An evaluation of passive properties of spastic muscles in hemiparetic stroke survivors.

We have reported earlier [1] a new method for estimating reflex threshold in spastic muscles of stroke survivors, using controlled amplitude taps superimposed on progressive and controlled muscle indentation of the bicipital tendon in the bicipital fossa. This muscle indentation is done with a linear actuator positioned over the biceps muscle tendon at the elbow. In the course of testing for increased stretch reflex responses, (a cardinal feature of spasticity), we have also observed that the intrinsic or passive stiffness of the muscle is often increased. This assessment is derived from recordings of the force generated by the tendon during progressive loading, and by the instantaneous force response to the tendon tap. Thus, it appears that passive properties of muscle are often also changed in parallel with the reflex abnormalities. While some of these mechanical features have been described in earlier studies of torque-angle relations of spastic joints, it appears that these features can also be recognized readily using a small actuator that loads the tendon progressively. These findings may help clinicians recognize early changes in muscle mechanical properties, and may help them prevent large-scale adverse changes in muscle function.