## 4 Discussion  
Racket head speeds and shuttlecock speeds by the participants showed good agreement with previously reported values by elite players [2; 27; 28]. Shuttlecock speed was greatest in the international players (\(94.4\pm 3.2\) m\(\cdot\)s\({}^{-1}\)), followed by national players (\(91.2\pm 2.6\) m\(\cdot\)s\({}^{-1}\)) and then regional players (\(86.4\pm 4.6\) m\(\cdot\)s\({}^{-1}\)), suggesting that the ability to achieve greater post-impact shuttlecock speeds is a good indication of playing level [29]. A one-way ANOVA revealed a significant difference in shuttlecock speed between groups (\(F_{(2,15)}=7.29\), \(p=0.006\)). Bonferroni post-hoc tests revealed the international group smashed significantly faster than the regional group (mean difference = 8.06 m\(\cdot\)s\({}^{-1}\), CI: 2.22, 13.90; \(p=0.006\)). The national group were not significantly different to either the international or regional groups. Five kinematic variables correlated significantly with shuttlecock speed: racket head speed at impact, peak wrist joint centre linear velocity, length of the acceleration phase, shoulder internal rotation angle at SC and X-factor (transverse) at ER. These results are indicative of proximal joint motions (shoulder and trunk) causing greater linear velocities in the distal segments (wrist and racket). Figure 6: Predicted shuttlecock speed against actual shuttlecock speed for the two-parameter stepwise regression equation (Table 5; Model b). With a higher percentage of the variation in shuttlecock speed explained the closer the data points lie to the line y = x (predicted shuttlecock speed = actual shuttlecock speed). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multirow{2}{*}{
\begin{tabular}{c} **Kinematic** \\ **Parameters** \\ \end{tabular} } & \multirow{2}{*}{**Coefficient**} & \multicolumn{3}{c}{**95\% CI**} & \multirow{2}{*}{**Variable**} & \multicolumn{3}{c}{**Model**} \\ \cline{4-9}  & & & **Lower** & & **Upper** & & **\(p\)** & **Percent** & **P** & **SEE** \\ \hline \multirow{2}{*}{a} & (Constant) & \(110.1\) & \(93.2\) & \(127.0\) & \(<\)0.001 & \(25.0\) & \(0.02\) & \(4.4\) \\  & AP Duration & \(-536.5\) & \(-976.8\) & \(-96.2\) & \(0.020\) & & & \\ \hline \multirow{2}{*}{b} & (Constant) & \(95.8\) & \(76.7\) & \(114.9\) & \(<\)0.001 & \(43.7\) & \(0.05\) & \(3.8\) \\  & AP Length & \(-514.0\) & \(-898.1\) & \(-129.9\) & \(0.012\) & & & \\ \multicolumn{2}{c}{X\(-\)Factor at ER} & \(-0.4\) & \(-0.7\) & \(-0.1\) & \(0.024\) & & & \\ \hline \hline \end{tabular} Abbreviations: CI: confidence intervals, SEE: standard error of the estimate, AP: acceleration phase, ER: end of retraction.  
\end{table} 
Table 5: Multiple regression equations explaining variance in shuttlecock speed. Racket head speed at impact, normal to the stringbed plane, correlated very highly (r = 0.903; CI: 0.753, 0.964; \(p<0.001\)) with instantaneous post-impact shuttlecock speed. This is a larger effect size than those previously reported [2; 27], which may be due to the more accurate calculation of post-impact shuttlecock speed i.e., at a precise time of impact, and an appropriate racket head speed (normal to the stringbed plane) interpolated to the time of initial contact with the shuttlecock in the present study. The non-perfect relationship between racket head and shuttlecock speed may be due to differences in racket specifications used by each player, where impact efficiency (location and coefficient of restitution), as well as mass properties of the racket, will affect the amount of linear momentum transferred to the shuttlecock, for a given impact velocity. Longitudinal impact locations, calculated using the previously described methodology of [PERSON] et al. [16], ranged between \(-29.4\) and \(24.9\) mm from the racket head centre, whereas medio-lateral impact locations ranged between \(-33.2\) and \(17.1\) mm from the racket head centre (medial-negative, lateral-positive; relative to the player).