Baseball batting is a skilful whole-body motion and hitting a ball properly requires both accurate spatial and temporal coordination; thus it is one of the most difficult skills in sport (DeRenne et al., 2008, Race, 1961). If the coordination of the bat-ball impact is maintained, improved batting performance, i.e., higher velocity of the batted ball, will result from higher bat-head speed at impact (Sawicki et al., 2003). Batters generate bat-head speed using a kinetic chain that transfers mechanical energy from the lower body to upper limbs and bat (Ae et al., 2017, Horiuchi et al., 2021, Welch et al., 1995). Therefore exploring the lower limb movements responsible for the generation of mechanical energy and bat-head speed can provide insight into how to improve batting performance. Biomechanical studies of baseball batting have determined that the lower body movements play an important role in the generation of mechanical energy and bat-head speed, and relationships between kinematic characteristics of the motion of the bat and body and bat-head speed have been identified (Escamilla et al., 2009a, Escamilla et al., 2009b, Dowling and Fleisig, 2016, Mcintyre and Pfautsch, 1982, Messier and Owen, 1984, Race, 1961, Welch et al., 1995). In addition, investigations of the ground reaction forces have revealed the kinetic characteristics of the batting motion and the contributions of each leg to the generation of bat-head speed (Fortenbaugh et al., 2011, Katsumata, 2007, Mason, 1987, Messier and Owen, 1985, Messier and Owen, 1986). More recently Ae et al. (2017) demonstrated the effect of lower limb joint kinetics (i.e., joint torque and mechanical work) on pelvis angular velocity about the vertical axis. The generation of pelvis and trunk angular velocity can be identified as the crucial outcome of the lower body movements, as the hitting motion is largely dependent upon the rotation of the torso about the longitudinal axis (Fleisig et al., 2013, Szymanski et al., 2007). Previous research has also indicated that the pelvis angular position at ball impact is a contributing factor to batting performance (Dowling and Fleisig, 2016; Escamilla et al., 2009a, Escamilla et al., 2009b, Welch et al., 1995). Although the importance of the relationship between lower body movements and bat-head speed have been demonstrated, no study has investigated optimum lower body movements to increase the pelvis angular velocity and the effects of pelvis angle at ball impact. In order to investigate the optimal motion to improve batting performance in baseball batting, Ae et al. (In press a,b) used a 10-segment angle-driven simulation model of the upper body to maximise the bat-head speed by manipulating the timing, or both timing and peak values of the joint angle time histories, revealing the previously overlooked role of the barrel-side (right-side) upper limb joint movements in the production of bat-head speed. In comparison to the direct effects of the upper body movements on bat-head speed, the lower body motion contributes indirectly. However the lower body movements have been shown to influence the generation of upper body joint torques and the transfer mechanical energy to the upper body and bat (Ae et al., 2017, Welch et al., 1995). The optimisation of batting technique and performance requires the exploration of the optimum lower body movements for the production of pelvis angular velocity about a vertical axis. The development of a lower body simulation model is also necessary for whole-body simulation research in baseball batting, an approach that is yet to be reported in biomechanical literature. The objective of this study was to optimise lower body movements in a computer simulation model to maximise the peak pelvis angular velocity about the vertical axis during baseball tee-batting. This study also investigated whether the production of pelvis angular velocity is influenced by the pelvis angle at ball impact
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