Learning a multi-joint throwing task: a morphometric analysis of skill development.

Changes in movement organization were examined during the learning of a multi-joint throwing task. Six participants threw a modified frisbee into the target area over an extended practice period (total of 1,300 trials). Throwing accuracy scores were recorded while 3-D arm motion was collected. Intrinsic shape and variability of end-point path and joint coordination pattern were assessed quantitatively by using generalized procrustes analysis (GPA) to remove extrinsic variability in location, orientation and size of movement configurations. Results indicated that throwing accuracy followed the power law of practice and had an inverse relationship with the actual variability of end-point path. GPA indicated that the intrinsic pattern of end-point path stabilized early during learning while the intrinsic pattern of joint coordination remained variable throughout practice. These findings support the proposal that skill acquisition is composed of two learning processes that occur at different rates. Topology (intrinsic pattern of end-point path) is acquired early during practice, while dynamic control (represented by joint coordination) occurs at a much slower rate.

The coupling between upper and lower extremity synergies during whole body reaching.

When performing whole body reaching movements, all four limbs participate in the task. We hypothesized that the synergies that characterize upper and lower extremity movement are flexible and become coupled into one functional unit to transport the body towards the target. To test this hypothesis, subjects reached to three targets, one within and two beyond arm's length. In addition, subjects reached at two speeds and either stopped at the target or returned to the original start position. To assess the coupling during the various whole body reaches, a principal component analysis was performed on the displacements of the five primary joints used to accomplish the task (ankle, knee, hip, shoulder and elbow). Analysis of the loadings from the principal component analysis indicated that the first component represented the reaching element of the task, while the second and third components represented the postural element. When reaching within arm's length the variance explained by the joint coupling was distributed between the first three principal components. However, as reach distance increased, the distribution shifted with most of the variance being explained by the first principal component. Neither movement velocity nor final joint configuration affected the coupling between the joints. Analysis of center of mass indicated that it shifted progressively forward as reached distance increased. We conclude that as target distance increased, the reach and postural synergies became coupled resulting in the arms, legs and trunk working together as one functional unit to move the whole body forward.

The effects of stance configuration and target distance on reaching. I. Movement preparation.

The aim of the present study was to investigate the relationship between the focal and postural components of a functional movement during the preparatory phase of a task. The contribution of the arms, trunk, and legs were varied by having subjects reach for two targets within and two beyond arm's length. In addition, the degree of postural stability was manipulated by varying the size of the base of support (BoS). Nine subjects reached and grasped a dowel placed at four locations while standing on a force plate with their feet in a parallel or step stance (right foot forward) under simple reaction-time (RT) conditions. Anticipatory postural adjustments (APAs) occurring prior to arm movement and RTs were analyzed. APAs varied depending on the demands of the task. For movements within arm's length, subjects selected different strategies to initiate the movement. However, for movements beyond arm's length, all subjects used the same strategy: the center of pressure (CoP) was shifted posteriorly, which resulted in the center of mass (CoM) moving towards the target. Target distance and BoS had no effect on the onset of APAs. In contrast, amplitude and duration of APAs increased linearly with target distance, and amplitude was always greater during the more posturally stable BoS configuration. Although wrist RT increased linearly with movement amplitude for both stance configurations, the rate of change was less under the more stable BoS. These results suggest that, during the performance of a functional task, dynamic changes that occur in the trunk and lower extremities prior to initiation of arm movement serve not only to stabilize the body, but are also used to initiate and assist whole-body reaching.

The coordination between trunk and arm motion during pointing movements.

The coordination between the trunk and arm of six subjects was examined during unrestrained pointing movements to five target locations. Two targets were within arm's length, three were beyond. The trunk participated in reaching primarily when the target could not be attained by arm and scapular motion. When the trunk did contribute to hand transport, its motion started simultaneously with arm movement and continued until target contact. Redundancy in the degrees of freedom used to execute the movement had no effect on the configuration of joints and segments used to attain a specified target; no difference in variability was noted regardless of whether redundancy existed. However, different configurations were used to achieve the same wrist coordinates along a common endpoint path, depending on the final position of the hand. The addition of trunk flexion, rotation and scapular motion did not alter the coupling between the elbow and shoulder joints and had no effect on the path of the hand or the smoothness of its velocity profile. Thus, trunk motion was integrated smoothly into the transport phase of the hand. As the trunk's contribution to hand transport increased, it played a progressively greater role in positioning the hand close to the target during the terminal stage of the reach. Of the movement components measured, trunk flexion was the last component to complete its motion when target reaches were made beyond arm's length. Hence, the trunk not only acts as a postural stabilizer during reaching, but becomes an integral component in positioning the hand close to the target.

A kinematic comparison of single and multijoint pointing movements.

Rapid pointing movements (no accuracy or reaction time requirements) were performed under three conditions which limited motion to the shoulder, elbow or a combination of these two joints. Velocity profiles of the hand's trajectory differed during single and multijoint movements. For the same magnitude of displacement, the hand always had a higher peak velocity, shorter rise time (time to peak velocity) and shorter movement time during single joint movements. However, when the profiles were normalized with respect to amplitude and movement time, no significant differences were observed between these three movement conditions. The velocity profiles of the elbow and/or shoulder were also compared under single and multijoint movement conditions. Analysis of these profiles revealed that the relationships between peak velocity and displacement and between movement time and displacement remained the same at the shoulder joint during single and multijoint movements. In contrast, the elbow joint velocity profiles were significantly affected by movement conditions. These relationships (peak velocity/displacement and movement time/displacement) changed during multijoint movements and became the same as those observed at the shoulder joint. The shape of the hand velocity profile and its invariance across movement conditions can best be explained by dynamic optimization theory and supports the notion that movement of the hand is of primary importance during rapid pointing. However, the consistency of the shoulder velocity profile and the highly significant relationships between the movement of the elbow and shoulder joints indicates that a subordinate joint planning strategy is also used. The purpose of this strategy is to functionally decrease the available degrees of freedom and to simplify coordination between the moving joints. Thus, the organization of arm movements is hierarchically structured with important, but different contributions being made on both the hand planning and joint planning levels.