Invariable H-reflex and sustained facilitation of stretch reflex with heightened sympathetic outflow

Stretch reflex shows sustained (3-min) increase with heightened sympathetic outflow [Hjortskov N, Skotte J, Hye-Knudsen C, Fallentin N. Sympathetic outflow enhances the stretch reflex response in the relaxed soleus muscle in humans. J Appl Physiol 2005;98:1366-70], but it is unknown if it accompanies a sustained increase in H-reflex. The purpose of the study was to test if there is a sustained facilitation in the H-reflex in the human soleus muscle during a variety of sustained tasks that are known to elevate sympathetic outflow. Mean arterial blood pressure, heart rate, and H- and stretch reflexes in the relaxed soleus muscle were obtained in healthy young adults who performed mental arithmetic, static handgrip exercise, post-handgrip ischemia, and cold stimulation. Each task lasted 3 min with a 3-min rest in between tasks. Data were analyzed for the initial 30 s and entire 3 min of each task. There was a heightened cardiovascular response in all tasks for both durations of analysis. An increase in H-reflex amplitude was not observed for either the initial or entire duration of the analysis. The tasks increased stretch reflex amplitude for both durations of analysis. Invariable H-reflex and sustained facilitation of stretch reflex with heightened sympathetic outflow would imply sympathetic modulation of muscle spindle sensitivity.