Biomechanics: Ulnar Collateral Ligament

[Last edited on Tuesday, January 14, 2009 at 2:47 AM; the "In the Delivery" section is being re-written from the ground up. It is not very clear, nor is it entirely accurate at this time. The re-write will fix this; however, I do not know when I will have it ready.]

Throwing a baseball is not nearly as simple as it seems at first glance. Different parts of the arm and body accelerate and decelerate at different times and different speeds. Analysis of the order and timing of these movements can help evaluate a pitcher's injury risk and isolate areas of inefficiency. When it comes to preventing injury, two areas are focused on above all others - the elbow and the shoulder.

Initial pitch velocity is created by the legs and core. As the arm accelerates, muscles of the upper body and arm add to this, sending a large amount of energy through the elbow. An inefficient transfer of this energy can lead to injury in soft tissue and bony tissue alike.

The most well known of these injuries is a tear of the elbow's ulnar collateral ligament (UCL).


3 bands of the ulnar collateral ligament: anterior (red), posterior (blue), transverse (yellow).

3 bands of the ulnar collateral ligament: anterior (red), posterior (blue), transverse (yellow).

Significant tearing of the UCL is often the result of an accumulation of micro-tears caused by repetitive near-limit stress, but it can also result from a single catastrophic event - almost never one "bad" pitch.

Valgus force, which is a prime component of valgus extension overload syndrome, is responsible for the tensile stress that causes these tears. In the elbow, valgus force is measured by its rotational translation valgus torque. This force "pushes" the forearm and hand laterally.

[Note: Dr. Mike Marshall believes that valgus extension overload syndrome does not exist. I tend to believe that it does exist but that it is potentially irrelevant with regard to pitching. There is more to come on this topic.]

The UCL connects the medial epicondyle of the humerus to the coronoid process and olecranon of the ulna, and valgus torque increases the separation between these bony structures. Increased tensile stress on the soft medial tissues of the elbow is the result of these bony structures being pulled apart. This tensile stress, caused by valgus force, is appropriately called valgus stress.

The flexor-pronator mass also anchors the forearm to the medial epicondyle. This collection of muscles is comprised of the pronator teres and the forearm flexors. Contraction of the pronator teres causes pronation - medial rotation of the forearm. Contraction of the forearm flexors creates wrist and finger flexion.

When these muscles contract, varus torque is generated. The varus torque contribution from these contractions varies from pitcher to pitcher and even from arm to arm based on the strength of the muscle group and the magnitude of the contractions. A strong enough contraction will hold the ulna firmly against the humerus effectively reducing valgus stress in the UCL.


Between 30° and 120° of elbow flexion, the UCL is the primary stabilizer against valgus force.[1] Outside of this range, bony structures and other soft tissues take larger roles in stabilization.

Morrey and An showed that, at 90° of flexion, up to 55% of the stabilizing force is contributed by the UCL.[5]

A study by Fleisig, Andrews et al. estimated the average peak valgus torque to be 64 Nm (Newton-meters), with a normal range between 52 and 76 Nm.[4] Building from the Morrey and An study, 55% of this is roughly 35 Nm, with a normal range between 28 and 42 Nm in UCL stress.

This number is in line with the study by Dillman et al. that showed an average failure load of 32 Nm.[3] This indicates that pitching frequently results in stress that is far in excess of the observed failure load of the UCL.

These widely cited studies, though, were performed on cadavers and do not measure the varus torque contribution of contracted flexor-pronator muscles. If a pitcher's arm actually relied primarily on the UCL for valgus stabilization, UCL tears would be routine, expected, and basically unavoidable.

In another cadaveric study, Park and Ahmad measured the valgus correction of individual muscles in the flexor-pronator mass. The muscles were electrically stimulated, and their valgus correction angles were measured. Park and Ahmad concluded that the flexor carpi ulnaris and the flexor digitorum superficialis were the primary and secondary stabilizers, respectively.

Somewhat surprisingly, their tests also showed the pronator teres to provide the least dynamic stability.[6] This may or may not be true for throwing athletes since they typically have stronger pronator teres muscles than an 'average' cadaver.

Arm wrestling is an example of a non-pitching activity that creates extreme valgus force. Arms are typically positioned at approximately 90° of elbow flexion, and the force is applied in far greater amounts over much longer periods of time than in pitching. Arm wrestlers have immensely strong forearm flexors which prevent most of this force from stressing the UCL. Arm wrestling seems to result in far more injuries to bony structures (avulsion fractures of the medial epicondyle) than to soft tissues (UCL tears), but I found no studies that provide evidence of this.


Valgus force is created by the inertia that opposes medial acceleration of the forearm which occurs after external rotation, during internal rotation, and to a lesser extent during horizontal shoulder adduction.

Physics tells us that valgus torque is greatest when the elbow is flexed to 90° (explanation in the following section). In the Morrey and An study, it was shown that the UCL handles its largest relative valgus load when the elbow is flexed to 90°. Combined, these two facts illustrate that valgus stress in the UCL is greatest when the elbow is flexed to 90°.

Rangers prospect Tommy Hunter at approximately 90° of elbow flexion (Photo Source: Scott Lucas)

Rangers prospect Tommy Hunter at approximately 90° of elbow flexion (Photo Source: Scott Lucas)

In the 'traditional' delivery, the pitching elbow is usually flexed near 90° from the late cocking phase through the early part of the acceleration phase. In fact, in a 1992 study by Conway, Jobe et al., 85% of their subjects experienced their greatest pain during the acceleration phase.[2]

The early part of the 'traditional' cocking phase involves active external rotation of the upper arm. When the active phase ends, external rotation continues in an inertial phase. The valgus torque of the inertial phase is only loosely opposed by the flexor-pronator mass because its muscles do not fully contract until the arm starts to accelerate the ball. Wrist extension during this phase increases valgus torque and prevents the forearm flexors from contracting effectively.

The inertial phase of 'traditional' cocking turns the forearm over. Because the forearm turns over after the body has begun to accelerate the arm, Dr. Mike Marshall calls this "late forearm turnover."

This inertia is increased by delayed forearm turnover - when "late forearm turnover" becomes "later" - which causes more forceful active external rotation and creates an opposition of forces with the elbow starting to move forward and the forearm trying to lay back.

Pitchers who pick up the ball with their elbows - hand below the elbow, as in the Inverted W, Inverted V, and Inverted L positions - are great examples of delayed forearm turnover. At foot plant, when the body begins to really drive the shoulder, these pitchers are still picking up the ball.

By delaying pick up, a pitcher must externally rotate his arm more aggressively because he is giving himself less time in which to properly position his arm for the throw. This delay also guarantees that the upper arm is already being accelerated by the body as the forearm lays back.

Right before the forearm muscles contract for acceleration, the upper arm begins to accelerate relative to the torso. In many cases, but not all, the pectoralis major contracts to start this acceleration. When it contracts, it horizontally adducts the humerus and causes incidental internal rotation. This internal rotation exaggerates the inertial effect of external rotation on the UCL in the instant right before the flexor-pronator mass contracts.

This event causes a "bounce" in the forearm that is often called "reverse forearm bounce" - another term coined by Dr. Mike Marshall. When the pectoralis major does not contract, internal rotation is delayed and "reverse forearm bounce" does not occur.


To a large extent, pitchers reduce UCL tension with a varus torque contribution from the flexor-pronator mass. Stronger contractions of this muscle group result in larger varus torque contributions. As the varus torque contribution of this muscle group increases, valgus stress in the UCL decreases.

Additionally, a strong pronator teres can powerfully pronate the forearm through pitch release. Doing so will help prevent hyper-extension and, as discussed above, reduce valgus stress. Powerful pronation also has positive performance implications for pitch velocity and ball rotation (pitch spin).

My conclusion: functional strengthening and conditioning of the flexor-pronator mass is beneficial to the health of the ulnar collateral ligament.

The best way to reduce UCL injury risk, though, is to minimize the valgus torque caused by external rotation and internal rotation of the upper arm. A little bit of physics will help explain how to do this.

According to mechanical physics, torque (t) is equal to force (F) multiplied by the distance (r) from the axis of rotation at which the force is applied, t = F*r. To reduce torque, F or r must be reduced. Since the mass being accelerated is constant (forearm, hand, and ball), the only way to reduce F is to decrease acceleration. This is clearly counterproductive to achieving maximum pitch velocity.

To decrease torque, then, r must be lowered. In other words, the mass being accelerated must be moved closer to the axis of rotation. In this case, the axis is a line extending through the length of the humerus.

With the elbow flexed to 90°, the mass can be moved no further from the axis, so r is greatest with the elbow flexed to 90°. With the arm fully extended, the mass can be moved no closer to the axis, so r is least with the arm fully extended. It then follows that the torque experienced during external rotation and internal rotation is greatest with the elbow flexed at 90° and is least with the arm fully extended. Plainly put, the valgus torque caused by external rotation or internal rotation is at a minimum when the arm is at full extension.

Additionally, at less than 20° of elbow flexion, the bony structures of the elbow provide primary stabilization1, dramatically reducing the relative stress in the UCL.

My conclusion: extending the arm as much as possible prior to external rotation and prior to internal rotation is beneficial to the health of the ulnar collateral ligament.

This conclusion implies early external rotation ("early forearm turnover") and delayed internal rotation, with elbow flexion and extension occurring in between. With the arm at full extension, the timing of the forearm turnover is less important, but it should still occur before the acceleration phase instead of in the middle of it.

Dr. Marshall's pitchers use a pendulum swing to cock their arms. This is a full extension arm action that externally rotates the upper arm, engages the muscles of the flexor-pronator mass, and turns the forearm over early.  When executed properly, reverse forearm bounce does not occur.

This has obvious implications for overhand curve balls - since most pitchers are taught to throw the pitch with a flexed elbow - and less obvious implications for pitch velocity.

Kevin Millwood extending prior to internal rotation (AP Photo - Dave Pellerin)

Kevin Millwood extending prior to internal rotation (AP Photo - Dave Pellerin)

Brandon Webb extending prior to internal rotation (AP Photo)

Brandon Webb extending prior to internal rotation (AP Photo)

Greg Maddux extending prior to internal rotation (AP Photo)

Greg Maddux extending prior to internal rotation (AP Photo)


Internal rotation and the inertia that follows active external rotation both create valgus torque in the elbow. These two actions create the most torque when they are in greatest opposition to each other at the end of the cocking phase and beginning of the acceleration phase. When the elbow is flexed, this torque creates near-limit stress in the ulnar collateral ligament which leads to structural damage. To limit this stress and ensure UCL health, I draw two conclusions: (1) improve functional strength and conditioning of the flexor-pronator mass and (2) extend the arm as much as possible prior to external rotation and prior to internal rotation.


  1. Cain EL Jr, Duga JR, Wolf RS, Andrews JR. Elbow injuries in throwing athletes: a current concepts review. Am J Sports Med. 2003; 31(4):621-635.
  2. Conway JE, Jobe FW, Glousman RE, et al. Medial instability of the elbow in throwing athletes. Treatment by repair or reconstruction of the ulnar collateral ligament. J Bone Joint Surg. 1992; 74A:67-83.
  3. Dillman C, Smutz P, Werner S, et al. Valgus extension overload in baseball pitching [abstract]. Med Sci Sports Exer. 1991; 23:S135.
  4. Fleisig GS, Andrews JR, Dillman CJ, Escamilla RF. Kinetics of baseball pitching with implications about injury mechanisms. Am J Sports Med. 1995; 23:233-9.
  5. Morrey BF, An KN. Articular and ligamentous contributions to the stability of the elbow joint. Am J Sports Med. 1983; 11:315-9.
  6. Park MC, Ahmad CS. Dynamic contributions of the flexor-pronator mass to elbow valgus stability. J Bone Joint Surg Am. 2004; 86-A(10):2268-74.