Cross-education of arm muscular strength is unidirectional in right-handed individuals

Purpose: Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training. The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals. Methods: Thirty-nine strongly right-handed females were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or nontraining control (CON) group. Strength training was 6 wk of maximal isometric ulnar deviation, 4X wk(-1). Peak torque, muscle thickness (ultrasound), and EMG activity were assessed before and after training in both limbs. Results: The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; P < 0.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4 and 12.2%). Strength training also increased trained limb strength in the LEFT (41.9%, P < 0.01) and the RIGHT (25.9%; P < 0.01) groups. Training groups increased trained limb muscle thickness (RIGHT and LEFT combined: 4.1 %) compared to CON (-4.0%) (P < 0.0 1). There were no changes in muscle thickness of untrained limbs compared to CON. Trained limb agonist EMG activation increased with training (P < 0.05) with no change for the antagonist. Changes in untrained limb EMG were not different compared to CON. Conclusions: Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. We conclude that cross-education of arm muscular strength is most pronounced to the nondominant arm.