In baseball, the role of the pitcher is critical and a high velocity of pitched balls is particularly important for game outcomes. The pitching motion is a highly demanding athletic skill involving fine coordination of all body segments (Atwater, 1979), and the mechanics of the lower limbs are also recognized as an integral part of the pitching motion (Mac Williams et al., 1998; Matsuo et al., 2001; Robb et al., 2010). The contributions of the lower extremities to baseball pitchers and their related motions have been described as the open kinetic chain (Kreighbaum and Barthels, 1985), in which all body segments are required to move the upper-extremity joints into appropriate positions in order to minimize the loads on each segment and transmit the generated force from the legs to more distal segments (Kibler, 1995). The lower extremities and trunk provide the beginning of the open kinetic chain that ends with force transmission to the baseball at the time of its release (Elliott et al., 1988; Mac Williams et al., 1998; Matsuo et al., 2001). Furthermore, the lower limbs have been considered to be important for constructing a stable base in which arm motion can be more efficiently and safely generated, along with providing rotational momentum (Burkhart et al., 2003; Kibler, 1991).
The mechanism of the kinetics of the lower limbs during pitching has been examined by measuring ground-reaction forces (GRF). Elliott et al. (1988) suggested that the ability to drive the body over a stabilized stride leg is a characteristic of high-ball-velocity pitchers. Mac Williams et al. (1998) reported that the maximum GRF values in the pitching direction were 0.35 and 0.72 per body weight for the pivot and stride legs, respectively, and wrist velocity at the time of ball release was related to both these variables. These findings indicate that greater GRF are necessary to throw a ball at a greater velocity. In high-ball-velocity pitchers, however, which joint of lower limbs contributes to generate a greater pitched ball velocity remains question. Thus, to clarify differences in the kinematics and kinetics of lower limbs as well as trunk during pitching between high- and low-ball-velocity pitchers may provide important knowledge concerning training and technical guidance for increasing ball velocity during pitching.
The purpose of this study was to clarify differences in the kinematic and kinetic profiles of the trunk and lower limbs during baseball pitching in collegiate baseball pitchers, in relation to differences in the pitched ball velocity.
Thirty male collegiate baseball pitchers voluntarily participated in this study. Descriptive data on the physical characteristics of the subjects are shown in Table 1. Twenty-five subjects were right-handed and the other five were left-handed. On the basis of pitching maximum ball velocity during testing, the subjects with ball velocity greater than 0.5 SD above the mean (> 36.2 m x [s.sup.-1]) were assigned to the high-velocity group, while the subjects with ball velocity lower than 0.5 SD below the mean (
The participants threw a baseball from a portable pitching mound towards a strike zone marked on a home plate. The force plate was attached to the rigid steel frame of the portable pitching mound. The distance between the portable pitching mound and the home plate was the same as the official pitching distance (18.44 m). Ball velocity was measured using a radar gun (2ZM-1035, Mizuno Corporation, Tokyo, Japan) positioned behind the strike zone and adjusted to the position of the ball release. Prior to the pitching trials, participants performed warm-up exercises including stretching. After the completion of the warm-up exercises, the subjects were asked to perform fastball pitches 10 times at maximal effort with an interval of about 15 seconds between the trials.
The GRF of the pivot and stride legs during pitching was measured using two multicomponent force plates (Z15907, 60 x 120 cm, Kistler Corporation, Winterthur, Switzerland), each of which had a sampling rate of 2000 Hz. Thirty-nine reflective markers aligned to specific body landmarks were attached directly onto the skin to minimize movement artifacts. Three-dimensional coordinates were measured using a motion analysis system (Eagle System, Motion Analysis Corporation, Santa Rosa, CA) with 12 Eagle cameras with a sampling rate of 500 Hz and a shutter speed of 2000 Hz. The root mean-square error in the calculation of the three-dimensional marker location was found to be less than 1.0 mm. The three-dimensional coordinates and the GRF were synchronized using software (Cortex 184.108.40.2068, Motion Analysis Corporation, Santa Rosa, CA) and then calculated. Marker position data were filtered using a fourth-order Butter-worth low-pass filter with a cut-off frequency of 13.4 Hz (Fleisig et al., 1999). The GRF and three-dimensional coordinates were defined as follows: Y-axis, throwing direction; Z-axis, vertical axis; X-axis, third-base direction, perpendicular to the Y- and Z-axes. The X-axis was reversed between right- and left-handers; the first-base direction for the left hander was defined as "+".
Kinematic and kinetic parameters were calculated with software (Motion musculous 1.51, Motion Analysis Corporation, Santa Rosa, CA), utilizing the inversedynamics computation of musculoskeletal human models using motion-capture data (Nakamura et al., 2005). Kinematic parameters were calculated using the same methods as previously described elsewhere (Fleisig et al., 1996; Ishida and Hirano, 2004; Milewski et al., 2012; Stodden et al., 2001). The joint angles in the lower extremities were...