Unilateral strength training with and without a mirror to improve motor function after stroke: Past, present, and future

Unilateral resistance training not only strengthens muscles on the trained side but also the homologous muscles on the untrained side. This phenomenon is called cross-education and was first observed in 1894. Since then, many studies have been conducted in neurologically intact persons but the potential of cross-education for people with unilateral orthopaedic and neurological impairments remains largely unexplored. In this commentary, we highlight the potential of cross-education in the rehabilitation after stroke. Current clinical practice in stroke rehabilitation is to train the more-affected side but people with a severe hemiparesis are often not able to train this side due to muscle weakness and limited range of motion. Resistance training of the less-affected side might be a great tool to improve muscle strength and motor function of the more-affected side. Mirror therapy, where a mirror reflection of the less-affected side creates the illusion that the more affected side is exercising, could even further augment these cross-education benefits. Three functional networks (i.e., attentional resources, the mirror-neuron system, and the motor network) play a role in cross-education with and without a mirror and induce neuroplasticity in the brain that can help people who have had a stroke in their recovery. The use of inter-limb mechanisms in the rehabilitation from unilateral impairments has to be exploited further and should be incorporated in the standard protocols for neurologic and musculoskeletal rehabilitation.