Movement Medicine

Elbow Flexor Mass Strains in Baseball Throwers

Eric Nichols
Apr 25, 2025By Eric Nichols

In baseball, particularly among pitchers, the elbow undergoes tremendous stress during the throwing motion. One common soft tissue injury that can sideline throwers is a flexor-pronator mass strain—an injury to the group of muscles responsible for stabilizing the elbow and assisting in wrist and finger flexion. Recognizing and properly managing these strains is crucial for ensuring a full recovery and minimizing time away from the field.

Frequency and Risk Factors in Baseball
Elbow flexor-pronator mass injuries are increasingly recognized among throwing athletes, particularly in high school, collegiate, and professional pitchers. In MLB, flexor-pronator injuries account for 8–10% of all elbow injuries (Erickson et al., 2016).

Risk factors include:

  • High pitch counts and velocity: Increased force generation stresses the medial elbow.
  • Poor throwing mechanics: Excessive valgus stress during late cocking and acceleration phases.
  • Previous UCL (ulnar collateral ligament) injury: Flexor mass strain may coexist with or compensate for UCL weakness.
  • Fatigue and inadequate recovery: Tired muscles are less able to absorb and dissipate throwing forces.


A 2014 study in The American Journal of Sports Medicine noted that flexor-pronator injuries are often seen in pitchers who throw high percentages of breaking balls and cutters.

What Is a Flexor-Pronator Mass Strain?

The flexor-pronator mass consists of muscles originating at the medial epicondyle of the elbow, including:

  • Flexor carpi radialis
  • Flexor carpi ulnaris
  • Palmaris longus
  • Flexor digitorum superficialis
  • Pronator teres

These muscles help stabilize the elbow against valgus forces during throwing. A strain occurs when these muscles or their tendons are overloaded or torn, typically during the acceleration phase of throwing.

Signs and Symptoms
Key symptoms of a flexor mass strain include:

  • Sharp pain at the medial elbow, especially during throwing
  • Painful gripping or resisted wrist flexion/pronation
  • Swelling or localized tenderness over the medial epicondyle or muscle belly
  • Decreased throwing velocity and control
  • Feeling of weakness or instability at ball release

In severe cases, athletes may describe a "pulling" sensation or audible pop at the time of injury.

Diagnosis and Imaging
A thorough clinical exam focuses on:

  • Palpation over the flexor-pronator mass
  • Resisted wrist flexion and pronation testing
  • Evaluation of UCL integrity (due to frequent association)

MRI is often used to:

  • Confirm the diagnosis
  • Grade the strain (Grade I–III)
  • Rule out concomitant UCL tears or other pathology

Ultrasound can also provide real-time dynamic imaging for muscle or tendon injuries.

Treatment Options
Conservative treatment is the standard for most flexor-pronator mass strains.

Phase 1: Acute Management (0–10 Days)

  • Rest from throwing
  • Ice, compression, and NSAIDs as needed
  • Protect the elbow with a sling or light brace if needed for comfort


Phase 2: Early Rehabilitation (1–3 Weeks)

  • Pain-free range of motion exercises
  • Begin gentle isometrics for forearm and grip muscles
  • Emphasize scapular and core stability

Phase 3: Strengthening and Dynamic Control (3–6 Weeks

  • Progressive wrist flexor and pronator strengthening
  • Plyometric training (e.g., medicine ball throws, rebounder drills)
  • Return-to-throwing program based on pain-free functional milestones

Phase 4: Return to Play (6–8+ Weeks)

  • Graduated throwing progression under therapist or coach supervision
  • Full return when throwing velocity, control, and strength are symmetrical and pain-free

Return to Sport Timelines

  • Grade I (mild strain): 3–4 weeks
  • Grade II (moderate tear): 5–8 weeks
  • Grade III (severe tear or avulsion): 8–12+ weeks; may require surgical intervention if associated with UCL damage

Research from Erickson et al. (2016) reported that 85–90% of professional pitchers successfully returned to play following non-operative management of isolated flexor-pronator mass strains.

However, recurrence is a concern if athletes do not address underlying throwing mechanics, workload management, or proximal stability deficits.

Risk of Recurrence and Prevention
To minimize reinjury risk:

  • Correct throwing mechanics (especially trunk rotation and arm slot issues)
  • Monitor pitch counts and ensure adequate recovery time
  • Build comprehensive forearm, shoulder, and core strength
  • Emphasize posterior chain mobility (hip and thoracic spine)
  • Early preseason screening to identify deficits before competition ramps up

Prevention Strategies

  • Forearm strength endurance training (eccentric wrist curls, pronation/supination drills)
  • Dynamic core training (planks, anti-rotation exercises)
  • Hip and thoracic mobility for kinetic chain efficiency
  • Pre-throwing warm-up routines targeting the arm, shoulder, and trunk
  • Strict adherence to pitch count and rest guidelines from youth to pro levels


Summary

Elbow flexor-pronator mass strains are a significant source of pain and downtime for baseball throwers but are highly treatable with early intervention and targeted rehab. A full recovery hinges on respecting the healing timeline, addressing the entire kinetic chain, and avoiding the temptation to rush back too soon.

Proper prevention strategies, mechanical corrections, and structured rehabilitation programs ensure that athletes not only return—but return better, stronger, and more durable.

References

  • Erickson BJ, et al. (2016). The impact of medial elbow injury on Major League Baseball pitchers. Orthopaedic Journal of Sports Medicine, 4(1): 2325967115627121.
  • Ahmad CS, et al. (2014). Elbow injuries in the throwing athlete. Journal of the American Academy of Orthopaedic Surgeons, 22(5): 315–325.
  • Cain EL, et al. (2003). Elbow injuries in throwing athletes: A current concepts review. The American Journal of Sports Medicine, 31(4): 621–635.
  • Nazarian LN, et al. (2003). Dynamic sonography of the flexor pronator mass and medial collateral ligament of the elbow. AJR American Journal of Roentgenology, 180(2): 357–363.
  • Osbahr DC, et al. (2010). Stress injuries of the elbow in the throwing athlete. Clinics in Sports Medicine, 29(4): 539–555.