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Two-Dimensional Video Analysis of Youth and Adolescent Pitching Biomechanics

A Tool For the Common Athlete

DeFroda, Steven F. MD, ME; Thigpen, Charles A. PhD; Kriz, Peter K. MD

doi: 10.1249/JSR.0000000000000295
Sports-Specific Illness and Injury: Section Articles
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Three-dimensional (3D) motion analysis is the gold standard for analyzing the biomechanics of the baseball pitching motion. Historically, 3D analysis has been available primarily to elite athletes, requiring advanced cameras, and sophisticated facilities with expensive software. The advent of newer technology, and increased affordability of video recording devices, and smartphone/tablet-based applications has led to increased access to this technology for youth/amateur athletes and sports medicine professionals. Two-dimensional (2D) video analysis is an emerging tool for the kinematic assessment and observational measurement of pitching biomechanics. It is important for providers, coaches, and players to be aware of this technology, its application in identifying causes of arm pain and preventing injury, as well as its limitations. This review provides an in-depth assessment of 2D video analysis studies for pitching, a direct comparison of 2D video versus 3D motion analysis, and a practical introduction to assessing pitching biomechanics using 2D video analysis.

1Department of Orthopaedics, Alpert Medical School of Brown University, Providence, RI; 2Department of Research and Analytics, ATI Physical Therapy, Greenville, SC; and 3Department of Orthopaedics and Pediatrics, Alpert Medical School of Brown University, Providence, RI

Address for correspondence: Steven F. DeFroda, MD, ME, Department of Orthopaedics, Alpert Medical School of Brown University, 593 Eddy Street, Providence, RI 02903; E-mail: sdefroda@gmail.com.

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Introduction

Stages of Throwing

Overhead throwing is one of the most complex activities in athletics, pushing the physiologic limits of the human body, as evidenced by injury rates in this population. Werner et al. (37) described the throwing motion in six phases: wind up, stride, arm cocking, arm acceleration, arm deceleration, and follow-through. Each phase places an emphasis on different muscles and joints. Ideally, force should be transmitted from the lower segment (legs) to the upper body (arm) throughout the throwing cycle with the optimization of body position leading to a more fluent transfer of kinetic energy.

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Biomechanics, Kinetics, and Kinematics

The accessibility and application of pitching motion analysis often uses biomechanics, kinetics, and kinematics interchangeably. However, understanding the unique definitions of these terms is important in the context of video pitching assessment. Biomechanics is the coordinated series of movements and muscular forces, which result in the ultimate goal of velocity and accuracy. Biomechanics is a function of kinematics and kinetics, involving the complex interplay of the lower extremity, trunk, pelvis, and upper extremity musculature (5,16). Simply stated, kinetics refer to the forces and torques of the throwing motion, while kinematics relate to the motions themselves (36). Kinetics explains why an object moves the way it does and quantifies both the forces and torques that contribute to motion (16). The kinetics of pitching allow potential energy stored in the legs during the early wind up and stride phase to be transferred through the trunk and upper extremity during late cocking, resulting in acceleration due to a combined torque through the shoulder and elbow, as well as a driving force from the lower extremity (5,13,15). Kinematics is how the body moves without stating the causes behind the motion. Kinetic data are collected during three-dimensional (3D) motion analysis of pitchers using high-speed videography in conjunction with inverse dynamics calculations, which estimate the net force or torque about a joint. Kinetic data cannot be ascertained with two-dimensional (2D) video alone (16). The kinetic chain is the interplay of all of these factors and involves the interaction of body segments in a sequential fashion from foot to hand (5).

When assessing a pitcher’s kinematics, it is important to consider that proper biomechanics are similar across different age groups. Fleisig et al. (15) found that youth pitchers 10 to 15 yr of age are capable of replicating mechanics similar to professional pitchers in 16 of 17 positional and temporal parameters. Only joint forces and torques varied, being greater in adults due to increased strength and muscle mass (15). Consequently, correcting mechanics at a young age to reduce the deleterious effect of biomechanical errors (e.g., repetitive microtrauma) may be as important an injury prevention in youth baseball as limiting overuse injuries through pitch count limits, days of rest, and discouraging year-round participation. By using video analysis to evaluate various kinematic parameters of the kinetic chain, it may be possible to diagnose and ameliorate the cause of the athlete’s arm pain at an early age to prevent long-term injury.

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2D Video Analysis Versus 3D Motion Analysis

Crucial movements in a pitcher’s delivery take place in 1/250-1/750th of a second (18). As the human eye is capable of processing images at a rate of only 32 frames per second (FPS), assessment of the pitching motion without high-speed motion analysis is an imprecise exercise which evaluates the entire sequence of events inefficiently (5). 2D video analysis is relatively inexpensive and appears to provide an effective means for coaches, players, and sports medicine providers to record and analyze the throwing motion and identify biomechanical flaws to correlate them to injury and reduced performance. The increasing popularity and accessibility to high-quality video cameras and smart phones have increased the number of platforms available for filming and analyzing the throwing motion ranging from very basic to more complex (Table 1). There are basic free applications which can be installed on a smart phone and allow users to record video and compare multiple videos side by side (34). These applications allow also for frame-by-frame review but lack the resolution of high-end motion analysis. Most phone cameras record at 30 FPS, whereas some action video cameras can record 240 FPS in high-definition resolution. More complex phone and camcorder applications allow the video to be uploaded to Internet-based software programs, which vary in the level of analysis. Software programs offer the ability to sync with up to eight cameras depending on the package purchased, and even include high-speed cameras and computers to analyze the data (Table 1) (34,22,28). Joint angles, distances (e.g., stride length), and biomechanical timing (e.g., first forward movement to foot strike) are all measurements that can be evaluated using 2D video analysis.

Table 1

Table 1

3D motion analysis is traditionally performed in an indoor laboratory setting with data captured using a motion analysis system (MAS), which includes 1) multiple (e.g., 6-12), high-speed, light-sensitive cameras with frame rates ranging from 200 to 1000 FPS; 2) reflective markers placed over bony prominences on the bare skin of a subject, which are automatically tracked by the MAS; 3) real-time 3D digitizing which is required for quantitative analysis; and 4) force plates built into the floor or simulated mound. 3D movement-space coordinates are reconstructed from the video images (2). Kinematic and kinetic data are generated from 3D motion analysis. Historically, 3D motion analysis has been available to elite athletes or those undergoing complex biomechanical studies or for an expensive fee (US $500–1000/assessment) (Table 2).

Table 2

Table 2

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Review of 2D Video Analysis Literature

Several studies have assessed the utility and reliability of 2D video analysis in the evaluation of youth and adolescent pitching mechanics. Studies have demonstrated the ability to differentiate several variables of pitching mechanics and demonstrated their association with performance, suggesting that 2D analysis may be a valuable tool (7,31). However, Nicholls et al. (23) evaluated 20 baseball pitchers aged 11 to 15 yr with a minimum of two seasons’ pitching experience. A qualitative analysis was performed using 24 kinematic variables. Pitchers were videotaped using three 60-Hz video cameras from frontal, lateral, and rear views, and two independent raters evaluated their technique using a qualitative analysis protocol (QAP). Additionally, each pitcher was assessed by a six-camera 200-Hz 3D MAS in an attempt to validate the QAP kinetic variables. Only variables from the stride, arm cocking, and arm acceleration/ball release phases were compared between QAP and MAS, as the windup and follow-through phases have been determined to be periods of low kinematic activity (8). Overall, 17 variables were quantified during validity analysis. Four (23.5%) of 17 QAP kinematic variables (elbow flexion at stride foot contact (SFC), sequence of hip-shoulder rotation during arm cocking, trunk flexion, and shoulder horizontal adduction at ball release) achieved statistical significance with good to excellent reproduction of MAS results. Three other variables approached statistical significance: lateral trunk tilt at ball release (P = 0.051), shoulder external rotation (ER) at SFC (P = 0.055), and shoulder abduction at SFC (P = 0.057). Interrater reliability showed agreement on only 8 (33%) of 24 QAP kinematic variables. Significant agreement levels were achieved for four stride phase variables (stride offset, foot angle, shoulder ER, shoulder abduction), two arm cocking phase variables (glove arm, maximum shoulder ER), and two ball release variables (lateral trunk tilt, shoulder abduction); none of the four windup phase variables achieved significant agreement. Limitations of the study included slow (60 Hz) video camera shutter speeds, which likely resulted in inaccurate analysis of the pitching action, as well as validity problems inherent to qualitative analysis (bias, human error, and subjectivity) (23).

Davis et al. (7) performed a laboratory study of 169 baseball pitchers aged 9 to 18 yr using 3D motion analysis (eight-camera 240-Hz system) and 2D video analysis using two 250-Hz cameras placed in frontal and lateral views to evaluate whether correct performance of five biomechanical pitching parameters considered to be key elements in youth pitching (leading with the hips, hand-on-top position, arm in throwing position, closed-shoulder position, and stride foot toward home plate). Correct performance of these parameters was associated with lower joint forces involving the shoulder and elbow. Three independent observers analyzed the same best pitch with respect to the five biomechanical parameters. Pitchers who performed both hand-on-top and closed-shoulder position correctly were more efficient (e.g., lower humeral internal rotation torque (HIRT)/velocity and lower elbow valgus load (EVL)/velocity) than those pitchers who performed both parameters incorrectly (P = 0.035 and 0.042, respectively). Overall, pitchers that demonstrated better biomechanics generated lower forces and demonstrated better mechanical efficiency than those with improper mechanics, illustrating the importance of teaching proper pitching mechanics in youth to reduce shoulder and elbow injuries. Limitations of the study included using a standard 25-ft distance from pitching mound to net rather than different distances for different age groups, selection of one pitch for motion and video analysis, limited assessment of trends in pitcher’s throwing mechanics, and utilization of video cameras with higher (240 Hz) frame rates compared with standard, commercially available camera rates.

Sgroi et al. (31) performed a single-episode, cross-sectional study of 420 youth and adolescent pitchers aged 12 to 18 yr during the preseason using 2D video analysis with two 210-Hz video cameras from frontal and lateral views. Demographic data were collected, and subjects pitched from a regulation practice mound appropriate for the subject’s level of play, and pitch velocity was measured with a radar gun. The single pitch most representative of the pitcher’s best effort was analyzed. A protocol containing kinematic variables previously validated with kinematic variables as well as observational mechanical measurements was used. After multivariate logistic regression analysis, age, height, separation of the hips and shoulders, and stride length were the most important correlates with pitch velocity (P < 0.001 for each variable). Limitations of the study included 1) use of an analysis system—2D video analysis—which lacks both validation and reliability compared with 3D motion analysis, 2) use of a single-episode study design rather than a prospective longitudinal study to measure improvements in velocity, and 3) strong covariance of height and weight (31). Together, these studies suggest 2D analysis provides an important assessment of pitching mechanics, but caution should be used in the comparison to 3D normative data, and careful attention to the process of obtaining the assessment should be used.

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2D Video Versus 3D Motion Analysis: Pros and Cons

2D video analysis is an attractive alternative to 3D motion analysis for many reasons. 2D video analysis is simpler and cheaper, requiring fewer cameras, hardware, and software to record and analyze sports movements. Setup time is minimal, and acceptable results are achieved for planar movements that are typically preselected (2). Conceptually, 2D video analysis is easier for athletes and coaches to comprehend. Limitations of 2D video analysis include 1) the relative subjectivity of kinematic measurements, such as angles, distances, etc., which are calculated by eye/hand rather than using reflective markers placed on the athlete’s body; 2) inferior image resolution and sampling (i.e., frame) rates of video cameras, which further reduces accuracy of kinematic measurements during movement analysis; 3) most digital video cameras cannot be “genlocked” to allow the shutter openings to be synchronized when using more than one camera to record movement, as in 3D motion analysis (2). While some action video cameras (e.g., GoPro™ series) allow multiple cameras to be synchronized with a single remote control, without genlock capability, cameras can take pictures up to half of a field (or 0.01 s) apart. Genlocking is feasible with action video cameras, but often requires purchasing of third-party hardware (e.g., MewPro™) to synchronize multiple cameras.

3D motion analysis has the advantage of capturing the body’s true 3D movements, with minimal distortion due to superior image resolution and high sampling rates. Angles between body segments can be calculated accurately due to multiple camera views (2). Limitations include expense, time, and resource-intensive requirements, including a laboratory setting and an onsite biomechanist to assist with analysis performance and interpretation. Although the computational complexity of 3D motion analysis allows reconstruction of movement and time synchronization of data, the statistical reports generated from analysis can be difficult for the layperson to understand and for biomechanists to translate into practical information that athletes and coaches can use to enhance performance and reduce injury risk.

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2D Video Analysis of Baseball Pitching: Applying to Youth and Adolescent Pitchers

Despite investigations of youth/adolescent pitching mechanics, which have led to better understanding of relationships between functional strength, mobility, and stability as they relate to mechanical efficiency, injury, and performance, pitching-related upper extremity injuries in youth and adolescent athletes have reached epidemic proportions (16,13,15,7,31). Recognizing disconnect between identification of biomechanical errors and implementation of injury prevention programs in youth baseball, sports medicine clinicians are currently directing their efforts toward multidisciplinary screening tools which utilize 2D video analysis to identify and correct at-risk mechanics and strength/flexibility deficits early in a pitcher’s development. Ideally, such a screening tool would include both 2D kinematic variables and observational measurements that have been validated against 3D kinematics and would possess excellent interrater and intrarater reliability. Although discussion of such a screening tool is beyond the scope of this review article, this section will focus on kinematic variables and observational measurements in the pitching delivery, which have been supported by the literature to be worthy of inclusion in a youth/adolescent pitching screening tool.

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Windup

During the windup, the pitcher maintains his center of gravity (COG) over his back leg, allowing generation of maximal momentum once forward motion is initiated with the lead (stride) hip (30). Maximal lead knee height (preferably with lead hip flexion ≥ 90°) is critical to generate potential energy (36). The back (stance) leg maintains slight knee flexion (36). The pitcher should appear stable and balanced during this phase, with good head stability, eyes focused on target (Fig. 1A). If the pitcher’s lead hip falls forward prematurely, the kinetic chain will be disrupted, resulting in greater shoulder force required to generate ball velocity (4,17). If COG is positioned too far posteriorly or anteriorly (Fig. 1B), the body segment sequence timing and torque transfer in the kinetic chain will be transferred to the upper extremity, thus predisposing the shoulder and elbow to injury (5). Leading with the hips, or premature forward momentum with the lead hip, can create a strong energy angle through early initiation of forward momentum, resulting in COG traveling toward home plate at maximum leg lift (Fig. 1C). Although this strong energy angle can be an integral component of pitching mechanics at the elite level, it has been shown to be associated with higher HIRT, higher EVL, and lower pitching efficiency in adolescent pitchers (7). Leading with the hips should therefore be discouraged in youth, adolescent, and high school pitchers.

Figure 1

Figure 1

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Stride

During the stride phase, the pitcher begins to generate linear velocity toward the target by lowering the lead (stride) leg and separating the arms/hands, with the throwing hand separating from the gloved hand so the throwing arm is synchronized with the stride leg motions (36,14). Pitchers should have the same angle of bend for both the pitching arm and the glove-side arm, so they appear opposite while mirroring each other (Figs. 2A, B) (18). This symmetry serves to preserve balance throughout the delivery.

Figure 2

Figure 2

Stance leg hip abduction initiates forward motion, and stance leg knee/hip extension further drive the body forward and lead to initiation of pelvic rotation (36,30). The stride leg hip begins ER, whereas the stance leg hip begins internal rotation (IR). Lack of stance hip IR can lead to “opening up” (premature pelvic rotation), leading to inefficient kinetic energy transfer from the pelvis to the trunk, increasing demands on the distal kinetic chain (shoulder, elbow) to maintain accuracy and ball velocity (36,14).

Hand-on-top position during arm separation has been associated with lower HIRT, lower EVL, and higher pitching efficiency (Fig. 2C). Hand-under-ball position (delayed glenohumeral abduction, early ER) during stride phase may lead to the throwing arm being “late” in the pitching motion (Fig. 2D). Excessive horizontal abduction, or hyperangulation, appears to contribute to throwing shoulder injuries (7).

At SFC, the throwing arm is semicocked with the elbow flexed, and the shoulder abducted and externally rotated (36,14). Both shoulders are horizontally abducted. Stride length should be approximately 75% to 85% of height (Fig. 3C), as Fleisig et al. (15) demonstrated in analysis of 23 youth (10–15 yr) and 33 high school (15–20 yr) healthy male pitchers; stride length is 85% ± 8% and 85% ± 9%, respectively (15).

Figure 3

Figure 3

The stride (lead) leg and stance leg should be roughly in line with each other and the target, ideally with the stride foot in a slightly closed position, with the stride foot angled slightly toward the third baseline for right handers, and slightly toward the first baseline for left handers. Lead shoulder position should be slightly closed at SFC (Fig. 4A), because this parameter, performed correctly together with hand-on-top position, has been shown to result in a biomechanical benefit with regard to pitching efficiency (youth and adolescent pitchers) and lower HIRT and EVL (youth pitchers) (7). Normative mechanics for stride foot position are 19 ± 14 cm closed at SFC, and 19° ± 11° closed for foot angle at SFC (Figs. 5A, B) (12).

Figure 4

Figure 4

Figure 5

Figure 5

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Arm Cocking

The arm cocking phase begins with SFC and ends at maximum shoulder ER (max ER) (30,14). Once the stride leg has created a steady base, the pelvis rotates to face the target, followed by lumbar spine hyperextension and upper torso rotation (36,14). Although the proximal kinetic chain is rotating and extending, the distal kinetic chain is externally rotating (shoulder) and flexing (elbow) (36). Throughout the arm cocking phase, shoulder abduction remains approximately 90°. As the upper torso rotates to face the target, the shoulder externally rotates and horizontally adducts (10°–20°). At the end of arm cocking phase (max ER), the throwing arm is positioned in 90° to 95° of elbow flexion, 150° to 180° of ER, approximately 90° of abduction, and 10° to 20° of horizontal adduction (Fig. 6A) (30,14).

Figure 6

Figure 6

Contralateral trunk tilt or lean (CTL) is a concept that has received increased attention over the past decade (Figs. 6B, C). During arm cocking phase, pitchers intuitively lean to the opposite side of their throwing arm, with peak CTL occurring between peak elbow varus moment and max ER (27). Oyama et al. (27) found that pitchers who exhibit increased CTL have statistically significant increases in both ball velocity and shoulder and elbow joint moments. More recently, Solomito et al. (32) found that the mean CTL angle in a cohort of 99 Division I/III college pitchers was 24° ± 10°; for every 10° increase over the median CTL at max ER, ball velocity increased by 1.1 mph (P = 0.003), elbow varus moments increased by 3.7 N·m−1 (P < 0.001), and glenohumeral IR moments increased by 2.5 N·m−1 (P < 0.001). For the same increase in CTL, ball velocity increased by 1.5%, elbow varus moments increased by 4.8%, and glenohumeral IR moments increased by 3.2%, demonstrating that CTL affected joint moments to a much greater extent than it did ball velocity (32). Given that cadaveric studies have shown that the moment at which the ulnar collateral ligament fails is around 34 N·m−1 (21), other studies have shown that up to 50% of the joint moment is transmitted to the UCL (1), and the Solomito study found the average elbow varus moment to be 75.6 ± 15.3 N·m−1 (32). Thus, even a 3.7 N·m−1 increase in elbow varus moment due to increase in CTL can bring the UCL dangerously close to the ultimate moment (failure):

Although excessive CTL can enhance performance (i.e., increased ball velocity), it should be discouraged in pitchers due to the increased shoulder and elbow torque that can result in significant injury (e.g., UCL tear). Instead, a more upright posture throughout the pitching cycle should be maintained. Improving core (hip abductor) strength and trunk control is critical.

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Arm Acceleration

Acceleration phase occurs between max ER and ball release (36,14). The throwing shoulder transitions from max ER into IR. The trunk transitions from its hyperextended position at max ER to a position of flexion (forward trunk tilt; mean, 32°–55°) at ball release (30). The throwing elbow, initially flexed from 90° to 120° at max ER, rapidly extends to near 25° just before ball release. Lead (stride) leg knee maintains flexion, then transitions to extension (mean, 58° knee extension) at ball release (30). The throwing shoulder maintains approximately 90° of abduction at ball release.

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Arm Deceleration

Arm deceleration begins with ball release and culminates with shoulder maximum IR (max IR) (36,14). The trunk and hips continue to flex, the stride (lead) knee, and throwing elbow continue to extend until near-full extension is achieved. The throwing arm horizontally adducts across the trunk to decelerate. Shoulder posterior force (infraspinatus, supraspinatus, teres major/minor, latissimus dorsi, and posterior deltoid) and horizontal abduction torque are produced to counteract anterior humeral translation and control horizontal adduction. Additionally, shoulder inferior force and adduction torque are generated to counteract superior humeral translation and shoulder abduction. The scapula derotates from an upwardly and externally rotated and posteriorly tilted position and returns to a downwardly rotated, protracted, and anteriorly tilted position through arm deceleration (26,20).

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Follow-through

The follow-through phase begins at max IR and ends with the pitcher in a fielding position. A long arc of deceleration from the throwing arm, trunk flexion, and stride (lead) knee extension allows kinetic energy to be absorbed by the large muscles of the trunk and legs rather than the upper extremity (14). With most of the weight and momentum of the body transferred to the stride leg, the throwing arm is spared excessive stress.

Stability, balance, and head control throughout the entire pitching delivery should be emphasized, culminating with the pitcher finishing in a balanced fielding position. If a pitcher does not stay dynamically balanced (e.g., head over COG) with minimum head movement or postural change throughout the delivery, every inch of inappropriate head movement will cause movement of up to two inches at ball release, resulting in an inefficient motion, discoordination of the kinetic chain, and ultimately increased injury risk and decreased performance (18).

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Timing and Sequencing

Biomechanical timing and kinematic sequencing of the critical elements of a pitcher’s delivery can be evaluated by 2D video analysis (Video 1, Addendum 1, http://links.lww.com/CSMR/A8). House and Thorburn (18) have determined through extensive 3D motion analysis research that a pitcher has approximately one full second (0.95–1.05 s) to get from his first forward movement during windup into foot strike (SFC). If a pitcher does not complete the initial links of the kinetic chain culminating with SFC in <1.05 s, the subsequent events following SFC are likely to fall out of sequence, resulting in decreased performance and increased injury risk (18). Delaying the initiation of trunk rotation until after SFC ensures that the hips have rotated far enough to generate hip-shoulder separation, which is thought to be responsible for 80% of ball velocity during the pitching cycle (Figs. 5C and D). Elite pitchers generate between 40° and 60° of hip-shoulder separation for every pitch (18).

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Conclusions

Pitching involves the complex interplay of muscles and joints from the foot to the hand, with crucial phases of the pitching delivery occurring at rates exponentially faster than the human eye can process. Costly, resource-intensive 3D motion analysis laboratories have traditionally been used to evaluate and study the throwing motion. Today, a wide array of less expensive, user-friendly platforms for 2D video analysis of the throwing athlete is available to the sports medicine clinician for assessing youth and adolescent throwing biomechanics. Although 2D video analysis currently has limitations including suboptimal interrater/intrarater reliability and the inability to assess kinetic variables, it is possible that a qualitative analysis using 2D filming with high camera frame rates and raters experienced in assessing pitching mechanics could yield a screening tool which reliably assesses kinematic variables and observational measurements of youth/adolescent pitching technique, with validity of certain kinematic variables comparable to 3D motion analysis. Until such a screening tool is available, coaches, players, and sports medicine professionals should recognize the limitations of 2D video analysis when attempting to identify proper and improper pitching mechanics, identifying causes of arm pain, and preventing injury in youth/adolescent baseball pitchers.

The authors have no disclosures or conflicts of interest to report.

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