The stability of M(max) and H (max) amplitude over time.

The stability of the maximal muscle response (M(max)) is critical to H reflex methodology. It has previously been reported that the amplitude of M(max) declines over time. If reproducible, this finding would have implications for all experimental studies that normalise the output of the motoneurone pool against the M wave. We investigated the effect of time on changes in M(max) and the maximal H reflex (H(max)) evoked at 4-s intervals over 60 min. To identify an influence of homosynaptic depression, we extended the interstimulus interval to 10 s and the time to 100 min. Two recording montages over soleus were used to ensure that interelectrode distance was not a critical factor. The soleus M(max) and H reflex were evoked by stimulation of the tibial nerve in the popliteal fossa in 7 subjects who sat with the knee flexed to 30° and the ankle plantar flexed by ~30°. We found no change in the pooled data for M(max), H(max), a reflex 50% of maximal, or the current required to produce it. However, one subject had a statistically significant increase in M(max) and a concurrent decrease in H(max) regardless of the interstimulus interval. On average, there was no change in the H(max)/M(max) ratio over time. While both M(max) and H(max) may change in response to many factors, these results suggest that, typically, time is not one of them.