The Cross-Education Phenomenon: Brain and Beyond

Abstract

Objectives: Unilateral resistance training produces strength gains in the untrained homologous muscle group, an effect termed “cross-education.” The observed strength transfer has traditionally been considered a phenomenon of the nervous system, with few studies examining the contribution of factors beyond the brain and spinal cord. In this hypothesis and theory article, we aim to discuss further evidence for structural and functional adaptations occurring within the nervous, muscle, and endocrine systems in response to unilateral resistance training. The limitations of existing cross-education studies will be explored, and novel potential stakeholders that may contribute to the cross-education effect will be identified.Design: Critical review of the literature.Method: Search of online databases.Results: Studies have provided evidence that functional reorganization of the motor cortex facilitates, at least in part, the effects of cross-education. Cross-activation of the “untrained” motor cortex, ipsilateral to the trained limb, plays an important role. While many studies report little or no gains in muscle mass in the untrained limb, most experimental designs have not allowed for sensitive or comprehensive investigation of structural changes in the muscle.Conclusions: Increased neural drive originating from the “untrained” motor cortex contributes to the cross-education effect. Adaptive changes within the muscle fiber, as well as systemic and hormonal factors require further investigation. An increased understanding of the physiological mechanisms contributing to cross-education will enable to more effectively explore its effects and potential applications in rehabilitation of unilateral movement disorders or injury.

Unilateral strength training leads to muscle-specific sparing effects during opposite homologous limb immobilization

“…Cross education (CE) of strength occurs when unilateral strength training produces performance improvement of the untrained contralateral limb (7,36). Within the last decade, there is renewed scientific interest in CE, on account of evidence of preserved strength and muscle size in an opposite immobilized limb (26). The allure of an immobilization model in CE research is that disuse causes accelerated loss in neural drive, strength, and muscle size (10,12,13,52) and the prospect of CE to attenuate these decrements can be explored.…”

“…Although ultrasound is a valid and reliable as a muscle size measure (8), it is not considered a gold standard measure. Revisiting these size sparing effects with a measure of muscle cross-sectional area (MCSA) from peripheral quantitative computed tomography (pQCT) is an important step to confirm or refute the previous findings (26).…”

“…Specificity effects are convincing indices of the involvement of neural mechanisms (24); therefore, the sparing effects of CE are hypothesized to demonstrate specificity. However, specificity effects may present differently for an immobilized limb due to inhibited movement, altered corticospinal excitability (4,15,38,43,54), muscle atrophy and altered protein synthesis (1,42), and potential systemic effects (26) that could contribute globally to muscle size sparing.…”

Fast-tracked Rehabilitation and Return to Sport of an Elite Rugby Player with a Complicated Posterolateral Corner Injury and Associated Peroneal Paralysis

“…This is a phenomenon where training of the non-immobilised limb was implicated to promote hypertrophy in the immobilised leg as well as cortical adaptations regarding functionality concerning both legs. 20 The paralysed peronei muscles, tibialis anterior and hallucis longus as well as the quadriceps underwent transcutaneous neuromuscular electrical stimulation (NMES) in two settings (30 min at 5 Hz, 400 μs and 30 min at 30 Hz, 160 μs) one to two times a day. Additionally, NMES was applied at 1-150 Hz on the posterior aspect of the fibular head to act as a neurostimulator for the paralysed peroneal nerve to stimulate function.…”

Unilateral dorsiflexor strengthening with mirror therapy to improve motor function after stroke: A pilot randomized study

“…The study also found a positive effect on functional mobility with improvements in timed up and go (TUG) scores. Although a meta-analysis of 31 crosseducation studies reports a more modest average strength increase of 16% of initial strength for the untrained lower limb (Manca, Dragone, Dvir, & Deriu, 2017), high intensity unilateral resistance training is believed to increase excitability in the untrained primary motor cortex (M1), mediate synaptic connectivity within neural circuits, and enhance neural drive to the contralateral homologous muscle, improving force output (Hendy & Lamon, 2017;Hortobagyi, 2005;Lee & Carroll, 2007). Because contralateral strength gains are facilitated via neural pathways damaged during stroke, cross-education seems appropriately suited to stroke rehabilitation (Zult, Howatson, Kadar, Farthing, & Hortobagyi, 2014).…”