Cross-Education Effects of Isokinetic Eccentric Plantarflexor Training on Flexibility, Strength, and Muscle–Tendon Mechanics

INTRODUCTION: Large increases in joint range of motion (ROM) have been reported after eccentric resistance training; however, limited data exist describing the associated mechanisms or potential cross-education effects in the contralateral limb. Therefore, the effects of a 6-wk isokinetic eccentric plantarflexor training program were examined in 26 participants. METHODS: Before and after the training program, dorsiflexion ROM, plantarflexor strength, and muscle-tendon unit (MTU) morphology and mechanics were measured in control ( n = 13) and experimental ( n = 13) young adult groups. Training consisted of 5 sets of 12 maximal isokinetic eccentric plantarflexor contractions twice weekly on the right limb. RESULTS: Significant ( P < 0.05) increases in dorsiflexion ROM (4.0-9.5°), stretch tolerance (40.3-95.9%), passive elastic energy storage (47.5-161.3%), and isometric (38.1-40.6%) and eccentric (46.7-67.0%) peak plantarflexor torques were detected in both trained and contralateral limbs in the experimental group. Significant increases in gastrocnemius medialis and soleus thickness (5.4-6.1%), gastrocnemius medialis fascicle length (7.6 ± 8.5%), passive plantarflexor MTU stiffness (30.1 ± 35.5%), and Achilles tendon stiffness (5.3 ± 4.9%) were observed in the trained limb only. Significant correlations were detected between the changes in trained and contralateral limbs for dorsiflexion ROM ( r = 0.59) and both isometric ( r = 0.79) and eccentric ( r = 0.73) peak torques. No significant changes in any metric were detected in the control group. CONCLUSIONS: Large ROM increases in the trained limb were associated with neurological, mechanical, and structural adaptations, with evidence of a cross-education effect in the contralateral limb being primarily driven by neurological adaptation (stretch tolerance). The large improvements in ROM, muscle size, and strength confirm that isokinetic eccentric training is a highly effective training tool, with potential for use in athletic and clinical populations where MTU function is impaired and current therapies are ineffective.