Reflex torque response to movement of the spastic elbow: Theoretical analyses and implications for quantification of spasticity

A parametric model of the human reflex torque response to a large-amplitude , constant angular velocity elbow extension was developed in order to help quantify spasticity in hemiparetic stroke patients, and to better understan d its pathophysiology. The model accounted for the routinely observed level ing of torque (i.e., a plateau) at a mean angular increment of 51 degrees+/ -10 degrees s.d. (n = 98) after the initial rise. This torque "plateau'' wa s observed in all eight subjects, and in 98 of 125 trials across 25 experim ental sessions. The occurrence of this plateau cannot be explained by decre ases in elbow flexor moment arms during elbow extension. Rather, the platea u is attributable to a consistent leveling in muscle activation as confirme d both qualitatively from recordings of rectified, smoothed electromyograph (EMG) activity, and quantitatively using an EMG coefficient model. A param etric model was developed in which the pattern of muscle activation in the stretch reflex response of elbow flexors was described as a cumulative norm al distribution with respect to joint angle. Two activation functions, one related to biceps and the other to brachioradialis/brachialis, were incorpo rated into the model in order to account for observations of a bimodal angu lar stiffness profile. The resulting model yielded biologically plausible p arameters of the stretch reflex response which may prove useful for quantif ying spasticity. In addition, the model parameters had clear pathophysiolog ical analogs, which may help us understand the nature of the stretch reflex response in spastic muscles. (C) 1999 Biomedical Engineering Society. [S00 90-6964(99)01206-0].