Submitted on April 4, 2008
Revised on September 19, 2008
Accepted on October 14, 2008
Post-activation depression of the soleus H reflex measured using threshold tracking
Penelope A. McNulty1, Stacey Kyle Jankelowitz2, Tanya M. Wiendels3, and David Burke4*
1 Prince of Wales Medical Research Institute
2 The University of Sydney
3 Prince of Wales Medical Research Institute and University of New South Wales
4 Institute of Clinical Neurosciences, The University of Sydney and Royal Prince Alfred Hospital
* To whom correspondence should be addressed. E-mail: d.burke{at}med.usyd.edu.au.
The interpretation of changes in the soleus H reflex is problematic in the face of reflex gain changes, a non-linear input/output relationship for the motoneuron pool and a non-homogeneous response of different motoneurons to afferent inputs. By altering the stimulus intensity to maintain a constant reflex output, threshold tracking allows a relatively constant population of 
-motoneurons to be studied. This approach was used to examine post-activation ("homosynaptic") depression of the H reflex (HD) in 23 neurologically healthy subjects. The H reflex was elicited by tibial nerve stimulation at 0.05, 0.1, 0.3, 1 and 2 Hz at rest and during voluntary plantar flexion at 2.5, 5 and 10% of maximum. A computerised threshold tracking procedure was used to set the current required to generate a target H reflex 10% of Mmax. The current required to produce the target reflex increased with stimulus rate but not significantly beyond 1 Hz. In three subjects the current required to produce H reflexes of 5, 10, 15 and 20% Mmax at 0.3, 1 and 2 Hz increased with rate and with the size of the test H reflex. HD was significantly reduced during voluntary contractions. Using threshold tracking, HD was maximal at lower frequencies than previously emphasized, probably because HD is greater the larger the test H reflex. This would reinforce the greater sensitivity of small motoneurons to reflex inputs.
