Specificity of Sparing Effects with Cross-Education after Eccentric Training

Abstract

Cross-education (CE) is the phenomenon that occurs after unilateral strength training whereby strength of the untrained contralateral limb is enhanced. A handful of studies have shown that CE can spare the loss of strength and size of an opposite immobilized limb, but specificity of these “sparing” effects is unknown. The purpose was to investigate specificity of CE sparing effects with immobilization. Sixteen participants were randomly assigned to a training (M=1, F=7; ht: 170.3±10.1 cm; wt: 77.2±19.2 kg) and control (M=2, F=6; ht: 169.3±8.5 cm; wt: 85.7±22.7 kg) group. Both groups wore a non-dominant forearm cast for four weeks. Two pre- and one post-testing session involved wrist flexors and extensors muscle thickness (ultrasound), eccentric (ECC), concentric (CON) and isometric (ISO) maximal voluntary contractions (dynamometer), electromyography (EMG) normalized to Mmax, and forearm muscle cross-sectional area (MCSA; peripheral quantitative computed tomography). Strength training was ECC wrist flexion 3 times per week. Group × time interactions for the immobilized and non-immobilized limbs revealed that only the training group showed strength preservation across all contractions in the wrist flexors of the immobilized limb (Training: pre=12.3±5.4 Nm, post=12.0±4.6 Nm vs. Control: pre=14.8±5.4 Nm, post=11.6±4.6 Nm; p=.04, η_p^2=.25), and increased wrist flexors strength of the non-immobilized limb (Training: pre=12.9±5.5 Nm, post=16.9±7.3 Nm vs Control: pre=14.9±5.5 Nm, post=13.8±7.3 Nm; p=.04, ηp2=.27). For MCSA there was a significant arm × time interaction for the control group only, p =.02, η_p^2 =.57, where the change in the left arm (pre: 35.2 ± 7.2 cm2; post: 34.4 ± 8.1 cm2; -2.3%) was different from the right arm (pre: 34.3 ± 7.7 cm2; post: 34.7 ± 8.0 cm2; 1.2%). Muscle thickness change differed between groups (Training: pre=3.3±0.5 cm, post=3.4±0.6 cm; control: pre=3.7±0.7 cm, post=3.7±0.6 cm) for the immobilized wrist flexors only (p=.01, η_p^2=.40). Analyses of normalized EMG data failed to reveal significant between group or co-activation differences regardless of muscle (flexors, extensors), task (flexion, extension) or contraction type (ECC, CON, ISO). Strength preservation was not specific to contraction type (p=.69, η_p^2=.03), yet sparing effects were specific to the trained muscle. The mechanisms of muscle size preservation remain unknown, but these data draw an important link between strength and muscle size sparing with CE and suggest that ECC training of the non-immobilized limb can preserve size of the immobilized contralateral homologous muscle and strength across multiple contraction types.