## Materials and Methods
### Variable Computation  
Twelve swing event frames were identified in each trial based on the club and lead arm (left arm) positions: BA (breakaway), MB (mid-backswing), LBA (late backswing, arm-based), LB (late backswing), EPR (end of pelvis rotation), TB (top of backswing), EDA (early downswing, arm-based), ED (early downswing), MD (mid downswing), BI (ball impact), MF (mid-follow-through), and MC (maximum cooking) [2]. BA is the instant the clubhead moves more than 3 cm away from the initial address position. EPR is the instant the direction of rotation of the pelvis changes from backward to downward. The ZYX (longitudinal-anteroposterior-mediolateral) rotation sequence was used in computing the orientation angles of the pelvis (rotation, lateral tilt, and anteroposterior tilt), and EPR was detected based on the first orientation angle. TB is the instant the club's direction of rotation changes from backward to downward in the frontal view. BI is the instant the clubface comes into contact with the ball. At the \"mid\" events, the club shaft is parallel to the ground in the frontal view. \"Early\" or \"late\" events are when the club shaft becomes vertical before or after the corresponding \"mid\" event. At the \"arm-based\" events, the lead arm is parallel to the ground. MC is the instant the wrist (the link between the upper lever and the club) is cocked the most on the FSP. BI was used as the zero-time event and all event times were expressed relative to BI. Five phases were defined from the events: backswing (BA-TB), downswing (TB-BI), transition (EPR-TB), early downswing (TB-EDA), and late downswing (EDA-BI). The position and normal vector of the FSP were calculated from the clubhead trajectory (MD-MF) following the method outlined by [PERSON] et al. [4]. A group of on-plane angles of the axle-chain system (i.e., HL, SL, UL, club, and wrist cock) were defined on the FSP (Figure 1) as outlined by [PERSON] et al. [2], and ranges of rotation (ROR) and angular velocities were computed from the angular positions. The angle between the HL and the SL was used as the X-factor (Figure 1). The increases in the X-factor beyond EPR observed during the transition (EPR-TB) and the downswing (TB to maximum) were labeled as the backswing and downswing X-factor stretches, respectively. The total stretch was computed as the sum of the backswing and downswing stretches. The instants the on-plane angular velocities of the body parts/club reached their respective peak values during backswing/downswing were identified as the peak points (Figure 2). The instants their on-plane rotations changed direction from backward to downward were identified as the transition points. All time data were expressed relative to BI. The actual clubhead speed (ACHS) was normalized to the body height (BH) to obtain the normalized clubhead speed (NCHS). Figure 1: The on-plane angles of the axle-chain system defined on the functional swing plane. The hip line (HL) and shoulder line (SL) angles are relative to the horizontal line (**a**), while the upper lever (UL) and lower lever (LL; club) angles are relative to the vertical line (downward) (**b**). The wrist cock (WC) angle is the relative angular position of the LL to the UL. Counterclockwise angles yield positive values in these angles. The X-factor is the angle formed by the HL and the SL. A more backward-rotated (more negative) SL position relative to the HL yields a positive X-factor. Figure 2: Ensemble-average on-plane angular position (**a**) and angular velocity (**b**) patterns of the axle-chain system (driver; \(n=66\)). Transition points and angular velocity peaks (backswing and downswing) are marked. Negative angular positions mean backward-rotated or cocked. Abbreviations: HL—hip line, SL—shoulder line, UL—upper lever, and WC—wrist cock angle.