Determining the Effects of Cross-Education on Muscle Strength, Thickness and Cortical Activation Following Limb Immobilization: A Systematic Review and Meta-Analysis

Abstract

Purpose:
 Cross-education (CE) increases strength of both the trained and untrained limb, with emerging evidence, suggesting CE could be used to attenuate muscle strength and thickness following periods of limb immobilization. This study examined the available evidence for the clinical efficacy of CE to attenuate muscle strength, thickness and neural activation during limb immobilization.
 Methods:
 We performed a systematic review and meta-analysis on the effects of CE on muscle strength, thickness and activation of an immobilized limb. The evidence from randomized controlled trials (RCTs) were pooled to assess effect estimates for changes in strength, muscle thickness and neural activation of the untrained immobilized limb.
 Results:
 CE attenuated muscle strength in 5 RCTs (n= 78) which reported a SMD of 1.60 (95% CI 0.62, 2.59; P = 0.001) and muscle thickness, with an SMD of 1.52 (95% CI 0.22, 2.81; P = 0.02) compared to control. There was no difference in muscle activation (SMD of 0.08; 95% CI -0.34, 0.50; P = 0.72), regions of cortical activation (MD 31.8; 95% CI -22.71, 86.31; P = 0.25) or corticospinal excitability (MD 5.2; 95% CI -2.38, 12.78; P = 0.18) compared to control. 
 Conclusions:
 These results show that strength training the free limb via cross-education maintains muscle strength and muscle thickness of the immobilized limb compared to control (immobilization only). Because there was no effect on muscle activation, but a large mean difference in cortical activation, it is likely that the attenuation of muscle strength is due to neural adaptations at a cortical level.