PROPRIOCEPTIVE COORDINATION OF DISCRETE MOVEMENT SEQUENCES - MECHANISM AND GENERALITY

A ''discrete'' movement sequence is defined as a movement with a singl e goal that involves a series of overlapping joint rotations. Reaching -and-grasping and throwing are examples of discrete movement sequences . The central nervous system (CNS) can use reafferent proprioceptive i nformation from one joint rotation in a sequence to coordinate subsequ ent rotations at other joints. The experiments reported in this paper demonstrate how the human CNS uses proprioceptive information to coord inate discrete movement sequences. We examined the mechanism (at an in formation processing level) underlying proprioceptive coordination and the generality (i.e., the boundary conditions) of these mechanisms as they apply to everyday movement sequences. Adult human subjects perfo rmed a discrete movement sequence that resembles backhand throwing: el bow extension followed by hand opening. The task was to open the hand as the elbow passed through a prescribed ''target'' angle. We eliminat ed visual information and made the arrival time at the target angle un predictable so that the available kinematic information was provided e xclusively by proprioception. The subjects were capable of performing this motor task with a high degree of precision, thereby demonstrating that the nervous system can use proprioceptive input to coordinate di screte movement sequences. Our data indicate that precise coordination is achieved by extracting kinematic information related to both the v elocity of elbow rotation as well as the elbow position during movemen t (i.e., ''dynamic position''). Dynamic position information appears t o be encoded as both absolute joint angle and angular distance, althou gh more precisely as angular distance. Although our experiments were c onducted under rather restrictive laboratory conditions, this mechanis m of motor coordination might also apply to everyday movement. Our res ults suggest that this mechanism could be employed for passive as well as active movement sequences, with and without opposing loads; it cou ld exert its influence in discrete movement sequences as brief as 210 ms or as long as 1.5 s; and it does not involve any significant degree of learning (this proprioceptive mechanism appears to be readily avai lable for use on the first attempt of a novel motor task).