Influence of Stance and Support Surface Variations on Soleus Presynaptic Inhibition

Abstract

Presynaptic inhibition (PI) is instrumental in enabling the nervous system to respond to environmental changes. Subjects who demonstrate little change in PI of the soleus muscle have been shown to have a decreased ability to adapt to environmental changes (i.e. uneven surface conditions). Little research has been done to examine changes in PI during various surface and stance conditions. PURPOSE: this study utilized H-reflex measures to examine extrinsic PI in the soleus muscle during single- and double-legged stance, under stable and unstable surface conditions. Design & settings: A 2 × 2 fully repeated measures design was employed with leg (single vs. double) and support surface (foam, no foam) serving as the independent variables. subjects: Ten healthy subjects (age = 23.±1.7 yr, mass = 75.2±6.4 kg, ht = 172.1±8.5 cm) with no known acute or chronic neurological or lower extremity musculoskeletal injury 6 months prior to the study served as volunteers. Measurements: The tibial nerve was stimulated using an extrinsic PI conditioning protocol. The ratio of the unconditioned H-reflex amplitude to the conditioned H-reflex amplitude represented extrinsic PI. All subjects completed a total of 7 trials for the conditioned and unconditioned soleus H-reflex response for each treatment. RESULTS: No interaction was found between leg and surface condition on soleus PI (p > 0.05). Main effects testing revealed that a significant increase in soleus PI for the single-legged stance compared the double-legged stance (P=0.001). Surface variation did not affect soleus PI (p > 0.05). CONCLUSIONS: The increase in soleus PI during a single-legged stance suggests inhibition of the motoneuron pool may aid in making postural corrections, while maintaining upright stance during less stable positions. Moreover, maintenance of upright stance on an unstable foam surface does not appear to influence extrinsic PI modulation of the soleus muscle.