Systematic nonlinear relations between joint mechanics and the neural reflex response with changes in stretch amplitude at the wrist.

The effects on both joint mechanics and the neural stretch reflex of changes in amplitude of stretch ranging from within the region of short range stiffness up to 3% of resting muscle length were quantified. The level of activation of the primary agonist was held constant but there was a small but systematic increase in activation of the other flexor and extensor muscles as stretch amplitude increased. The patterns of change with stretch amplitude in both the joint mechanics and stretch reflex properties were non-linear but systematic and were well described by power relationships that accounted for high proportions of variance. With an increase in stretch amplitude, joint stiffness, viscosity, damping ratio and natural frequency decreased. In parallel with these mechanical changes, the neural reflex coherence increased and the phase advance and gain decreased. Multiple regression analyses partitioned the variation in the joint mechanics that was attributable to the various factors and it was found that the variation of the mechanics was almost entirely attributable to the variation of the neural reflex gain, which accounted for 93% of the changes in the stiffness, viscosity and natural frequency and 82% of the changes in the damping ratio. It is concluded that joint mechanics are significantly affected by changes in the neural reflex gain.