The purpose of this study is to investigate neural strategies used to make point-to-point arm movements in a desired direction. Using a variety of targets that encompass many directions, the applicants will examine patterns of muscle activation at the onset of movement in three-joint motions (shoulder, elbow, and wrist), restricted to one plane. The investigator's plan to use three-joint motions is not simply to add another segment; more importantly, three joints are the minimum number of joints needed to provide redundant degrees of freedom, and consequently, a target location can be achieved by multiple final configurations of the arm. This is unlike the more prevalent two-joint studies of shoulder and elbow movements, in which the location of the tip of the arm determines the only possible configuration of the arm. The applicants are interested in the nervous system's choice of flexor or extensor muscle activity at each joint as related to target direction, when subjects are also free to choose the final configuration to reach a target. While a small number of studies have reported kinematics for three-joint arm movements, fewer have recorded muscle activities (EMG), and none have recorded simultaneously from reciprocal muscles at the three joints. Two prediction will be tested from two-joint studies: (1) a simple rule of flexor -vs-extensor-initiated movements exists for each joint; (2) the relative timing of muscle onset among the different joints will follow a proximal to distal sequence, regardless of the sequence of joint-rotational onset observed at the three joints. In addition, they will investigate whether the nervous system modifies the pattern of muscle initiation when the expected coordination among the joints is disrupted. To do this, they will immobilize the elbow joint, and then determine changes in the choice and timing of muscle onset for point-to-point movements with similar directions as before. Results of this study will significantly add to the understanding of the control of multi-jointed limbs and can aid in assessment of patients who have difficulty in producing accurate and coordinated arm movements due to orthopedic or neurological impairments.
| Koshland, G F; Hasan, Z (1994) Selection of muscles for initiation of planar, three-joint arm movements with different final orientations of the hand. Exp Brain Res 98:157-62 |
