## 4 Discussion
Modelling the racket as two segments (handle and frame) and assuming a frame transverse angular velocity of \(80\) rad-s\({}^{-1}\) at impact, consistent with experienced players [28], the difference in racket head speed at the most proximal and distal longitudinal impact locations would be \(4.3\) m-s\({}^{-1}\), suggesting that impact location can have a large effect on subsequent shuttlecock speed, even given identical input in terms of racket angular velocities. As well as racket head speed, peak linear velocity of the wrist joint centre, was found to be a strong predictor of shuttlecock speed (r = 0.767; CI: 0.467, 0.908; \(p<0.001\)), indicating that players should ultimately aim to achieve high linear velocities within distal components of the kinetic chain. This correlation was stronger than previous reported by [PERSON] et al. [4] in elite international players, who reported a low positive relationship (r = 0.454) despite mean values in the present study showing good agreement (14.2 vs. 11.7 m-s\({}^{-1}\)) with differences attributable to the variety in player ability in the present study and differences in capture frequency (400 vs. 50 Hz), where 50 Hz may be inadequate for the badminton smash [28]. Greater shuttlecock speeds were also produced when AP was shorter in duration (r = -0.543; CI: -0.805, -0.101; \(p=0.020\)), demonstrated in Figure 4(b), where the racket head speed becomes positive much closer to impact, when comparing the \"fastest\" to the \"slowest\" participant. Furthermore, two kinematic technique factors significantly correlated with shuttlecock speed. First, players who were in a position of greater shoulder internal rotation at impact produced greater shuttlecock speeds (r = 0.508, moderate; CI: 0.054, 0.788; \(p=0.031\)), where a clear difference in position between the \"fastest\" and \"slowest\" player is shown in Figure 4(b), despite very similar minimum positions. This suggests that greater shoulder internal rotation allows more work done by this joint rotation, which [PERSON] et al. [6] found contributed 66% towards racket head velocity. However, correlational analysis revealed a non-significant relationship between shoulder internal rotation range of motion and shuttlecock speed (r = 0.403, low; CI: -0.079, 0.732; \(p=0.097\)). When accounting for the time in which this range was completed, i.e., average angular velocity, the correlation came closer to significance (r = 0.444, low; CI: -0.029, 0.755; \(p=0.065\)). Second, a low negative correlation was found between X-factor at ER and shuttlecock speed (r = 0.484; CI: -0.755, -0.022; \(p=0.042\)), where players who produced a greater pelvis-thorax separation angle in the transverse plane at ER produced greater shuttlecock speeds. Again, a clear difference is seen between the \"fastest\" and \"slowest\" participants in Figure 4(b). X-factor (maximum pelvis-thorax separation) has previously been found to be a strong correlate (r = 0.60, \(p<0.01\)) with shuttlecock speed [12]. A greater rotational countermovement of the torso presumably allows greater trunk rotation contribution to racket head speed, which [PERSON] et al. [9] found to contribute 57% to racket head speed. It has been previously reported that both peak vertical ground reaction force and jump height significantly correlate with shuttlecock speed (r = 0.548 and 0.508, respectively) [2]. The present study found a non-significant correlation, with CI marginally crossing zero, between jump height and shuttlecock speed (r = 0.454; CI: -0.017, 0.760; p = 0.059); however, a similar effect size. Perhaps greater jump heights are a characteristic of more able players, who typically produce greater shuttlecock speeds [12; 29], and serves as a tactical factor allowing players to produce steeper smash strokes as opposed to producing greater shuttlecock speeds. Note that jump height was significantly correlated with racket head speed in the present study (r = 0.494; CI: 0.035, 0.781; \(p=0.037\)).