Bilateral neuromuscular plasticity from unilateral training of the ankle dorsiflexors

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Training a muscle group in one limb yields strength gains bilaterally—the so-called cross-education effect. However, to date there has been little study of the targeted application of this phenomenon in a manner relevant to clinical rehabilitation. For example, it may be applicable post-stroke, where hemiparesis leads to ankle flexor weakness. The purpose of this study was to examine the effects of high-intensity unilateral dorsiflexion resistance training on agonist (tibialis anterior, TA) and antagonist (plantarflexor soleus, SOL) muscular strength and H-reflex excitability in the trained and untrained limbs. Ankle flexor and extensor torque, as well as SOL and TA H-reflexes evoked during low-level contraction, were measured before and after 5 weeks of dorsiflexion training (n = 19). As a result of the intervention, dorsiflexor maximal voluntary isometric contraction force (MVIC) significantly increased (P < 0.05) in both the trained and untrained limbs by 14.7 and 8.4%, respectively. No changes in plantarflexor MVIC force were observed in either limb. Significant changes in H-reflex excitability threshold were also detected: H@thresh significantly increased in the trained TA and SOL; and H@max decreased in both SOL muscles. These findings reveal that muscular crossed effects can be obtained in the ankle dorsiflexor muscles and provide novel information on agonist and antagonist spinal adaptations that accompany unilateral training. It is possible that the ability to strengthen the ankle dorsiflexors bilaterally could be applied in post-stroke rehabilitation, where ankle flexor weakness could be counteracted via dorsiflexor training in the less-affected limb.

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