Differential effects of low-frequency depression, vibration-induced inhibition, and posttetanic potentiation on H-reflexes and tendon jerks in the human soleus muscles

1. A comparison was made between the effects of repetition rate, muscle vibration, and tetanic stimulation of the tibial nerve on H-reflexes and tendon (T) jerks elicited in the relaxed soleus muscle of normal human. H- and T-reflexes with electromyographic (EMG) potentials of equivalent amplitude were elicited alternately in the same leg to ensure identical experimental conditions. 2. H- and T-reflexes of identical EMG amplitude showed isometric twitch tensions of identical amplitude and time course. 3. H- and T-reflexes were elicited at different repetition rates ranging from once every 6 s to 2/s. H-reflexes showed a significantly stronger low-frequency depression than T-reflexes. 4. Inhibition of H- and T-reflexes was induced by 2-min vibration at 100 Hz of ankle extensors, ankle flexors, or knee flexors. Vibration of ankle extensors and flexors induced a stronger inhibition than vibration of knee flexors. In all three conditions, inhibition of H-reflexes was stronger than inhibition of T-reflexes. The difference was relatively greater during vibration of ankle extensors and flexors than during vibration of knee flexors. 5. When tested together, the effects of low-frequency depression and vibration-induced inhibition of H- and T-reflexes were found to be independent if reflex amplitudes were expressed as a percentage of the control reflex amplitude. 6. The cessation of vibration of ankle extensors was followed by a gradual recovery of H-reflexes from inhibition. On the contrary, T-reflexes showed a marked potentiation. Such postvibratory effects were generally not observed following vibration of ankle or knee flexors. When postvibratory effects were tested during low-frequency depression, percent postvibratory depression of H-reflexes and percent postvibratory potentiation of T-reflexes were found to be independent of the degree of low-frequency depression. 7. Tetanic stimulation of the tibial nerve for 20 s at 200 Hz resulted in posttetanic potentiation (PTP) of H- and T-reflexes, the former being stronger than the latter. 8. The extent of the changes in H- and T-reflex EMG amplitudes, due to changes in repetition rate, vibration, or tetanic stimulation, was generally inversely related to the intensity of the electrical or mechanical reflex stimuli. 9. The observed discrepancies between the induced changes in H- and T-reflex amplitudes are interpreted in terms of the differences in afferent inputs between the two reflexes that were suggested by Burke and his colleagues (9-11). The results presented here are consistent with their tentative conclusions that H- and T-reflexes are not simple monosynaptic reflexes elicited by single volleys in group Ia afferent fibers and that they may show differences in central transmission.