Heteronymous feedback from quadriceps onto soleus is influenced by limb loading and task context

Abstract

Heteronymous reflexes from quadriceps can increase and/or decrease soleus activity; yet few studies have examined factors influencing reflex strength. This study examined the independent influence of limb loading, posture, and task context on heteronymous feedback from quadriceps onto soleus. The influence of limb loading and posture was determined by comparing femoral nerve elicited heteronymous excitation and inhibition of soleus in a semi-recumbent position with and without 50% body weight limb loading and while standing with back support (n = 16). Task context was examined by comparing heteronymous reflex magnitudes while standing with back support to maintaining an unsupported squat posture which requires tonic soleus activity to maintain the posture (n = 12). Heteronymous inhibition decreased by 20% with limb loading in both semi-recumbent and standing postures, while excitation remained unchanged suggesting that limb loading, rather than postural orientation, independently modulates heteronymous inhibition. Inhibition decreased by 50% and excitation by 90% when maintaining the squat posture compared to supported standing. The pronounced suppression of both excitation and inhibition during the squat is considered a task-appropriate reflex modulation that aids in maintaining the posture. The results of this study highlight an important modulatory influence of limb loading afferents and task context on heteronymous reflex circuits.

Keywords: Femoral nerve; Heteronymous reflex; Limb loading; Proprioceptive feedback; Task modulation.

Fig. 1

Schematic depicting the reflex circuits relevant for this study (A), and an example soleus EMG trace showing heteronymous excitation and inhibition in the semi-recumbent limb unloaded condition from a representative participant (B). (A) Femoral nerve stimulation was used to evoke heteronymous excitatory Ia (continuous line), inhibitory Ib, and recurrent inhibition from antidromic motor axon propagation acting on Renshaw inhibitory interneurons (i.e., recurrent inhibition, dashed lines) from Q onto SOL motoneurons. (B) Ensemble average of a rectified SOL EMG trace (mean of 30 repetitions) showing an excitatory period starting ~ 25 ms after FN stimulation followed by an inhibitory period starting ~ 32 ms. Excitation and inhibition onset were determined by the rectified trace moving above or below 1 standard deviation (horizontal dashed lines) of the mean SOL background EMG (horizontal solid line), respectively. Q, quadriceps; MN, motor neuron; FN, femoral nerve; SOL, soleus.

Fig. 2

Comparison of heteronymous inhibition area (top) and duration (bottom) when semi-recumbent with the limb unloaded, semi-recumbent with the limb loaded 50% of body weight, and standing with wall support. A significant reduction of inhibition area (p = 0.01) and duration (p = 0.042) was observed in the semi-recumbent limb loaded and standing posture compared to the semi-recumbent limb unloaded condition.

Fig. 3

Heteronymous inhibition (top panel) and heteronymous excitation (bottom panel) magnitudes when standing with back support against the wall and when maintaining a squat posture. Heteronymous inhibition (*p = 0.009) and excitation (*p = 0.011) were significantly decreased when maintaining the squat posture compared to standing with back against the wall.