## 3 Results  
While a number of classification schemes have been used to describe the phases of the hammer throw, our results show that the skill should be divided into five phases: (1) initiation, (2) transition, (3) turns (normally three), (4) throw, and (5) follow through (Figure 2). The phases are defined by initial posture and trunk rotation, i.e., phase 1: from the beginning to the start of body rotation, phase 2: the 1 st 360\({}^{\circ}\) trunk twist, phase 3: the 2 nd-4 th 360\({}^{\circ}\) trunk twists, phase 4: from the end of phase 3 to the release of hammer, and phase 5: from the release of the hammer to the end of the performance. The goal of initiation is to set the hammer into rotation and create the proper conditions to perform the transition. Our data shows that initiation is characterized by stationary-leg control as well as dynamic-arm control with two arm swings (i.e., forward and backward) and two overhead arm rotations (Figures 2 and 3). The transition is a period in which the control of the trunk and lower limbs shift from the stationary-leg control (i.e., upright standing) to the dynamic body turns. Our synchronized data unveils that the double-feet support (DFS) is much longer than single-foot support (SFS) during the transition phase, and the wire-tension displays the first big jump (Figure 2). The three turns demonstrate a gradual increase of the wire-tension's peak- and trough-value, with relatively equal DFS and SFS. The DFS aids the uphill development of the tension, while the SFS creates the downhill tension. The 3D motion analysis further reveals that the rotatory plane of the hammer tilts gradually up, preparing for an optimal release (Figure 3). Following the turns is the throwing phase. The throwing phase is characterized by DFS and the second big jump of the wire-tension (Figure 2). The last phase is the follow through, which is used to dissipate the residual body angular momentum. Based on numerous analyses of our synchronized data, the following variables have been selected for illustrating the characteristics of the quick and complex throw skills in order to establish a practical, digital way to optimize the practice:  
* timing
* the tension development/force-generation
* peak-to-trough values during turns
* upper-limb control/vertical variation of hands
* trunk control/trunk tilt angle
* lower-limb control/vertical variation of hip. Such focused communication would help practitioners understanding the complex motor control in a timely, efficient way. Table 1 represents the timing characteristics of the hammer throw. In general, the national-level subject could finish the throw faster than the college-level subject. Our data illustrates that, compared to the latter, the elite thrower took more time for initiating the rotatory movement of the hammer (i.e., slower initiation), but could perform a faster transition, quicker body spins, and more explosive throws (i.e., shorter throw phase). Further, the timing data indicates that the movement control of the elite thrower became consistently faster, while the college-level subject was unable to keep such a trend. Figure 3: The spatial trace of a hammer in various phases and some dynamic postures on key time points. Figure 2: The development of wire-tension during a throw and the phase division based on both wire-tension variation and 3D motion analysis. The 3D biomechanical models reveal the postures at key instances (dotted lines). DFS—double feet support, SFS—single foot support. Tables 2 and 3 show the kinetic characteristics of the hammer throw. The tension developments of both subjects evidence two different strategies of throw-force generation. The elite thrower demonstrated three explosive-force generations (i.e., the transition, the third turn and the throw), while the college-level thrower had only two (i.e., the transition and the throw).