Sequential motions of body segments in striking and throwing skills: descriptions and explanations.

The motions of segments participating in striking and throwing skills are generally sequenced in a proximal-to-distal fashion. These sequences are often described in terms of the linear velocities of the segment endpoints, joint angular velocities or segment angular velocities. While each method of description has its own merit, the latter is recommended since it leads to an intuitively pleasing way of explaining segment motions. Explanations of segment motion sequences are dependent not only on a knowledge of the joint moments driving the system of linked segments, but on the way the segments interact as functions of their motions and orientations. The motion-dependent interaction among segments is significant and offers an explanation of the sequencing of segment motions. As illustrated by the thigh and lower leg in kicking and by the upper arm and forearm in overarm pitching, the forward acceleration of the proximal segment plays a large role in causing the distal segment to lag behind. The subsequent forward acceleration of the distal segment is largely a result of the way the proximal segment interacts with the distal segment as a function of the proximal segment's angular velocity. The proximal segment is subsequently slowed down largely due to the motion-dependent effect of the distal segment on the proximal segment. Differences in the way segments interact in striking and throwing skills can account for variations in the timing of segment actions and these differences need to be examined before establishing general principles governing striking and throwing.