Late Responses

Late responses are a group of responses occurring after the compound muscle action potential (M-wave) in motor nerve conduction studies and include F-waves, H-reflexes, and axon reflexes. These responses can indirectly measure aspects of proximal conduction that are not obtained with routine motor nerve conduction studies and may be useful in assessing diseases that affect proximal nerves, such as polyradiculopathies. A thorough understanding of the physiological basis, sources of error, and clinical applications and limitations enhances the usefulness of F-waves and H-reflexes as one means to evaluate proximal functioning of the motor and sensory nerves. This chapter will discuss the study methods and applications of F-waves and H-reflexes.