The Role of the Flexor-Pronator Mass in Protecting the UCL

It’s no secret that elbow injuries are a big problem in baseball. One study conducted in 2017 found that 70% of 17-year-old baseball players reported a history of elbow pain (7). Of the elbow injuries reported in baseball, ulnar collateral ligament (UCL) injuries are one of the most common pathologies (7). A survey conducted by Stan Conte in 2015 showed that 25% of MLB pitchers had undergone UCL reconstruction surgery (2). With a recovery time of 12-15 months, these are devastating injuries, not only to the player involved, but also the player’s team. For these reasons, it is necessary to explore different ways to protect the UCL is warranted. There are lots of factors at play when it comes to preventing an elbow injury while performing the fastest motion in all of sports; throwing a baseball. Some of these factors include pitching mechanics, pitch count, shoulder mobility, and even hip mobility (click here). Today we will focus on just one; the role of the flexor-pronator mass in UCL protection.

The Role of the UCL in the Pitching Elbow

Because of the kinetics at play during the pitching delivery, pitchers tend to subject themselves to the greatest risk of an elbow injury in baseball. The pitching motion consists of six different phases; the wind-up, stride, arm cocking, arm acceleration, arm deceleration and follow through (8). Maximal valgus force is applied across the elbow during the cocking and acceleration phases. The cocking phase begins as the pitcher’s lead leg contacts the ground and ends when the pitcher achieves maximal external rotation at the shoulder. The acceleration phase begins at maximal external rotation and ends at ball release.

During these phases of the pitching motion, the elbow is flexed between 80 and 120 degrees (3). This is important because the UCL plays a critical role in resisting valgus stress at the elbow in these same ranges. One study showed that when the UCL was sectioned off, the elbow was unstable between 20 and 140 degrees of elbow flexion (3). Another study was able to confirm these findings, showing that release of the UCL caused about 6 degrees of valgus instability at 30 degrees of elbow flexion and about 5 degrees of valgus instability at 90 degrees of elbow flexion (7). These studies were able to conclude that the anterior bundle of the UCL was the primary stabilizer of the elbow against valgus forces in these ranges of elbow flexion.

Repetitive valgus forces provide a traction force to the medial structures of the elbow. Over time, these forces can lead to microtrauma and failure of the UCL that prevent a baseball pitcher from being able to throw (2,6). When the pitching elbow’s UCL is ruptured, the anterior bundle is the part of the ligament that is damaged, while the posterior bundle tends to remain intact (1). This once again confirms the importance of the anterior bundle of the UCL in resisting the valgus forces experienced while pitching.

Pitching Elbow Kinetics

Overuse injuries, such as UCL tears, are believed to be caused by large forces and torques at the elbow and shoulder joints during the pitching delivery (4). These forces and torques are what we refer to as kinetics. During the pitching delivery, kinetics at the pitching elbow are greatest near the end of the arm cocking phase, as the shoulder approaches 165+ degrees of external rotation (4).

In this position, a valgus force is applied to the medial elbow that must be resisted in order to prevent damage to these structures. This is where things get interesting. As we discussed earlier, the UCL plays a critical role in resisting valgus stress at the medial elbow, but the UCL isn’t strong enough to resist these forces alone. Towards the end of the cocking phase, it is estimated that a varus torque between 64 and 120 Nm is needed to resist the valgus forces present (4,8). Several studies, however, have shown that the UCL fails around 34 Nm (1,4,8). This means that other structures are needed to prevent valgus stress as well. The flexor-pronator mass is one of the structures that may play an important role in providing varus torque to create stability at the elbow during the pitching motion.

The Flexor-Pronator Mass

The flexor-pronator mass is made up of four different muscles: the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and flexor carpi ulnaris. These four muscles are able to provide stability at the elbow “by means of direct muscle action with vectors that are optimally positioned to resist valgus” (6). In fact, EMG activity for all four muscles of the flexor-pronator mass have been shown to increase as the elbow progresses from the cocking phase to the acceleration phase (1). While these four muscles may all play a role in providing varus torque, there are two muscles that have a particularly important role. The flexor carpi ulnaris and flexor digitorum superficialis have been identified as the muscles optimally positioned to provide secondary medial elbow support while throwing (3,6).

The flexor carpi ulnaris is positioned the most posterior out of the four muscles of the flexor-pronator mass. This positioning may place it in the most functionally advantageous position for baseball pitchers. In fact, during the phase of the pitching cycle when valgus loads at the elbow are greatest, the flexor carpi ulnaris is positioned directly over the anterior aspect of the UCL (3). One study performed by Park et al. was able to help confirm this hypothesis in a study they conducted in 2004. In this particular study, they were able to demonstrate that contraction of the flexor carpi ulnaris at both 30 and 90 degrees of elbow flexion provided the greatest amount of stability in the UCL-insufficient elbow (6). For these reasons, the flexor carpi ulnaris is often times referred to as the primary dynamic stabilizer of the medial elbow (the UCL would be considered a static stabilizer).

The flexor digitorum superficialis is considered the secondary dynamic stabilizer of the medial elbow during the pitching motion (6). Interestingly, the flexor digitorum superficialis actually originates from the anterior aspect of the UCL and lies either slightly anterior to the UCL or over 1-2 mm of the ligament during the phase of the pitching cycle when valgus forces are greatest (3). Due to its size and force potential, the flexor digitorum superficialis is an important muscle to consider as well when attempting to protect the medial pitching elbow.


As we know, the UCL plays an important role in resisting valgus forces at the elbow during the pitching delivery. Unfortunately, the UCL isn’t capable of resisting these forces by itself. Luckily, there are other structures that we can influence to help stabilize the elbow joint and prevent missed playing time. The flexor-pronator mass, specifically the flexor carpi ulnaris and flexor digitorum superficialis, is one of those structures that warrant our attention. These muscles have the potential to act as dynamic stabilizers to assist with dissipating the forces and torques that are experienced during the pitching delivery.

A study published by Saito et al. in 2021 does a great job of summarizing the topics we have discussed so far (7). In their study, 26 collegiate baseball players performed 7 sets of 15 pitches for a total of 105 pitches. The pitcher was allowed 15 minutes of rest between sets to simulate a game-like situation. Between each set, the pitcher’s medial elbow joint space and pronator muscle elasticity, or stiffness, was measured. Below are the results:

  • Medial elbow joint space significantly increased after 60 pitches

  • Flexor digitorum superficialis elasticity significantly increased after 45 pitches

  • Flexor carpi ulnaris elasticity significantly increased after 60 pitches

The enlargement in medial elbow joint space that was identified after 60 pitches may be a risk factor for UCL injury (7). Elasticity of the flexor-pronator mass, however, has the potential to influence joint flexibility and range-of-motion. Although the elasticity of the flexor digitorum superficialis had no correlation with the enlargement in medial elbow joint space, the elasticity of the flexor carpi ulnaris did. The flexor carpi ulnaris elasticity had a negative correlation with medial joint space. In other words, this marked increase in elasticity of the flexor carpi ulnaris after an increased number of pitches may actually help to reduce the flexibility of the elbow joint and suppress an increase in medial elbow joint space! This is one more example of the significance of the flexor-pronator mass for the pitching elbow, and its significance likely increases as pitch counts and elbow laxity increases.

When considering injury prevention, surgical techniques, and rehabilitation for overhead throwing athletes, we must pay special attention to optimizing the function the flexor carpi ulnaris and flexor digitorum superficialis. Therapy and training directed specifically at these muscles has the potential to provide valgus support to the elbow and reduce the risk of injury in this population. One of my favorite ways to strengthen the flexor-pronator mass is by performing resisted forearm pronation/supination. I will typically have the athlete use a long, weighted object, such as a baseball bat, to perform this exercise. The long lever of a baseball bat helps to slow the athlete down as they alternate between pronation and supination. By slowing the athlete down, we are able to increase time under tension during the eccentric and concentric phases, which can help to improve tissue quality and tendon health in addition to the strength benefits seen with resistance training.


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7. Saito A, Namiki Y, Okada K. Elasticity of the Flexor Carpi Ulnaris Muscle After an Increased Number of Pitches Correlates With Increased Medial Elbow Joint Space Suppression. J Strength Cond Res. 2021 Jul 13.

8. Werner SL, Fleisig GS, Dillman CJ, Andrews JR. Biomechanics of the elbow during baseball pitching. J Orthop Sports Phys Ther. 1993 Jun;17(6):274-8.