Baseball pitching, throwing, and hitting rely on proper sequencing of the lower extremities, trunk, and upper extremities to impart force to a ball or a bat.
It is the only throwing sport that uses an elevated pitching mound, forcing the pitcher to both generate and dissipate ground reaction forces (GRFs). To the authors' knowledge, there have only been 2 studies examining GRFs in baseball and 1 study in softball. Elliot et al. were the first to document GRFs in 8 international baseball pitchers (1). These authors examined the vertical and horizontal forces generated under the pivot limb. MacWilliams et al. measured the GRFs in 6 collegiate and 1 high school baseball pitcher under both the pivot and stride limbs (3). These authors demonstrated an association between lower-extremity performance, wrist angular velocity, and ball speed (3). Guido et al. examined the GRFs under the stride leg in 53 youth windmill softball pitchers (2). Because there has only been 1 study examining GRF data under the stride limb in baseball pitchers, additional data are necessary to understand the relationships between lower-extremity kinetic and full-body kinematic and kinetic parameters. The examination of GRFs is critical for both performance enhancement and injury prevention in baseball and windmill softball pitchers. The purpose of this study was to investigate GRFs in collegiate baseball pitchers and their relationship to pitching mechanics.
Experimental Approach to the Problem
This study was descriptive in nature and investigated collegiate baseball pitchers. High-speed video and 3D GRF and full-body kinematic and kinetic data were collected. Correlation analysis was carried out between GRFs and kinematic and kinetic variables related to pitching mechanics.
Fourteen collegiate baseball pitchers participated in this study. Eleven pitchers were right-hand dominant, and 3 were left-hand dominant. Written consent was obtained from each participant (approved by the Institutional Review Board of the Tulane University Health Sciences Center). Anthropometric data including height, weight, radius, and humerus length were obtained. Mean age, height, and mass were 20 ± 1 years, 177.8 ± 5 cm, and 79.5 ± 6 kg, respectively. Range of motion measurements were taken on the dominant and nondominant shoulder and elbow. Pitchers were excluded from the study if they had a history of injury or surgery during the prior year. At the time of data collection, all the pitchers were engaged in regular season collegiate team activities with no limitations.
Thirty-three spherical markers were then placed on relevant anatomical landmarks. The specific landmarks, procedure, and methodology for kinematic and kinetic data collection have been described elsewhere (5). The pitchers then performed their normal warm-up routine and were allowed to acclimate to an indoor pitching mound. The mound itself was specially constructed to meet the specifications of collegiate mounds to gain data representative of actual pitching. The mound is 10 in. high and slopes down 1 in. per foot (Figure 1).
Using a global coordinate system, the slope was 4.76° about the y-axis relative to the pitching rubber and contained a 60 × 120-cm Bertec force plate. The width of the force plate was within the width of the pitching rubber because the landing foot should always fall within this width when practicing proper pitching mechanics. The force plate was supported by a sloped platform that was mounted to the ground. The pitching rubber was placed 6 in. behind the beginning of the slope as in regulation mounds. The pitchers were able to throw the regulation 18.4 m (60 ft., 6 in.) distance to a catcher.
Six electronically synchronized high-speed video cameras surrounded the pitching mound and were attached to the walls of the laboratory via a sliding track positioned approximately 2.5 m above the ground. Fastball trials were recorded at 240 Hz by all cameras. Data collection continued until the athlete was satisfied that 10 representative fastball trials thrown for strikes had been gathered. Force plate data were sampled at 1,200 Hz and synchronized in time with the video data. Force data were filtered with a fourth-order zero-phase shift low-pass Butterworth filter with a cutoff frequency of 20 Hz and then normalized by body weight (BW). A radar gun was used to assess ball velocity for each pitcher. Ground reaction forces for the stride limb were calculated in 3 dimensions, anterior and posterior, horizontal (medial and lateral), and vertical directions.
Pitching mechanics data included the following: kinematic variables shoulder abduction at stride foot contact (SFC), SFC to maximum shoulder external rotation (MER), shoulder abduction at acceleration (ACCEL), and kinetic variables maximum elbow flexion torque and MER torque.
Data from the 3 fastball trials with the highest ball speed, thrown for strikes, were averaged for each pitcher. A statistical package (Systat, Evanston, IL, USA) was used to calculate descriptive statistics for kinematic and kinetic parameters (Table 2). Correlation analysis was carried out between the GRF variables and the kinematic and kinetic variables related to pitching mechanics. An alpha level of 0.05 was used to test statistical significance.
The average ball speed for the 14 pitchers was 35 ± 3 m/sec (78 ± 7 mph). As the stride foot contacted the ground, the knee angle was approximately 53 ± 9° of flexion. Stride length averaged 66 ± 5% of body height (HGT). Peak GRFs of 245 ± 20% BW were generated in an anterior or braking direction to control descent. These forces increased rapidly after SFC. Horizontal GRFs tended to occur in a laterally directed fashion, reaching a peak of 45 ± 63% BW. The maximum vertical GRF averaged 202 ± 43% BW approximately 45 milliseconds after SFC. The 3 components of the GRF are displayed in Figure 2.
Shoulder abduction angle at SFC (102 ± 18°) demonstrated a negative correlation in the medial direction and a positive correlation with the horizontal GRF in a laterally directed fashion. Just the opposite occurred during ACCEL with shoulder abduction angle (81 ± 17°) having a positive correlation in the medial direction and a negative correlation in the laterally directed horizontal GRF. Time from SFC to maximal external rotation (MER) (0.26 ± 0.04 seconds) was negatively correlated with the vertical GRF. Maximum elbow flexion torque (3 ± 0.4% BW × HGT) was positively correlated to the maximum horizontal GRF in the lateral direction and MER torque (3 ± 1% BW × HGT) was negatively correlated with time from the SFC to the maximum horizontal GRF in the lateral direction.
This study measured GRFs in collegiate baseball pitchers and investigated the relationships between these forces and full-body kinematics and kinetics. Ground reaction forces in excess of 200% BW were generated under the stride limb. The results of this study disagree with the MacWilliams et al. data. They documented a resisting force of 72% BW with the landing leg (3). The study by MacWilliams et al. failed to document anthropometric and ball velocity data. The athletes in our sample may have demonstrated higher body mass indexes and heights, and higher ball velocities, which could account for the larger GRFs found in our study (Table 1). Methodological differences included video data collection at 240 vs. 200 Hz and force plate sampling at 1,200 vs. 1,000 Hz. The results of this study were also higher than those generated by windmill softball pitchers in both the vertical (139% BW) and braking (115% BW) directions (2). Higher GRFs found in this study vs. the GRF in our softball study may be because of pitching from an elevated mound and having to control the descent. Horizontal GRFs are generated to control the rotational nature of the pitching motion, reaching a peak of 45 ± 63% BW. The results of this study were higher than the MacWilliams et al. data, which documented a maximum of <10% BW at ball release (3).
The maximum vertical GRF averaged 202 ± 43% BW approximately 45 milliseconds after SFC. This result was higher than the MacWilliams et al. data of 150% BW, peaking just before ball release (3).
The negative correlation between the time from SFC to MER may mean that a longer time to peak vertical GRF was associated with a higher ball velocity. In other words, the pitchers with the highest ball velocity also demonstrated higher breaking GRF. Although there was a strong correlation between shoulder kinematics, shoulder and elbow kinetics, and GRFs, at the present time, further study is needed to elucidate the relationship between these parameters.
From a performance stand point, exercises that mimic the force profile of the stride limb, or potentially reduce the GRF, should be included in a sports specific strength and conditioning program. Lower-extremity plyometrics may be critical for pitchers to develop the force profiles documented in this study to allow these athletes to manage the large loads occurring under the stride limb. A unique drill designed to dissipate the GRFs in the stride leg begins with the athlete standing in his balance position on the pivot limb. The athlete then moves toward the plate during his pitching motion and rapidly pushes back up to the balance point using the stride limb. The balance position is held 2–3 seconds, and then the drill is repeated until the technique breaks down, fatigue occurs, or the athlete is unable to maintain a solid balance point. This activity creates a stiff front side to manage the GRFs under the stride limb and does not allow the athlete to collapse over the front leg with increased knee flexion.
High school, collegiate, and professional pitchers may throw >100 pitches in a game. The vertical GRFs of 200% BW are comparable with that which occurs during level walking (2–4 times BW) (5). Running and jumping can sometimes reach up to 20× BW (4). However, because of the downward inclination and rotation of the pitching motion, in addition to the volume, shear forces may occur in the musculoskeletal tissues of the stride limb. Shear stress on connective tissue may be responsible for some of the overuse injuries seen in this athletic population, including meniscal tears in the knee and intraarticular pathology and degenerative joint disease in the hip.
A thorough understanding of the profile of the GRFs for a collegiate baseball pitcher can assist the entire medical team in the rehabilitation of injuries suffered by the stride limb. Ground reaction forces in excess of 2 times the BW are generated in an extremely short period of time. Strength and conditioning specialists can replicate the impulsive loading conditions to prevent injury and enable the athlete to reach peak performance. Further study is needed to understand the relationships between GRFs and upper extremity kinematic and kinetic parameters.
The content is solely the responsibility of the authors, and no funding or grant was used to complete this study.
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