A localized depression or dent in the bicep muscle suggests a change in the underlying soft tissues. This concavity, or loss of volume, can be triggered by a variety of causes, ranging from sudden mechanical trauma to gradual neurological or tissue-related changes. Understanding the origin of the dent is important because it can indicate anything from a benign cosmetic alteration to a medical issue requiring attention. The location, speed of onset, and accompanying symptoms like pain or weakness provide significant clues about the potential source of the bicep’s altered contour.
Dents Caused by Acute Injury and Muscle Tears
A sudden, visible dent in the bicep often results from an acute injury, most commonly a bicep tendon rupture. This injury typically occurs during heavy, forceful activities like lifting, leading to an immediate “pop” sensation in the arm. The bicep muscle is connected to bone by tendons at both the shoulder (proximal) and the elbow (distal).
When one of these tendons tears completely, the muscle belly retracts, shortening its length and bunching up. This retraction creates a prominent bulge, frequently referred to as the “Popeye sign,” with a corresponding indentation or gap where the tendon detached. If the tear is proximal, the muscle belly rolls down toward the elbow, leaving a dent high on the arm. A distal tear causes the muscle to retract upwards toward the shoulder, leaving a dent near the elbow crease.
The acute injury is usually accompanied by sharp pain, swelling, and bruising around the upper arm or elbow. While a proximal tear may not cause significant weakness, a distal tear can result in a considerable loss of strength, particularly when trying to turn the forearm palm-up (supination). This combination of sudden onset, pain, bruising, and the characteristic change in muscle shape clearly distinguishes it from more gradual causes.
Dents Resulting from Gradual Muscle Atrophy
When a dent develops slowly over time, it may signal muscle wasting, or atrophy, caused by a disruption in the nerve signals to the bicep. The bicep brachii muscle is primarily innervated by the musculocutaneous nerve, which originates from the brachial plexus. Damage to this nerve pathway prevents the muscle from receiving the necessary electrical impulses to contract and maintain its mass.
Conditions that compress or damage the musculocutaneous nerve or its roots in the neck, such as cervical radiculopathy (a pinched nerve), can lead to neurogenic atrophy over weeks or months. As the muscle fibers shrink, the bicep loses volume, creating a depression that becomes increasingly noticeable. This type of atrophy is generally accompanied by motor and sensory changes, including weakness in elbow flexion and numbness or tingling along the outer forearm.
The weakness and volume loss may progress gradually, making the change in contour less dramatic than an acute tear. Unlike an injury, the dent from atrophy typically lacks the initial event of sharp pain or bruising. Chronic nerve constriction causes the muscle to thin out, resulting in a visible depression that reflects the deterioration of the muscle tissue itself.
Localized Tissue Changes and Indentations
Some bicep indentations are not related to the main muscle structure but rather to changes in the layers of fat and connective tissue above it. A common cause in this category is lipoatrophy, which is the localized loss of subcutaneous fat. Lipoatrophy presents as a depression in the skin that can range from a subtle dimple to a pronounced concavity.
This condition is frequently linked to past subcutaneous injections in the area, such as those used for insulin, corticosteroids, or certain vaccines. Corticosteroids, if injected too superficially, can inhibit the growth of cells that produce collagen and elastin, leading to a breakdown of fat and connective tissue beneath the skin at the injection site. The resulting dent is usually cosmetic, lacking the pain and weakness associated with muscle or tendon issues, and may appear weeks or months after the injection.
Rarer causes involve connective tissue disorders like localized scleroderma, also known as Morphea. Morphea causes a hardening and thickening of the skin and underlying tissues. When this process affects the fat, fascia, or even the muscle beneath the skin, it can tether the tissue down, creating a linear or circular indentation and loss of volume in the arm.
When to Seek Medical Evaluation
A medical evaluation is warranted for any new or changing bicep dent, particularly if certain accompanying symptoms are present. You should seek prompt attention if the dent appeared suddenly following a traumatic event, such as heavy lifting or a fall, especially if you felt a “pop” or have significant, acute pain. This set of symptoms is highly suggestive of a tendon rupture requiring diagnosis, often from an orthopedic specialist.
Consult a physician if the dent is accompanied by progressive weakness, difficulty moving the elbow or shoulder, or any sensory changes like numbness or persistent tingling in the arm or hand. These signs could indicate a neurological issue or nerve entrapment, which would require evaluation by a neurologist or physical medicine specialist. Even dents that are not painful should be assessed if they are rapidly deepening or if they occurred after a medical injection.