## 4 Discussion  
This research began with the question of how upper limb biomechanical behavior could influence the optimization of throwing. Within the relevant literature, many researchers debate the difference between different types of throwing in handball. In our study, we proposed a two-phase throwing design and evaluated the role of trunk rotations in throwing velocity based on analyzing the kinematic parameters of the upper limb angular velocity of the trunk rotation, the angular velocity of the arm rotation, and wrist velocity in two types of throwing: standing throws without a run-up and jump throws. Our results demonstrate a variability in the kinematic parameters that could be explained by anthropometric aspects, the possibility of developing different strategies for optimizing throwing, and also some different movement patterns. Our results demonstrate a relationship between the movement of the trunk, the movement of the arm, and the wrist velocity during throwing, which was more obvious due to the two-phase analysis of throwing. Regarding trunk torsion, these results indicate that exo- and endorotation have variability. Their variability could be explained by the fact that in Phase F1, the pre-stretch of the abdominal muscles and the exorotation were strong, and this increased explosive movement in the trunk during endorotation. In Phase 2, the angular velocity of trunk torsion increased, and there was also an increase in the wrist velocity in Phase F2. This aspect aligns with the results of [PERSON] et al. [4], who also found a large correlation of 0.78 between trunk torsion and wrist velocity. Another aspect is arm rotation, which had a great range of motion in terms of exorotation based on a pre-stretch of the pectoralis major and the subscapular muscles in Phase F1. This contributed to an increase in the throwing velocity of the ball. This finding demonstrates a good correlation between the angular velocity of the arm rotation and wrist velocity. The same result was found by [PERSON] et al. [7] and [PERSON] et al. [19]. These two aspects conclude that wrist velocity depends on angular arm rotation velocity and trunk torsion. Therefore, the development of explosive force in the muscles in the trunk and arm can improve wrist velocity and ball velocity, which optimizes throwing. An explanation for this could be that the biomechanical principles were reliable and enabled the athlete to have a high throwing velocity. Therefore, the ball's velocity is the result of complex movements based on distor-proximal activation, which is a specific phenomenon because distal joints have a short delay in their response. In conclusion, training specific muscles, such as core training, could increase the flexibility and explosive muscle force of the shoulder and trunk. This confirms the importance of the shoulder and arm in throwing movements because the rotator cuff muscles generate acceleration and deceleration after throwing. One of the strengths of this paper is its ability to provide a more nuanced understanding of the throwing mechanisms in handball by highlighting the distinct contributions of different phases. By specifically delineating the preparatory and propulsive stages, our study seeks to reveal insights that might have been obscured in a single-phase analysis. A biomechanical analysis of the upper limbs could be used for the development of specific training addressing the trunk and upper limbs in terms of glyometric training. Some limitations of this study are that our research was focused on one subject and that the experiment was conducted in a laboratory.