The phenomenon where an ipsilateral strength training intervention increases strength of homologous untrained muscle is called cross-education. A Meta-Analysis on contralateral effects of unilateral training (UT) has shown an increase of 7.8% in strength of the contralateral limb and 35.1% gains on the trained limb (Munn, Herbert & Gandevia, 2004). Another large study (n=115) performing elbow flexion with different contraction velocities has shown improvements of 7% on the untrained limb. The cross education effect might be relevant while treating injured athletes as it has shown to prevent the loss of strength on an immobilized limb and ithas significantly improved the strength (P<0.01) on the free arm, from pre-test peak torque 14.5 Nm to 17.7 Nm post-test with no changes in strength of the immobilized cast armpre-test 13.9Nm to 14.2 Nm post-test coupled with a muscle thickness pre-test 3.61 to 3.57cmpost-test.
Shima et al., (2002) conducted a 6 week training intervention and have found a 7.8% strength gains oncontralateral limb and after 6weeks of detraining the effects on cross-education appears to not have changed significantly. It appears that the individual rate of strength gain by cross education on the contralateral limb are significantly correlated with rate of strength loss after detraining and the faster is the individual gains in cross education the faster is the loss. Apparently cross education transfer effects appears to be evoked quite fast as a single unilateral surface electrical stimulation was sufficient to enhance the strength of the contralaterallimb. Additionally, cross-education appears to not solely occur in the agonist muscle as a study from Dragert and Zehr (2010) have shown a decrease on the recruitment of the high threshold motor units of the antagonist muscle with a reduction on spinal excitability of the trained and untrained limb. Furthermore, the manipulation of strength variables such as load, velocity and the precision of movements appear to improve the effectiveness of neural transmission along the corticospinal pathway to the trained and untrained musculature when compared to slow training. These findings are supported in a study of unilateral ballistic training which has been suggested to enhance bilateral corticospinal adaptations.
The precise mechanism responsible for muscular and neural adaptations seems to be complex and it is beyond this work to explain. At the muscular level, it appears that UT is unlikely to yield enough motor unitactivation to elicit muscular adaptations on the untrained limb and generate hypertrophy, changes in muscle enzymeand changes in contractile protein composition. The proposed candidates for neural mechanisms for force production on the contralateral limbs appear to come from the cortical, subcortical and spinal cord mechanisms. Thus, this knowledge about the mechanisms underlying cross education could offer a more effective way to use of resistance training protocols to improve recovery from an injury that predominantly affect one side of the body.