## 5 Conclusions  
The main purpose of this study was to investigate the association between the on-plane angular motion parameters of the axle-chain system and the clubhead speed in three different club conditions (driver, 5-iron, and pitching wedge) in a group of skilled male golfers. It was concluded that:  
* Higher normalized clubhead speed is associated with shorter downswing phases (total, early downswing, and late downswing) and more delayed onset of downswing events (TB and thereafter) closer to impact. In addition, a shorter early downswing phase is associated with a longer transition phase.
* Higher normalized clubhead speed is associated with larger backward/downward on-plane rotations (hip/shoulder lines and upper lever) on the functional swing plane. Normalized clubhead speed is not significantly associated with the wrist-cock angles or wrist motion ranges. In the driver swing, more wrist cocking at EDA (keeping the club closer to the body) is associated with higher normalized clubhead speed.
* Higher normalized clubhead speed is associated with a larger X-factor at BI (with the shoulder line trailing the hip line). Normalized clubhead speed is not significantly associated with other X-factor/stretch parameters.
* Higher normalized clubhead speed is associated with more delayed onset of backswing angular velocity peaks (shoulder line) and more delayed backswing-to-downswing transitions (hip/shoulder lines, upper lever, and club). Normalized clubhead speed is not significantly associated with the timings of the backswing angular velocity peaks.
* Only the transition sequence exhibits consistent proximal-to-distal sequences with a minor deviation in the pitching wedge condition. The backswing and downswing sequences deviate from the proximal-to-distal sequence significantly and are inconsistent across club conditions. The backswing sequence revealed a dominant separation between the axle (hip/shoulder lines) and the chain (upper lever and club), while the downswing sequence exhibited a dominant separation between the body and the club. The downswing angular velocity peaks of the body (hip/shoulder lines and upper lever) typically occur within a narrow time window.
* Higher normalized clubhead speed is associated with faster backward rotations (hip/shoulder lines and upper lever). Higher normalized clubhead speed is also associated with faster downward rotations (hip/shoulder lines and upper lever), more slowdowns after reaching the peak (hip line and upper lever), and faster rotations at BI (shoulder line and upper lever). Faster backward rotations are associated with faster downward rotations (hip/shoulder lines).
* While larger/faster backward and downward rotations of the axle-chain system on the functional swing plane are the overarching pre-requisites of a high clubhead speed, the correlation and kinematic sequence profiles fundamentally highlight two key aspects of a biomechanically robust golf swing: body-driven active (fast) backswing and active/elongated backswing-to-downswing transition.  
Conceptualization, M.V.M., K.A.B. and Y.-H.K.; methodology, all; software, Y.-H.K.; validation, M.V.M. and Y.-H.K.; formal analysis, M.V.M., M.A.A., N.A.L. and N.J.T.; investigation, M.V.M., M.A.A., N.A.L. and N.J.T.; resources, M.V.M. and Y.-H.K.; data curation, M.V.M., M.A.A., N.A.L. and N.J.T.; writing--original draft preparation, M.V.M. and Y.-H.K.; writing--review and editing, all; supervision, K.A.B. and Y.-H.K.; project administration, M.V.M. and Y.-H.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors would like to thank Qualisys AB for providing an optical motion capture system.  
The authors declare no conflict of interest.