Neural control: novel evaluation of stretch reflex sensitivity

We evaluated the stretch reflex activities of the elbow flexor and extensor muscles considering the relationship between the reflex electromyographic (EMG) responses and their corresponding standardized muscle stretch velocit ies. Specifically, muscular stretch velocity was estimated by using ultraso nograms. Stretch reflex EMG responses were elicited in the biceps brachii, brachioradialis and triceps brachii with a ramp-and-hold rotation at the el bow joint, which consisted of various angular velocities for the extension- or flexion-direction. The whole muscle stretch velocity induced by each ra mp-and-hold rotation was calculated on the basis of fibre length changes as sociated with the elbow joint angle. A linear regression equation was fitte d to the relation between the whole muscle stretch velocity and the reflex EMG responses, and the variables from the equation were used to quantify se nsitivity of each reflex EMG component. The reflex EMG responses were incre ased as the ramp-and-hold rotational velocity increased. There were no sign ificant differences in the recorded magnitudes of reflex EMG responses with equivalent joint rotational velocity between the brachioradialis and the t riceps brachii medial head. These muscles showed the highest reflex respons es in the flexor and extensor muscles, respectively. To the contrary, the r eflex EMG response elicited by the standardized muscle stretches was signif icantly greater in the extensor muscles, indicating a higher reflex sensiti vity. This was because of the lower muscle stretch velocity of the triceps brachii with an equivalent elbow joint rotation. The stretch reflex sensiti vity in both the elbow flexor and extensor muscles might be regulated so as to make the reflex responses the same when the equivalent joint rotational velocity is applied to these muscles.