Variability in Hoffmann and tendon reflexes in healthy male subjects

There is a time dependent decrease in amplitude of H- and T-reflexes during Zero-G exposure and subsequently an increase in the amplitude of the H-reflex 2-4 hours after return to a 1-G environment. These alterations have been attributed to the adaptation of the human neurosensory system to gravity. The Hoffman reflex (H-reflex) is an acknowledged method to determine the integrity of the monosynaptic reflex arc. However deep tendon reflexes (DTR's or T-reflexes), elicited by striking the tendon also utilize the entire reflex arc. The objective of this study was to compare the variability in latency and amplitude of the two reflexes in healthy subjects. Methods: Nine healthy male subjects, 27-43 years in age, 161-175 cm in height plus 60-86 Kg in weight, underwent weekly testing for four weeks with a Dan-Tec EMG counterpoint EMG system. Subjects were studied prone and surface EMG electrodes were placed on the right and left soleus muscles. The H-reflex was obtained by stimulating the tibial nerve in the politeal fossa with a 0.2 msec square wave pulse delivered at 2 Hz until the maximum H-reflex was obtained. The T-reflex was invoked by tapping the achilles tendon with a self triggering reflex hammer connected to the EMG system. The latencies and amplitudes for the H- and T-reflexes were measured. Results: These data indicate that the amplitudes of these reflexes varied considerably. However, latencies to invoked responses were consistent. The latency of the T-reflex was approximately 3-5 msec longer than the H-reflex. Conclusion: The T-reflex is easily obtained, requires less time, and is more comfortable to perform. Qualitative data can be obtained by deploying self triggering, force plated reflex hammers both in the 1-G and Zero-G environment.