The Examination of Potential Mechanisms Underlying the Cross Education Phenomenon

Abstract

Cross education is the strength or skill gain that is found in the contralateral, homologous limb following a unilateral training program or practice. This phenomenon provides a beneficial rehabilitation model for unilateral injuries or neurologic disorders, such as stroke. Although the cross-body transfer of strength and skill are each widely studied, they are rarely examined concurrently, despite each contributing to the goal of functional movement rehabilitation. Therefore, the overall purpose of this thesis was to examine the neuromuscular adaptations of unilateral resistive exercise training contributing to the transfer of strength and skill, while employing the necessary methodological controls that have been under-examined and under-used. The assessment of neuromuscular mechanisms requires both voluntary and evoked contractions to be performed simultaneously. Therefore Manuscript 1 examined a novel electrode configuration, consisting of one electrode on the electrically identified motor point and the second electrode directly adjacent in a bipolar configuration. Both voluntary surface electromyography measures and evoked potentials were found to be reliable (ICCs > 0.75) and effective across multiple test sessions. Manuscript 2 was a comprehensive review of 90 unilateral training studies in young and older able-bodied participants and in patient populations. The cross education strength gain was estimated at 18% in young, and 17% in older able-bodied participants. The cross education strength gain was 29% in patient populations consisting of post-stroke, multiple sclerosis, osteoarthritis, and neuromuscular disorder patients. The meta-analysis identified the efficacy of electromyostimulation (EMS) training over voluntary training modalities. The magnitude of strength transfer was similar between upper and lower and between males and females. Lastly, manuscript 3 consisted of a 6-week unilateral training program resulting in contralateral strength gains of 11% in the wrist flexors and 15% in the dorsiflexors. A continued increase in contralateral strength at retention demonstrated the persistence of cross education following 6-weeks of detraining. Skill transfer in the contralateral limbs was evident in the force variability measures calculated during contractions without concurrent feedback (noKR). Agonist RMS amplitude, V-wave amplitude, and central activation ratio indicated neuromuscular adaptations; however, there was no change in motor unit firing rates at 60% of maximal force.