The question of whether shoulders can be “double jointed” is a widespread misconception that arises when people exhibit extraordinary flexibility. It is anatomically impossible to possess two joints where only one should exist. The correct scientific term for this phenomenon is Joint Hypermobility, which refers to joints that move beyond the typical, expected range of motion. This excessive laxity, especially noticeable in the shoulder, can create the illusion of a joint that is somehow doubled or extra flexible. The underlying cause is not extra anatomy but a difference in the supporting biological structures.
The Reality of Joint Hypermobility
Joint hypermobility is defined by a joint’s ability to exceed the normal limits of movement. It is a common trait, estimated to occur in up to 20% of the adult population. The colloquial term “double-jointed” describes this inherent flexibility, but clinically, the condition is viewed as a spectrum, ranging from asymptomatic flexibility to a systemic disorder.
Hypermobility itself is not inherently a medical problem; many individuals experience no discomfort and find their flexibility advantageous in activities like gymnastics or dance. The clinical reality focuses on the degree of excessive laxity in the supporting tissues around the joint. When this excessive range of motion leads to symptoms like chronic pain, frequent injuries, or joint instability, it is classified as a Hypermobility Spectrum Disorder (HSD).
Anatomy and Susceptibility of the Shoulder Joint
The shoulder is the most mobile joint in the human body, which inherently makes it highly susceptible to hypermobility. This complex structure, known as the glenohumeral joint, is a classic ball-and-socket joint. Its primary function is to prioritize an enormous range of motion—allowing the arm to move through nearly 360 degrees—over stability.
The “ball” is the head of the humerus, or upper arm bone, and the “socket” is the glenoid fossa, a shallow indentation on the shoulder blade. Unlike the deep socket of the hip joint, the glenoid fossa is relatively flat and small, covering only about a quarter of the humeral head’s surface area. This minimal bony contact requires the surrounding soft tissues, including the joint capsule and ligaments, to provide most of the joint’s static stability. If the ligaments are already lax due to hypermobility, this lack of deep bony support means the shoulder is far more prone to instability, subluxation, or complete dislocation.
Causes of Excessive Joint Flexibility
The biological basis for excessive joint flexibility is rooted in genetics and the composition of connective tissue. The primary cause is an inherited variation in the structure of collagen, the most abundant protein that provides strength and elasticity to skin, tendons, and ligaments. In hypermobile individuals, the collagen fibers in the ligaments and joint capsule are structurally more elastic and less rigid than usual. This laxity allows the ligaments to stretch farther than normal, permitting the joint to exceed its typical range of motion.
These inherited differences can manifest as generalized joint hypermobility, affecting multiple joints throughout the body. In more pronounced cases, this laxity is a feature of specific genetic connective tissue disorders, such as Hypermobility Spectrum Disorder (HSD) or hypermobile Ehlers-Danlos Syndrome (hEDS). The condition is characterized by a systemic defect in connective tissue, resulting in weakened ligaments that cannot adequately restrain the joints. This biological foundation explains why the condition often runs in families and why the flexibility reflects the body’s overall tissue architecture.
Identifying and Managing Shoulder Hypermobility
Identifying generalized joint hypermobility often begins with a physical examination that includes the Beighton Score. This nine-point scoring system assesses laxity in specific joints by measuring the ability to perform maneuvers like touching the thumb to the forearm or hyperextending the knees and elbows. A score of four or more out of nine points in adults is often used as an indicator of generalized hypermobility, suggesting that shoulder laxity may also be present.
Management of a hypermobile shoulder focuses entirely on enhancing dynamic stability rather than increasing flexibility. Since the static stabilizers (ligaments) are already lax, the goal is to strengthen the muscles that actively hold the joint in place. Physical therapy is the recommended approach, specifically targeting the rotator cuff muscles and the scapular stabilizers to create a muscular “brace” around the inherently loose joint. Activities that involve deep stretching or extreme ranges of motion are avoided, as they can further stress the lax ligaments and increase the risk of subluxation or dislocation.