Cross-education of lower limb muscle strength following resistance exercise training in males and females: A systematic review and meta-analysis.

Cross-education describes the training of one limb that leads to performance enhancements in the contralateral untrained limb, driven by neural changes rather than muscle adaptation. In this systematic review and meta-analysis, we aimed to evaluate the efficacy of cross-education (vs. a control group) via resistance exercise training (RET) for improving muscle strength in the untrained lower limb of healthy males and females. A literature search from inception to September 2023 was conducted using MEDLINE (via PubMed), the Cochrane Library (CENTRAL), Web of Science (Core Database), Scopus, EBSCO-host, and Ovid-EMBASE. Independent screening, data extraction and quality assessment were conducted. The measured outcomes were change in one-repetition maximum (1-RM) load, maximum voluntary contraction (MVC), and concentric, eccentric and isometric peak torque. Change in muscle structure (pennation angle and muscle thickness) was also analysed. A total of 29 studies were included. The pooled effect size from the random-effects model shows that cross-education significantly increased 1-RM compared to the control group (standardised mean difference (SMD): 0.59, 95% CI: 0.22-0.97; P = 0.002). Cross-education also significantly improved MVC (SMD: 0.55, 95% CI: 0.16-0.94; P = 0.006), concentric (SMD: 0.61, 95% CI: 0.39-0.84; P < 0.00001), eccentric (SMD: 0.39, 95% CI: 0.13-0.64; P = 0.003) and isometric (SMD: 0.45, 95% CI: 0.26-0.64; P < 0.00001) peak torque, each compared to the control group. When RET was categorised as eccentric or concentric, subgroup analysis showed that only eccentric training was associated with significantly increased isometric peak torque via cross-education (SMD: 0.37, 95% CI: 0.13-0.61; P = 0.003) (concentric, SMD: 0.33, 95% CI: -0.09 to 0.74; P = 0.12). This systematic review and meta-analysis emphasise the potency of cross-education for improving lower limb muscle strength. These findings have potential implications for clinical situations of impaired unilateral limb function (e.g., limb-casting or stroke). Future work exploring the mechanisms facilitating these enhancements will help to develop optimised rehabilitation protocols.