A number of recent studies have measured the extent and timing of segment rotation during the golf swing. A promising technique, instantaneous screw axis (ISA) theory, could provide a better expression of segment rotation. In Part 1 of this two-part study, the objectives are to identify the ISA of the pelvis, shoulders and left arm during the downswing, compute segment angular velocity relative to that segment's ISA and verify that ISA theory is a valid tool to analyse segment rotation during the golf swing. Results indicate that for all subjects, at least 71% of marker velocity is a result of rotation about their respective ISA, when averaging results over the duration of the downswing, confirming that motion is primarily rotational. Furthermore, ISA position and orientation of each segment approaches, on average, the expected gross axis of rotation, confirming that motion about the ISA is representative of joint motion.

Recent studies have measured body segment rotation to study the kinematic sequence of the downswing. However, this sequence has yet to be determined relative to an instantaneous screw axis (ISA), free to change position and orientation during motion to reflect shifts in a segment's dominant axis of rotation. In Part 2 of this two-part study, the objectives were to compute the amplitude of segment angular velocity relative to the corresponding ISA of that segment and verify if the magnitude of segment angular velocity followed the proximal to distal sequence of the summation of speeds principle. Results indicate that the kinematic sequence of 2 of the 5 subjects analyzed supports the summation of speeds principle, where the sequence in which the maximum angular velocity about the pelvis, shoulders and left arm occurred, for one subject, at 68.2 ± 3.2, 72.8 ± 1.7 and 100 ± 0.0% of the downswing.