Loading modulates monosynaptic transmission from spindle primary afferents to motoneurons in humans

Abstract

The literature does not provide a consistent account of how mechanical loading influences H-reflex excitability. Given the methodological diversity across previous studies, the present study investigated how different levels of mechanical load affect soleus H-reflex excitability during quiet stance in healthy adults. It incorporated several experimental controls to enhance the reliability and comparability of results. Eighteen participants were tested under five load conditions (10-100% of body weight) while maintaining a consistent M-wave amplitude and a relaxed muscle posture. H-reflex amplitude decreased progressively with increasing load, reaching significant suppression at full weight-bearing (F (4, 68) = 7.04, p < 0.001, partial η² = 0.293), whereas background EMG activity showed an opposite trend (χ² (4) = 26.97, p < 0.001). This dissociation suggests that muscle spindle-based spinal reflex excitability does not scale linearly with muscle activation, indicating enhanced premotoneuronal modulatory control under higher loading. These findings highlight that spinal circuits dynamically adjust reflexes to stabilise posture, prevent excessive contractions and fine motor control in response to increasing mechanical demands.