Biomechanics: Ulnar Collateral Ligament

Trip Somers • December 18, 2008 • Research Review

[Updated January 2020 to reflect evolution of thought in the 11+ years since the post was originally published.]

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 theoretically help estimate a pitcher's injury risk and isolate areas of inefficiency.

In the throwing motion, the rotation of the shoulders and the drive forward allow the arm to exceed forces that it can generate on its own. Arm action translates these forces into a throw and then must powerfully slam on the brakes in fractions of a second.

In part because the arm is tasked with handling forces beyond what it alone can generate -- engineers might call this a design flaw -- a number of injuries can result from repetitive throwing. 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 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, stressing the medial stabilizers of the elbow joint.

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 medial soft tissues in 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, wrist, and hand to the medial epicondyle. This collection of muscles is comprised of the pronator teres and the wrist and finger flexors. Contraction of the pronator teres causes pronation -- medial rotation of the forearm and wrist. Contraction of the wrist and finger flexors creates wrist and finger flexion, respectively (and hopefully obviously).

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, the magnitude of the contractions, and the pitcher's arm action. A strong enough contraction will hold the ulna firmly against the humerus effectively minimizing (potentially even eliminating!) 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, however, 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 for every single pitch!

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 presumably 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 magnitude over a much longer period of time than in pitching. Arm wrestlers have immensely strong forearm flexors which prevent most of this force from stressing the UCL.

Based on a cursory review of media regarding arm wrestling injuries, 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 study that provides concrete evidence.

Those previous two paragraphs are a long-winded way of saying that it can't be valgus torque alone that leads to UCL injuries.


Valgus force can be created a number of ways in the delivery including but not limited to:

The equation for calculating torque (t = F*r) tells us that, assuming equal force (F), valgus torque is greatest when the elbow is flexed to 90° because that maximizes the distance (r) from the axis of rotation (humerus). Additionally, the Morrey and An study discussed above also showed that the UCL handles its largest relative valgus load 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)

This paints a very alarming picture of 90° of elbow flexion, but would you be surprised to learn that, due to the ridiculously large number of factors involved, there's nothing magical about it? One arm action might hit its peak valgus torque at 75° of flexion. Another might hit its peak at 100° of flexion. And those peaks might not even represent the greatest load on the UCL during the throw!

With 90° representing the position of theoretical maximum valgus torque, however, it is helpful to look at the parts of the delivery that generally involve that level of elbow flexion.

In many different styles of delivery, the pitching elbow is commonly flexed near 90° starting late in the cocking phase and into the early part of 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

Generally speaking, the late cocking phase is when active external rotation has ended and passive (inertial) external rotation sets the arm in the fully cocked position at or near maximum external rotation. At this point in a typical delivery, the muscles that cross the elbow are generally not doing much to stabilize the elbow, leaving the UCL to deal with a significant portion of the valgus torque generated by the inertia of passive external rotation that turns the forearm over, laying it back behind the elbow.

When it happens after the pitching shoulder starts moving forward, this is called "late forearm turnover". Dr. Mike Marshall originally coined the term "late pitching forearm turnover", and this action occurs in nearly every non-Marshall-approved arm action.

The later in the delivery that active external rotation begins, the later and more violently the late cocking phase will start, leaving less time to get the arm into position for the acceleration phase.

If it occurs late enough -- usually with intent via inverted arm positions -- there's a visible violence to it that can resemble a bounce. This is called "reverse forearm bounce", and it's another term from Dr. Marshall, originally "reverse pitching forearm bounce".

Such a delay also practically guarantees that the upper arm is already being accelerated by the body as the forearm lays back. If the forearm is "outside" of the flexed elbow -- in other words, if the elbow is flexed between 30° and 90° -- when this happens, there is an additional distraction at the elbow due to the centripetal force of the upper arm moving through space.


To a large extent, pitchers reduce UCL tension with a varus torque contribution from the flexor-pronator mass. Though it isn't entirely clear which of the muscles of the flexor-pronator mass provide the best support, it should be safe to conclude that stronger contractions of this muscle group will result in larger varus torque contributions. As the varus torque contribution of this muscle group increases, valgus stress in the UCL decreases.

Conclusion #1: 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 create as little valgus torque as possible. Going back to the equation for torque -- torque (t) = force (F) * radius (r) -- there are two variables that we can manipulate to effect torque.

Since force is equal to mass (m) times acceleration (a) and mass will be constant since we're talking about a regulation baseball and not planning to chop off any of our fingers, the only way to manipulate force is to reduce acceleration. This is counterintuitive since we're trying to throw the ball pretty hard. Stay with me.

Our other variable is the radius. When calculating torque, the radius is the distance of the force variable from the axis of rotation. In this case, the axis of rotation is the humerus.

With the elbow flexed near 90°, the mass of the forearm, hand, and ball cannot be moved any further from the axis of rotation, so the radius's greatest possible value is when the elbow is flexed near 90°. With the elbow fully extended, the mass of the forearm, hand, and ball cannot be moved any closer to the axis of rotation, so the radius's smallest possible value is when the elbow is fully extended.

Near to full extension gets bonus UCL protection as the joint orientation alters the relative contributions of the joint stabilizers. At less than 20° of elbow flexion, the bony structures in the elbow provide primary stabilization1, dramatically reducing the relative stress in the UCL.

Original Conclusion #2: 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.

While this conclusion makes sense from a logical standpoint, at the very least, it is incomplete, overly simple, and too idealistic.

First of all, it fails to consider transitional improvements such as moving to 60° from 90° instead of all the way to 0°, but more importantly, it fails to consider two crucial factors: (1) a reduction in radius could also be achieved by increasing elbow flexion to 120° from 90°, and (2) when the elbow is moving in an arc around the body, an elbow extended to less than 90° of flexion can result in additional valgus torque independent of that created by internal and external rotation as well as a centripetal joint distraction force.

To understand Factor #2, think about the torque equation but realize that the axis of rotation in this case is the thoracic spine. (This is somewhat simplistic since we're focusing on torque at the elbow and not torque at the spine, but it will do for now.) With that in mind, it can be shown that the increased elbow flexion that reduces valgus torque in Factor #1 also reduces the valgus torque described in Factor #2 because it moves the forearm, hand, and ball closer to the spine.

With that, two new conclusions have replaced the original second conclusion.

New Conclusion #2: avoiding 90° of elbow flexion during external and internal rotation is beneficial to the health of the ulnar collateral ligament.

New Conclusion #3: flexing the elbow beyond 90° while the elbow is moving in an arc around the body is beneficial to the health of the ulnar collateral ligament.

Now, if you're trying to get your elbow flexed "inside" of 90° for thoracic rotation, it doesn't make particularly obvious sense to be "outside" of 90° during external rotation. You may not be surprised to find out that Dr. Marshall developed a "pendulum swing" for nearly this precise purpose.

Dr. Marshall's pendulum swing 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 to prevent both late formearm turnover and reverse forearm bounce. He instructs pitchers to pendulum swing their arms to driveline height -- "slightly above the top of the head" -- and allow the forearm to trail behind the elbow. [NOTE: Dr. Marshall's delivery aims to eliminate the arc of the elbow in space during thoracic rotation, so he makes no specific recommendation for elbow flexion with respect to that movement.]

A simpler and more compact arm action that satisfies both new conclusions is the elbow spiral -- described here on the Driveline Baseball blog. The elbow spiral action picks up and turns over the forearm with the elbow flexed "inside" of 90°, setting the pitcher up to fairly easily stay "inside" of 90° during thoracic rotation.

The last implication from these new conclusions is a concept of delayed internal rotation which proposes that internal rotation is not initiated until after elbow extension. This concept may or may not hold water, but here are three photos that appear to indicate the sequence occurs in some deliveries.

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)



Valgus torque in the pitching elbow is of great concern when it comes understanding ulnar collateral ligament injuries. External and internal rotation when the elbow is flexed near 90° has the potential to create near-limit stress in the UCL and lead to structural damage. To limit this stress and help protect the UCL from injury, I draw the following conclusions:

  1. Improve functional strength and conditioning of the flexor-pronator mass.
  2. Avoid elbow flexion near 90° during external and internal rotation of the upper arm.
  3. Avoid elbow flexion "outside" of (less than) 90° during thoracic 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.