A muscle strain is a soft tissue injury involving the tearing of muscle fibers. When this damage occurs, the body initiates a healing process that can result in a noticeable, palpable mass under the skin. A muscle strain can cause a lump, which is a common byproduct of the natural repair mechanism. Understanding the nature and timeline of these lumps is helpful, as they represent different stages of the body’s response to the injury.
Acute Lumps: Hematomas and Inflammation
The immediate cause of a lump following a muscle strain is the leakage of blood and fluid from damaged tissue. When muscle fibers are torn, blood vessels within the muscle belly also rupture, leading to internal bleeding. This localized collection of blood outside the vessels is known as a hematoma, which forms a soft, tender mass within the first few hours to days of the injury.
This acute lump is frequently accompanied by visible bruising, or ecchymosis, as the blood spreads beneath the skin, and significant localized swelling called edema. The presence of this fluid collection, combined with the body’s inflammatory response, causes the area to feel bulky and firm to the touch. Hematomas generally begin to resolve as the body reabsorbs the blood components, causing the lump to gradually decrease in size over one to two weeks.
The size of the hematoma is often proportional to the severity of the strain, with larger, deeper tears creating more significant blood pooling. In some cases, a very large hematoma may not be easily reabsorbed and can persist, potentially requiring medical intervention such as aspiration or drainage. Early management of the acute injury focuses on limiting the size of this initial lump through rest, ice, compression, and elevation to control the bleeding and swelling.
Persistent Lumps: Scar Tissue and Fibrosis
Lumps that appear weeks or months after the initial injury, or those remaining after the initial hematoma dissipates, result from tissue repair and remodeling. The body heals torn muscle fibers by bridging the gap with fibrous connective tissue, a process called fibrosis. This non-contractile tissue, primarily composed of type I collagen, becomes a solid structure felt as a firm, sometimes rigid, lump or band within the muscle.
This fibrous scar tissue is laid down by fibroblasts in the weeks following the injury and is a permanent part of the muscle structure. While necessary for structural integrity, it is less elastic than the original muscle, which can lead to reduced flexibility and function. The scar mass tends to decrease in volume over time as the tissue matures, but it may remain as a mildly tender or painless nodule.
A rare, persistent, hard lump is Myositis Ossificans (MO), which involves the abnormal formation of bone tissue within the muscle. This complication usually begins to calcify two to four weeks after a severe strain or contusion. MO presents as a hard, sometimes painful mass that restricts joint movement. It occurs when the body mistakenly replaces damaged muscle cells with bone-forming cells instead of muscle or fibrous tissue, resulting in a very firm, fixed lump.
When the Lump is Not a Strain Byproduct
While most post-injury lumps relate to the strain, not all soft tissue masses in the area are a direct byproduct of the muscle tear. Several other conditions can present as a lump, and differentiating these masses is paramount for accurate diagnosis and appropriate treatment.
A common non-strain-related mass is a lipoma, a benign tumor made of fat cells. It typically presents as a soft, doughy, and highly mobile lump just beneath the skin. Unlike a strain-related mass, a lipoma is usually painless and is not the result of a traumatic injury, though the injury may bring an existing one to attention.
Another possibility is a muscle herniation, which occurs when a tear in the fascial sheath allows the underlying muscle tissue to bulge through the defect. This hernia often appears more prominent when the muscle is actively contracted or stressed, then recedes when the muscle is relaxed.
A cyst or an abscess can also present as a lump, characterized by a fluid-filled or pus-filled pocket that may feel warm, tender, and show signs of localized infection like redness. Abscesses are usually associated with systemic symptoms like fever and chills, which are not typical of an uncomplicated strain.
Finally, a soft tissue sarcoma is a malignant tumor that must be considered when evaluating an unexplained lump. Sarcomas are often painless in their early stages, but they tend to be firm, fixed, and grow rapidly. Masses larger than five centimeters carry a higher risk of being malignant.
Warning Signs and When to Seek Medical Care
While a small, resolving lump is a normal part of the healing process, certain signs should prompt an immediate medical evaluation. Professional assessment is required for any lump that rapidly increases in size over days or weeks, especially if the initial injury was minor. A lump that becomes significantly more painful, warm, or red suggests a potential secondary issue, such as an infection or an inflammatory process like Myositis Ossificans.
You should seek care if the lump is accompanied by systemic symptoms, such as an unexplained fever or chills, which can point toward an abscess or infection. Persistent numbness or tingling in the limb, or pain that continues to worsen, may signal pressure on adjacent nerves or vessels.
Generally, a healthcare professional should evaluate any soft tissue mass that is fixed, deep, or larger than a golf ball. Assessment is also necessary if the lump does not show signs of improvement after seven to ten days.