Hemiparesis after stroke decreases ability to dorsiflex the more-affected ankle during walking. Increased strength would be beneficial, but the more-affected limb is often too weak to be trained. In neurologically intact participants, training one limb induces strength gains in the contralateral, untrained limb. This approach remains unexplored post-stroke. The aim of this study was to test the hypothesis that unilateral dorsiflexor high-intensity resistance training on the less-affected side increases strength and motor output bilaterally following stroke. 19 participants (84.1 ± 77.6 months post-infarct) performed 6 weeks of maximal isometric dorsiflexion training using the less-affected leg. Voluntary isometric strength (dorsiflexion torque, muscle activation), reciprocal inhibition (RI), walking ability (gait speed, kinematics, EMG patterns), and clinical function were measured within 1 week before and 4 days following training. Post-intervention, dorsiflexion torque increased by ~31 % (p < 0.05) in the more-affected (untrained) and by ~34 % (p < 0.05) in the less-affected (trained) legs. Muscle activation significantly increased bilaterally, by ~59 and ~20 % in the trained and untrained legs, respectively. Notably, 4 participants who were unable to generate functional dorsiflexion on the more-affected side before training could do so post-intervention. Significant correlations between muscle activation and size of RI were noted across muscle groups before and after training, and the relation between size of RI and level of muscle activation in the more-affected tibialis anterior muscle was significantly altered by training. Thus, significant gains in voluntary strength and muscle activation on the untrained, more-affected side after stroke can be invoked through training the opposite limb. We demonstrate residual plasticity existing many years post-stroke and suggest clinical application of the cross-education effect where training the more-affected limb is not initially possible.

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.

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Cutaneous feedback from the hand could assist with coordination between the arms and legs during locomotion. Previously we used a reduced walking model of combined arm and leg (ARM&LEG) cycling to examine the separate effects of rhythmic arm (ARM) and leg (LEG) movement. Here we use this same paradigm to test the modulation H-reflexes with and without interlimb cutaneous conditioning evoked by stimulating a nerve innervating the hand (superficial radial, SR). It was hypothesized that both ARM and LEG would contribute significantly to suppression of H-reflex amplitude during ARM&LEG. We also predicted a conservation of interlimb cutaneous conditioning during movement and an interaction between arm and leg rhythmic movement control. Subjects were seated in a recumbent ARM&LEG cycle ergometer and maintained a low-level soleus contraction for all tasks. H-reflex amplitude was facilitated by cutaneous conditioning evoked by stimulation of the SR nerve. H-reflex amplitudes were taken from recruitment curves and included modulation of 50% H max and H max. The suppressive effect of arm was less than that for LEG and ARM&LEG, while suppression during LEG and ARM&LEG were generally equivalent. For H-reflexes conditioned by cutaneous input, amplitudes during ARM&LEG instead were in between those for ARM and LEG modulation. Multiple regression analysis revealed a significant contribution for arm only in trials when SR stimulation was used to condition H-reflex amplitudes. We suggest that there is a measurable interaction between neural activity regulating arm and leg movement during locomotion that is specifically enhanced when cutaneous input from the hand is present.

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