July 27, 2018
Justin W. Andrushko 1, Layla A. Gould 1,2, Jonathan P. Farthing 1
1 College of Kinesiology, University of Saskatchewan, Saskatoon, Canada
2 College of Medicine, University of Saskatchewan, Saskatoon, Canada
Evidence for the opposite limb (i.e. contralateral) effect of single limb training has existed for well over a century. This inter-limb transfer effect is commonly described as cross-education whereby single limb strength or skill training leads to an enhancement in the opposite untrained limb. In young healthy persons, the cross-education effect after strength training usually amounts to 50% of the gains observed on the trained side – although this can vary somewhat depending on the experimental conditions (e.g. training duration, limb dominance, upper vs. lower body, training history, etc.). Admittedly, the phenomenon has little relevance for healthy individuals without injury or impairment, especially if training goals are to maintain body symmetry. Cross-education becomes relevant for those experiencing a neurological or orthopedic injury affecting one side.
Surprisingly it is only within the last decade that well-controlled, clinically relevant studies have emerged. A handful of these have employed an immobilization model, where a group of healthy volunteers wear a cast or splint on the left limb, and half of them engage in intense strength training of the opposite right arm. Andrushko, Gould, and Farthing (2018) surveyed the five studies to date that have applied this model and report quite remarkable congruence. On average, strength training the right limb can completely spare strength (~13%) and muscle size (~4%) of the opposite immobilized left limb after 3-4 weeks. The newest evidence points towards site-specificity, where the “sparing effects” appear to be localized to the muscle that was trained on the opposite side. The authors note that the precise mechanisms of these sparing effects are not clear, particularly for the muscle size effects. Cross-education studies that do not involve immobilization suggest the mechanisms of untrained limb strength increase reside in the brain or central nervous system – likely related to how the brain hemispheres share information, or simultaneously adapt, after single limb training. In these studies there is no evidence of muscle adaptation or hypertrophy in the untrained limb. Yet, somehow, single limb strength training can prevent atrophy when the opposite limb is immobilized.
Andrushko et al. (2018) highlight the possible causes of strength and muscle sparing effects of cross-education, including evidence of adaptation in both hemispheres of the brain and central nervous system after single limb training, and the possible role of low level contractions of the resting, immobilized limb during training – known as “mirror” contractions. The authors discuss the possibility that strength training the opposite limb can influence the balance of protein synthesis in the immobilized limb, by some yet unknown mechanism.
Lastly, Andrushko et al. (2018) highlight the need for continued work involving clinical populations such as those recovering from fracture, surgery, or stroke. Evidence is emerging but still sparse, with only one or two randomized clinical trials. Current evidence from clinical studies suggests cross-education effects are beneficial post-fracture and post-stroke, but less potent or null after knee surgeries.
Andrushko JW, Gould LA, Farthing JP. Contralateral effects of unilateral training: sparing of muscle strength and size after immobilization. Appl Physiol Nutr Metab. 2018 May 25. doi: 10.1139/apnm-2018-0073. [Epub ahead of print]
This article is a summary of an article published in Applied Physiology, Nutrition & Metabolism. If you intend to cite any information in this article, please consult the original article and cite that source. This summary was written for the Canadian Society for Exercise Physiology and it has been reviewed by the CSEP Knowledge Translation Committee.