The infraspinatus is a thick, triangular muscle that resides in the large hollow on the back of the shoulder blade, known as the infraspinous fossa. As one of the four muscles that form the rotator cuff, it is a frequent focus of clinical assessment when shoulder function is compromised. Understanding the health and integrity of this muscle is important for diagnosing shoulder issues, ranging from weakness to a structural tear.
Understanding the Muscle’s Function
The infraspinatus muscle attaches from the infraspinous fossa of the scapula to the greater tubercle on the upper part of the humerus. This positioning allows it to perform its main biomechanical role: the external rotation of the shoulder joint. It is considered the primary muscle responsible for this outward twisting motion of the arm.
The muscle also plays a significant part in stabilizing the shoulder joint. By contracting, it helps keep the head of the humerus centered within the shoulder socket during arm movements. This stabilization is a key function of all rotator cuff muscles. Therefore, any assessment of the infraspinatus is designed to isolate its ability to perform external rotation.
Testing Muscle Strength (Manual Resistance Methods)
Manual Muscle Testing (MMT) is the primary method used to gauge the functional strength of the infraspinatus. This procedure isolates the muscle’s strength by applying controlled resistance against its specific action. The test is most often performed with the patient seated or standing, with the arm positioned to minimize the contribution of other muscles.
A common technique involves positioning the patient’s arm with the elbow bent to 90 degrees and the shoulder held in a neutral position, close to the torso. The examiner stabilizes the shoulder blade while applying resistance to the forearm, just above the wrist. The patient is instructed to push outward against the resistance, performing external rotation.
The strength is then quantified using a standardized grading scale, typically ranging from 0 to 5. A grade of 5 indicates the muscle can hold the test position against strong resistance, which is considered “normal” strength. A grade of 3 signifies the muscle can complete the movement against gravity but cannot tolerate any added resistance.
Grades 0, 1, and 2 are used when the muscle cannot overcome gravity or can only complete the movement in a gravity-eliminated position. Weakness during this test, especially when compared to the opposite side, suggests a possible strain, nerve issue, or partial tear. MMT focuses on the muscle’s capacity to generate force, distinct from tests that assess for structural failure.
Testing for Tears and Pathology (Diagnostic Signs)
Beyond simple strength measurement, specialized clinical assessments are used to identify structural damage, such as a full-thickness tendon tear. These diagnostic signs look for the inability to actively hold a position against gravity, which indicates a compromise of the tendon’s integrity. The Infraspinatus Test, sometimes referred to as the External Rotation Lag Sign (ERLS), is a common test used for this purpose.
To perform the Infraspinatus Test, the patient sits while the examiner passively positions the arm. The examiner holds the elbow at 90 degrees of flexion and rotates the shoulder fully outward into maximum external rotation. The examiner then releases the patient’s wrist, asking the patient to actively maintain the rotated position.
A positive result, known as a “lag,” occurs if the patient is unable to hold the position and the arm rotates inward. The degree of this inward drop is often measured to quantify the extent of the structural damage. This suggests a functional failure of the infraspinatus tendon, often due to a significant tear.
Another related test is the Drop Sign, performed similarly but with the arm elevated to 90 degrees of abduction in the scapular plane. The examiner externally rotates the arm and asks the patient to hold the position when the wrist is released. An inability to maintain the external rotation is a positive sign, indicating a compromise to the infraspinatus or supraspinatus tendons.
A positive result on these structural tests, especially when combined with weakness on MMT, raises suspicion of a full-thickness rotator cuff tear. The clinician will often recommend further investigation, such as magnetic resonance imaging (MRI) or ultrasound, to confirm the diagnosis.