Inverting a model of neuro-muscular control to estimate descending activation patterns that generate fast reaching movements.

Reaching movements generally show smooth kinematic profiles that are invariant across varying movement speeds even as interaction torques and muscle properties vary non-linearly with speed. How the brain brings about these invariant profiles is an open question.We develop an analytical inverse dynamics method to estimate descending activation patterns directly from observed joint angle trajectories based on a simple model of the stretch reflex, and of muscle and biomechanical dynamics. We estimate descending activation patterns for experimental data from eight different planar two-joint movements performed at two movement times (fast: 400 msec; slow: 800 msec). The temporal structure of descending activation differed qualitatively across speeds, consistent with the idea that the nervous system uses an internal model to generate anticipatory torques during fast movement. This temporal structure also depended on the co-contraction level of antagonistic muscle groups. Comparing estimated muscle activation and descending activation revealed the contribution of the stretch reflex to movement generation that was found to set in after about 20 percent of movement time.