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-week 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 (GM) and soleus thickness (5.4-6.1%), GM 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.

Conclusion:
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.