Postactivation depression of the soleus H reflex measured using threshold tracking

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Interpreting soleus H reflex changes is complex due to reflex gain, nonlinear motoneuron pool input/output, and nonhomogeneous motoneuron responses.
• Threshold tracking offers a method to maintain a constant alpha-motoneuron population for study by adjusting stimulus intensity for a constant reflex output.

Purpose of the Study:

• To examine postactivation depression of the H reflex (HD) in healthy subjects using threshold tracking.
• To investigate the influence of stimulus frequency and voluntary contraction on HD.

Main Methods:

• Utilized computerized threshold tracking to maintain a target H reflex (10% of Mmax) by adjusting stimulus current.
• Elicited H reflexes via tibial nerve stimulation at various frequencies (0.05–2 Hz) during rest and submaximal voluntary plantar flexion (2.5–10% MVC).

Main Results:

• The current needed for a target H reflex increased with stimulus rate, but not significantly beyond 1 Hz.
• Postactivation depression (HD) was significantly reduced during voluntary contractions compared to rest.
• HD magnitude increased with the size of the test H reflex, suggesting greater sensitivity in smaller motoneurons.

Conclusions:

• Threshold tracking facilitates the study of H reflex dynamics by stabilizing the motoneuron pool.
• Postactivation depression of the H reflex is modulated by voluntary activation levels.
• HD is maximal at lower frequencies than previously thought, particularly with larger test H reflexes, highlighting differential motoneuron sensitivity.