Bidirectional modulation of soleus spinal excitability induced by upper limb movement practice.

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.