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Structure
Intermediate
8 min read

The shoulder

The most mobile joint in the body, and the one that holds least by its shape. A wide head resting on a shallow socket, retained by soft tissue.
3D scene
The shoulder, bones and joint tissues
The scene shows the shoulder blade, the collarbone and the upper humerus, with the capsule, the glenoid labrum and the glenohumeral ligaments highlighted. Without the 3D, the idea fits in one sentence: a large head resting against a small socket, held by a supple wrapping and by the muscles around it.
Open in the explorer

The word shoulder covers several joints

Saying "the shoulder" really names a set. Four linkages take part, and one arm movement brings them all into play.
  • The glenohumeral joint links the head of the humerus to the glenoid cavity of the shoulder blade. That is the one described here.
  • The acromioclavicular joint links the end of the collarbone to the acromion.
  • The sternoclavicular joint attaches the collarbone to the breastbone, the only point where the upper limb touches the skeleton of the trunk.
  • The scapulothoracic gliding is not a joint in the strict sense: the shoulder blade slides on the ribs, with no articular surface.
That last one supplies roughly a third of the range of arm elevation, driven by serratus anterior and the trapezius.

A socket too small for its head

Glenoid cavity
The hollow articular surface of the shoulder blade that receives the head of the humerus. It is shallow and markedly smaller than the head it receives.
glenoid
The contrast with the hip is striking. There, a sphere set deep into a cup. Here, a wide head resting against an almost flat surface, of which it covers only a fraction at a time.
This arrangement gives the shoulder the greatest range in the body. It also gives it its dependence on everything that is not bone.

The labrum and the capsule

A ring of fibrocartilage, the glenoid labrum, borders the cavity. It deepens the rim and increases the contact area, without changing the essential point: the retention stays modest.
The joint capsule wraps the whole. It is loose — it has to allow the range through — and three thickenings reinforce it in front: the superior, middle and inferior glenohumeral ligaments. The coracohumeral ligament completes the arrangement from above.
Certainty level · Established
The stability of the glenohumeral joint rests more on soft tissue than on the shape of the bony surfaces.
Descriptive anatomy and kinesiology describe it the same way: the cavity is shallow, the labrum adds only a limited amount, and the retention comes from the capsule, the ligaments and above all the [rotator cuff](/en/body/muscles/rotator-cuff). The relative contribution of each element varies with arm angle and with the measurement method.
Standring S (2020) · Neumann DA (2016)

What this implies

Two joints, two trade-offs

Criterion
Shoulder
Hip
Socket depth
Shallow
Deep
Range
The greatest in the body
More limited
Source of stability
Soft tissue and muscle
Bone shape and ligaments
Cost of holding
Muscular, continuous
Ligamentous, cheap when standing
The practical consequence is that the shoulder depends on muscles working continuously. The rotator cuff keeps the humeral head centred while the deltoid moves the arm. That is no side role: without that centring, the movement does not proceed normally.
Certainty level · Probable
Shoulder characteristics differ in people who regularly do resistance training.
The study covers a population of recreational lifters and describes differences in range and in muscle characteristics. It is an observation on a given group: it establishes no cause and effect and supports no conclusion about any particular person.
Kolber MJ, Beekhuizen KS, Cheng MS, Hellman MA (2010)

Seeing the shoulder

The shoulder, bones and joint tissues

The scene shows the shoulder blade, the collarbone and the upper humerus, with the capsule, the glenoid labrum and the glenohumeral ligaments highlighted. Without the 3D, the idea fits in one sentence: a large head resting against a small socket, held by a supple wrapping and by the muscles around it.
Current step
The shoulder
The head of the humerus is far wider than the socket that receives it. The glenoid labrum deepens the rim, the capsule wraps the whole, and three glenohumeral ligaments reinforce it in front. That is the price of mobility: little bone to hold, a great deal of soft tissue.
Scene description
A three-dimensional model of four joints from the left side of the body, presented one after another on a paper background. The shoulder shows the shoulder blade, collarbone and upper humerus in bone tones, with the joint capsule, the glenoid labrum and the glenohumeral ligaments in olive grey. The hip shows the hip bone and femur, the capsule wrapping the neck, the acetabular labrum and the three ligaments wound around it. The knee shows the femur, tibia, fibula and kneecap, with the two menisci resting on the tibial plateau, the cruciate ligaments at the centre and the collaterals on either side. The lumbar spine, seen from the side, shows the five vertebrae and the sacrum separated by five discs, with the longitudinal ligaments running in front of and behind the vertebral bodies. The joint tissues are highlighted at each step, the bones staying in the background.
Visible structures
  • Glenoid labrum
    A fibrocartilaginous rim fixed around the edge of the glenoid cavity. It deepens the socket and increases the contact area with the head of the humerus, whose diameter far exceeds that of the cavity.
  • Ligaments of the hip
    Three bands wound around the capsule — iliofemoral, pubofemoral, ischiofemoral. They tighten as the hip extends, which allows standing without continuous muscular effort.
  • Cruciate ligaments
    Two ligaments stretched across the centre of the knee, crossing one in front of the other. They limit the tibia sliding under the femur — forwards for the anterior, backwards for the posterior.
  • Intervertebral discs
    Five discs separate the lumbar vertebrae and the sacrum. Each pairs a tough fibrous ring with a more deformable core, which spreads load and allows slight movement between two neighbouring vertebrae.
  • Ligaments of the elbow
    Two collateral ligaments hold the elbow from the sides, and a ring encircles the head of the radius so it can pivot without leaving its place. The elbow thus combines a hinge and a pivot.
  • Lateral ligaments of the ankle
    Three slender bands link the fibula to the talus and the heel bone. They are markedly thinner than their medial counterparts, which is why the ankle is more easily forced inwards.
Guided steps
  • 1/6
    The head of the humerus is far wider than the socket that receives it. The glenoid labrum deepens the rim, the capsule wraps the whole, and three glenohumeral ligaments reinforce it in front. That is the price of mobility: little bone to hold, a great deal of soft tissue.
  • 2/6
    Unlike the shoulder, the femoral head sits deep in the socket, which the acetabular labrum extends further. The three ligaments wind around the neck and tighten in extension: standing, they carry part of the work the muscles no longer have to supply.
  • 3/6
    Two poorly matched surfaces — the rounded condyles of the femur on an almost flat tibial plateau. The menisci fill the gap, the cruciates hold the tibia front to back, the collaterals hold it side to side. Remove any one of these parts and the load shifts onto the others.
  • 4/6
    Five vertebrae, five discs, and a set of ligaments running the whole height. Each level moves only a few degrees; it is their sum that gives the trunk its range. The longitudinal ligaments limit flexion and extension, the ligamenta flava close the back of the canal.
  • 5/6
    The elbow does two things inside one capsule. The humerus and ulna form a hinge that flexes and extends in a single plane; the head of the radius pivots in place inside a ligamentous ring, which turns the palm over. The two collateral ligaments hold it all from the sides.
  • 6/6
    The talus is wedged between the two malleoli like a tenon in a mortise. On the inner side, a thick fan-shaped ligament; on the outer side, three markedly thinner bands. That asymmetry is no detail: it explains which way the ankle most often gives.
Model licence · Z-Anatomy et BodyParts3D — CC-BY-SA 4.0

What the model simplifies

Limit
What the image is missing
The cartilage and the bursae are not represented. The subacromial bursa in particular occupies the space between the rotator cuff and the acromion, and that space sits at the heart of many discussions about the shoulder.
The gliding of the shoulder blade on the ribcage does not appear as a joint, since strictly it is not one. It nonetheless supplies a large share of arm range, and ignoring it would give an incomplete picture of the movement.
The shoulder is stabilised by the rotator cuff and moved by the deltoid, pectoralis major and the latissimus dorsi. The orientation of the socket depends on serratus anterior and the trapezius. It works in vertical pushing and pulling.

Sources

Main sources

  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Kolber MJ, Beekhuizen KS, Cheng MS, Hellman MA (2010). Shoulder joint and muscle characteristics in the recreational weight training population. Journal of Strength and Conditioning Research.
Put it into practice in Shapier

Practise the shoulder press

The shoulder press, a vertical push that loads the deltoid, is practised in Shapier; Body Lab only explains the joint it puts into play.
Practise the shoulder press
Body Lab explains; Shapier lets you act and track.

Check my understanding

What mainly retains the head of the humerus in the glenoid cavity ?
Why does scapulothoracic gliding matter in an arm movement ?
What can be concluded from the study of shoulders in recreational resistance trainers ?
Choose an answer

Read next

  • Rotator cuff
    Four short muscles link the shoulder blade to the upper arm and hold the head of the humerus against its socket. They orient the shoulder more than they move it.
    With a 3D scene
  • The hip
    A sphere set deep in a socket, locked by three ligaments that tighten in extension. That is what makes standing possible without continuous muscular effort.
    With a 3D scene
  • Deltoids
    The deltoid caps the shoulder with three heads pointing in different directions. It raises the arm in every direction, and its shape explains why the same shoulder changes function depending on the angle of the movement.
    With a 3D scene

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • The model shows neither the cartilage nor the bursae, including the subacromial bursa: they nonetheless occupy the space between the cuff and the acromion.
  • The shoulder is not one joint but a complex: the shoulder blade also glides on the ribcage, and that gliding is not depicted as a joint in its own right.
  • This page describes an anatomy. Shoulder pain, loss of strength or a shrinking range belong to a health professional: no page can attribute them to a structure.
Sources
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • Kolber MJ, Beekhuizen KS, Cheng MS, Hellman MA (2010). Shoulder joint and muscle characteristics in the recreational weight training population. Journal of Strength and Conditioning Research.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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