Passive, intrinsic and reflex-mediated stiffness in the ankle extensors of hemiparetic patients.

The total mechanical response to a stretch in a contracting muscle is the sum of the response from the properties of the fibres contracting prior to the stretch (intrinsic properties), the response from the passive tissues and the response from the stretch reflex-mediated contraction of the muscle fibres. The passive, the intrinsic and the reflex-mediated mechanical response to a 4 degrees stretch of the ankle extensors in both legs of nine spastic hemiparetic patients and, as control, the left ankle extensors in eight healthy subjects were measured. The stretch was imposed at the top of a maintained contraction, which was varied from zero to maximal contraction. The torque increments, measured 200 ms after stretch, were converted into stiffness by dividing by the amplitude of the stretch. We found that the reflex stiffness of the spastic and contralateral leg was within the normal range of the reflex stiffness in healthy subjects, and that the passive stiffness of the spastic and the contralateral leg is larger than the passive stiffness in healthy subjects, whereas the intrinsic stiffness is unchanged in the spastic extensor muscles.