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.