Parallels in control of voluntary and perturbation-evoked reach-to-grasp movements: EMG and kinematics

To determine the potential differences in control underlying compensatory and voluntary reach-to-grasp movements the current study compared the kinematic and electromyographic profiles associated with upper limb movement. Postural perturbations were delivered to evoke compensatory reach-to-grasp in ten healthy young adult volunteers while seated on a chair that tilted as an inverted pendulum in the frontal plane. Participants reached to grasp a laterally positioned stable handhold and pulled (or pushed) to return the chair to vertical. The distinguishing characteristic between the two behaviors was the onset latency and speed of movement. Consistent with compensatory balance reactions, the perturbation-evoked reach response was initiated very rapidly (137 vs. 239 ms for voluntary). As well the movement time was shorter, and peak velocity was greater for PERT movements. In spite of the profound differences in timing, the sequence of muscle activity onsets and the order of specific kinematic events were not different between maximum velocity voluntary (VOL) and perturbation-evoked (PERT) reach-to-grasp movements. Peak velocity and grasp aperture occurred prior to hand contact with the target for PERT and VOL movements, and wrist trajectory was influenced by the direction of perturbation relative to the target. To achieve such target specific control for responses initiated within 100 ms of the perturbation, and when characteristics of body movement were unpredictable, the perturbation-evoked movements would need to incorporate sensory cues associated with body movement relative to the target into the earliest aspects of the movement. This suggests reliance on an internal spatial map constructed prior to the onset of perturbation. Parallels in electromyographic and kinematic profiles between compensatory and voluntary reach-to-grasp movements, in spite of temporal differences, lead to the view they are controlled by common neural mechanisms.