Cross-education: Is it a viable method for rehabilitation?

In 1894, psychologists Edward W. Scripture, Theodate L. Smith, and Emily M. Brown reported for the first time the curious observation that practicing a motor skill with one hand also dramatically improved the non-practiced hand, giving rise to the phenomenon now coined as cross-education. Cross-education is the increase in motor output (i.e., force generation, skill) of the opposite, untrained limb following a period of unilateral motor training. 1 The potential to exploit such inter-limb adaptations for the purposes of rehabilitation of unilateral neurological or orthopedic injuries has captured the attention of scientists and therapists for years. The magnitude of cross-education varies greatly between muscles and participants. In healthy adults, resistance training improves maximal voluntary force of the untrained limb by up to ~20%, usually half of the trained limb's improvement, but mirror training, 2 non-invasive brain stimulation 3 and neuromuscular electrical stimulation 4 can augment the transfer effects. Although not completely unraveled, researchers agree that the effects are likely driven by neuroplasticity in the primary and supplementary motor brain regions. 1 Until recently, it has remained unclear if cross-education could aid rehabilitation of patients after a (unilateral) fracture, surgical intervention, a stroke, or multiple sclerosis (MS). During experimental arm immobilization of healthy adults, cross-education offset declines in strength and muscle cross-sectional area. 5 Cross-education appears to be amplified in clinical settings, with evidence for improved grip strength and range of motion after wrist fracture, 6 wrist and ankle strength in chronic post-stroke hemiparesis, 7 and ankle strength and mobility in persons with MS. 8 Cross-education effects in MS were similar to direct training of the more affected side; 8 efficacious for scenarios where the more affected limb is unable to train or becomes fatigued.