Soleus H-Reflex Gain, Threshold, and Amplitude as Function of Body Posture and Load in Spinal Cord Intact and Injured Subjects

Abstract

In this study, we established parameters of the soleus H-reflex excitability in response to changes of posture and load in 8 chronic spinal cord injured (SCI) and 10 spinal-intact subjects. The soleus H-reflex recruitment curve was established in all subjects while they were supine, seated, and standing on a stable treadmill. During standing, body weight support (BWS) was provided via an upper body harness and ranged in SCI subjects from 20%–50% and in spinal-intact subjects was set at 0% and 50%. Stimuli corresponding to the H-threshold (Hth), maximal H-reflex amplitude (Hmax), and 50% of Hmax as well as the reflex gain were estimated based on a sigmoid function of the ascending limb of the soleus H-reflex recruitment curve. The soleus H-reflex gain, Hmax amplitude, and stimuli corresponding to Hth, 50% of Hmax, and Hmax were increased in SCI subjects regardless of the body position and loading. Further, the reflex gain was not modulated appropriately during conditions of weight bearing in SCI subjects. Impaired spinal reflex excitability in SCI subjects is accompanied by changes of the H-reflex recruitment curve parameters regardless of presence or absence of body loading.