Effect of muscle biomechanics on the quantification of spasticity

The impact of muscle biomechanics on spasticity was assessed by comparison of the reflex responses of the elbow and metacarpophalangeal (MCP) flexor muscles in individuals with chronic spastic hemiplegia following stroke. Specifically, methods were developed to quantify reflex responses and to normalize these responses for comparison across different muscle groups. Stretch reflexes were elicited in the muscles of interest by constant velocity ramp-and-hold stretches at the corresponding joint. The muscles were initially passive, with the joint placed in a midrange position. Estimates of biomechanical parameters were used to convert measured reflex joint torque and joint angle into composite flexor muscle stress and stretch. We found that the stretch reflex response for the MCP muscle group had a 74% greater mean stiffness modulus than that for the elbow muscle group, and that the reflex threshold was initiated at an 80% shorter mean muscle stretch. However, we determined that initial normalized fiber length was significantly greater for the experiments involving the MCP muscles than for those involving the elbow muscles. Increasing the initial composite fiber length of the elbow flexors produced significant reduction of the reflex threshold (p<0.001), while decreasing the initial length of the MCP flexors significantly reduced their measured reflex stiffness (p<0.001). Thus, biomechanical parameters of muscle do appear to have an important effect on the stretch reflex in individuals with impairment following stroke, and this effect should be accounted for when attempting to quantify spasticity.