Methods of eliciting and measuring the Hoffmann reflex (H-reflex) have been reported previously.19,20 The H-reflex is an electrically elicited monosynaptic and oligosynaptic reflex (laafferents to alpha motoneurons) that provides a measure of the excitability of
the alpha motoneuron pool. The stimulating electrode sites were prepared in the manner described for the recording electrodes. A silver-silver chloride plate anode electrode was taped in place justproximal to the patella. A cathode disk electrode was located in the popliteal fossa with an adjustable elastic band and Velcro strap. A constant voltage stimulator delivered rectangular pulses (1-msec duration) through an isolation unit. The largest diameter axons, the la afferents, have the lowest resistance to external electrical stimulation and reach threshold first, resulting in the propagation
of action potentials and facilitory input to triceps surae muscle alpha motoneurons.
If the facilitative input is sufficient to cause the motoneurons to fire, the result will be an H-reflex, which is recorded by electrodes over the soleus muscle. If the intensity of the electrical stimulation also is sufficient to cause the propagation of action potentials in motoneuronal axons, an M-response (wave) will occur before the H-reflex. A 1.1 × H-reflex threshold voltage was calculated for use as the experimental
stimulus intensity. This intensity has been shown to be optimal in demonstrating both facilitation and inhibition of motoneuron excitability21 and was sufficient to produce consistently small M-responses. The H-reflexes were identified on the basis of latency (≈ 30 msec) and their characteristic triphasic waveform. 20 Control and experimental
H-reflexes were elicited every 10 seconds, avoiding depression of the H-reflex amplitude that occurs at shorter interstimulus intervals (ISIs).22 Stimulus artifacts were recorded on FM tape and used in triggering and timing the samples of H-reflex and EMG data.