The glenohumeral joint capsule is a flexible sleeve of connective tissue surrounding your main shoulder socket joint. It helps contain the joint while leaving enough slack for the arm to move through a much larger range than most other joints.

Location

Encircles the ball-and-socket shoulder joint
Attaches around the rim of the glenoid, the shallow socket on the shoulder blade
Attaches around the anatomical neck of the humerus, the upper arm bone
Blends with the glenoid labrum, glenohumeral ligaments and nearby rotator cuff tendons
Has a loose fold at the bottom called the axillary pouch

The capsule is not a rigid bag. Different areas tighten or relax as your arm changes position.

Function

Helps keep joint fluid inside the shoulder
Provides passive support near the limits of movement
Allows overhead reach and rotation through its normal folds and slack
Carries sensory information that contributes to joint position awareness
Works with the labrum, ligaments and rotator cuff to control the humeral head

The capsule cannot stabilize the shoulder by itself. Your muscles provide active control, while the capsule and ligaments become more important near the end of your available range.

Structures It Connects To

Glenohumeral joint — the joint enclosed by the capsule
Shoulder ligaments — thicken and reinforce parts of the capsule
Rotator cuff — blends with the capsule and provides active stability
Glenoid labrum — forms an attachment around the socket rim
Axillary nerve — passes close to the lower capsule

How It Changes With Movement

With your arm resting by your side, the lower capsule is relatively folded.
As you raise the arm, the axillary pouch unfolds.
Rotation tensions different parts of the capsule and its ligament thickenings.
Normal shoulder movement depends on both flexibility and controlled tension.

This is why a capsule that is unusually stiff or unusually loose can produce very different movement problems.

Common Problems

Frozen shoulder — pain and marked loss of motion are associated with thickening and contracture of capsular tissue
Shoulder instability — excessive laxity or injury may reduce passive restraint
Capsular sprain — can occur with dislocation or forceful end-range movement
Post-immobilization stiffness — may develop when the shoulder has moved very little for an extended period

These problems cannot be identified from one painful movement or scan finding alone. Clinicians compare your history, active movement, assisted movement and strength.

Clinical Relevance

A stiff capsule does not mean you should forcefully stretch the shoulder. The right amount and direction of mobility work depend on why motion is limited and how irritable the joint is.

Likewise, a flexible shoulder is not automatically unstable. Instability involves symptoms and loss of control, not flexibility alone.

Glenohumeral joint — the main shoulder socket joint
Shoulder ligaments — the capsule's reinforcing bands
Rotator cuff overview — active muscular support
Axillary nerve — travels near the lower joint

References

StatPearls. "Anatomy, Shoulder and Upper Limb, Glenohumeral Joint." https://www.ncbi.nlm.nih.gov/books/NBK537018/
StatPearls. "Shoulder Arthrogram." https://www.ncbi.nlm.nih.gov/books/NBK580562/
Lugo R, Kung P, Ma CB. Shoulder biomechanics. Eur J Radiol. 2008;68(1):16-24. https://pmc.ncbi.nlm.nih.gov/articles/PMC5039561/

Glenohumeral Joint Capsule