Restoring symmetry: clinical applications of cross-education.

The "restoring symmetry" hypothesis poses that cross-education of strength--a crossed-limb adaptation after unilateral training--is best applied to clinical conditions presenting with asymmetries. Cross-education mechanisms should be viewed as evolutionarily conserved circuits that have a small impact on daily life but a meaningful impact for rehabilitation. Two recently published examples are hemiparesis after stroke and unilateral orthopedic injury.