Improper Trunk Rotation Sequence Is Associated With Increased Maximal Shoulder External Rotation Angle and Shoulder Joint Force in High School Baseball Pitchers

Abstract

Background:

In a properly coordinated throwing motion, peak pelvic rotation velocity is reached before peak upper torso rotation velocity, so that angular momentum can be transferred effectively from the proximal (pelvis) to distal (upper torso) segment. However, the effects of trunk rotation sequence on pitching biomechanics and performance have not been investigated.

Purpose:

The aim of this study was to investigate the effects of trunk rotation sequence on ball speed and on upper extremity biomechanics that are linked to injuries in high school baseball pitchers. The hypothesis was that pitchers with improper trunk rotation sequence would demonstrate lower ball velocity and greater stress to the joint.

Study Design:

Descriptive laboratory study.

Methods:

Three-dimensional pitching kinematics data were captured from 72 high school pitchers. Subjects were considered to have proper or improper trunk rotation sequences when the peak pelvic rotation velocity was reached either before or after the peak upper torso rotation velocity beyond the margin of error (±3.7% of the time from stride-foot contact to ball release). Maximal shoulder external rotation angle, elbow extension angle at ball release, peak shoulder proximal force, shoulder internal rotation moment, and elbow varus moment were compared between groups using independent t tests (α < 0.05).

Results:

Pitchers with improper trunk rotation sequences (n = 33) demonstrated greater maximal shoulder external rotation angle (mean difference, 7.2° ± 2.9°, P = .016) and greater shoulder proximal force (mean difference, 9.2% ± 3.9% body weight, P = .021) compared with those with proper trunk rotation sequences (n = 22). No other variables differed significantly different between groups.

Conclusion:

High school baseball pitchers who demonstrated improper trunk rotation sequences demonstrated greater maximal shoulder external rotation angle and shoulder proximal force compared with pitchers with proper trunk rotation sequences.

Clinical Relevance:

Improper sequencing of the trunk and torso alter upper extremity joint loading in ways that may influence injury risk. As such, exercises that reinforce the use of a proper trunk rotation sequence during the pitching motion may reduce the stress placed on the structures around the shoulder joint and lead to the prevention of injuries.

Keywords

pitching technique biomechanics injury shoulder elbow

Shoulder and elbow injuries are common among baseball pitchers, affecting their sports participation and activities of daily living.^17,20,33 In particular, injuries to the glenoid labrum and ulnar collateral ligaments often require surgery and result in long recovery times.^14,28 Improper pitching biomechanics are among the intrinsic factors that are considered to increase pitchers’ predisposition to these injuries.^3,15,26 The specific joint loads that have been linked to injuries include shoulder external rotation moment, shoulder proximal force, and elbow varus moment.^3,15,26 These mechanical loads increase the stress within the structures that support the shoulder and elbow joints, and thus these structures are commonly injured in baseball pitchers.^{18,19,23,24,29,35} Joint kinematics, such as greater maximal shoulder external rotation angle^34,39 and horizontal abduction (ie, hyperangulation), has also been linked to greater stress on the joint structures.^4,22

The importance of trunk movement in pitching has been recognized since Feltner^11-13 first described the influence of trunk movement on shoulder and elbow joint kinematics. It was described that the forces that result from rapid trunk rotation about its longitudinal axis and shoulder horizontal adduction moment are primarily responsible for producing shoulder external rotation and elbow valgus after stride-foot contact.^11-13 Furthermore, the rotation of the throwing shoulder about the contralateral shoulder is considered the primary force driving elbow extension during the arm-cocking and acceleration phases of pitching.^11-13 This influence of trunk motion on upper extremity kinematics suggests that trunk kinematics must be examined to identify biomechanical factors that increase loads at the shoulder and elbow joints.

Trunk rotation about its longitudinal axis during pitching involves rotations of the pelvis and upper torso.^1,21,36,41 Transfer of momentum in movements that involve sequential rotation of multiple segments is optimized with a proximal-to-distal sequence of segment angular velocities.^30,31 Therefore, the literature^30,31 suggests that achieving peak pelvic rotation velocity before peak upper torso axial rotation velocity will lead to efficient pitching performance, whereas reversal in the order of peak rotation velocities will result in decreased ball speed and/or compensation in the upper extremity joint movements, which may lead to increased joint loading and ultimately injury.^2,41 Despite its perceived importance in baseball pitching performance and injury risk, the effects of the sequence of pelvic and upper trunk rotation on ball speed and upper extremity biomechanics linked to common pitching injuries have not been clearly demonstrated. Therefore, the purpose of this study was to investigate the effects of trunk rotation sequence on ball speed and upper extremity kinematics and kinetics linked to injuries in high school baseball pitchers. We hypothesized that pitchers with improper trunk rotation sequences would demonstrate slower pitch velocities, higher joint loading, and kinematic patterns that have been linked to greater joint loading and stress on the structures around the shoulder and elbow joints.

Materials and Methods

Participants

A total of 72 high school pitchers aged 13 to 19 years participated in this study (mean age, 15.5 ± 1.2 years; mean pitching experience, 6.1 ± 1.9 years; mean body mass, 72.7 ± 9.9 kg; mean height, 179.4 ± 7.3 cm). All pitchers had pitched in at least 2 baseball seasons as starters or in relief. Exclusion criteria included having any ongoing injury, pain, or muscle soreness that prevented them from pitching as they normally would and pitching with underarm (submarine) or sidearm techniques. A history of previous injury was not considered an exclusion criterion because the focus of this study was simply to describe the relationship between pitching technique and joint loading. The use of human subjects in this study was approved by the Biomedical Institutional Review Board at the University of North Carolina at Chapel Hill.

Instrumentation

A pitching mound that met high school baseball regulations was constructed.²⁵ Pitches were performed into a backstop with a rectangular strike zone (0.64 m high × 0.38 m wide) that was placed 18.4 m from the pitching rubber. The slope of the mound was instrumented with a force plate (model 4060-NC; Bertec Corp). Ground-reaction forces from the stride foot were captured at 900 Hz and were synchronized with the kinematic data.²⁷ The position of the force plate was adjusted to the pitchers’ stride lengths.²⁷

Kinematic data were captured using a 7-camera motion analysis system (model MX-40; Vicon Systems) with Vicon Nexus automatic digitization software (version 1.6; Vicon Systems). Kinematic data were captured at 300 frames/s.⁹ A radar gun (model SR3600; Sports Radar Ltd) was used to capture ball speed.

Procedures

Data collection took place inside a research laboratory. Upon arrival, pitchers and one of their parents read and signed the informed consent and assent forms. After consent was obtained, participants changed into tight-fitting clothing and went through measurements of height and mass to normalize the kinetic variables and the length, breadth, and circumference of the dominant limb to estimate the inertial characteristics of their body segments.⁵ Subsequently, the participants were given unrestricted time to warm up as they normally would before practices or games. After the warm-up, 40 reflective markers were secured onto anatomic landmarks, on the basis of recommendations from the International Society of Biomechanics.^43,44 The specific marker placements have been described previously.²⁷ After the marker setup, participants performed 5 to 10 submaximal pitches to become comfortable pitching with the reflective markers attached to the body.

The pitchers were instructed to perform fastball pitches from a wind-up and to pitch as fast and as accurately as possible while aiming at an X marked at the center of the strike zone. Pitches were considered qualified for analysis if they hit the backstop and were considered strikes if they hit the strike zone on the backstop. Each pitcher continued to pitch until a minimum of 5 qualified pitches and a minimum of 3 strike pitches were captured. The pitchers, on average, performed 10 pitches before meeting this requirement. Biomechanical data and ball speed were captured during each pitching trial. The 3 fastest strike pitches were used for data analysis.

Data Processing

Raw 3-dimensional coordinate data were filtered through a fourth-order Butterworth low-pass filter using an estimated optimal cutoff frequency of 17 Hz.⁴⁵ The kinematic and kinetic variables of interest were calculated from the filtered coordinate data.²⁷ The anatomic coordinate systems of the pelvis, upper torso, arm, and forearm segments and the shoulder and elbow joints were defined on the basis of International Society of Biomechanics recommendations.^43,44 The shoulder joint angles were calculated using a Euler-Cardan angle method with rotation orders recommended by the International Society of Biomechanics.⁴⁴

The pelvic and upper torso orientations in the global reference frame were calculated using a Euler-Cardan angle method with a rotation order of axial rotation, contralateral flexion, and flexion. The pelvic and upper torso axial rotation velocities were calculated as the first time derivative of the axial rotation angle. To describe the timing of pelvic and upper torso rotations, the velocity data were time normalized to the total pitch time (from stride-foot contact to ball release) and expressed as percentage pitch times. Stride-foot contact was identified as the instant when the vertical ground-reaction force from the stride-foot exceeded 10 N.^6,7 The instant of ball release was determined as the fourth frame after the wrist surpassed the elbow in the direction of the throw (in a global reference frame).²⁷ In addition, the timing of the initiation of upper torso rotation was identified as the instant of maximum separation in pelvic and upper torso axial rotation angles.

Pitchers were defined to have improper trunk rotation sequences when the peak pelvic rotation velocity was reached later than the peak upper torso rotation velocity by more than the standard error of measurement (SEM; standard deviation × [1 − intraclass correlation coefficient]^0.5) of the peak upper torso rotation velocity (3.7% of pitch time). The SEM of peak upper torso rotation velocity was used as a cutoff criterion because it was larger than the SEM of the timing of peak pelvic rotation. Pitchers were defined to have proper trunk rotation sequences when peak pelvic torso rotation velocity was reached earlier than peak upper torso rotation velocity by >3.7% of pitch time. Subjects with differences in the timing of peak velocities within ±3.7% of pitch time were not included in the analyses to ensure that 2 comparison groups were different beyond error.

The internal joint forces and moments at the shoulder and elbow joints were calculated using an inverse dynamics solution.⁴² The forces and moments at the shoulder and elbow joints were calculated as the forces and moments acting on the distal segment transformed into the respective joint coordinate system. Segment mass and the location of the center of mass of the hand, forearm, and arm segments were estimated using the method developed by Clauser et al.⁵ The moment of inertia of each segment in the frontal and sagittal planes were estimated as described by Dempster et al.⁸ The moment of inertia about the longitudinal axis of each segment was considered negligible. The linear acceleration of the marker on the third metacarpal head was used as an estimation of the linear acceleration of the ball. Specific kinetic variables calculated were peak proximal forces at the proximal ends of the humerus, shoulder internal rotation moment, and elbow varus moment. The joint forces were normalized to the pitcher’s body weight, and joint moments were normalized to the product of the pitcher’s height and body weight. All data processing and reductions were conducted using MATLAB software (The MathWorks Inc).

Data Analysis

Independent t tests were used to compare maximal shoulder external rotation angle, shoulder horizontal abduction angle at maximal shoulder external rotation, timing of peak pelvic rotation velocity, timing of peak upper torso rotation velocity, timing of the initiation of upper torso rotation, shoulder proximal forces, shoulder internal rotation moment, and elbow varus moment between the pitchers with proper and improper trunk rotation sequences. An a priori α level of 0.05 was used to determine statistical significance.

Results

Among the 72 pitchers, 22 (mean age, 15.7 ± 1.3 years; mean pitching experience, 6.2 ± 2.0 years; mean body mass, 73.7 ± 8.6 kg; mean height, 179.6 ± 6.3 cm) demonstrated proper trunk rotation sequence, and 33 (mean age, 15.5 ± 1.1 years; mean pitching experience, 6.0 ± 1.7 years; mean body mass, 73.6 ± 10.0 kg; mean height, 180.2 ± 8.4 cm) demonstrated improper trunk rotation sequences. A total of 17 pitchers were excluded from analysis because the temporal difference between the peak pelvic and upper torso rotation velocities was within the margin of the SEM. There were no statistically significant differences in demographics (age, years of pitching, body mass, and height) between the 2 groups (P < .05).

Peak pelvic rotation velocity occurred later in the pitchers with improper trunk rotation sequences (mean difference, 23.3% ± 3.4%; 95% confidence interval [CI], 15.3% to 27.3%; t ₅₄ = 6.9; P < .001) (Table 1). On the other hand, the initiation of upper torso rotation (mean difference, −9.5 ± 3.6%; 95% CI, −17.8% to −3.9%; t ₅₄ = −2.6; P = .011) and peak upper torso rotation velocity (mean difference, −4.9% ± 2.4%; 95% CI, −10.5% to −1.1%; t ₅₄ = −2.0; P = .046) occurred earlier in the pitchers with improper trunk rotation sequences (Table 1). Subjects with improper trunk rotation sequences demonstrated greater maximum shoulder external rotation angles (mean difference, 7.2° ± 2.9°; 95% CI, 0.87° to 12.7°; t ₅₄ = 2.5; P = .016) and greater shoulder proximal force (mean difference, 9.2% ± 3.9% body weight; 95% CI, 1.3% to 17.2% body weight; t ₅₄ = 2.4; P = .021) compared with those with proper trunk rotation sequences (Table 1). There were no group differences in shoulder horizontal abduction at maximal shoulder external rotation, ball speed, or other joint kinetic variables (Table 1).

Table 1

Kinematic and Kinetic Variables Between Pitchers With Proper and Improper Trunk Rotation Sequences^a

	Trunk Rotation Sequence
Variable	Proper	Improper	95% CI	t	P
Ball speed, m/s	31.4 ± 2.5	31.1 ± 2.9	–0.83 to 2.2	.96	.340
Kinematics
Maximum shoulder external rotation angle, deg	170.7 ± 11.5	177.9 ± 10.1	0.87 to 12.7	2.5	.016^b
Shoulder horizontal abduction at maximal shoulder external rotation, deg	8.5 ± 6.4	5.2 ± 10.8	–8.2 to 2.1	–1.3	.205
Timing of peak pelvic rotation velocity, % pitch	34.5 ± 15.3	57.8 ± 9.9	15.3 to 27.3	6.9	<.001^b
Timing of peak upper torso rotation velocity, % pitch	47.6 ± 8.9	42.7 ± 8.7	–10.5 to −1.1	–2.0	.046^b
Timing of initiation of upper torso rotation, % pitch	22.3 ± 15.1	12.9 ± 11.7	–17.8 to −3.9	–2.6	.011^b
Kinetics
Shoulder proximal force, %BW	93.0 ± 15.2	102.2 ± 13.5	1.3 to 17.2	2.4	.021^b
Shoulder internal rotation moment, %(BW × height)	6.3 ± 1.1	6.5 ± 1.2	–0.45 to 0.83	1.2	.250
Elbow varus rotation moment, %(BW × height)	3.9 ± 0.80	4.0 ± 0.73	–0.26 to 0.57	.75	.457

BW, body weight.

Statistically significant (α = 0.05).

Discussion

Peak pelvic rotation velocity occurred later and the initiation of upper torso rotation and peak upper torso rotation occurred earlier in pitchers with improper trunk rotation sequences. We observed that pitchers who exhibited improper trunk rotation sequences demonstrated greater maximal shoulder external rotation angles and peak shoulder proximal force compared with pitchers with proper distal-to-proximal trunk rotation sequences. However, ball speed, shoulder horizontal abduction at maximal shoulder external rotation, peak shoulder internal rotation moment, and peak elbow varus moment were equivalent between the pitchers with proper and improper trunk rotation sequences.

Our hypothesis that the proper trunk rotation sequence would be associated with more efficient transfer of momentum to the upper torso, and thus greater ball speed, was not supported by the results of this study. A possible explanation for this lack of group difference may be that there are multiple strategies pitchers adopt to achieve ball speed. Pitchers who did not demonstrate proper trunk rotation sequences may have compensated for the contribution from trunk rotation using other strategies or had greater muscular strength, which was not measured in this study.

Pitchers with improper trunk rotation sequences demonstrated greater maximal shoulder external rotation angles compared with those with proper trunk rotation sequences. A greater maximal shoulder external rotation angle has been linked to higher ball velocity in previous studies.^10,21,40 However, ball speed was not different between pitchers with and those without improper trunk rotation sequences in this study. This observation may be due to the young age of the pitchers in this study. The pitchers in the previous studies, in which the association was demonstrated, were either college or professional pitchers. From an injury development perspective, greater shoulder external rotation may be harmful to the shoulder, as it would exaggerate posterior impingement^4,38 and pull the long head of the biceps posteriorly to increase shear and tensile stress on the superior labrum.^4,18,29 This pattern of stress is commonly associated with the mechanism of superior labrum anterior posterior lesions (“peel-back” mechanism).⁴

Pitchers with improper trunk rotation sequences also experienced greater shoulder proximal force. Shoulder proximal force resists distraction of the upper arm from the trunk segment. Therefore, higher magnitude of proximal force would increase tensile loading on soft tissues around the shoulder, which include the rotator cuff, joint capsule, and biceps-labral complex. These observations suggest that an improper trunk rotation sequence may put pitchers at risk for various injuries. Greater shoulder horizontal abduction, or hyperangulation of the arm relative to the upper torso, increases stress on the anterior aspect of joint capsule and exacerbates posterior impingement at maximal shoulder external rotation. However, we did not observe any group difference in shoulder horizontal abduction angle. We also did not observe differences in elbow varus and shoulder internal rotation moments between groups. It has been demonstrated that Little League pitchers experience lower shoulder kinetic values compared with professional pitchers, even after their body size (height and weight) has been taken into account.³² It is possible that the lack of differences in elbow varus and shoulder internal rotation moments between groups was due to the age of the pitchers in this study. Further investigation is needed to understand why some kinetic variables were affected while others were unaffected by the trunk rotation sequence.

Several studies have investigated the relationship between pelvic and/or upper torso rotation kinematics and joint loading. Wight et al⁴¹ examined the effects of pelvic rotation style on upper extremity kinetics and reported that pitchers who initiated pelvic rotation later achieved peak pelvic rotation velocity later (10.8% vs 26.7% pitch) and experienced higher upper extremity joint kinetics compared with the early pelvis rotators. In our study, pitchers with improper trunk rotation sequences also achieved peak pelvic rotation velocity later (34.5% vs 57.8%) and exhibited kinetic and kinematic characteristics that may be harmful to the joints compared with pitchers with proper trunk rotation sequences. The systematic differences in temporal values between studies are likely due to the differences in the definition of stride-foot contact. Stride-foot contact was defined as the instant when the heel or toe touched the force plate in this study, whereas it was defined as the instant when both heel and toe markers fell below a certain height in the previous study.⁴¹ Despite the systematic difference in values between these studies, these observations are in agreement that delayed pelvic rotation may be associated with greater stress on the upper extremity joints.

Aguinaldo and Chambers² examined the effects of timing of the initiation of upper torso rotation on pitching biomechanics and reported that pitchers who initiated upper torso rotation before stride-foot contact experienced greater elbow varus moment. In another study,¹ the investigators also demonstrated that less experienced (ie, high school and youth) pitchers initiated upper torso rotation earlier and experienced higher normalized internal rotation moment compared with higher level (ie, professional) pitchers. In our study, pitchers with improper trunk rotation sequences demonstrated earlier initiation of upper torso rotation (12.9% vs 22.3%) and peak upper torso rotation velocity (47.6% vs 42.7%) compared with pitchers with proper trunk rotation sequences. These observations suggest that premature initiation of upper torso rotation may be harmful to the upper extremity joints.

Last, a recent study by Urbin et al³⁷ examined the relationship between various temporal variables and joint kinetics. Although it is difficult to compare the results of that study to those of the present study because the temporal variables were reported in absolute units (seconds), Urbin et al reported that a longer time between peak pelvic and upper torso rotation velocities was associated with lesser shoulder distraction force, which is in agreement with what we observed in this study. However, all pitchers in the study of Urbin et al achieved peak pelvic rotation velocity before peak upper torso rotation velocity, so the effects of sequence were not examined in the study. The fact that none of the pitchers in the previous study achieved peak upper torso axial rotation velocity before peak pelvis rotation velocity may have been due to the difference in subjects’ ages and skill levels. The pitchers in Urbin et al’s study were older and more skilled than those in our study.

Although these studies all examined the effects of pelvis and upper torso rotation kinematics on joint loading, the present study is the first to examine the effects of the order of peak pelvic and upper torso rotation velocities on joint kinetics. Considering that an improper trunk rotation sequence occurs as a combination of delayed pelvic rotation and early upper torso rotation, this study and the current literature^1,2,37,41 support the idea that proper timing of upper torso and pelvic motion may be a critical factor in limiting the stress on a pitcher’s joints. The pelvic and upper torso rotation sequence is achieved by the sequential activation of abdominal muscles.¹⁶ Therefore, weakness or fatigue of trunk musculature may lead to an improper trunk rotation sequence during pitching and thus to injuries. On the other hand, strengthening the abdominal muscles, and incorporating drills and exercises that facilitate proper activation of the abdominal muscles may help pitchers achieve proper trunk rotation sequence during pitching and possibly reduce the risk for shoulder injuries. However, studies that examine the effects of such training on pitching biomechanics must be conducted in the future.

The results of this investigation should be interpreted with caution, as data collection took place inside a research laboratory, in which pitching was performed with reflective markers while wearing rubber-soled shoes (instead of cleats). These factors may have influenced performance, thus limiting the generalizability of the findings. However, collecting the data in a controlled environment was necessary to compare the biomechanical data across pitchers. Although the observations of this study have implications for injury development, all participants were free of injuries at the time of data collection. A prospective cohort study is needed to determine if improper trunk rotation increases the risk for injury. Last, the study needs to be replicated in pitchers of different skill levels and ages, as it has been demonstrated that joint kinetics varies among pitchers of different age groups.³²

Conclusion

High school pitchers with improper trunk rotation sequences exhibited increased maximum shoulder external rotation angles and greater shoulder proximal force compared with pitchers with proper trunk rotation sequences, which may be harmful to their shoulders. Achieving a proper trunk rotation sequence using proper activation of abdominal muscles may be important in protecting pitchers’ shoulders.

Footnotes

Acknowledgements

The authors thank the baseball coaches, who were instrumental in recruiting the pitchers, and the undergraduate research assistants (Jake Whitley, Kelly Yerkes, Corey Shelton, Michelle Ikoma, and Bhavesh Patel) for their contribution to the project.

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution.

References

Aguinaldo

Buttermore

Chambers

. Effects of upper trunk rotation on shoulder joint torque among baseball pitchers of various levels. J Appl Biomech. 2007;23(1):42-51.

Aguinaldo

Chambers

. Correlation of throwing mechanics with elbow valgus load in adult baseball pitchers. Am J Sports Med. 2009;37(10):2043-2048.

Anz

Bushnell

Griffin

Noonan

Torry

Hawkins

. Correlation of torque and elbow injury in professional baseball pitchers. Am J Sports Med. 2010;38(7):1368-1374.

Burkhart

Morgan

Kibler

. The disabled throwing shoulder: spectrum of pathology. Part I: pathoanatomy and biomechanics. Arthroscopy. 2003;19(4):404-420.

Clauser

McConville

Young

. Weight, Volume, and Center of Mass of Segments of the Human Body. Dayton, OH: Wright-Patterson Air Force Base; 1969.

Decker

Torry

Noonan

Riviere

Sterett

. Landing adaptations after ACL reconstruction. Med Sci Sports Exerc. 2002;34(9):1408-1413.

Decker

Torry

Wyland

Sterett

Richard Steadman

. Gender differences in lower extremity kinematics, kinetics and energy absorption during landing. Clin Biomech (Bristol, Avon). 2003;18(7):662-669.

Dempster

Sherr

Priest

. Conversion scales for estimating humeral and femoral lengths and the lengths of functional segments in the limbs of American Caucasoid males. Hum Biol. 1964;36:246-262.

Elliott

Grove

Gibson

. Timing of the lower limb drive and throwing limb movement in baseball pitching. Int J Sports Biomech. 1988;4(4):59-67.

10.

Escamilla

Fleisig

Barrentine

Andrews

Moorman

III . Kinematic and kinetic comparisons between American and Korean professional baseball pitchers. Sports Biomech. 2002;1(2):213-228.

11.

Feltner

Dapena

. Dynamics of the shoulder and elbow joints of the throwing arm during a baseball pitch. Int J Sport Biomech. 1986;2(4):235-259.

12.

Feltner

. Three-dimensional interactions in a two-segment kinetic chain. Part I: general model. Int J Sport Biomech. 1989;5(4):403-419.

13.

Feltner

. Three-dimensional interactions in a two-segment kinetic chain. Part II: application to the throwing arm in baseball pitching. Int J Sport Biomech. 1989;5(4):420-450.

14.

Fleisig

Andrews

Cutter

. Risk of serious injury for young baseball pitchers: a 10-year prospective study. Am J Sports Med. 2011;39(2):253-257.

15.

Fleisig

Barrentine

Escamilla

Andrews

. Biomechanics of overhand throwing with implications for injuries. Sports Med. 1996;21(6):421-437.

16.

Hirashima

Kadota

Sakurai

Kudo

Ohtsuki

. Sequential muscle activity and its functional role in the upper extremity and trunk during overarm throwing. J Sports Sci. 2002;20(4):301-310.

17.

Krajnik

Fogarty

Yard

Comstock

. Shoulder injuries in US high school baseball and softball athletes, 2005-2008. Pediatrics. 2010;125(3):497-501.

18.

Kuhn

Lindholm

Huston

Soslowsky

Blasier

. Failure of the biceps superior labral complex: a cadaveric biomechanical investigation comparing the late cocking and early deceleration positions of throwing. Arthroscopy. 2003;19(4):373-379.

19.

Lin

Kohli

Perlmutter

Lim

Nuber

Makhsous

. Muscle contribution to elbow joint valgus stability. J Shoulder Elbow Surg. 2007;16(6):795-802.

20.

Lyman

Fleisig

Waterbor

. Longitudinal study of elbow and shoulder pain in youth baseball pitchers. Med Sci Sports Exerc. 2001;33(11):1803-1810.

21.

Matsuo

Escamilla

Fleisig

Barrentine

Andrews

. Comparison of kinematic and temporal parameters between different pitch velocity groups. J Appl Biomech. 2001;17(1):1-13.

22.

Mihata

McGarry

Kinoshita

Lee

. Excessive glenohumeral horizontal abduction as occurs during the late cocking phase of the throwing motion can be critical for internal impingement. Am J Sports Med. 2010;38(2):369-374.

23.

Morrey

. Articular and ligamentous contributions to the stability of the elbow joint. Am J Sports Med. 1983;11(5):315-319.

24.

Morrey

. Functional anatomy of the ligaments of the elbow. Clin Orthop Relat Res. 1985;201:84-90.

25.

National Federation of State High School Associations. Field diagram. Available at: http://nfhs-baseball.arbitersports.com/front/105406/Site/Baseball-Resources/Baseball-Field-Diagram.

26.

Oyama

. Prevention of pitching-related upper extremity injuries through pitching technique instruction. J Sport Health Sci. 2012;1(2):80-91.

27.

Oyama

Blackburn

Padua

Myers

. Effects of excessive contralateral trunk tilt on pitching biomechanics and performance in high school baseball pitchers. Am J Sports Med. 2013;41(19):2430-2438.

28.

Petty

Andrews

Fleisig

Cain

. Ulnar collateral ligament reconstruction in high school baseball players: clinical results and injury risk factors. Am J Sports Med. 2004;32(5):1158-1164.

29.

Pradhan

Itoi

Hatakeyama

Urayama

Sato

. Superior labral strain during the throwing motion. A cadaveric study. Am J Sports Med. 2001;29(4):488-492.

30.

Putnam

. A segment interaction analysis of proximal-to-distal sequential segment motion patterns. Med Sci Sports Exerc. 1991;23(1):130-144.

31.

Putnam

. Sequential motions of body segments in striking and throwing skills: descriptions and explanations. J Biomech. 1993;26(Suppl 1):125-135.

32.

Ramappa

Chen

Hawkins

. Anterior shoulder forces in professional and Little League pitchers. J Pediatr Orthop. 2010;30(1):1-7.

33.

Oyama

Marshall

Mueller

. Pitching practices and self-reported injuries among youth baseball pitchers. A descriptive study. Athl Train Sports Health Care. 2011;4(1):11-20.

34.

Sabick

Torry

Lawton

Hawkins

. Valgus torque in youth baseball pitchers: a biomechanical study. J Shoulder Elbow Surg. 2004;13(3):349-355.

35.

Seiber

Gupta

McGarry

Safran

Lee

. The role of the elbow musculature, forearm rotation, and elbow flexion in elbow stability: an in vitro study. J Shoulder Elbow Surg. 2009;18(2):260-268.

36.

Stodden

Langendorfer

Fleisig

Andrews

. Kinematic constraints associated with the acquisition of overarm throwing part I: step and trunk actions. Res Q Exerc Sport. 2006;77(4):417-427.

37.

Urbin

Fleisig

Abebe

Andrews

. Associations between timing in the baseball pitch and shoulder kinetics, elbow kinetics, and ball speed. Am J Sports Med. 2013;41(2):336-342.

38.

Walch

Boileau

Noel

. Impingement of the deep surface of the supraspinatus tendon on the posterior superior glenoid rim: an arthroscopic study. J Shoulder Elbow Surg. 1992;1:238-243.

39.

Werner

Gill

Murray

Cook

Hawkins

. Relationships between throwing mechanics and shoulder distraction in professional baseball pitchers. Am J Sports Med. 2001;29(3):354-358.

40.

Werner

Suri

Guido

JAJ

Meister

Jones

. Relationships between ball velocity and throwing mechanics in collegiate baseball pitchers. J Shoulder Elbow Surg. 2008;17(6):905-908.

41.

Wight

Richards

Hall

. Influence of pelvis rotation styles on baseball pitching mechanics. Sports Biomech. 2004;3(1):67-83.

42.

Winter

. Biomechanics and Motor Control of Human Movement. 3rd ed. Hoboken, NJ: John Wiley; 2005.

43.

Siegler

Allard

. ISB recommendation on definitions of joint coordinate system of various joints for the reporting of human joint motion—part I: ankle, hip, and spine. International Society of Biomechanics. J Biomech. 2002;35(4):543-548.

44.

van der Helm

Veeger

. ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion—part II: shoulder, elbow, wrist and hand. J Biomech. 2005;38(5):981-992.

45.

Hay

. Angular momentum and performance in the triple jump: a cross-sectional analysis. J Appl Biomech. 1995;11(1):81-102.