Quantitative relations between hypertonia and stretch reflex threshold in spastic hemiparesis

The relative contributions of variations in stretch reflex threshold and total joint stiffness to changes in stretch-evoked torque were assessed in the spastic elbow muscles of 14 hemiparetic spastic subjects. For a given subject, variations in torque, measured after a constant angular deflection, were mediated largely by changes in stretch reflex threshold, rather than by changes in reflex stiffness. Between-subject comparisons were sensitive to stiffness differences between limbs, but reflex thresholds were still broadly correlated with torque magnitude, suggesting that reductions in stretch reflex threshold are uniformly present in spastic muscles. These findings, coupled with the apparent similarity of reflex stiffness estimates in voluntarily activated spastic and normal muscles, suggest that the central disturbance in spasticity is a reduction in the threshold of the stretch reflex, without a significant enhancement of reflex gain.