A compilation of registration numbers from all 5 governing bodies of softball in the United States indicates that over 2 million girls between the ages of 12 and 18 competed in fast pitch softball in 2003. Despite its popularity, however, little research has been carried out in this sport. To the authors' knowledge, no published report exists examining the relationships between ground reaction forces and full-body kinematics and kinetics in softball pitching. Even for baseball pitching, which has been well studied for the upper extremity, minimal data are available on ground reaction force characteristics. MacWilliams et al. (4) did attempt to correlate force data and full-body kinematics in baseball pitching and found a link between wrist velocity and leg drive.
Ground reaction forces are important in pitching because the muscles of the lower extremity and trunk are larger than those in the upper extremity, and the only external contact a pitcher has is between the feet and the ground. In softball, pitchers throw from a flat pitching surface without the aid of gravity (i.e., not from a raised mound as in baseball pitching). Werner et al. (5) reported that peak ground reaction forces created by windmill pitchers are similar, and in some cases higher, than those reported for baseball. Clinically, windmill softball pitchers are seen for lower-extremity injuries directly related to the throwing motion. Hill et al. (2) documented 16 acute and 14 chronic/overuse injuries to the lower extremities in sophomore-senior collegiate pitchers. Therefore, knowledge of the magnitudes and timing sequences of ground reaction forces in softball pitching is important for clinicians working with these athletes. Because of the lack of research in this area, additional data are necessary to draw conclusions about lower-extremity kinetic (i.e., ground reaction forces, joint loads, etc.) parameters and their relationships to full-body kinematics and kinetics, particularly at the youth level. The purpose of this study was to investigate the relationships between ground reaction forces and throwing mechanics in youth windmill pitchers.
Experimental Approach to the Problem
This study was descriptive in nature and investigated youth windmill softball pitchers. High-speed video and 3-dimensional ground reaction force data were collected. Correlation analysis was carried out between ground reaction forces and kinematic variables related to pitching mechanics.
Fifty-three youth softball pitchers participated in this study. All but 2 of the pitchers were right-hand dominant. After obtaining written consent (approved by the Institutional Review Board of the Tulane University Health Sciences Center) from each participant and her parent/guardian, height, weight, radius length, humerus length, and elbow and shoulder range of motion measurements were taken. Mean age, height, and mass were 14 ± 2 years, 165 ± 8 cm, and 59 ± 9 kg, respectively. 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 pitchers were engaged in regular season high school or select team activities with no limitations.
Twenty-six spherical reflective markers were placed on relevant anatomic landmarks. The landmarks, procedures, and methodology have been described elsewhere and will not be repeated here (5). The pitcher then performed her normal warm-up routine and was allowed to acclimate to an indoor pitching mound.
The mound was positioned so that the stride foot of the pitcher would land on top of a 60 × 120 cm Bertec force plate that was anchored and recessed in the ground underneath the pitching mound. The pitchers were able to throw the regulation 12.2 m (40 ft) distance to a catcher. Six electronically synchronized high-speed video cameras surrounded the pitching mound were attached to the walls of the laboratory by way of 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 had been gathered. Force plate data were sampled at 1200 Hz and synchronized in time with the video data. All pitches were charted for location from behind the athlete, and a radar gun was used to assess ball velocity for each pitch. Ground reaction force measures were calculated in 3 dimensions, anterior/posterior, horizontal, and vertical directions.
Data from the 3 fastball trials with highest ball speed, thrown for strikes, were averaged for each pitcher. A standard statistical package (Systat, Evanston, IL, USA) was used to calculate descriptive statistics for the kinematic and kinetic parameters. Correlation analysis was carried out between the GRF variables and the kinematic variables related to pitching mechanics. An alpha level of 0.05 was used to test statistical significance.
The average ball speed at release for the 53 pitchers was 25 ± 1 m/sec (55 ± 3 mph). As the stride foot contacted the ground, the knee angle was approximately 30° of flexion. Stride length averaged 103 ± 10 cm for the 53 subjects, and was 62 ± 5 % height (HGT). The stride foot tended to be oriented toward a closed (toe pointing 3rd base for a right-handed pitcher and toward 1st base for a left-handed pitcher) position. The braking force peaked quickly after SFC to an average magnitude of 115 ± 46 %BW and then reversed its direction. This anterior/posterior component of the ground reaction force reached 0 at REL and continued to move toward a peak propulsive force of 24 ± 11 %BW just after REL. The medially directed component of the ground reaction force also increased rapidly after SFC and reached a peak in the medial direction of 42 ± 27 %BW. The maximum vertical ground reaction force averaged 139 ± 43 %BW approximately 61 milliseconds after SFC. The three components of the ground reaction force are displayed in Figure 1.
Table 1 contains a summary of peak ground reaction force components and the time from SFC to peak force. All times to peak force were shorter than 62 milliseconds. Correlation analysis was carried out between the kinematic and kinetic variables. Parameters with correlation coefficients greater than or equal to 0.650 are listed in Table 2. Longer times to peak braking and vertical ground reaction force components were associated with greater ball velocity and the times from TOB to REL and SFC to REL. Time to peak braking force was highly correlated with stride length. A longer stride was also associated with higher ball velocity.
This is the first study to investigate the relationships between ground reaction forces generated by the lower extremities and full-body kinematics and kinetics of windmill softball pitchers. These athletes generate large braking and vertical forces as they drive toward the plate and decelerate body weight with the stride limb.
Peak ground reaction forces in the anterior and vertical directions were greater than body weight and were created quickly after stride foot touchdown. The medially directed force component had a low magnitude but also peaked quickly after SFC. In general, these stride foot ground reaction force patterns appear to be different from those found in baseball pitching. A comparison of stride foot kinetics for the 2 types of pitching is presented in Table 3. The magnitudes of the peak vertical forces found for the softball pitchers were similar to that reported for the stride foot in baseball pitching; however, the rate of force development was different for the 2 pitching styles. It appears that windmill softball pitchers need to be able to create force extremely quickly as the stride foot contacts the ground. These forces act to brake, or decelerate, the body as the back foot drives the hips closed toward the release point.
The explosive nature of the pitching motion and the large ground reaction forces may be responsible for the lower-extremity injuries reported in the literature. Clinically, we have treated windmill softball pitchers for patellofemoral dysfunction, patellar tendinitis, and meniscal pathology affecting the knee of the stride limb. In a rehabilitation setting, knowledge of the magnitude and loading rates (time to peak force) is critical. A softball pitcher should not be released to return to her sport until she is able to generate the force profiles displayed in Figure 1.
From a performance enhancement standpoint, lower-extremity training should target these same force profiles. Force components in excess of body weight need to be created in less than 62 milliseconds by these athletes. The results of this study also have training implications for the nonstride, or drive, leg. Windmill pitchers who had longer strides reached stride foot contact later in the pitch and had higher ball velocities. Thus, it appears that higher propulsive forces under the drive leg would result in increased ball speed. Because of technical issues related to the pitching rubber, a force plate was not placed under the nonstride foot in the current study. Future study into the forces created between the nonstride foot and the ground is needed.
Ground reaction force data for the stride leg also has implications for core training. MacWilliams et al. (4) noted that the landing leg serves as an anchor in transferring the forward and vertical momentum into rotational components. Everything that occurs in softball is rotational in nature, whether it be throwing, hitting, or pitching. Strength and plyometric training of the core musculature, including the transverse abdominus and the internal/external obliques, may aid in the transfer of the forces from the lower extremity to the throwing arm.
In summary, it appears that as the stride foot contacts the ground in windmill pitching, the stride leg creates an anteriorly, medially, and vertically directed force on the ground to brake the forward motion of the body. This force peaks quickly and acts to provide resistance, against which the body is rotated and moved toward the release point. Energy from the lower extremity then appears to be transferred to the torso as the trunk, throwing shoulder, and elbow reach high rates (500-1,300°/sec) of angular velocity as the arm accelerates through the bottom half of the downswing.
Knowledge of the magnitudes and loading rates of the forces produced/absorbed by the stride foot provides a scientific basis for preventive and rehabilitative programs for clinicians working with windmill pitchers. Overuse injuries to the knee have been reported for windmill pitchers (1-3). Physical therapists and athletic trainers rehabilitating lower-extremity injuries for a softball pitcher now have quantitative information regarding the demands placed on the stride leg. In addition, it appears prudent for strength and conditioning specialists to begin to incorporate exercises that replicate the impulsive loading conditions endured by the shoulder, elbow, and stride leg for these athletes.
The authors thank Ryan McNiece, Jasper Richardson, and Sidney Stokes for their assistance with data collection and analysis; data were collected at the Tulane Institute of Sports Medicine.
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Keywords:© 2009 National Strength and Conditioning Association
softball; pitching; lower extremity; kinetics; biomechanics