Lateral Trunk Lean in Pitchers Affects Both Ball Velocity and Upper Extremity Joint Moments

Abstract

Background:

The incidence of upper extremity injuries in baseball pitchers is increasing. Over the past decade there has been a great deal of research attempting to elucidate the cause of these injuries, focusing mainly on the mechanics of the pitching arm with no examination of other key segments, such as the trunk. This is surprising, as coaches will often comment on trunk position in an effort to improve pitching outcomes.

Purpose:

To determine the association between contralateral trunk lean and ball velocity and the moments about the elbow and glenohumeral joint.

Study Design:

Descriptive laboratory study.

Methods:

A total of 99 pitchers were recruited for this study and underwent a pitching analysis using 3-dimensional motion analysis techniques. A random intercept mixed-effects regression model was used to determine if statistically significant associations existed between contralateral trunk lean (away from the pitching arm side) and ball velocity, as well as the elbow varus moment and glenohumeral internal rotation moment.

Results:

The results demonstrated that the greatest contralateral trunk lean occurs around the time of the peak elbow varus moment. Statistically significant associations were found between contralateral trunk lean and increased ball velocity (P = .003) indicating that for every 10° increase in contralateral lean, ball velocity increased 0.5 m/s. Results also indicated that for every 10° increase in contralateral lean, elbow varus moments increased by 3.7 N·m and glenohumeral internal rotation moments increased by 2.5 N·m (P < .001 for both).

Conclusion:

Study findings indicate that the positioning of the trunk plays a substantial role in pitching performance and pitcher injury potential. This work helps to demonstrate the importance of proper trunk mechanics in pitching and highlights the need for future research to understand the contribution of the trunk to pitching mechanics.

Clinical Relevance:

Pitching coaches and trainers can use the results of this study to stress the importance of proper trunk mechanics in pitching. Specifically, improving core strength and trunk control in an effort to maintain a more upright posture through the pitching cycle can reduce upper extremity joint stresses.

Keywords

trunk biomechanics pitching elbow moments

Over the past 3 decades, there has been a continual rise in the incidence of elbow and shoulder pain experienced by pitchers,^10,11 which can limit or ultimately end pitching activities. There are a number of theories that suggest a possible cause for this increased incidence of injuries, including poor pitching mechanics, overuse, and insufficient muscle strength and flexibility.^5,6,14 The rise in injury rates has led a number of researchers to investigate the biomechanics of baseball pitching using 3-dimensional motion analysis, with the intention of elucidating the causes of pitching injury.^3,14,17 Because pitching-related injuries occur to the upper extremity, much of the research has been focused on the mechanics of the pitching arm, while the contribution of the trunk to these injuries has been largely ignored.

Baseball pitching is an explosive, coordinated event that begins with the initial windup and ends with the follow-through of the pitch. A large portion of this event is dictated by the motion of a pitcher’s trunk. The trunk helps transfer energy from the legs during the drive and helps to develop power after foot contact. This power is then transferred to the pitching arm to deliver an accurate pitch at high velocity. Coaches often direct pitchers to modify their pitching motion in terms of their trunk mechanics to improve performance or prevent injury to the pitching arm.

To date, only a few studies have been conducted to investigate the role of the trunk in upper extremity injury. Aguinaldo et al¹ investigated how the motion of the trunk could affect the shoulder joint torque of baseball players at various skill levels. In their study, the authors collected data on 38 pitchers—from youth levels through professional baseball pitchers—and focused on how the rotation of the trunk affected the internal shoulder rotation moments. The results showed that those pitchers who initiated trunk rotation later in the pitching cycle had lower internal rotation shoulder moments.¹ The Aguinaldo et al study was one of the first to specifically look at the influence of trunk motion on joint stresses in baseball pitchers and the first to indicate that there could be an association between trunk movement and risk of injury.

Matsuo and Fleisig¹² performed an optimization-based study on the influence of both shoulder abduction and lateral trunk tilt on the peak elbow varus torque in collegiate baseball pitchers. While the primary focus of these authors’ work was to find the optimal positioning of both the shoulder and the trunk to reduce elbow varus moment, a secondary regression-based analysis was performed that showed that lateral trunk tilt was not related to increased elbow varus moments.

More recently, a study published by Oyama et al¹⁶ looked at the relationship between lateral trunk lean on ball velocity and joint moments in high school–aged pitchers. Oyama et al recruited 73 high school pitchers and used motion analysis techniques to evaluate trunk lean at the instant of maximum external rotation (MER) of the glenohumeral joint. The results of this work showed that those pitchers who exhibited increased contralateral trunk lean had statistically significant increases in both ball velocity and shoulder and elbow joint moments.¹⁶ These findings led the authors to conclude that contralateral trunk lean was associated with increased joint moments and that, with further research, they could help to develop a strategy to reduce injury risk in adolescent pitchers.

Interestingly, the work by Oyama et al¹⁶ directly contradicts that of Matsuo and Fleisig,¹² who stated that lateral trunk tilt was not related to increased joint moments. Further study is required to provide consistent evidence describing the trunk’s role in upper extremity joint stresses so that coaches and trainers can more effectively teach injury prevention strategies to baseball pitchers. Therefore, the purpose of this study was to investigate the association between contralateral trunk lean on ball velocity and glenohumeral and elbow joint moments. It was hypothesized that increased contralateral trunk lean increases the moments at both the glenohumeral and elbow joints. It was also hypothesized that ball velocity will be unaffected by increased contralateral trunk lean.

Methods

The study was approved by the Connecticut Children’s Medical Center’s Institutional Review Board, and all study participants signed consent before the start of the pitching analysis. A total of 99 college-aged pitchers currently pitching for Division I and Division III schools of the National Collegiate Athletics Association were recruited for this study. None of the participants involved in this study had sustained a serious injury to their pitching arm—that is, an injury that caused them to miss pitching in at least 1 game or practice—within 6 months of the analysis. Additionally, none of the participants had a history of surgery to their pitching arm.

Data Collection

Before the pitching analysis was started, anthropometric measures were taken to properly scale the inertial properties of the model. A total of 38 reflective markers were attached to specific anatomic landmarks to create a 16-segment biomechanical model as previously described by Nissen et al.¹⁵ An additional 2 markers were placed on the ball to determine the instant of ball release, calculate ball velocity, and allow for the computation of joint kinetics.

Before the start of data collection, the participants were given as much time as they required to warm up and become comfortable pitching within the data collection space. All participants pitched from a 10-inch mound toward a pitching target with a designated strike zone 60 feet 6 inches away. All participants pitched multiple pitch types, in random order, to simulate a game setting; however, this work is limited to the results of the fastball pitches only. Motion data were collected for a total of 7 fastball pitches. Motion data were collected at 250 Hz using a 12-camera motion system (Vicon 512; Vicon Motion Systems). The first 3 trials in which all marker data were present throughout the pitch cycle were analyzed for each participant.

Data Analysis

The pitching motion was divided into 4 major phases, as described by Fleisig et al⁴ (Figure 1). The pitching cycle begins at the instant the lead foot makes contact with the mound, and it ends with the maximum internal rotation of the glenohumeral joint. The pitching cycle is further divided by 2 intermediate time points: the instant of MER and ball release. Joint angles were computed using the Euler equations of motion in Vicon Workstation and BodyBuilder (Vicon Motion Systems) as previously described.¹⁵ Joint kinetics were computed in custom Matlab code (Mathworks) using standard inverse dynamic techniques.⁹ All kinetic data presented are internal moments.

Figure 1.

Depiction of the major events in the pitching cycle.

For data analysis, we used the first 3 trials for each pitcher in which all markers were present. Although data were computed for all joints for each participant, the specific variables of interest for this study were ball velocity, lateral trunk lean, glenohumeral internal rotation moment, and elbow varus moment. The trunk was defined using the Vicon Plug-in Gait marker configuration, which uses a total of 4 markers to construct the trunk segment. These markers were placed over the C7 vertebra, the T10 vertebra, and the sternal notch and at the base of the xiphoid process. The primary axis for the trunk was the long axis, described by the z-axis, which was defined as a vector passing through the midpoints of the vectors between the C7 and sternal notch marker and the vector between the T10 and xiphoid marker. The x-axis was perpendicular to the z-axis following the direction of forward progression, and the y-axis was constructed as the cross-product of the z- and x-axes. The lateral trunk lean angle was defined as the angle between the vertical position of the laboratory coordinate system and the long axis of the trunk segment within the y-z plane (Figure 2).

Figure 2.

Angle definition for contralateral trunk lean.

Statistical Analysis

Descriptive statistics were computed for all parameters of interest; means and standard deviations are presented here. To determine the association between lateral trunk lean, ball velocity, and joint moments, a random intercept mixed-effects regression model was used.^7,8 This model is capable of taking into account repeated measures as well as making use of all the trials available rather than using a singular averaged trial, which increases the precision of the model. In the cases in which <3 trials were available for a pitcher, this model can account for variations in the number of trials for each pitcher by calculating the correct standard error based on the degrees of precision available. All statistical testing was performed using SAS software version 9.3 (SAS Institute Inc).

Results

A total of 99 pitchers with a mean age of 19.9 ± 1.4 years and a minimum 2 years of pitching experience completed this study. The average fastball velocity was 32.1 ± 1.9 m/s. The total average lateral trunk lean range of motion for the pitchers was 24° ± 10°, with the greatest change in lean occurring between initial foot contact and MER. The data suggested that at foot contact pitchers were nearly neutral or leaning slightly toward their pitching arm (–3° ± 7°) and as the pitchers progressed through the pitch cycle, they leaned away from their pitching arm. By MER they were near the maximum lean (18° ± 10°) and quickly reached the maximum lean (19° ± 10°) around the time that the maximum elbow varus moment occurred. There was very little trunk motion between MER and at the instant of ball release. After ball release the pitchers on average began to correct their trunk positioning toward a more neutral posture (12° ± 11°) (Figure 3).

Figure 3.

Lateral trunk position over the entire pitching cycle. The black thick line is the group mean; the gray band is ±1 standard deviation; the dotted vertical line is the time of maximum external rotation; and the solid vertical line is the ball release time. +ve, positive.

The results of the regression analysis showed statistically significant associations at MER and ball release between contralateral trunk lean and ball velocity, peak elbow varus moment, and glenohumeral internal rotation moment. Results of this analysis indicated that the effect of contralateral lean was statistically significant (P = .003) and had a low standard error (SE = 0.02); however, in terms of the effect size, the beta value (β = 0.05) was minor, suggesting that for every 10° increase over the median contralateral trunk lean at MER, ball velocity increased by 0.5 m/s.

Results also indicated a statistically significant positive association between the elbow varus moment and the glenohumeral internal rotation moment with contralateral trunk lean (P < .001 for both). However, unlike with ball velocity, the beta values were much larger, suggesting that contralateral lean affected the joint moments to a much greater extent than it did ball velocity, with a beta value of 0.37 for the elbow varus moment and a beta value of 0.25 for the glenohumeral internal rotation moment. This suggests that for every 10° increase over the median contralateral trunk lean at MER, the elbow varus moment increased by 3.7 N·m and the glenohumeral internal rotation moment increased by 2.5 N·m. Again the standard errors for these analyses were low, with values of 0.1 for both the elbow varus moment and the glenohumeral internal rotation moment. Similar results were seen at the instant of ball release where for every 10° increase in lateral trunk lean over the median, ball velocity increased by 0.4 m/s (P = .023, β = 0.04, SE = 0.02), elbow varus moment increased by 3.5 N·m (P < .001, β = 0.35, SE = 0.10), and glenohumeral internal rotation velocity increased by 2.5 N·m (P < .001, β = 0.35, SE = 0.10).

Discussion

The purpose of this article was to discuss the implications of contralateral trunk lean on both pitchers’ ball velocity and their joint moments, specifically the elbow varus and glenohumeral internal rotation moments. The pitchers in this study were found to move through a very predictable pattern of lateral trunk lean, starting with a nearly neutral trunk position at foot contact and eventually reaching the maximum lean away from their pitching arm around the instant of maximum elbow varus moment and MER, then returning toward a more neutral position at maximum internal rotation and late follow-through. We also found contralateral trunk lean away from the pitching arm to be significantly associated with increased ball velocity as well as increased elbow varus and internal glenohumeral moments.

The results of this work are consistent with those of Oyama et al,¹⁶ who found that high school–aged pitchers with increased contralateral lean while pitching demonstrated higher ball velocities and joint moments when compared with those who did not lean to any great extent. Furthermore, this work describes the associations between lateral trunk lean and ball velocity and moments, which were not presented in the work by Oyama et al. This work also extends the findings of Oyama et al to collegiate pitchers, indicating that the trunk plays an important role in pitching mechanics regardless of age or skill level.

Interestingly, Matsuo and Fleisig¹² did not show that trunk lean was associated with larger elbow varus moments in collegiate-aged pitchers; however, this could be due to a number of reasons. First, the model used by Matsuo and Fleisig was different from both the Oyama et al¹⁶ model and our model. Both rely on the Vicon plug-in gait trunk model described above, whereas the trunk segment in the Matsuo and Fleisig work is defined using markers on the greater trochanters and lateral superior tips of the acromions. Second, Matsuo and Fleisig based their results on 33 pitchers, whereas our study used 99 pitchers and Oyama et al used 73 pitchers; therefore, there is a possibility that the Matsuo and Fleisig study was not sufficiently powered to find associations between trunk lean and the elbow varus moment. Finally, the Matsuo and Fleisig study was focused on determining how the interaction between coronal plane shoulder motion and trunk lean affected the elbow varus moment, while our work focused solely on the effect of lateral trunk lean.

This work demonstrated that increases in lateral trunk lean away from the pitching arm are capable of increasing a pitcher’s ball velocity and therefore provide a benefit to their performance. This is logical since the shift in the pitchers’ trunk axis of rotation away from their pitching arm results in an increased distance between the ball and the center of rotation, which allows the pitcher to increase the forward velocity of their arm. Essentially, the radius between the ball and the trunk’s axis of rotation increases allowing for the increase in velocity. In the 2 extreme cases, the pitcher with the least amount of lean during the pitch showed an increase in 12 cm between the ball and trunk axis of rotation while the pitcher with the greatest lean showed an increase of nearly 25 cm. Although these changes in distance are not entirely caused by the lateral lean of the trunk but incorporate changes in elbow and glenohumeral position, the lean away from the pitching arm requires modifications to the pitching arm for it to remain in an appropriate position to deliver a powerful and accurate pitch. However, the increase in lateral trunk lean away from the pitching arm also increases the moments at the elbow and glenohumeral joint. Again this makes physical sense in that the same increase in distance that is allowing the pitcher to increase the linear velocity of the arm is also increasing the lever arm, which is taken into account in the calculations for moment. Therefore, when the distance between the mass of the trunk and the joints of the upper extremity is increased, the moment at both of the joints is increased, especially at the elbow since it is more distal.

The most important finding of our study is the fact that the increase in ball velocity is minimal when compared with the increase in joint loads at the elbow and glenohumeral joint. The results showed that for the same increase in contralateral lean, ball velocity increased by 1.5%, whereas the glenohumeral joint moment and elbow varus moment increased by 3.2% and 4.8%, respectively. Although these increases in the joint moment may seem trivial, cadaveric studies have shown that the ultimate moment of the ulnar collateral ligament (UCL)—that is, the moment at which the UCL fails—is around 34 N·m,² and other studies have shown that up to 50% of the joint moment is transmitted by the UCL.¹³ Therefore, with an average joint moment in this study of 75.6 ± 15.3 N·m, the moment placed on the UCL is very close to the ultimate moment, and any additional stress caused by an increase in moment by an increase in lateral trunk lean away from the pitching arm can increase the risk of injury to these pitchers. Further work is needed to find the optimal position to recommend to pitchers that will provide the best ball velocity with minimal increases in joint loads.

Based on the results of this work, pitching coaches and trainers can provide recommendations to pitchers to pitch with a more upright posture, thus reducing the risk of injury in this population. Given the similar results of Oyama et al¹⁶ in younger pitchers, coaches should stress from an early age the importance of trunk positioning in pitching. Coaches should also incorporate core strengthening and coordination drills into their training regimens for pitchers to improve trunk mechanics by reducing contralateral trunk lean.

This study has several limitations. This is a laboratory study, and the effects of pitching with reflective markers, as well as the controlled conditions of the laboratory, may affect participant performance. However, this level of analysis is not viable in the field and is comparable with other motion-based studies currently in the literature. To simulate actual pitching conditions, we used a regulation mound and a full-length pitching space, and we allowed pitchers time to warm up and feel comfortable pitching in the laboratory environment.

Conclusion

This study showed that contralateral trunk lean can have significant effects on the upper extremity joint moments as well as on ball velocity in college-aged baseball pitchers. The findings provide evidence indicating that coaches and trainers should instruct proper trunk positioning to reduce the moments placed on the glenohumeral and elbow joints.

Footnotes

One or more of the authors has declared the following potential conflict of interest or source of funding: This study was funded in part by a grant from Major League Baseball.

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