The H-reflex as a probe: pathways and pitfalls.

The Hoffmann (or H) reflex is considered a major probe for non-invasive study of sensorimotor integration and plasticity of the central nervous system in humans. The first section of this paper reviews the neurophysiological properties of the H-reflex, which if ignored create serious pitfalls in human experimental studies. The second section reviews the spinal inhibitory circuits and neuronal pathways that can be indirectly assessed in humans using the H-reflex. The most confounding factor is that reciprocal, presynaptic, and Ib inhibition do not act in isolation during movement. Therefore, characterization of these spinal circuits should be more comprehensive, especially in cases of a neurological injury because neurophysiological findings are critical for the development of successful rehabilitation protocols. To conclude, the H-reflex is a highly sensitive reflex with an amplitude that is the result of complex neural mechanisms that act synchronously. If these limitations are recognized and addressed, the H-reflex constitutes one of the major probes to assess excitability of interneuronal circuits at rest and during movement in humans.