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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 < 0.01). 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.