Dynamics of patellar tendon reflex in spastic multiple sclerosis patients

Patellar tendon reflexes were studied in fourteen normal subjects and ten multiple sclerosis (MS) patients. An instrumented hammer was used to tap the patellar tendon and record the tapping force, while the quadriceps EMG and knee extension torque were recorded isometrically as the reflex responses. The relationship between tapping force and reflex torque was characterized using several system measures: the tendon reflex gain (G_tr), contraction rate (R_c), half-relaxation rate (R_hr), contraction time (t_c), half-relaxation time (t_hr), and reflex loop delay (t_d). G_tr, R_c, and R_hr of the MS patients were significantly higher than their counterparts in normal controls (p<0.001 for all cases), t_d of the MS patients was significantly shorter than that of the normal group (p<0.005). G_tr, R_c, and R_hr values correlated more closely with clinical measures of spasticity (Ashworth scale and tendon reflex scale) than did pure output measures (peak reflex torque and quadriceps EMG signals). Tendon reflexes were more repeatable in the MS population than in normal controls. The higher correlation to clinical measures and lower variation of G_tr, R_c, and R_hr indicate the potential values of these dynamic system measures in clinical practice as a more accurate and consistent approach towards quantification of the tendon jerk and spasticity.