Int. J. Sport Health Sci. Paper : Biomechanics Contribution of Hip Joint Kinetics to Rotate the Pelvis during Baseball Pitching Arata Kimura*, Shinsuke Yoshioka and Senshi Fukashiro Department of Life Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Corresponding author [email protected] [Received June 9, 2019; Accepted January 24, 2020; Published online February 10, 2020] This study examined the effects of hip joint kinetics on pelvic rotation about the superior- inferior (SI) axis during baseball pitching from the viewpoint of energetics. Twelve right- handed males participated and all used an overarm style. Five participants were active colle- giate baseball players and seven participants were former collegiate baseball players. Each participant was instructed to try their maximum effort pitch from an indoor pitching mound. Three pitches per participant that passed through the strike zone were selected for analysis. A motion capture system consisting of 13 cameras and two force platforms were used to collect data and calculate joint torques. Pelvic rotation torque, mechanical energy generation, and transfer were calculated. The hip external rotation torque transferred the mechanical energy from the thigh to the pelvis in the pivot leg, which mainly increased the mechanical energy of the pelvis about the SI axis. Regarding the stride leg, the hip adduction torque generated the mechanical energy, which mainly increased the mechanical energy of the pelvis about the SI axis. The findings highlight the importance of these torques in rotating the pelvis about the SI axis. Keywords: pelvic rotation, hip joint torques, mechanical energy, baseball pitching 1. Introduction upper torso rotates from the viewpoint of energetics during baseball pitching. They showed that the upper Baseball pitchers are usually required to produce a torso rotation about the SI axis was mainly caused by high ball velocity when throwing the ball. Gray (2002) the pelvic rotation about the SI axis. This finding sug- found that high ball velocity led to an error in the gests that the rapid rotation of the pelvis caused by contact of the bat and ball. This suggests that the abil- hip joint kinetics allows the rapid rotation of the upper ity to produce high pitch velocity is critical for base- torso, possibly resulting in high pitch velocity at the ball pitchers. Therefore, it is valuable to investigate time of ball release. Therefore, it is valuable to exam- the underlying mechanisms of how baseball pitchers ine the effects of hip joint kinetics on pelvic rotation produce ball velocity and many researchers in the field during baseball pitching. of biomechanics have investigated this topic (Matsuo For this purpose, the pelvic rotation torque should et al., 2001; Naito et al., 2011; Stodden et al., 2001). likely be considered. The pelvic rotation torque is de- Previous studies indicated the importance of upper fined as the torque acting on the pelvis about the SI torso rotation about the superior-inferior (SI) axis and axis by the hip joint force and torque. This can clarify showed that the angular velocity of the upper torso which hip joint force and torque rotate the pelvis contributed to producing the distal endpoint velocity about the SI axis. Several previous studies have of the throwing arm (Hirashima et al., 2007; Urata et quantified the pelvic rotation torque (Akutagawa and al., 2014). Hirashima et al. (2008) revealed that base- Kojima, 2005; Iino et al., 2014; Iino and Kojima, ball players accelerated the distal endpoint during the 2001). Moreover, a mechanical energy analysis was late phase by using the angular velocity of the upper considered in addition to the pelvic rotation torque in torso during the early phase. These studies indicated this study. Previous studies examined the pelvic rota- that generating high ball velocities can be expected by tion torque (Shimada et al., 2000) and mechanical rotating the upper torso quickly about the SI axis. energy created by the hip joint torques in baseball Therefore, Kimura et al., (2019) investigated how the pitching (Shimada et al., 2004; Hirayama et al., 2010; 16 International Journal of Sport and Health Science Vol.18, 16-27, 2020 http://taiiku-gakkai.or.jp/ Hip Joint Kinetics during Baseball Pitching Kageyama et al., 2015; Uchida et al., 2018). However, it has not been shown that how much mechanical en- ergy of the pelvis about the SI axis is increased by the hip joint force and torque. The analysis of either the pelvic rotation torque or mechanical energy alone can- not show how much mechanical energy of the pelvis about the SI axis is increased. This can be shown by combining the two analyses. Therefore, the purpose of this study was to examine the effect of hip joint kinet- ics on pelvic rotation during baseball pitching from the viewpoint of energetics. Figure 1 The location of reflective markers. 2. Methods (Motion Analysis Corporation, Santa Rosa, CA, USA) 2.1. Participants recorded the three-dimensional coordinates of the position of the reflective markers (sampling rate: The participants included 12 male baseball players 200 Hz). Ground reaction force (GRF) was recorded (age: 22.4±2.3 years, height: 1.73±0.06 m, mass, using two force platforms (Force Plate 9281E, Kistler, 67.6±7.4 kg, playing experience: 10.9±3.1 years), Switzerland, 0.6 m×0.4 m), at a sampling rate of who were not injured. Five participants were active 2000 Hz, and was synchronized with the motion data. collegiate baseball players and seven participants were The X, Y, and Z axes of the global coordinate system former collegiate baseball players, and all used an (GCS) were defined rightward-leftward, forward- overarm style. The experimental procedure was in backward and upward-downward directions, respec- accordance with the Declaration of Helsinki and was tively. approved by the ethical committee of the Graduate School of Arts and Sciences of the University of 2.4. Phases of the pitching motion Tokyo. The study participants gave written informed consent to participate in this study and to publish The pitching motion was divided into three phases these case details. as previously defined (Fleisig et al., 1996) Figure( 2). The stride phase was from the maximal knee height 2.2. Procedure (MKH) of the stride leg to the stride foot contact (SFC). The arm cocking phase was from the SFC to The experiment was performed on an indoor mound the maximal external shoulder rotation (MER) of the designed in conformance with baseball mound criteria. throwing arm. The arm acceleration phase was from Participants wore close-fitting clothing and their shoes. the MER to ball release (BR). The SFC was defined Forty-eight reflective markers (diameter: 20 mm) were as the instant when the vertical GRF exceeded 10 N attached to anatomical landmarks on each participant (Oyama et al., 2013). The BR was defined as the in- and three reflective markers were attached to the ball stant when the distance between any of the markers (Figure 1). After a warm-up, the participants were in- on the ball and the marker on the throwing hand in- structed to try their maximum effort pitch from the in- creased by more than 2 cm (Nissen et al., 2007). The door pitching mound to a strike zone (height: 0.64 m, time from MKH to BR was divided into 100 parts and width: 0.38 m) positioned at a distance of 5 m away. normalized (MKH: 0%, BR: 100%). A cubic spline They were allowed enough rest (60 s) between pitches function was used to normalize the data. to avoid the effects of fatigue. Three pitches per par- ticipant that passed through the strike zone were se- 2.5. Data analysis lected for analysis. The data analysis was performed using MATLAB 2.3. Data collection 2015a (MathWorks Inc., Natick, MA, USA). The posi- tion coordinates of the markers were smoothed by ap- A motion capture system consisting of 13 cameras plying a bidirectional fourth-order Butterworth low- 17 Arata Kimura, et al. Figure 2 The main phase and events of the pitching. The stride phase was defined from the maximal knee height (MKH) to the stride foot contact (SFC). The arm cocking phase was defined from the SFC to the maximal external shoulder rotation (MER). The arm acceleration phase was defined from the MER to the ball release (BR). These defi- nition conformed to the previous studies (Fleisig et al., 1996). pass filter. A residual analysis (Winter, 2009) was The joint angular velocity was calculated by sub- performed to identify the optimal cut-off frequency for tracting the angular velocity of proximal segment from each of the three-dimensional positions of each marker that of the distal segment in the GCS. Then, each joint in each trial. A range of cut-offs between 7 and 15 Hz angular velocity was transformed into the right-handed was used for the dataset. The GRF data were orthogonal local coordinate system at each joint. The smoothed using a Butterworth low-pass digital filter joint force and torque of the ankle, knee, and hip with a cut-off frequency of 15 Hz to prevent artifacts joints were calculated using inverse dynamics in the from appearing in the joint torque (Bisseling and Hof, GCS. Then, each joint torque was transformed into the 2006; Kristianslund et al., 2012). right-handed orthogonal local coordinate system at The position coordinates of the middle point of the each joint and normalized by the body mass. The hip ball were calculated using the position coordinates of joint force and the hip joint center velocity were the left and right side of the ball. Then, the ball veloc- transformed into the pelvis coordinate system (xpel: ity was calculated by differentiating the middle point lateral-medial axis; ypel: anterior-posterior axis; zpel: of the ball.