## 4 Discussion  
Strength of the trunk muscles contributes significantly to an athlete's ability to transfer forces to the extremities [28]. For instance, the execution of a golf swing or the stroke in tennis or baseball requires rotational power of the trunk, as well as rapid movements of the extremities. Among trunk muscles that are active during the acceleration phase of the golf swing, the highest level of activity was observed in the trail-side abdominal oblique muscles [29]. Preliminary measurements revealed that mean power and velocity in the acceleration phase of seated trunk rotations with 1 and 20 kg are sensitive variables able to discriminate between- and within-group differences [25; 26]. This study identified significant between-group differences in mean power in the acceleration phase of trunk rotations, especially at higher weights (\(\geq\)10.5 kg) or lower velocities (\(\leq\)334.2 rad/s). The highest values of power were produced by fighting sports athletes, followed by those of water sports, combat sports and ball sports. Between-group comparisons also revealed that fighting sports athletes achieved the highest power of the three remaining groups at 10.5 kg; however, values at a higher weight of 20 kg were similar to those produced by water sports athletes. Furthermore, ball sports athletes achieved higher values of power at lower weights (or at higher velocities) than combat and water sports athletes, but the latter two groups performed better at higher weights. Similarly, angular velocity at lower weights was the highest in fighting sports athletes but water sports athletes produced higher velocity at higher weights. In spite of the lower force at lower velocities in athletes of fighting as opposed to water sports, they were able to produce higher force at higher velocities. These between-group differences in the velocity and power produced during trunk rotations may be ascribed to specificity of their training involving movements of the trunk at different velocities under various load conditions. While ball and fighting sports athletes who have to generate high force outputs in a short amount of time achieved the highest power at lower weights or at higher velocities, water sports athletes who exert a high force against the water and those of combat sports who require high explosive strength of both the lower and upper body musculature to lift and throw the opponent were able to produce the highest power at higher weights. Indeed, the training of canoeists was shown to significantly increase mean power in the acceleration phase of trunk rotations with weights \(\geq\)10 kg after the preparatory period, whereas no significant changes after the competitive period were observed [30]. Similarly, values significantly increased in hockey players during trunk rotations with weights \(\geq\)12 kg only after the preparatory period [30]. Figure 4: Force during trunk rotations across different velocities in ball, combat, fighting and water sports athletes. However, there was a significant increase in mean power in tennis players after the preparatory (at weights from 10 to 26 kg) as well as the competitive period (at weights from 6 to 26 kg) [30]. This indicates that changes in power produced during trunk rotations reflect their sport-specific training. Therefore, testing conditions should be close to those used during competition and training. Commonly, athletes perform resistance exercises with increasing weights up to one repetition maximum (1 RM). Then individual power-velocity and force-velocity curves can be provided. In addition, the relationship between power or velocity and the load can be analyzed. While the velocity in the concentric phase of lifting decreases with increasing weights, the power increases from lower weights and, after reaching a peak, decreases again toward higher weights. The optimal velocity at which the highest power is produced depends on the ratio of slow and fast twitch muscle fibers [31] and, therefore, it may be hardly changed with training.