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Biomechanics of Throwing and Shoulder Injury in Baseball Pitchers

MLB pitchers strive relentlessly for the prestigious achievement of a 100 mph pitch. Once deemed a mere luxury, it has evolved into an indispensable element of the game. Nearly every MLB team now boasts a formidable fireballer, capable of captivating the crowd with this extraordinary feat. While home runs once held spectators spellbound, their focus now gravitates towards the stadium’s radar gun, eagerly awaiting the electrifying instant when a 100-mph pitch streaks across the plate—a genuine baseball grand slam that all enthusiasts long to behold.

The toll of rigorous physical demands in baseball is apparent, with a notable percentage of competitive players, ranging from 40% to 60%, encountering shoulder pain at some stage. What raises further concern is that one in every five injured athletes opts to persevere and continue playing despite the discomfort. It is not uncommon for a significant portion of these athletes to undergo shoulder surgery, and in some cases, even multiple surgeries to address the problem.

Overhead athletes face considerable risk to shoulder injuries. Terminologies such as “impingement,” “bursitis,” “tendinitis,” “labral tear,” “SLAP tear,” “hyperlaxity,” “instability,” and “rotator cuff tear” have permeated households due to the prevalence of these diagnoses. The repetitive motion of the arm involved in pitching and throwing puts immense strain on the rotator cuff muscles and tendons.

The likelihood of shoulder injury is directly proportional to the number of throws and the velocity at which they are thrown. This combination of throw volume and intensity determines the point at which the shoulder may fail. For example, a player who typically throws at 80 mph but suddenly delivers a line drive at 90 mph may be at risk for an injury due to the increased intensity of that particular throw. Likewise, a pitcher who comfortably maintains an average velocity of 90 mph over 80 pitches but then increases to 100 pitches may also suffer an injury due to the increased volume of throws.

The speed at which the arm can move is directly linked to the stability of the shoulder joint. The shoulder joint acts as a solid platform from which the arm can accelerate, much like a runner pushing off from the starting block at the beginning of a 100-yard dash. The greater the stability of the shoulder joint, the faster the acceleration can be achieved. Rotator cuff engagement plays a critical role in maintaining shoulder stability. Similar to a race car, where the engine initiates the process but the performance relies on a set of high-quality tires, the rotator cuff serves as the “tire” to the race car that is the shoulder. It provides the necessary support and stability for optimal arm velocity and performance.

To ensure shoulder stability, all four muscles must contract synergistically and in a balanced manner, exerting the right amount of tension from each muscle. This coordinated effort keeps the humeral head centered and compressed against the glenoid keeping the shoulder in the “socket” during active shoulder motion. This effect is referred to as “glenohumeral joint compression” or “centration.”

If there is an imbalance or weakness in one of the rotator cuff tendons, it can lead to asymmetric compression, causing a subtle wobble of the shoulder. This condition is known as “microsubluxation” of the shoulder. When the shoulder undergoes repetitive motions such as pitching and throwing, this imbalance can increase the risk of peripheral tearing of the labrum, shoulder impingement, bursitis, tendinitis, and tendon tears.

Biomechanics of Pitching

The throwing motion in baseball involves several distinct phases that rely on specific biomechanical movements.

Understanding these shoulder throwing biomechanics provides insight into the tremendous forces and complexities involved in delivering high-speed pitches and the importance of proper rotator cuff function for injury prevention and optimal performance

When a pitcher throws with a velocity exceeding 90 mph, these complex movements are executed in a fraction of a second. In the acceleration phase, elite pitchers can achieve remarkable shoulder rotational angular velocities, reaching up to 7,600 degrees per second. This means that their arm can spin more than 20 times in a single second or approximately 1,200 revolutions per minute. To put this into perspective, it is akin to the spin cycle speed of a front-loading washing machine, which is considerably faster than the typical top-loading washing machine. The incredible speed and rotational forces generated during these movements highlight the extraordinary abilities and physical demands placed on the shoulder joint during high-velocity throwing in baseball.

Shoulder Injury

During the initial phase of the throwing sequence, when the arm is moving backward, the leverage of the entire arm tends to push the humeral head forward, causing it to slip off the front edge of the glenoid. To prevent this anterior displacement of the humeral head, the subscapularis muscle, located at the front of the shoulder, undergoes a strong contraction while being stretched by the backward movement of the arm (referred to as an “eccentric” contraction). This forceful contraction of the subscapularis acts as a protective wall, compressing the humeral head and maintaining its central position. It counteracts the forward pressure exerted by the leverage effect of the arm moving backward. Simultaneously, the three posterior rotator cuff muscles work synergistically, contracting in a balanced manner, to ensure that the humeral head does not excessively move either toward the front or the back of the glenoid.

Before the onset of shoulder acceleration during the late cocking phase, it is crucial to establish absolute shoulder stability. The success of this phase, as well as the overall performance and velocity of the arm, relies heavily on the rotator cuff muscles’ ability to maintain a stable glenohumeral joint that is both centered and compressed. Similar to how a starting block provides a stable platform for foot races or swim races, a stable shoulder joint serves as the foundation for effective acceleration. To prevent shoulder injuries, it is essential to have a robust rotator cuff that can withstand the powerful forces exerted during the deceleration phase, effectively keeping the shoulder joint compressed against forces exceeding 100 lbs within a fraction of a second. Maintaining this stability and engagement is a critical component of shoulder injury prevention in high-velocity throwing activities.

During the forward acceleration phase of the throw, a different dynamic comes into play. As the arm moves forward diagonally across the body, a leverage force is generated that initially pushes the humeral head posteriorly, followed by a subsequent forward shear force as the entire arm is propelled forward. The rotator cuff muscles surrounding the shoulder joint must rapidly adapt to these momentary changes in direction. If the rotator cuff muscles are weak or imbalanced, the result is an abnormal wobbling of the humeral head with each overhead movement. This microsubluxation causes asymmetrical loading stress on the humeral head against the rim of the glenoid. Over time, this can lead to shoulder pain, labral tears, impingement tendinitis, bursitis, fraying of tendons, and even tendon tears in overhead athletes.

The role of the rotator cuff in stabilizing and engaging the shoulder is of utmost importance, particularly during high-velocity throws and the overall quantity of throws. While elite baseball players typically possess strong arms capable of generating substantial throw speeds, the key factor that influences the risk of injury is the cumulative volume of throws they perform.

In order to achieve maximum velocity, the shoulder must exhibit an exceptional range of motion, allowing for the execution of movements through an extensive arc. However, this extensive range of motion comes with the inherent challenge of shoulder instability and the potential for subluxation, or partial dislocation.

A robust and dynamically engaging rotator cuff plays a vital role in enhancing shoulder performance, arm velocity, and safeguarding against shoulder injuries.


SLAP tears pose a distinctive challenge primarily observed in athletes involved in throwing sports. The acronym “SLAP” represents Superior Labrum Anterior and Posterior tear, indicating the detachment of the upper portion of the fibrocartilaginous rim from the shoulder socket (glenoid) where the biceps tendon attaches. Several factors contribute to the occurrence of SLAP tears, including 1) direct falls onto an outstretched arm, 2) repetitive overhead activities without adequate shoulder stability, or 3) biceps muscle contractions exerting tension on the unstable shoulder.

Athletes participating in overhead sports activities are vulnerable to SLAP tears due to the repetitive and forceful nature of their movements. The risk of developing SLAP tears escalates with the intensity and frequency of motion.

Among professional baseball players, only around one-third can fully recover and regain their previous level of performance after undergoing SLAP tear repair. However, when examining different player groups, positional players exhibit a higher success rate compared to pitchers. Approximately 80% of positional players can successfully resume their sport, whereas fewer than one in six pitchers are able to do so. This disparity highlights the tremendous challenges faced by pitchers due to the demanding intensity and volume of their throwing activities.

A weakened rotator cuff is the most significant predictor of both shoulder pain and SLAP tears. The rotator cuff muscles play a critical role as eccentric decelerators in maintaining proper “centration” of the shoulder joint. During shoulder movement, especially at extreme ranges such as fully cocking the arm backward or stretching it forward at the end of a throw, the shoulder joint becomes particularly susceptible to instability. The centration effect provided by the rotator cuff muscles becomes crucial in preserving the alignment of the shoulder joint, preventing slippage, dislocation, shoulder pain, and SLAP tears.

To effectively prevent shoulder injuries in baseball pitchers, it is crucial to prioritize the care and attention given to the rotator cuff.