Modulation of spastic ankle stiffness dynamics with voluntary contraction in spinal cord injury

A parallel-cascade system identification technique was used to examine the intrinsic and reflex contributions to overall ankle stiffness in normal (control) and spastic spinal cord injured (SCI) subjects as a function of voluntary contraction level. Intrinsic dynamics were modeled as a linear, 2nd-order system relating intrinsic torque to joint position. Reflex dynamics were described by a linear, 3rd-order system relating half-wave rectified velocity and reflex-torque. Intrinsic stiffness was similar in magnitude in both groups and increased with voluntary contraction at similar rates. In contrast, reflex stiffness dynamics behaved differently in the two groups: (1) reflex stiffness gain was significantly greater in SCI than control subjects at all contraction levels, (2) the modulation of reflex gain with voluntary contraction was abnormal, and (3) the reflex frequency parameter was lower in SCIs and decreased with contraction level while it increased in controls. These differences were significant across a wide range of contraction levels with the gain difference being largest at low levels of contraction and the frequency difference being largest at high levels of contraction.