Hip

What it is

A ball-and-socket joint between the head of the femur and the acetabulum of the pelvis.

It is the direct counterexample to the shoulder. Here the socket is deep, enclosing well over half the head of the femur, and it is deepened further by the acetabular labrum. The capsule is thick and reinforced by three of the strongest ligaments in the body, the iliofemoral among them.

The result is a joint that is genuinely stable and correspondingly less mobile. Dislocating a hip takes major trauma; dislocating a shoulder does not.

The neck of the femur projects at an angle, which offsets the joint from the shaft and gives the hip muscles their leverage. That angle, and the degree to which the neck is rotated forward, varies considerably between individuals — which has real consequences below.

What it does

Moves in all directions: flexion, extension, abduction, adduction, rotation and circumduction. Range is smaller than the shoulder's in every direction.

It also transmits your body weight from the trunk to the legs, and does so on one side at a time through most of walking.

What loads it

Everything upright. Squatting, hinging, walking, running, stairs. The goblet squat and box squat load it heavily, particularly at depth.

What is commonly got wrong about it

That everyone should squat with the same stance. Hip socket orientation and femoral neck version vary substantially between people. That variation changes which stance width and degree of toe-out lets a given person reach depth without the joint running out of room. A stance that works beautifully for one person can be genuinely restrictive for another, and this is anatomy rather than mobility.

That anyone can achieve a deep squat with enough mobility work. Soft tissue can change. The shape of the bones cannot. Some people have a bony end-range earlier than others.

Hip impingement as a straightforward diagnosis. Femoroacetabular impingement is a real described condition, and the shape features associated with it are also found in large numbers of people without symptoms — particularly athletes. The relationship between shape, symptoms and outcome is actively debated.

That hip pain is a muscle problem. It has many possible sources and needs examination.