The neural control of movement must contend with trajectory-specific and nonlinearly distorted manifolds of afferent muscle spindle activity

We introduce the concept of trajectory-specific sensory manifolds. They are the unique multidimensional and time-varying combinations of afferent signals that obligatorily emerge during a limb movement. We use the example of muscle spindles (i.e., the muscle's proprioceptors for length and velocity) that arise during movements of an arm (a planar 2-DOF 6-muscle model) during the production of straight, curved and oscillatory hand movements. Through the use of parallel coordinates, we visualize the high-dimensional evolution of the afferent signaling across muscles and tasks. We demonstrate that a given movement gives rise to a distinct sensory manifold embedded in the 12-D space of spindle information that is largely independent of the choice of muscle coordination strategy. Given that muscle lengths and velocities are fully determined by joint kinematics, such manifolds provide a rich set of information to use in its control.