The shoulder is one of the most complex and mobile joints in the human body, allowing for an extraordinary range of movement (ROM). ROM describes the distance and direction a joint can move, and the shoulder’s ball-and-socket structure enables its large operational arc. This unique anatomical design provides the mobility needed for daily activities, from reaching overhead to intricate manual tasks. Understanding the established limits of this movement is the foundation for assessing shoulder health and function.
The Mechanics of Shoulder Movement
The shoulder complex is capable of movement in multiple planes, which combine to create fluid, three-dimensional motion. These movements are categorized into six primary directions that occur at the glenohumeral joint.
Flexion involves lifting the arm straight forward and upward. Extension, the opposite motion, moves the arm straight backward. Abduction is the action of raising the arm directly out to the side, away from the midline of the body. Adduction is the reverse of abduction, bringing the arm back toward or across the body.
The rotational movements are internal rotation and external rotation, which involve spinning the upper arm bone (humerus) around its long axis. Internal rotation turns the arm inward, such as reaching behind the back, while external rotation turns the arm outward.
Defining Normal Range of Motion
Normal shoulder range of motion is expressed in degrees, representing the arc of movement possible for a healthy adult. These measures represent the maximum active range of motion (AROM), which is the distance an individual can move the joint using only their own muscles.
For shoulder flexion, the standard measurement is between 170 and 180 degrees, allowing the arm to move fully overhead. Abduction, lifting the arm out to the side, also ranges from 170 to 180 degrees. The backward movement of extension is significantly smaller, usually measuring between 50 and 60 degrees.
Adduction, the movement of the arm toward or across the chest, is approximately 30 degrees beyond the midline. Rotational movements are typically measured with the elbow bent to 90 degrees and held at the side. External rotation generally measures around 90 degrees, while internal rotation measures between 70 and 90 degrees.
Assessing Shoulder Mobility
The assessment of shoulder mobility distinguishes between two types of movement. Active range of motion (AROM) is measured when a person moves their limb independently, reflecting the function of muscles and tendons. Passive range of motion (PROM) is measured when an external force moves the limb, assessing the flexibility of the joint capsule and ligaments.
A significant difference, where passive range is greater than active range, often indicates a problem with the muscles, such as a rotator cuff tear or weakness. If both active and passive ranges are similarly limited, this suggests restriction within the joint capsule itself, as seen in frozen shoulder. The most common tool used to precisely measure these angles is a goniometer, a protractor-like device.
Accurate measurement necessitates stabilizing the shoulder blade (scapula) and the trunk to ensure only true movement at the glenohumeral joint is recorded. Without this stabilization, movements can be artificially inflated by the accompanying motion of the scapula on the rib cage. Standardized testing protocols are important for reliable results.
When Range of Motion is Restricted or Excessive
When the shoulder’s mobility falls outside of the normal parameters, it can lead to functional limitations or instability. Restricted range of motion, or hypomobility, is a common issue that makes everyday tasks difficult. This can be caused by conditions like adhesive capsulitis (frozen shoulder), which involves the painful tightening of the joint capsule.
Other common causes of restricted movement include arthritis, which causes joint surface damage and stiffness, or a rotator cuff injury resulting in pain and weakness. A limitation in motion can significantly impair the ability to reach into a cupboard, fasten a seatbelt, or perform personal hygiene.
Excessive range of motion, or hypermobility, is a condition where the joint moves beyond the typical healthy limits. While this may be advantageous in some sports, it can also indicate underlying joint instability or a connective tissue disorder. Hypermobility can make the shoulder more susceptible to dislocations or subluxations. Consult a healthcare professional if you experience a sudden loss of motion, pain with movement, or a noticeable difference in mobility between your two shoulders.