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

Cross education (CE) occurs after unilateral training whereby performance of the untrained contralateral limb is enhanced. A few studies have shown that CE can preserve or “spare” strength and size of an opposite immobilized limb, but the specificity (i.e., trained homologous muscle and contraction type) of these effects is unknown. The purpose was to investigate specificity of CE “sparing” effects with immobilization. The nondominant forearm of 16 participants was immobilized with a cast, and participants were randomly assigned to a resistance training (eccentric wrist flexion, 3 times/week) or control group for 4 weeks. Pre- and posttesting involved wrist flexors and extensors eccentric, concentric and isometric maximal voluntary contractions (via dynamometer), muscle thickness (via ultrasound), and forearm muscle cross-sectional area (MCSA; via peripheral quantitative computed tomography). Only the training group showed strength preservation across all contractions in the wrist flexors of the immobilized limb (training: −2.4% vs. control: −21.6%; P = 0.04), and increased wrist flexors strength of the nonimmobilized limb (training: 30.8% vs. control: −7.4%; P = 0.04). Immobilized arm MCSA was preserved for the training group only (training: 1.3% vs. control: −2.3%; P = 0.01). Muscle thickness differed between groups for the immobilized (training: 2.8% vs. control: −3.2%; P = 0.01) and nonimmobilized wrist flexors (training: 7.1% vs. control: −3.7%; P = 0.02). Strength preservation was nonspecific to contraction type (P = 0.69, 𝜂2