Interlimb neural coupling allows upper limb (UL) activity to influence lower limb (LL) spinal excitability, yet the direction and magnitude of this modulation can vary across individuals. While rhythmic, locomotor-like UL tasks typically suppress soleus H-reflex amplitude, the effects of discrete voluntary UL movements remain unclear. The objective of this study was to determine whether rhythmic UL movements modulate soleus H-reflex excitability. Twenty-four able-bodied adults completed a single session including a 20-min paradigm of four symmetric, bilateral UL movements performed at 1 Hz. Soleus H-reflexes were measured using peripheral nerve stimulation while participants stood upright (two baselines and post UL task). Hmax/Mmax ratios were compared across time points, and k-means clustering was applied to individual change scores to identify responder subgroups. No significant group-level modulation of H-reflex excitability was observed following UL movement practice (p = 0.564). However, clustering revealed two distinct and opposite response profiles. One subgroup (n = 13) demonstrated a significant decrease in H-reflex excitability (22.1% ± 11.0% suppression), while the other subgroup (n = 11) exhibited a significant increase (19.5% ± 12.5% facilitation). Fatigue ratings increased post-intervention, whereas energy ratings remained stable. UL movements can modulate spinal excitability bidirectionally, with individuals showing either facilitation or suppression of the soleus H-reflex. This individualized responsiveness highlights heterogeneity in interlimb neural coupling, suggesting that UL-based neuromodulation strategies may require personalized tailoring. The classification of facilitatory and suppressive responders offers a methodological framework for developing adaptive rehabilitation approaches that leverage natural interlimb circuit interactions.
Doshi et al. (Mon,) studied this question.