Synovial fluid

Synovial fluid is the viscous liquid inside joint capsules. It lubricates the joint surfaces, reduces friction to a level lower than ice on ice, absorbs shock, and delivers nutrients to cartilage, which has no blood supply of its own.

That last function explains a great deal of practical joint advice. Cartilage is fed by diffusion driven by the compression and release of movement, so a joint that is not moved is a joint that is not being nourished, which is why stiffness follows inactivity and why gentle motion is part of managing almost every joint problem.

Its main components are hyaluronic acid, which gives it viscosity, and lubricin, which handles boundary lubrication at the surface. Both are produced by the synovial membrane lining the joint.

It behaves in a useful way under load. The fluid thins under shear, so it flows easily when a joint moves quickly, and thickens at rest, which is a property that engineered lubricants imitate rather than surpass.

The warm up effect people notice is partly this. Movement raises joint temperature and reduces fluid viscosity, which is one physiological component of why the third set feels better than the first.

When joints are inflamed the volume increases, producing swelling, and the composition changes, which is why aspirating and analysing joint fluid is a standard diagnostic step in an acutely swollen joint.

In practice, the actionable content is simple: joints do better when they are moved regularly through their range, and a hot swollen joint is a reason to see a doctor rather than to stretch it.