## 4 Discussion  
There are several new aspects revealed by the current study. All the aspects would be vital for training and optimizing the hammer throw. Due to the limited studies on hammer throw, there is currently no review article available to summarize the key factors influencing the quality of the throw. An equivalent one is a book chapter published in 2000 [30]. Hence, these new aspects would also play a relevant role in developing digital method in coaching the throw skill. Firstly, the initiation phase is totally overlooked by researchers. Our data indicate that elite thrower completes 13.0% (i.e., 371/2853 in Table 2) of maximum force in the initiation, while the college-level athlete finishes 17.5% (i.e., 411/2350 in Table 2). Generating more percentage force, i.e., setting the hammer rotation faster, during the initiation may affect the entrance to the next phase, i.e., setting the body into spin to follow the hammer. An unsuitable condition for entrance may delay the increase of the tension, e.g., the tension plateau of college-level athlete in Figure 4. a right pace of the initiation should be individualized based on the physical ability and training experience. a real-time biofeedback device may help the individualized optimization of the initiation phase [4]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hip Vertical Height (m)** & **National-Level** & **College-Level** & **Difference** \\ \hline P-T value of Transition & 0.12 & 0.14 & \(-\)16.7\% \\ P-T value of 1 st Turn & 0.09 & 0.07 & 22.2\% \\ P-T value of 2 nd Turn & 0.11 & 0.15 & \(-\)36.4\% \\ P-T value of 3 rd Turn & 0.12 & 0.09 & 25.0\% \\ Release & 1.11 & 1.06 & 4.5\% \\ \hline \hline \end{tabular}  
* P-T value: peak-to-trough value in each phase or turning cycle.  
\end{table} 
Table 6: The characteristics of the lower-limb control and their difference between the national-level and the college-level athlete. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hand Vertical Height (m)** & **National-Level** & **College-Level** & **Difference** \\ \hline P-T value of Transition & 0.78 & 0.85 & \(-\)9.0\% \\ P-T value of 1 st Turn & 0.59 & 0.41 & 30.5\% \\ P-T value of 2 nd Turn & 0.66 & 0.44 & 33.3\% \\ P-T value of 3 rd Turn & 0.72 & 0.62 & 13.9\% \\ Release & 1.45 & 1.44 & 0.7\% \\ \hline \hline \end{tabular}  
* P-T value: peak-to-trough value in each phase or turning cycle.  
\end{table} 
Table 4: The characteristics of the upper-limb control and their difference between the national-level and the college-level athlete. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**P-T Value of Trunk Tilt (\({}^{\circ}\))** & **National-Level** & **College-Level** & **Difference** \\ \hline Initiation & 33.6 & 35.7 & \(-\)6.2\% \\ Transition & 29.9 & 38.6 & \(-\)29.1\% \\
1 st Turn & 6.3 & 18.8 & \(-\)198.4\% \\
2 nd Turn & 11.8 & 12.6 & \(-\)6.8\% \\
3 rd Turn & 14.4 & 14 & 2.8\% \\ Throw & 29.7 & 16.8 & 43.4\% \\ \hline \hline \end{tabular}  
* P-T value: peak-to-trough value in each phase or turning cycle. Trunk tilt: the orientation of the trunk, i.e., the angle between trunk and horizontal plane.  
\end{table} 
Table 5: The characteristics of trunk control and their difference between the national-level and the college-level athlete. Secondly, the transition phase is inappropriately defined in the previous literature. The widely accepted definition for hammer throw analysis is the four rotation-release model [6, 30]. However, both our kinetic/tension and kinematic/3D data reveal that the first body spin is essentially different from the rest of the body spins (Figure 4). Both athletes perform their first explosive-force generation, reaching 34.2% of max tension for the national-level athlete (i.e., 975/2853 in Table 2) and 40.3% of max tension for the college-level athlete (i.e., 947/2350 in Table 2). Such a rapid increase is not repeated in the following body turns. Additionally, the first turn has ca. three times longer DFS than SFS, while the rest turns have ca. equal DFS and SFS.