CHANGES IN REFLEX EXCITABILITY FOLLOWING ISOMETRIC CONTRACTION IN HUMANS

Enhancement of muscle stretch following isometric contraction has been thought to occur as a result of inhibitory reflex mechanisms. Experim ents with electrical stimulation (H-reflex) have demonstrated maximal H-reflex suppression during force relaxation followed by gradual recov ery over the following 20 s. There has been considerable speculation a s to whether electrical and mechanical stimulation elicit similar resp onse behaviour. The present study examined postisometric reflex modula tion following both stimulation modalities. In ten subjects dorsiflexi on stimuli varying in speed and amplitude were applied after 30% and 6 0% maximal voluntary contraction (MVC). Modulation of the mechanically and electrically evoked responses following isometric plantarflexion was investigated. Reflex responses following both stimulation modaliti es were depressed during the course of force relaxation. A rather fast recovery was observed in mechanical stimulation. Postisometric respon se modulation was neither altered by the amount of isometric plantarfl exion, nor by the amplitude of the applied stretch stimulus. With incr easing velocity of the applied dorsiflexion, however, the shape of the reflex modulation persisted, but the magnitude of the responses was s ignificantly enhanced. In electrical stimulation, however, recovery wa s delayed. It is suggested that postisometric reflex modulation is due to presynaptic inhibition. Moreover, possible peripheral mechanisms r esulting from alpha-gamma-coactivation may also affect the stretch rec eptor itself because of inherent stiffness properties. The latter poss ibility particularly would explain the differences between mechanical and electrical stimulus modalities. With respect to practical implicat ions, the very fast recovery (< 400 ms) of the stretch responses to co ntrol values strongly contradicts the interpretation that after isomet ric precontraction, suppression of reflex activity might be used for m ore efficient stretching of the tendomuscle system.