Forearm Muscle Strength and Inner Elbow Injury in Baseball: Is the Solution That Simple?

Well, no. Injuries are multifactorial and it's impossible to reduce the solution to the injury epidemic to a single solution. But more strengthening of the flexor carpi ulnaris would likely help!

The game of baseball submits the body to significant force. Breaking news, I know. Swinging, throwing, sliding into home, and crashing into the outfield wall strain the body’s soft (muscles, ligaments, tendons, cartilage) and hard (bone) tissues. Over time, these tissues adapt and get stronger or break down. This is the heart of MLB’s pitcher injury epidemic.

I’ve written elsewhere that, based on the current construction and performance motivations of the game, there isn’t a practical fix to MLB’s pitcher injury epidemic. Velocity is king and youth travel teams, particularly in the South and the West Coast, are playing year-round. That toothpaste can’t be put back in the tube, at least not without significant reform that will take years to implement. (To nip this in the bud: Yes, increased pitching velocity generally is associated with greater torque (i.e. force) values at the inner elbow.¹ Yes, this is not necessarily true when comparing individual pitchers against each other. Yes, it is true when looking at a broad population.)

As a result, injury prevention and rehabilitation specialists are left to deal with myriad variables contributing to pitcher injuries, each controllable to inconsistent extents. One such variable—the entire catalog is beyond the scope of this article—is muscle strength.

Generally two types of muscle contraction are involved in sports: concentric and eccentric. When lay folk think of muscle strength, they likely envision in their mind’s eye a concentric contraction. Think the “press” portion of the bench press or the “curl” portion of the biceps curl. During these movements, the individual muscle fibers glide past each other, causing the muscle belly to shorten. However, while concentric contractions produce significant torque and allow athletes to throw hard and run fast, it isn’t the “strongest” muscle contraction.

Eccentric contractions involve the muscle belly lengthening while the individual fibers produce force (i.e. “contract). This occurs during the lowering portion of the lifts mentioned above (lowering the bar to the chest, for example).

Most muscles in the human body work at a mechanical disadvantage, meaning they have to produce more torque than is induced to move an object during a concentric contraction. (For example: to lift a 50-pound weight, the muscles involved need to generate greater than 50 pounds of torque.) Additionally, force increases within the muscle when the fibers are forcibly stretched while they are trying to contract (i.e. an eccentric contraction). As a result, humans can lower much heavier weights than they can lift, making eccentric contractions the “stronger” of the two.

The rotator cuff—which is comprised of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis)—has become the golden child of so-called arm care. Studies have shown that rotator cuff strength (or lack thereof²³) is associated with shoulder and elbow injury occurrence amongst pitchers, making it rightfully a primary component of injury risk mitigation and rehabilitation programs.⁴⁵⁶

However, one muscle group that has received relatively less attention is the flexor-pronator mass. This group is comprised of the flexor carpi ulnaris, flexor carpi radialis, flexor digitorum superficialis, flexor digitorum profundus, and the pronator teres, which share a common attachment at the inner elbow near the attachments of the ulnar collateral ligament (UCL). These muscle flex the fingers, wrist, and, to a lesser extent, elbow and pronate the forearm, motions necessary for throwing a baseball, particularly at high speed. The flexor-pronator mass is frequently injured⁷; often a strain of the musculature serves as an initial diagnosis before a UCL tear because of their close proximity. (UCL pain can refer to the flexor-pronator mass and it isn’t uncommon for the ligament to be damaged concurrently with the muscles.)

Arguably the two most important of the flexor-pronator muscles are the flexor carpi ulnaris and flexor digitorum superficialis. These muscles are the primary dynamic stabilizers of the inner elbow, resisting valgus force and relieving pressure off the UCL during throwing and other activities.⁸ (The UCL is a static stabilizer as ligament fibers do not contract but hold a joint together; muscle fibers do contract and therefore dynamically hold together joints.)

The UCL experiences the greatest amount of force when the arm is maximally externally rotated during a pitch (see image below).⁹¹⁰ As the arm transitions from maximal external rotation to internal rotation (i.e. bringing the ball “forward”), the flexor carpi ulnaris and flexor digitorum profundus transition from an eccentric “stabilizing” contraction to a concentric “torque producing” contraction. It’s been theorized that this transition period—known as the stretch-shortening cycle—is one cause of muscle and tendon injury and rupture, although training it has been proven to improve athletic performance during tasks like jumping, running, and throwing.¹¹¹²

Citation: The Clinician's Guide to Baseball Pitching Biomechanics

Research has shown that repeated high intensity baseball throwing not only fatigues the flexor-pronator muscles, specifically the flexor carpi ulnaris and flexor digitorum superficialis, but also impairs their elasticity, or ability to return to their “shape” at rest; this likely places increased force through the UCL and ulnar nerve, leading to injury.¹³¹⁴¹⁵¹⁶ Impaired eccentric strength and elasticity due to fatigue, specifically of the flexor carpi ulnaris, has been shown to increase joint gapping during throwing, which places more pressure on the UCL to maintain inner elbow joint integrity. On the flipside, improved elasticity, again of the flexor carpi ulnaris, has been shown to reduce joint gapping, theoretically reducing injury risk.¹⁷¹⁸¹⁹

So, do professional (really, all) baseball pitchers just need to improve their flexor-pronator muscle strength, particularly during eccentric muscle contractions, and the injury curse will be broken?! Well, it isn’t that simple.

Injury occurrence is multifactorial and it is all but impossible to reduce the solution to the injury epidemic to a single factor. More emphasis on training the forearm musculature is likely a solution but not the solution.

It has been shown that certain exercises can increase the activation of the flexor carpi ulnaris and flexor digitorum superficialis above that of the other flexor-pronator muscles.²⁰ Repeated completion of these exercises and others like them following strict strength training principles would improve the strength of these muscles.

It would likely benefit pitchers to throw less, particularly when they are young and their bodies are still developing, and complete forearm strengthening exercises more. Additionally, increasing the volume (sets plus repetitions) of forearm exercises between starts may reduce inseason losses of forearm strength and elasticity, serving to protect the UCL.

More research needs to be conducted, but previous research into rotator cuff training programs (such as the Thrower’s 10) has shown an associated reduction in injury occurrence and improved athletic performance.²¹ Perhaps studies into forearm strengthening programs would find the same.

1. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C24&q=pitching+velocity+AND+elbow+torque&btnG=

2. https://journals.sagepub.com/doi/abs/10.1177/0363546509360404

3. https://journals.sagepub.com/doi/abs/10.1177/0363546514535070

4. https://journals.lww.com/jaaos/abstract/2007/03000/understanding_shoulder_and_elbow_injuries_in.3.aspx

5. https://link.springer.com/chapter/10.1007/978-3-030-69567-5_35

6. https://pmc.ncbi.nlm.nih.gov/articles/PMC4832224/

7. https://journals.sagepub.com/doi/abs/10.1177/0363546518778252

8. https://pubmed.ncbi.nlm.nih.gov/15466738/

9. https://www.drivelinebaseball.com/2016/10/elbow-stress-pulse-velocity/#:~:text=Explaining%20Elbow%20Stress%20and%20Torque&text=The%20arm%20cocking%20phase%20is,contact%20and%20max%20external%20rotation.&text=%E2%80%9CIn%20the%20arm%2Dcocked%20position,is%2099%2B%2D17%20Nm.

10. https://pubmed.ncbi.nlm.nih.gov/28806094/

11. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C24&q=stretch+shortening+cycle+AND+injury+occurrence&oq=stretch+shortening+cycle+AND+injury+oc

12. https://www.frontiersin.org/research-topics/10437/the-stretch-shortening-cycle-of-active-muscle-and-muscle-tendon-complex-what-why-and-how-it-increases-muscle-performance/magazine

13. https://pmc.ncbi.nlm.nih.gov/articles/PMC10399083/

14. https://www.tandfonline.com/doi/abs/10.1080/00913847.2021.1954861

15. https://www.mdpi.com/2227-9032/11/1/50

16. https://www.proquest.com/openview/d645f39dfe994ff9440d7e334dc2ac03/1?pq-origsite=gscholar&cbl=18750&diss=y

17. https://journals.lww.com/nsca-jscr/fulltext/2021/09000/Elasticity_of_the_Flexor_Carpi_Ulnaris_Muscle.31.aspx

18. https://pubmed.ncbi.nlm.nih.gov/34265815/

19. https://www.tandfonline.com/doi/abs/10.1080/00913847.2021.1954861

20. https://pubmed.ncbi.nlm.nih.gov/36793573/

21. https://scholar.google.com/scholar?hl=en&as_sdt=0%2C24&q=thrower%27s+ten+AND+injury&btnG=