Quantitative features of the stretch response of extrinsic finger muscles in hemiparetic stroke

Despite its potential importance in hand dysfunction, spasticity in the finger muscles following stroke has not been well described. To explore this area, we assessed the role of finger flexor spasticity, along with that of passive mechanical forces, in resisting finger movement in 13 chronic stroke subjects. Subjects were tested with a device that stretched the extrinsic finger muscles through imposed rotation of the metacarpophalangeal (MCP) joints. Both maintained and constant-velocity stretches were imposed. For the constant-velocity stretches, eight of the 13 stroke subjects exhibited strong stretch reflexes, as determined by electromyography and net work. The net work of this reflex response, calculated from the integral of the torque-angle plots, increased proportionally with increasing velocity, indicating a contribution from flexor muscle spasticity. Conversely, nine of the 13 stroke subjects did not possess distinctly greater passive, mechanical resistance to MCP rotation than control subjects. While extensor spasticity was not observed, stretch of the extrinsic finger flexors also produced some reflex activity in the finger extensors concomitant with reflex excitation of the flexors. These findings suggest that resistance to muscle stretching following stoke is mediated primarily by neurological rather than biomechanical disturbances, although changes in muscle fiber length may exaggerate the resistance.