A lat strain is a stretch or tear in the latissimus dorsi, the large, flat muscle that spans most of your back from your lower spine to your upper arm bone. It’s one of the widest muscles in the body, and when it’s strained, you’ll typically feel a burning pain along your side, under your shoulder blade, or deep in your back that gets noticeably worse when you reach overhead or pull something toward you. These injuries are uncommon in everyday life but show up regularly in athletes who throw, climb, or lift heavy weight.
What the Latissimus Dorsi Actually Does
The latissimus dorsi (often just called the “lat”) fans out from the lower half of your spine, your pelvis, and your lowest three or four ribs, then narrows into a flat tendon that twists and attaches to the front of your upper arm bone near the shoulder. That wide origin gives it enormous leverage. The muscle is responsible for pulling your arm down and back, rotating it inward, and drawing your shoulder blade downward. Every time you do a pull-up, row a boat, swim a stroke, or throw a ball, your lat is doing heavy work.
Because it connects the lower back to the arm, the lat also helps stabilize the spine during movements that load your trunk, like swinging a bat or carrying something at your side. This dual role as both a shoulder mover and a spinal stabilizer is part of why a strain can produce pain in surprisingly different locations.
How Lat Strains Happen
The most common cause is a forceful overhead or pulling motion that overloads the muscle fibers. In baseball pitchers, tears most often occur during the late cocking and early acceleration phases of a throw, when the lat is contracting hard while also being stretched. In non-throwing athletes, the typical mechanism is forceful resisted pulling (bringing the arm down against resistance) or a sudden forced stretch where the arm is yanked overhead or outward beyond its normal range.
Professional baseball pitchers account for the largest share of documented lat injuries, but the injury has also been reported in water skiers, rock climbers, steer wrestlers, and track athletes. Weight lifters and CrossFit athletes are an increasingly common group. The higher the competitive level, the more frequently these injuries appear, likely because of the greater forces involved. Outside of sports, lat strains are rare.
Symptoms and Where the Pain Shows Up
A strained lat can produce pain in several areas because the muscle is so large. The most common complaints are:
- Burning or sharp pain along the side of your back or under the armpit, especially when reaching overhead or pulling
- Shoulder pain near the back of the armpit, sometimes mistaken for a rotator cuff problem
- Swelling or stiffness in the mid to lower back or along the side of the torso
- Skin discoloration (bruising) over the affected area in more severe tears
- A palpable lump or mass in the muscle belly where torn fibers have bunched together
Some people also report abdominal pain with a lat strain, which makes sense given the muscle’s attachment points along the lower ribs. Pain that gets worse with any arm movement involving pulling, pushing down, or rotating inward is a strong signal. In a significant tear, you may hear or feel a pop at the moment of injury, followed by immediate weakness when trying to use the arm.
Grading the Severity
Lat strains follow the same three-grade system used for all muscle injuries. The grade determines how long you’ll be sidelined and what kind of treatment you’ll need.
A Grade 1 (mild) strain means the muscle fibers have been stretched or only minimally torn, with less than 5% loss of function. Pain is localized and relatively minor, swelling is minimal, and you can still move your arm through a nearly full range of motion. These typically heal within a few weeks.
A Grade 2 (moderate) strain involves tearing of a larger portion of the muscle fibers without a complete rupture. You’ll have more significant pain that’s harder to pinpoint, noticeable swelling, and a meaningful loss of strength and range of motion (roughly 10 to 25 degrees of restriction). You won’t be able to continue the activity that caused the injury. Recovery takes several weeks to a few months.
A Grade 3 (severe) strain is a complete or near-complete rupture of the muscle or its tendon. Pain is severe and diffuse, swelling develops rapidly, and you may be able to feel a gap or defect in the muscle. Strength loss is dramatic, often more than 50%. These injuries may require surgical repair, and total recovery, including post-surgical rehabilitation, can take four to six months.
How It’s Diagnosed
A physical exam is the starting point. A clinician will check for tenderness along the muscle, test your strength against resistance in pulling and arm rotation movements, and look for visible swelling or bruising. Pain or weakness when you try to pull your arm down or rotate it inward against resistance points toward the lat specifically.
MRI is the most useful imaging tool for confirming the diagnosis and grading the tear. On MRI, a partial tear shows up as thickening of the tendon with uneven signal patterns and irregular contours. A complete tear (avulsion) appears as a bright signal on fluid-sensitive sequences, roughly 2 centimeters wide at the point of separation. In some cases, the swelling and disrupted tissue around the tendon can look surprisingly like a tumor on imaging, which is why accurate reading matters. Ultrasound can also detect tears but is more operator-dependent.
Treatment by Grade
Mild and moderate lat strains are almost always treated without surgery. The initial phase focuses on rest, ice, and avoiding any movements that reproduce the pain, particularly overhead reaching, pulling, and internal rotation of the arm. As pain decreases, you gradually reintroduce range-of-motion exercises, then light strengthening, and finally sport-specific movements. For throwers, that means a structured throwing program with incremental increases in distance and intensity.
Severe tears, especially those where the tendon has pulled away from the arm bone, are more likely to need surgical reattachment, particularly in athletes who need to return to high-demand overhead sports. After surgery, the arm is typically immobilized for up to six weeks before a rehabilitation program begins. Return to full competition after a Grade 3 tear generally falls in the four-to-six-month range.
Conditions That Mimic a Lat Strain
Because the lat sits near several other structures, the pain can be confused with other injuries. The teres major, a smaller muscle that runs right alongside the lat and attaches in nearly the same spot on the arm, can strain independently and produce very similar symptoms. Rotator cuff injuries, rib stress fractures, and thoracic spine problems can also overlap in how they present. Imaging is often the clearest way to tell these apart, since the physical symptoms can be almost identical. If your pain started with a specific forceful pulling or throwing motion, a lat injury should be high on the list of possibilities.