviation (9) and, as expected, exceed the 8% value reported in meta analyses for familiar movements (3, 29). During effortful unilateral muscle contractions, the contralateral homologous muscle can exhibit as high as ∼20% mirror EMG activity and involuntary force (40).…”

“…The evolution of cross education was linked to a strong dependence on persistent and large gains (49.9%) in motor output of the exercised, right, FDI because the strength gains in each FDI became more strongly correlated as the study progressed from session 5 (r = 0.01), 10 (r = 0.17), 15 (r = 0.55), and to session 20 (r = 0.81). Because there were no changes in the twitch-evoked forces produced by electrical stimulation in the untrained, left, FDI, cross education, was the result of neural mechanisms and not muscle hypertrophy, as suggested previously (3, 9, 29). The progressively stronger association between the increases in EMG activity recorded during test MVCs of the untrained, left FDI and the increases in cross education suggests that an increase in gross neural drive to the untrained, left, FDI contributed to cross education, strengthening the argument for a neural, possibly supraspinal mechanism mediating cross education (23).…”

“…In addition, in cross sectional experiments unimanual serial reaction-time task transferred implicit knowledge and unimanual pinching task transferred speed and accuracy to the non-practiced hand so that the transferred motor behaviors were associated with the magnitude of decrease in IHI (2, 32). It thus appears reasonable to hypothesize that chronic unimanual practice involving, as in numerous studies before (3, 9, 29), effortful isometric contractions would produce cross education and IHI would mediate this inter-limb transfer of the ability to exert maximal force. Therefore, the purpose of the present study was to determine if increases in excitability of the non-task M1 or reductions in IHI from the task to the non-task M1 contribute to cross education produced by chronic unimanual motor practice comprising serial effortful isometric contractions of a finger muscle.…”

“…For over 100 years, the majority of studies on cross education have reported an increase in motor function in the contralateral homologous muscle after unilateral practice involving strong voluntary contractions with no skill (3, 9, 29) or involving complex motor skills (24). The present results extend these findings by showing that serial voluntary isometric contractions at submaximal intensity involving minimal skills can also produce functionally and statistically significant inter-limb transfer of maximal voluntary force.…”

“…Historical and recent observations suggest that unimanual motor practice increases the motor output not only in the practiced but also in the unpracticed homologous muscle (3, 9, 29). Neural mechanisms, which are unknown, must mediate such cross education because it occurs in the absence of muscle hypertrophy.…”