Identification of static and dynamic components of reflex sensitivity in spastic elbow flexors using a muscle activation model

Static and dynamic components of the stretch rifles were studied in elbow f lexors of 13 hemiparetic brain-injured individuals. Constant-velocity joint rotations were applied to the elbow, and the resulting stretch reflex torq ue and electromyographic responses were recorded in the biceps brachii and brachioradialis muscles. Ten elbow extension velocities between 6 and 150 d eg s(-1) were applied in random order. The resulting reflex torque response was plotted as a function of elbow angle and fitted with a mathematical mo del designed to depict elbow flexor activation. We found that four of the s ix model parameters were essentially independent of test velocity. Converse ly, 73% (19/26) of cases involving tile other two modal parameters were dep endent on velocity of joint extension (p <0.05). We conclude from these res ults that four of the model parameters reflect the static reflex response w hile the two remaining velocity-dependent parameters reflect the dynamic re flex response. To describe overall velocity dependence of stretch reflexes in spastic elbow muscles, tile two dynamic reflex parameters were fitted to a fractional exponential function of velocity, similar to a model previous ly used to describe spindle firing rate in the cat hindlimb. We found that the mean velocity exponent of the dynamic reflex parameters was 0.24 + 0.17 (s.d.) (N = 13, a value similar to that for muscle spindle velocity sensit ivity in reduced animal preparations. We conclude that both static and dyna mic reflex sensitivities can be measured by examining different aspects of the torque/angle relation associated with the reflex response to a large-am plitude lamp stretch of the elbow. (C) 2001 Biomedical Engineering Society.