Relationship of Biomechanical Factors to Baseball Pitching Velocity: Within Pitcher Variation

Transcription

1 44 JOURNAL OF APPLIED BIOMECHANICS, 2005, 21, Human Kinetics Publishers, Inc. Relationship of Biomechanical Factors to Baseball Pitching Velocity: Within Pitcher Variation David F. Stodden 1, Glenn S. Fleisig 2, Scott P. McLean 3, and James R. Andrews 2 1 Bowling Green State University; 2 American Sports Medicine Institute; 3 Southwestern University To reach the level of elite, most baseball pitchers need to consistently produce high ball velocity but avoid high joint loads at the shoulder and elbow that may lead to injury. This study examined the relationship between fastball velocity and variations in throwing mechanics within 19 baseball pitchers who were analyzed via 3-D high-speed motion analysis. Inclusion in the study required each one to demonstrate a variation in velocity of at least 1.8 m/s (range m/s) during 6 to 10 fastball pitch trials. Three mixed model analyses were performed to assess the independent effects of 7 kinetic, 11 temporal, and 12 kinematic parameters on pitched ball velocity. Results indicated that elbow flexion torque, shoulder proximal force, and elbow proximal force were the only three kinetic parameters significantly associated with increased ball velocity. Two temporal parameters (increased time to max shoulder horizontal adduction and decreased time to max shoulder internal rotation) and three kinematic parameters (decreased shoulder horizontal adduction at foot contact, decreased shoulder abduction during acceleration, and increased trunk tilt forward at release) were significantly related to increased ball velocity. These results point to variations in an individual s throwing mechanics that relate to pitched ball velocity, and also suggest that pitchers should focus on consistent mechanics to produce consistently high fastball velocities. In addition, pitchers should strengthen shoulder and elbow musculature that resist distraction as well as improve trunk strength and flexibility to maximize pitching velocity and help prevent injury. Key Words: throw, kinetics, trunk, shoulder, elbow 1 Kinesiology Division, 214 Eppler South, Bowling Green State University, Bowling Green, OH 43403; 2 American Sports Medicine Institute, th St. South, Birmingham, AL 35205; 3 Dept. of Kinesiology, PO Box 770 Southwestern University, Georgetown, TX

2 Biomechanical Factors and Pitching Velocity 45 The ability to consistently maximize fastball velocity is an important factor for most baseball pitchers. Theoretically, an individual s maximum pitching velocity potential is a product of optimal pitching mechanics. The notion of optimal pitching mechanics for anyone is a concept that is difficult to address due to the dynamic and complex nature of the movements involved in throwing and the inherent differences in the anatomical, neuromuscular, and physiological makeup of each individual. A pitcher s maximal velocity is indicative of kinematics, kinetics, and relative timing of segmental interactions that lead to effective transfer of momentum to the baseball. Slight changes in a pitcher s mechanics may result in higher or lower ball velocity. Many studies have compared mechanics between pitchers in an attempt to understand variables related to pitched ball velocity (Elliott, Grove, & Gibson, 1988; Fleisig, 1994; Fleisig, Barrentine, Zheng, Escamilla, & Andrews 1999; Matsuo, Escamilla, Fleisig, Barrentine, & Andrews 2001). However, the study of within-pitcher variability has been limited, due in part to previous studies which stated that mechanics within pitchers were remarkably consistent and showed little variability among pitches (Feltner & Dapena, 1986; Pappas, Zawacki, & Sullivan, 1985). Stodden, Fleisig, McLean, Lyman, and Andrews (2001) addressed variability within individual pitching motions and indicated that they may not be as consistent as previously reported. Theoretically, increased pelvis and upper torso velocities would allow more momentum to be transferred from the trunk to the throwing arm, and ultimately to the ball, leading to increased pitch velocity. This idea is supported by Fleisig et al. s (1999) finding that more advanced pitchers (college and professional) generally achieved higher upper torso velocities than their less-developed counterparts (youth and high school). College level pitchers also showed increased pelvis velocities over both high school and youth pitchers. Matsuo et al. (2001) found that kinematic parameters early in the pitching movement influenced pitch velocity. Specifically, lead knee movement, maximum external rotation, and forward trunk tilt at release were associated with differences in pitching velocity between high and low velocity groups. Additive effects of trunk angular velocities and anthropometric factors were also suggested to be related to group velocity differences. Increases in momentum transfer from proximal to distal segments may imply a demand for increased kinetics at the shoulder and elbow during arm acceleration. Changes in temporal parameters may indicate that momentum was transferred in a more effective manner, thus limiting the demand for joint kinetics to produce high ball velocities. This alternative argument is supported by Herring and Chapman (1992), who used a three-segment computer model that simulated the throwing motion in a sagittal plane. Their study indicated that variations in the timing of torque reversal at the shoulder, elbow, and wrist produced variations in ball velocity. The purpose of the present study was to examine the relationship between fastball velocity and variations in kinematic, kinetic, and temporal parameters within individual pitchers. Methods Participants The current study utilized the same group of 19 healthy male baseball pitchers as did Stodden et al. (2001). Participants had an average age of 20.9 ± 2.1 years, height of

3 ± 5.1 cm, and mass of 83.0 ± 6.8 kg. To be considered for the study, pitchers were required to throw a fastball pitch at least 33.5 m/s (75 mph) during testing. In addition, they were required to have at least 1.8 m/s (4 mph) of variation in ball velocity among their maximal effort pitch trials. A 1.8 m/s variation in velocity was chosen because it represents a variation in ball velocity that is considered by many in baseball to be large enough to affect the pitcher s performance. Fleisig et al. (1999) also showed a difference of 2.0 m/s between college and elite pitchers. Procedure and Design After completing informed consent and history forms, each participant was tested with a procedure previously described (Escamilla, Fleisig, Barrentine, Zheng, & Andrews, 1998; Fleisig, Escamilla, Andrews, et al., 1996). Each one completed his warm-up and stretching routine in accordance with his individual preference and was then asked to complete 10 maximal effort throws from a pitching mound. Some pitchers used more than one type of pitch within their 10 maximal effort throws (e.g., curve, slider, change-up). Since only fastball throws were used for data analysis in this study, each pitcher had a total of 6 to 10 fastball trials that were used in the analysis. Ball velocity was recorded with a Jugs Tribar Sport radar gun (Jugs Pitching Machines Co., Tualatin, OR) from behind home plate. The radar gun was accurate to ±.22 m/s (0.5 mph). Each participant was marked with retroreflective 2.5-cm diameter balls bilaterally on the distal end of the third metatarsal, lateral malleolus, lateral femoral epicondyle, greater trochanter of the femur, lateral tip of the acromion, and lateral humeral epicondyle. A reflective band wrapped around the wrist on the throwing arm was used to mark the joint center of the wrist. A reflective marker was also placed on the ulnar styloid of the glove hand. Participants wore spandex shorts and no shirts so as to limit movement of the markers from their anatomical landmarks during the pitching motion. The reflections of these markers were tracked individually by four electronically synchronized 200-Hz charged-coupled device (CCD) cameras (Motion Analysis, Corp., Santa Rosa, CA). Three-dimensional marker locations were calculated with Motion Analysis ExpertVision 3D software, utilizing the direct linear transformation (DLT) method (Abdel-Aziz & Karara, 1971). The locations of the midhip, midshoulder, elbow joint center, and shoulder joint center were calculated (see Figure 1) in each frame as described by Dillman, Fleisig, and Andrews (1993). In each frame, local reference frames were calculated at the shoulder (R s ), the elbow (R e ), and the trunk (R t ). Kinematic Parameters Angular displacements of the shoulder (i.e., glenohumeral joint), elbow, and trunk were calculated as described by Fleisig et al. (1996) (see Figure 2a). Twelve kinematic parameters were calculated from front foot contact to ball release (see Table 1). Figure 3 depicts each stage of the pitching motion and instances separating the phases. Stride foot contact was defined as the time when velocity of the lead ankle joint marker decreased to less than 1.5 m/s. These parameters were chosen because they define important segmental positions during critical moments within a pitch. Angular velocities of the pelvis and upper torso were calculated with a method published by Feltner and Dapena (1989) (see Figure 2f). Angular

4 Biomechanical Factors and Pitching Velocity 47 Figure 1 Markers attached to (1) the leading hip, (2) leading shoulder, (3) throwing shoulder, (4) throwing elbow, and (5) throwing wrist. Virtual markers calculated at the mid-hips (MH), mid-shoulders (MS), throwing shoulder joint center (S), and throwing elbow joint center (E). Unit vectors for the pelvis (P), upper torso (U), and trunk (T). Reference frames shown for the shoulder (X S Y S Z S ) and elbow (X E Y E Z E ). velocity of the pelvis was the cross-product of the pelvis vector and its derivative. Angular velocity of the upper torso was the cross-product of the upper torso vector and its derivative. Kinetic Parameters Resultant joint forces and torques were calculated at the shoulder and elbow using kinematic data, documented cadaveric body segment parameters (Clauser, McConville, & Young, 1969; Dempster, 1955), and inverse dynamics equations (Fleisig, 1994). The calculations of forces and torques began at the distal end of the system where the force of the ball and hand, together as one unit, acted on the wrist. The subsequent masses, forces, and torques associated with the forearm were used to calculate the forces and torques acting on the elbow, and the subse-

5 48 Figure 2 Definition of kinematic variables: (a) shoulder abduction, (b) horizontal adduction, (c) external rotation, (d) elbow flexion, (e) trunk tilt, and (f) pelvis angular velocity and upper torso angular velocity. Adapted from Fleisig et al. (1996), Kinematic and kinetic comparison between baseball pitching and football passing, Journal of Applied Biomechanics, Vol. 12, pp quent masses, forces, and torques associated with the upper arm were used to calculate the forces and torques acting on the shoulder. Eleven kinetic variables were calculated throughout the pitch (see Figure 4). Force applied to the arm at the shoulder was separated into three components: anterior-posterior, superior-inferior, and proximal. Shoulder torque was separated into horizontal abduction-adduction, adduction-abduction, and internal-external rotation components. Force applied to the forearm at the elbow was divided into three components: medial-lateral, anterior-posterior, and proximal. Elbow torque was separated into only two components: flexion-extension and varus-valgus. Forces were normalized as percent body weight, and torques were normalized as percent body weight height. Maximum values for 7 of the kinetic variables were used (see Table 2). Kinetic variables were analyzed between front foot contact and just after ball release (see Figure 3). This interval has been shown to produce the largest forces and torques at the shoulder and elbow during pitching (Fleisig, Andrews, Dillman, & Escamilla, 1995). Temporal Parameters Eleven temporal parameters primarily related to joint or segment angular and linear velocities were calculated (see Table 3). These temporal parameters were shown as relative values where 0% corresponded to stride foot contact and 100% corresponded to the instant of ball release. The specific temporal event times were cho-

6 Biomechanical Factors and Pitching Velocity 49 Table 1 Kinematic Parameter Data and Factors Associated With Ball Velocity (N = 166) Variable Mean SD Ball velocity 35.2 m/s 1.6 m/s Shoulder abduction at SFC 96 14º Shoulder horizontal adduction at SFC** 17 12º External rotation at SFC 63 32º Stride leg knee angle at SFC 131 6º Elbow angle at SFC 96 20º Maximum shoulder horizontal adduction 21 9º Maximum external rotation º Average abduction during acceleration** 99 10º Trunk tilt forward at release** 32 9º Trunk tilt sideways at release º Elbow angle at release º Shoulder horizontal adduction at release 12 9º Note: All parameters were initially entered in the model and then were individually removed if not significantly contributing to the model. SFC = stride foot contact. **Significant differences, p <.01 Figure 3 The six phases of pitching. Images represent the instances separating the phases: initial motion, balance point, stride foot contact, maximum external rotation, release, and maximum internal rotation. Adapted from Fleisig et al. (1996), Kinematic and kinetic comparison between baseball pitching and football passing, Journal of Applied Biomechanics, Vol. 12, pp

7 50 Figure 4 Definition of kinetic variables: (a) shoulder forces: superior, proximal, and anterior; (b) shoulder torques: internal rotation and horizontal abduction; (c) elbow forces (medial); and (d) elbow torques (varus). Adapted from Fleisig et al. (1996), Kinematic and kinetic comparison between baseball pitching and football passing, Journal of Applied Biomechanics, Vol. 12, pp Table 2 Kinetic Parameter Data and Factors Associated With Ball Velocity (N = 166) Variable Mean SD Shoulder anterior force (%BW) Shoulder proximal force (%BW)** Elbow proximal force (%BW)** Shoulder horizontal adduction torque (%BW H) Shoulder internal rotation torque (%BW H) Elbow varus torque (%BW H) Elbow flexion torque (%BW H)** Note: All parameters were initially entered in the model and then were individually removed if not significantly contributing to the model. Significant differences: *p <.05; **p <.01

8 Biomechanical Factors and Pitching Velocity 51 Table 3 Temporal Parameter Data and Factors Associated With Ball Velocity (N = 166) Variable Mean SD Total pitch time (sec) Maximum pelvis angular velocity (% pitch) Maximum mid-pelvis linear velocity (% pitch) Maximum upper torso angular velocity (% pitch) Maximum mid-upper torso linear velocity (% pitch) 93 5 Maximum trunk tilt angular velocity (% pitch) Maximum horizontal adduction angular velocity (% pitch) Maximum horizontal adduction (% pitch)* Maximum external rotation (% pitch) 81 6 Maximum elbow extension angular velocity (% pitch) Maximum internal rotation angular velocity (% pitch)* Note: Total pitch time measured in seconds. Other 10 parameters measured from stride foot contact until particular event, expressed in time or percentage of pitch (where 0% corresponds to instant of stride foot contact and 100% corresponds to instant of ball release). All parameters were initially entered in the model and then were individually removed if not significantly contributing to the model. *Significant differences, p <.05 sen because they represent theoretical windows for the transfer of momentum from proximal segments to more distal segments during the delivery of the ball. Total pitch time (SFC to ball release, in seconds) was also reported. Statistical Analysis Three separate mixed models (MANOVAs) were used to assess the independent effects of the parameters within kinetic, positional, and temporal groups since the data structure included multiple pitch trials for each participant (Stodden et al., 2001). The initial kinetic, positional, and temporal models were then reduced using a stepwise modeling procedure, which eliminated nonsignificant variables without a reduction in model fit. The stepwise regression was a combination of a backward and forward modeling procedure. The modeling procedure reduced the full model by the least significant variable (VAR1). The models were then reevaluated and the next least significant variable was removed (VAR2). At this point VAR 1 was reentered into the model to see if significance was then obtained. If not, it was dropped again. At each step the overall model significance was then evaluated to see if there had been a significant reduction in model fit. This continued until all remaining variables were significant and/or the removal of an additional variable significantly reduced model fit. Significance at p <.05 and p <.01 are reported. SAS Version 8.0 was used for all analyses.

9 52 Results A total of 166 pitches were collected from the 19 participants for data analysis. Total pitches analyzed from a single pitcher ranged from 6 to 10. Means and standard deviation values for the parameters and ball velocity are shown in Tables 1, 2, and 3. The average ball velocity in this study (35.2 m/s) was comparable to other studies involving elite pitchers (Fleisig et al., 1995: 38.3 m/s; Dillman et al., 1993: 38 m/s; Feltner & Dapena, 1986: 33.5 m/s). Results of the kinetic, temporal, and positional mixed models all indicated strong model fitness (kinetic parameter model, χ 2 = ; temporal parameter model, χ 2 = ; positional parameter model, χ 2 = , all p <.0001). The analysis of the full kinetic model, with all 7 variables, indicated that only elbow flexion torque increased as ball velocity increased. However, when the model was reduced from 7 variables to eliminate nonsignificant variables and to improve model fit, elbow flexion torque combined with two additional parameters in separate models. Consequently, both models improved the overall model fit. When introduced into the model separately with elbow flexion torque, both shoulder proximal force and elbow proximal force increased the model fit and attained the.05 significance level. Both shoulder proximal force and elbow proximal force increased with increasing ball velocities. When introduced together into the model with elbow flexion torque, neither met the.05 significance level. The use of two separate models can be rationalized by recognizing that shoulder proximal force and elbow proximal force are highly correlated (r =.79). When these variables are introduced in the same model with elbow flexion torque, their contributions to the model are not independent and thus must be examined in separate models. Results of the reduced temporal model indicated that as ball velocity increased, time to maximum horizontal adduction and time to maximum internal rotation velocity were significantly associated with ball velocity. Specifically, as the pitchers velocity increased, time to maximum horizontal adduction increased. Conversely, time to maximum internal rotation velocity was inversely related to ball velocity. As ball velocity increased, time to maximum internal rotation velocity decreased. The reduced positional model indicated that three variables were significantly associated with increased ball velocity: horizontal adduction at stride foot contact, shoulder abduction during the acceleration phase, and trunk tilt forward at release. Two parameters were inversely related to ball velocity. As ball velocity increased, shoulder horizontal adduction at stride foot contact and shoulder abduction during the acceleration phase decreased. Conversely, as a pitcher s ball velocity increased, trunk tilt forward at release increased. Discussion The purpose of this study was to examine the relationship between fastball velocity and variations in throwing mechanics within individual pitchers. Overall, 8 of 30 kinetic, temporal, and kinematic parameters were significantly associated with increased pitched ball velocity within an individual pitcher. The complex relationship of the three significant kinetic parameters is important for discussion of both performance and injury concepts. As pitchers ve-

10 Biomechanical Factors and Pitching Velocity 53 locities increased, elbow flexion torque, shoulder proximal force, and elbow proximal force all increased. Increases in these three kinetic variables were required in order to resist distraction of both the upper arm from the glenohumeral joint and the forearm at the elbow joint, as well as control the rate of elbow extension. Increase in shoulder proximal force is provided by the musculature that supports the glenohumeral joint as well as capsular and ligament structures (Fleisig et al., 1995). The increase in elbow proximal force is provided by the musculature supporting the elbow joint as well as the ligaments. The increased proximal force at both the shoulder and the elbow is directly related to the increase in pelvis and upper torso rotational velocities (Stodden et al., 2001) and opposes the resultant increases in centrifugal force acting at both the glenohumeral and elbow joint. The mass of the forearm, hand, and ball are common aspects of the centrifugal force acting at both the shoulder and elbow to cause distraction at both joints. These two forces are at their maximum almost simultaneously (elbow first and then shoulder) near or at the end of the arm acceleration phase (Fleisig et al., 1995). Therefore, the proximal forces acting at both joints to resist this centrifugal force should be highly correlated. In fact, the high correlation between shoulder and elbow proximal force (r =.79) is the primary reason why a model, which indicates that both variables nonsignificantly contribute to velocity, does not justify their practical importance. With respect to implications for injury, the unique biarticular nature of the biceps brachii allows this muscle to contribute to both shoulder and elbow proximal stability during the arm acceleration phase. During the early part of the arm acceleration phase, the eccentric contraction of the biceps brachii, along with the other two principal elbow flexors (brachialis and brachioradialis), provides a large elbow flexion torque to control the rate of elbow extension (Feltner, 1989; Fleisig et al., 1995). Controlling the rate of extension serves to enhance the effect of the shoulder internal rotation torque on the velocity of the distal aspect of the forearm/ hand during the rapid internal rotation of the humerus. The secondary function of the biceps brachii is to resist both distraction of the humerus from the glenohumeral joint and distraction of the forearm from the elbow joint (Fleisig et al., 1995). Fleisig et al. (1995) suggested that, with improper mechanics, the loads sustained by the biceps due to shoulder proximal force and elbow flexion torque may be demanded closer in time (simultaneously), thus requiring a greater total force by the biceps. This increased load on the biceps may be one factor leading to the common injury pathology known as a SLAP lesion (tear to the superior labrum anterior and posterior). One other interesting interaction between the three variables was that both shoulder and elbow proximal force were inversely related to elbow flexion torque. Both correlations were modest, although the relationship does support the argument that proper timing of elbow extension would serve to limit increases in elbow flexion torque and shoulder and elbow proximal force. The dual role of the biceps brachii is another reason why it is necessary to present two separate models to elucidate the complexity of shoulder and elbow joint dynamics. Two kinematic parameters and one temporal parameter provided further evidence of the roles of the trunk and shoulder in the pitch. As an individual pitcher threw faster, pelvis and upper torso angular velocities increased (Stodden et al., 2001) and pitchers increased their trunk tilt forward. This combination of movements of the trunk induces a lag effect that can be defined as horizontal abduction

11 54 of the humerus relative to the trunk (Dillman et al., 1993; Feltner & Dapena, 1986; Hong, Cheung, & Roberts, 2001). As pitchers threw faster, they were in a position of greater horizontal abduction at stride foot contact, which occurred before the rotation of the upper trunk and before the lag effect. Greater horizontal abduction at foot contact has been identified to be a significant component in why pitchers from certain countries generate greater ball velocity (Escamilla, Fleisig, Barrentine, Andrews, & Moorman, 2002; Escamilla, Fleisig, Zheng, Barrantine, & Andrews, 2001). When pitchers began to rotate their upper trunk, the humerus had to overcome an increased horizontal abduction angle, which may have led to the increase in time it took to reach maximum horizontal adduction. Additionally, decreased horizontal adduction at foot contact and increased trunk tilt forward at ball release suggest that the distance the ball traveled from stride foot contact to release increased as ball velocity increased. The increase in distance traveled in conjunction with the applied muscular forces at the shoulder would serve to enhance ball velocity. The increased eccentric loading of the horizontal adduction musculature may facilitate increased storage and recovery of the elastic energy component in the stretch-shortening cycle. However, an increase in the forces applied to the throwing arm would not necessarily serve to increase horizontal adduction velocity or internal rotation velocity, because kinematics are a function of complex interactions of the shoulder, elbow, and wrist, as was discussed in the explanation of increased kinetics. The influence of the inertial properties of the forearm, hand, and ball on the humerus, in conjunction with rapid elbow extension, may lead to a more effective transfer of momentum without showing an increase in the velocity of the proximal segment (humerus). Neal, Snyder, and Kroonenberg (1991) also found that highly skilled throwers were able to move the arm segments through a greater range of motion compared to less skilled throwers. The average time to reach maximum internal rotation velocity actually occurred just after ball release (104% of total pitch time). As a pitcher s ball velocity increased, maximum internal rotation velocity was reached earlier in the pitch and closer to the instant of ball release. This result agrees with Matsuo et al. (2001), who found that faster pitchers reached maximum internal rotation velocity sooner than slower pitchers. One positional parameter, shoulder abduction during acceleration, was associated with ball velocity. As abduction decreased, ball velocity increased. DiGiovine, Jobe, Pink, and Perry (1992) showed that the pectoralis major and latissimus dorsi are most active during arm acceleration. Thus, high activity from these two muscles might not only create horizontal adduction and internal rotation velocity, but also reduce abduction. Increasing the time that the internal rotation and horizontal adduction musculature are active would further increase the ball/hand velocity generated during the rapid internal rotation phase of arm acceleration. Overall, the data from this study and previous studies indicate that elite pitchers produce large forces and torques at the shoulder and elbow, as well as high velocities and extensive ranges of motion in the trunk and upper extremity. An understanding of the kinematics and kinetics of pitching can assist in technique and strength-training programs that focus on performance enhancement and injury prevention. Trunk (core) strength is a very important consideration when training for a complex ballistic movement that demands effective momentum transfer

12 Biomechanical Factors and Pitching Velocity 55 through the kinetic chain. Additionally, training the rotator cuff and surrounding musculature of the shoulder and elbow is paramount to maintaining shoulder and elbow joint integrity and stabilizing the humeral head within the glenoid fossa as extreme distraction forces are applied during the pitch. One methodological limitation in this study was the inherent error associated with generating kinetic data from kinematic data. Isolating joint movement about one axis, summing forces that contribute to arm acceleration (soft tissue forces, bone on bone forces, and cumulative muscle forces), and identifying the built-in error of the motion analysis system all suggest a cautious interpretation of the results. The analysis of an individual s pitching motion yielded kinematic patterns that were consistent to a certain extent, which supports previous literature. However, mechanics varied enough within each pitcher such that parameters associated with ball velocity could be identified. Because the RMS error in the current study is larger than typically reported, the likelihood of a Type II error is increased. Conversely, our chance of making a Type I error would be decreased, suggesting that the differences we found were real. In summary, the effects of increased pelvis and upper torso rotational velocities (Stodden et al., 2001), trunk tilt forward at ball release, increased shoulder and elbow proximal force, increased elbow flexion torque, decreased horizontal adduction at foot contact, and changes in relative temporal parameters suggest that when a pitcher increased ball velocity, it was due to a more effective transfer of momentum in the kinetic chain. The complex interaction of the three increased kinetic variables suggests that increased elbow flexion torque serves to limit the increases in shoulder and elbow proximal forces. When attempting to improve velocity, the notion of throwing harder or more effort may not be the correct terms to use with a pitcher who is already throwing with maximal effort. Avoiding injuries is a high priority with baseball pitchers, and this study does not address all the mechanisms that are factored into injury. Slight changes in mechanics taught by knowledgeable instructors, and improvements in strength and range of motion of the shoulder, elbow, and trunk may be a more appropriate strategy for (a) improving momentum generation and transfer within the trunk, (b) protecting the integrity of the glenohumeral and elbow joints, and (c) producing consistent maximal velocities while limiting increases in loads at the shoulder and elbow. References Abdel-Aziz, Y.I., & Karara, H.M. (1971). Direct linear transformation from comparator coordinates into object space coordinates in close-range photogrammetry. In Proceedings of the ASP UI Symposium on Close-Range Photogrammetry (pp. 1-18). Falls Church, VA: American Society of Photogrammetry. Clauser, C.E., McConville, J.T., & Young, J.W. (1969). Weight, volume, and center of mass of segments of the human body. Dayton, OH: Wright-Patterson Air Force Base, Aerospace Medical Research Lab. (AMRL-TR-69-70). Dempster, W.T. (1955). Space requirements of the seated operator. Dayton, OH: Wright- Patterson Air Force Base, Wright Air Development Center (WADC-TR ). Dillman, D.J., Fleisig, G.S., & Andrews, J.R. (1993). Biomechanics of pitching with emphasis upon shoulder kinematics. Journal of Orthopaedic and Sports Physical Therapy, 18,

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