The internal oblique (IO) is a deep muscle in the abdominal wall, often the source of pain described in the side or flank. This muscle is one of the four main components of the core musculature, providing a layer of support beneath the more superficial external oblique. The internal oblique is important for both trunk stability and dynamic movements, particularly those involving twisting and bending. Pain originating from this area can range from a minor muscle ache to a symptom of a deeper, more serious condition.
Locating the Internal Oblique and Identifying the Pain
The internal oblique muscle is situated in the lateral and anterior abdominal wall, lying immediately beneath the external oblique and superficial to the transversus abdominis. Its fibers run upward and toward the midline, an orientation perpendicular to the fibers of the external oblique muscle. This crisscross pattern provides strength and integrity to the torso. The IO originates from the thoracolumbar fascia, the iliac crest, and the inguinal ligament, inserting onto the lower ribs and the midline aponeurosis.
This muscle’s primary actions are to rotate the trunk to the same side and assist in lateral flexion. It also assists in forced exhalation and helps to maintain intra-abdominal pressure during activities like coughing or lifting. Pain associated with the internal oblique is felt along the side or flank, often between the lower ribs and the hip bone. The discomfort may be diffuse across the side of the abdomen, sometimes radiating toward the front or the lower back, and usually intensifies with movements that require twisting or bending.
Direct Muscular Strain and Overuse Injuries
The most frequent cause of internal oblique pain is a direct strain, which involves tearing a portion of the muscle fibers. These acute injuries result from a sudden, forceful contraction or overstretching, often during activities requiring rapid, powerful rotational force.
Athletes in sports like baseball, golf, tennis, and javelin throwing are particularly susceptible due to the high-velocity, repetitive twisting movements involved. During a powerful swing or throw, the internal oblique on the side opposite the dominant arm is under eccentric load, which can lead to fiber tearing. Improper technique during weightlifting, such as twisting while performing a deadlift or squat, can also acutely strain the IO.
Overuse injuries develop more gradually from chronic fatigue and repetitive stress. Intense core workouts, such as excessive side planks, Russian twists, or bicycle crunches, can lead to micro-tears and inflammation. The pain from an overuse injury may present as a dull, persistent ache that worsens toward the end of a training session. This pain is often localized and tender to the touch, and it may be accompanied by stiffness, especially when attempting to rise from a seated or lying position.
Non-Muscular and Referred Sources of Pain
Not all pain felt in the internal oblique region originates from the muscle tissue itself; some discomfort is referred from nearby structures. Nerve irritation, such as intercostal neuralgia, can mimic muscular pain by causing sharp, burning, or aching sensations along the path of the nerves that run along the ribs. This condition involves the nerves that innervate the abdominal wall, including the IO, and the pain is often triggered by simple motions like coughing or deep breathing.
An abdominal wall hernia, specifically a Spigelian or flank hernia, is another non-muscular cause. A Spigelian hernia occurs when abdominal contents push through a weakness in the connective tissue layers. Because the protrusion often happens between the muscle layers, it may not produce a visible bulge but can cause localized pain that increases with muscle contraction.
Pain can also be referred from internal organs located in the flank area, such as the kidneys. Conditions like kidney stones or a kidney infection can cause flank pain that is sometimes mistaken for a deep muscle strain. Kidney pain presents as a deep ache or a severe, wave-like pain that does not change with movement of the torso. Unlike a muscle strain, kidney-related pain is frequently accompanied by other symptoms, which helps distinguish it from an isolated IO injury.
Recognizing Serious Symptoms and Initial Management
While most internal oblique pain is due to a muscle strain that will resolve with self-care, certain symptoms warrant immediate medical attention. Pain accompanied by a high fever, shaking chills, persistent vomiting, or blood in the urine suggests a potential kidney infection or other serious internal issue. If the pain is sudden and severe, or if a noticeable, non-reducible bulge appears, it could indicate a complicated hernia requiring evaluation.
For a suspected, uncomplicated muscle strain, initial management focuses on protecting the area and controlling inflammation. Temporarily modifying activities that aggravate the pain, particularly twisting and heavy lifting, allows the muscle fibers to heal. Applying a cold compress to the painful area for short periods in the first 48 hours can help reduce swelling and discomfort. After the initial acute phase, switching to heat therapy can help relax the muscle and improve blood flow for recovery. Over-the-counter pain relievers can manage symptoms, but if the pain persists beyond a few days or if any concerning symptoms develop, professional diagnosis is required.