Outside the Strike Zone: What Is a Flexor Mass Strain?

najarianRobert G. Najarian, MD, is board certified in orthopaedic sports medicine and orthopaedic surgery. He is a member of Inova Sports Medicine, the official sports medicine partner of the Washington Nationals and Washington Redskins. He has particular interest in treating injuries and disorders of the knee, shoulder and elbow. Read Dr. Najarian’s profile.

Every now and then, a little-known sports injury pops into public awareness. This week, sports fans are standing around the water cooler talking about something called “flexor mass strain.”

So, what is it? The flexor mass is a collection of muscles and tendons that all come together in the forearm near the elbow. They allow you to flex and pronate (turn) your wrist.

The flexor mass can become strained when it is stretched or slightly torn. This type of injury can be acute—that is, a sudden injury that occurs during a single event, such as a baseball pitch gone wrong. Or, it can develop over time through overuse.

A flexor mass strain isn’t exactly rare, but it’s not a terribly common injury. When we do see it, it’s usually in professional pitchers—a result of the repetitiveness and extreme velocity of their throws.

shutterstock_144630395Diagnosis: Listen and Learn

A lot of things can go wrong in the arm of a pitcher, and diagnosing injuries like flexor mass strains can be an interesting challenge.

Often, high-level pitchers have a variety of muscle and tendon changes due to overuse. Those changes aren’t necessarily problematic, but they can make it hard to see what’s going on in an MRI.

Doctors hone in on the problem with a physical exam that stresses various muscles in the arm. But with these types of injuries, it’s also critical to listen carefully to the athlete’s description of the problem.

With flexor mass strains, pitchers usually don’t feel pain until the follow-through phase of the pitch. That’s when you need the flexor mass to snap the wrist to get the ball in the strike zone. When the flexor mass is injured, the ball often ends high and pitchers can have problems locating their pitch.

Rest and Recover

Sports injuries are always frustrating for players and fans, but flexor mass strains aren’t usually among the worst-case scenarios. We typically treat them with rest, anti-inflammatory medications and a rehab/strengthening program. Rarely do they require surgery.

Still, it’s impossible to predict how long any given player might take to recover. It depends on the player and the severity of the strain. Some players are back on the field after 2 weeks of rest; for others it can take 6 to 8 weeks.

The important thing with flexor mass strains—and indeed, all sports injuries—is not to do too much too soon. Playing before an injury has healed can create a bigger problem, such as a tendon tear or ligament injury. It could also expose nearby structures to injury. That’s always an outcome we strive to avoid.

At Inova Sports Medicine, we treat patients of all age and activity levels, providing personalized care to get you back to the games and activities you love. To make an appointment, call 703-970-6464.


  1. Tim on February 25, 2018 at 12:43 pm

    “Playing before an injury has healed can create a bigger problem, such as a tendon tear or ligament injury. It could also expose nearby structures to injury.” Doctor, could you expand upon that? Are you saying it could possibly lead to damaging the UCL?

    • InovaNewsroom on April 12, 2018 at 12:00 pm

      Posted on behalf of Dr. Robert Najarian: Hi Tim. Thank you for your questions. In response to your first query, the flexor pronator mass is a significant muscle/tendon structure on the medial side of the elbow that sits just on top of the UCL (ulnar collateral ligament). Although most of the stability of elbow joint comes from the bony architecture and the ligaments that surround it, the flexor pronator mass also provides “dynamic” or muscle stability to the medial side of the elbow. With a weakened or torn flexor pronator tendon, the medial side of the elbow (most importantly the UCL), is exposed to even greater tensile forces during the throwing motion due to the inability of the injured tendon to bear/dissipate some of the force and protect the surrounding structures. This, in turn, can expose the UCL to a greater chance for injury and/or tearing. Another important consideration is that pitchers may often experience some discomfort on the inside of the elbow after a strain of their FP tendon, not mention it to anyone, and choose to “play through it”. To avoid the pain, the pitcher will often make subtle changes to their pitching mechanics, “open up” early in the delivery of their pitch and drop their arm as they deliver the baseball. This change in arm angle can also expose the medial side of the elbow and the UCL to greater tensile forces, potentially leading to injury of the ligament.

  2. Tim on February 26, 2018 at 11:55 am

    Good Morning, Dr. Najarian
    I have one more question on this article. You say that some players are back on the field in 2 weeks while some take 4-6 weeks. In the case of a pitcher are you saying 2 weeks of not throwing or 2 weeks of back on the field pitching in a game/intrasquad situations? Thanks for you feedback.

    • InovaNewsroom on April 12, 2018 at 12:00 pm

      Posted on behalf of Dr. Robert Najarian: In response to your second question, even the most mild flexor pronator strains are given at least 7-10 days of rest from all throwing. With a gradual return to throw program that takes at least another 7-10 days of pain free pitching, the best case scenario is that the he/she can be back to full game activity at about 2 weeks. This amounts to missing a minimum of 2-3 starts if the pitcher is part of a 5-man rotation. Realistically, however, even for the most mild strains, normal pain-free throwing does not happen until the second week post injury. So once the pitcher finishes his/her throwing rehab, they are not truly “game-ready” until at least week 3.

      As a general rule, we always err on the side of caution and never rush pitchers back until they are truly pain free with all of their pitches and their strength is equal to their opposite/uninjured side.

  3. […] The flexor mass is a collection of muscles and tendons which come together in the forearm near the elbow. They function by allowing you to turn and flex your wrist. The injury as it affects a pitcher such as Hunter is described as follows by Robert G. Najarian, MD with Inova: […]

  4. Jerry on January 30, 2020 at 9:20 am

    Good Morning, Dr. Najarian

    Question about flexor pronator strains. My son had a negative MRI for the UCL in October 2019, but his forearm was very tight. The doctor suggested 6 weeks of rest after the MRI. He did PT and took 10 weeks away of throwing. The doctor officially diagnosed him with a “mild UCL strain” even though the MRI revealed the UCL was completely intact.

    After getting released from PT in November, he returned to normal weight training activities like trap bar dead lift, variation of rows, pull-ups, and etc. A lot of gripping activities. Periodically, during that time, he would experience some tightness in the forearm as well. Also, with doctor recommendation, he performed daily wrist twist with therapy bar. Upon returning to throwing (January), his forearm continues to experience tightness.

    My questions is do you think he had a flexor strain all along and continued to slightly aggravate it during lifting not letting it fully heal. And then returning to throwing, the injury presented itself once again? The symptoms are dull pain in wrist, tightness in forearm, “pulling” in the funny bone part of inside the elbow, and soreness in tricep.

Leave a Comment