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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?

  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.

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