An Overview of the Biomechanics and Risk Factors with UCL Injury in Baseball Pitchers

In 1974, orthopedic surgeon Frank Jobe marked a pivotal movement in sports medicine history by performing the world’s first reconstruction of the ulnar collateral ligament of the elbow (UCL) on Los Angeles Dodgers pitcher Tommy John. This revolutionary surgery, now coined “”Tommy John’s surgery””, has become now commonplace amongst individuals pitching at all levels of baseball since. The prevalence of successful reconstruction of the UCL has grown tremendously since Jobe’s pioneering surgery.

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However, the incidence of injury to the UCL in Major League Baseball (MLB) pitchers has significantly increased despite notable improvements in diagnostic measures, athlete conditioning, and surgical advancements (3,5).

Compared to other popular American sports, Baseball is considered to be relatively safe with an extremely low injury rate per games played (5). Despite being characterized with a low injury rate, 56-75% of pitching injuries require time lost from practice and gameplay (5). Of those injuries, 67% are localized to the upper extremities, with UCL injuries accounting for 25% of total injuries to MLB pitchers (5). As the incidence of UCL injuries continues to rise, investigations into the epidemiology of the injury have also progressed. Several investigations focus on the anatomical analysis of the UCL, the biomechanics of the kinetic chain of events during the pitching motion, and risk factors associated with the injury.

Section II: Biomechanics

Investigation into the mechanisms contributing to UCL damage requires adequate knowledge in the biomechanics underlying the kinetic chain of motion during the pitching motion. The pitching motion consists of 6 phases: windup, early cocking/stride, late cocking, acceleration, deceleration, and follow through (6). The complex integration between these phases results in the efficient generation and transfer of energy from the body into the arm and, ultimately, the hand and ball (6). Though an overhead throwing motion, the lower extremities and core musculature in the kinetic chain reduces contribution from the shoulder joint (6). With this in mind, the pitching motion is regarded as an integrated motion of the entire body that culminates with rapid motion of the upper extremity, rather than solely an upper extremity action (6).

The Phases of the Pitching Motion:

The Wind-up

The pitching motion initially begins with the wind-up phase. This phase is initiated with movement of contralateral lower extremity, with elevation of the lead leg to its highest point (1,6). The pitcher must keep his center of gravity over his back leg to ensure maximum momentum once forward motion is initiated (6). The windup and stride position the body to optimally generate the forces and power required to achieve top velocity (6).

Figure 1: The windup and stride of the throwing motion (6).

Early Cocking/ Stride

Once the pitcher has reached the maximum lead leg height during the wind-up phase and removes the ball from the glove, the early cocking phase is initiated (6). The stride of this phase, functions to increase the distance over which linear and angular trunk motions occur (6). This allows for adequate energy production for the transfer of kinetic energy from the lower extremities to the upper extremity (6). During the stride, the stance knee and hip extend and lead to the initiation of pelvic rotation and forward tilt, followed by upper torso rotation (1,6). The pelvis achieves maximum rotational velocities of 400 to 700 degrees per second during this phase (6). Supraspinatus, infraspinatus, and teres minor are active late, to initiate shoulder external rotation (6). This phase then ceases when the lead foot contacts the pitching mound (6).

Figure 2: The early cocking/stride of the pitching motion with emphasis displayed on pelvis external rotation (6).

Late Cocking

The late cocking phase occurs between lead foot contact and the point of maximal external rotation of the throwing shoulder (6). Once the lead foot is planted creating a stable point for pelvis and trunk rotation, the scapula retracts and the elbow flexes (6). This causes the humerus to undergo abduction and external rotation (6). As the torso rotates, the anterior deltoid and pectoralis major contract to bring the throwing extremity into horizontal adduction to achieve the 15° to 20° for the launch position of late cocking (6). Maximum shoulder internal rotation torque occurs just before maximum shoulder external rotation (6). Near the end of arm cocking, maximum valgus torque is experienced at the elbow (6). The flexor and pronator muscles of the forearm generate a counter varus torque of 64 Nm (6). As the pelvis reaches max rotation, rotational and angular velocities are transferred through the torso and into the throwing arm during external rotation. At termination of the late cocking phase, the arm is positioned in 95° of elbow flexion, 165° to 175° of external rotation, 90° to 95° degrees of abduction, and 10° to 20° degrees horizontal adduction (6).

Figure 3: The late cocking phase of the throwing motion (6).


The acceleration phase of the pitching motion is defined as the time between maximum external rotation of the shoulder and ball release (1,6). Trunk rotation and tilt continues through the acceleration phase, causing the transfer of potential energy during the horizontal adduction and extreme internal rotation of the humerus (1,6). This rapid motion delivers the arm from as much as 175° of external rotation to 100° of internal rotation (at ball release) in only 42 to 58 milliseconds (6). During acceleration, the elbow initially flexes from 90° to 120°, then rapidly extends to near 25° just before ball release (6). Maximum elbow extension angular velocity occurs just before ball release and may reach mean angular velocity of 2251 degrees per second (6).

Figure 4: The acceleration phase of the throwing motion (6).

Deceleration / Follow Through

The deceleration phase is considered the most violent phase of the pitching motion, and occurs between ball release and maximum humeral internal rotation and elbow extension (6). During this phase, extraordinary glenohumeral joint loading is encountered during the throw. Excessive posterior (400 N) and inferior shear forces (300 N) occur, along with elevated compressive forces (> 1000 N) and adduction torques (6). Posterior soft tissues structures such as teres minor, infraspinatus, and the posterior deltoid function to dissipate these forces as the arm continues to adduct and internally rotate (6). After ball release, the upper extremity is outstretched to home plate; the elbow is flexed 25°; and the arm is abducted an average of 93° and set in 6° of horizontal adduction (6). During deceleration, there is marked biceps and brachialis activity decelerating the rapidly extending elbow and pronating forearm (6). FIGURE

Figure 5: The deceleration phase of the throwing motion (6).

As follow-through proceeds, the body continues to move forward with the arm until motion has ceased (6). Horizontal adduction increases to 60°, and muscle firing decreases in general (1,6). The follow-through phase culminates with the pitcher in a fielding position (6).

Anatomical & Biomechanical Metrics of the UCL during the Throwing Motion:

Anatomically, the UCL functions as a soft tissue restraint that stabilizes the elbow joint from valgus stress associated with the throwing motion (3, 5). During the incidence of valgus instability, the main dynamic stabilizer is the flexor-pronator mass and the main static stabilizers are the UCL and the medial joint capsule (3). The ligament itself is composed of three separate bundles: anterior, posterior, and transverse (3). The anterior bundle is the primary restraint to valgus stress, and subsequently is the portion most prone to injury. The anterior is broken down further into two seperate bands, the anterior and posterior (3). The anterior band functions as the primary valgus stabilizer from 30° to 90° of flexion (3). Whereas, the posterior band functions as the primary stabilizer from 90° to 120° of flexion (3).

Figure 6: An anatomical view of the ulnar collateral ligament and bundles (3).

Structurally, the UCL is dynamic and undergoes hypertrophy with training and stress (3). The mean thickness of the UCL was 6.2 ± 1.6 mm in the throwing arm compared with 4.8 ± 1.3 mm on the non-throwing arm (3). In previous studies, it was shown that the UCL was only able to resist an ultimate load to failure of ~32 Nm of valgus stress (5). During the late cocking and early acceleration phases, the medial elbow experiences significant force, approximately ~64 Nm in professional pitchers, which causes impeccable stress on the UCL (3). As a result, the valgus stress placed on the UCL during the late cokcing phase of every pitch is suffcient to cause a tear in the ligament (3). Since the UCL does not rupture with each consecutive pitch thrown, it is shown that the cumulative microtrauma of repetitive near-failure tensile stress leads to UCL tears and failure (5).

Section III: Risk Factors

In an effort to reduce the amount of microtrauma accumulation that leads to the UCL failure, and subsequently Tommy John’s surgery, research has turned to identify risk factors in order to aid in prevention strategies. Notable risk factors discussed by clinicians thus far have focused on; pitch count, pitching mechanics, pitching velocity, and glenohumeral range of motion (2,5).

Pitch Count

The incidence of cumulative microtrauma, as a result of repetitive throwing, has been well documented as a risk factor for UCL damage and failure (3,5). The prevalence of baseball pitchers accumulating throws by way of warm-ups, innings pitched, and pitching for extended months of the year is a risk factor by way of overuse. Though there is limited information on pitch count contributing to increased risk of UCL in MLB pitchers, there are previous studies in youth and high-school populations linking high pitch counts to an increased risk of shoulder and elbow pain (5). In these studies, it was shown that the risk of elbow pain increases by 6% for every ten pitches thrown and increases 50% when greater than 75 pitches are thrown (5). Most findings for MLB pitchers demonstrating little significant association with single-game pitch count as a risk factor for UCL damage (5). However, investigation into the relationship between accumulation of repetitive throws from youth participation through professional pitching has yet to be conducted, and would provide an overview on the cumulative effects of microtrauma through the athlete’s complete career.

Pitching Mechanics

The kinetic chain of movement involved in the pitching motion is one that produces extreme force from the body through the upper extremity (6). This complex movement is one that places significant strain on the soft-tissue restraints of the medial elbow (5). The basic phases of the pitching motion have been clearly defined in literature, however, independent differences in one’s specific throwing motion are often unique to the athlete or their specific pitching coach (5).

Although many individuals follow the similar phases of motion outlined by the standard pitching motion, there has been investigation on specific mechanics that can increase the valgus stress placed on the elbow, thus increasing risk of UCL injury (5). Previously outlined in prior studies, pitchers displaying the deviations of increased maximum shoulder external rotation angle, increased elbow extension at peak valgus torque, late lateral trunk rotation, >10° contralateral trunk lean, and a sidearm delivery place further valgus stress at the elbow (5). The incidence of a pitcher displaying one or more of the deviations can greatly increase the valgus stress placed on the elbow, possibly leading to microtrauma or failure of the UCL (5).

Pitching Velocity

In previous studies, evidence was shown that high pitching velocity has been associated with increased stress and incidence of elbow injury in both professional and high-school pitchers (5). However, the incidence of increased medial elbow stress due to high-velocity throws was shown to have no association in collegiate baseball pitchers (5). These findings display the need for further investigation into the correlation between high velocity throws and its effects on valgus stress and UCL injury at all populations.

However, the pitching mechanic deviations prone to increasing valgus stress on the elbow share a relationship in producing throws with greater velocity (5). Research has suggested that pitchers with greater shoulder external rotation were classified as high-velocity pitchers, as compared low-velocity pitchers not displaying the deviation (5). The mechanism behind the relationship has been hypothesized as increased stored elastic energy of the internal rotators and an improved stretch-shortening cycle which increases the concentric strength of the internal rotators during the acceleration phase of pitching as a result of the increased shoulder external rotation (5). Additionally, the mechanics deviation of an increased contralateral trunk tilt has also been associated with increased pitching velocity (5). The authors of the study cite the contralateral trunk tilt indirectly produces greater maximum shoulder external rotation, leading to increased pitching velocity (5).

Glenohumeral Range of Motion

Previous research has found a link to adaptations in glenohumeral range of motion and an increased incidence of UCL failure. The evidence found comes as a result of Glenohumeral internal rotation deficit (GIRD), which involves the loss of internal rotation range of motion in the throwing shoulder as a result of repetitive throwing (2,4,5). In one study, it was found that in baseball players with UCL microtrauma and injury had 28.5° of GIRD, while healthy controls had 12.7° of GIRD (2,5). These results suggested that the incidence of GIRD causes in increase in valgus instability, creating a physiological link between the shoulder and elbow in the throwing motion (2,5). Other cross-sectional studies investigating this link have found that baseball players with injuries to their UCL displayed a 7° difference in total rotational range of motion, with uninjured players displaying only a 1° difference between throwing and non-throwing arms (5). Additionally, previous research has also observed a 2.6 times greater risk of UCL injury in pitchers displaying a total glenohumeral range of motion deficit of 5° or more (2,5). As a result of GIRD’s effects on the biomechanics and kinetics during the throwing motion, in conjunction with repetitive throwing, it can be seen that the disorder causes increased valgus stress on the UCL and it’s stabilizing structures, potentially leading to greater injury risk (2,5).

Section V: Conclusions

In reviewing the notable literature on the biomechanics behind the throwing motion, the anatomical and biomechanical effects of the throwing motion on the UCL, and the risk factors associated with injury, it can be seen that challenges exist in optimal management of the UCL in pitchers at all levels. As pitching velocities continue to increase in today’s MLB, further understanding into the tremendous stress placed on the elbow and the mechanisms at work during pitch delivery should prioritized and monitored to properly manage the incidence of UCL microtrauma and injury.

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