In baseball, the velocity of the pitched ball is one of the most important factors for the outcomes of games. At present, many conditioning programs such as pitching, long-distance running, sprint running, and resistance training are adopted to improve the pitched ball velocity. These training programs are applied to not only collegiate players but also adolescent ones, in spite of large differences between these two populations in terms of body size, strength capability, and training experience. In addition, kinetic profiles in pitching motion differ considerably between adolescent and collegiate players. From the findings of Fleisig et al. (1999), while nearly all of the kinematic and temporal parameters during pitching are quite similar between young players aged 10 to 15 years and either high school or collegiate players, ball velocity and kinetic parameters differ among these age groups. As a reason for the observed differences, they suggested the age-related difference in muscle strength capability. However, it should be noted that the prior study focused on the motions of the upper limbs and trunk, although knee flexion angles in front foot contact and ball release were determined. To our knowledge, no studies have examined how kinematics and kinetics of the lower limbs during pitching motion differ between adolescent and collegiate pitchers.
Pitching motion is a high-demand athletic skill involving fine coordination of all body segments (Atwater, 1979), and the mechanics of the lower limbs are recognized as an integral part of the pitching motion (Elliott et al., 1988; Kageyama et al., 2014; Mac Williams et al., 1998; Matsuo et al., 2001; Milewski et al., 2012; Robb et al., 2010). The contributions of the lower extremities to baseball pitchers and their 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 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). Thus, 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 contribution of lower limbs for producing high pitched ball velocity has been examined by measuring kinetic and kinematic parameters. Elliott et al. (1988) have suggested that the ability to drive the body over a stabilized stride leg is a feature of high-ball-velocity pitchers. Mac Williams et al. (1998) reported that the maximum ground-reaction forces (GRFs) 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. In addition, Kageyama et al. (2014) indicated that collegiate high-ball-velocity pitchers could generate greater momentum by hip and knee joints in pivot and stride leg. These findings indicate that greater momentum of lower limbs during pitching plays an important role to throw ball with high velocity. However, less information on how the kinematics and kinetic on lower limbs during pitching motion differs between adolescent collegiate baseball pitchers is available from previous studies. To clarify this may provide useful information on training and technical guidance for adolescent baseball pitchers.
The purpose of this study was to clarify the differences between adolescent and collegiate baseball pitchers in the kinematic and kinetic profiles of lower limbs as well as trunk during the pitching motion.
Thirty-two adolescent baseball pitchers aged 12-15 years (ApG; right-handed, n = 29; left-handed, n = 3) and thirty collegiate baseball pitchers aged 18-22 years (CPG; right-handed, n = 25; left-handed, n = 5) voluntarily participated in this study. Descriptive data on the physical characteristics of the subjects are shown in Table 1. This study was approved by the Ethics Committee of the National Institute of Fitness and Sports in Kanoya and was consistent with their requirements for human experimentation. Prior to the measurements, all subjects and the parents of the adolescents were fully informed of the purpose as well as the procedures of this study and possible risks of the measurements, and gave their written informed consent.
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, Osaka, 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 only fastball pitches 10 times at maximal effort with an interval of about 15 seconds between the trials. In the present study, the kinematic and kinetic data in the fastest pitch passing the strike zone were used for detailed analysis.
The GRFs were collected with two multicomponent force plates (Z15907, 60 x 120 cm, Kistler Corporation, Winterthur, Switzerland) attached to the rigid steel frame of a custom-built pitching mound. To simulate the sloped geometry of a regulation pitching mound in official baseball rules, the inclination angle of the portable pitching mound was set at 4.8[degrees]. The GRFs of the pivot and stride legs during pitching was measured using two multicomponent force plates, each of which had a sampling rate of 2000 Hz. One force plate was set below the rubber to record push-off forces during the windup and initial portions of delivery, and a second force plate recorded the landing force.
Thirty-six reflective markers aligned to specific body landmarks (Figure 1) 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...