How Joints Work: Cartilage, Synovial Fluid, Tendons and Movement Explained

In this guide: bones and joint types, articular cartilage, the synovial membrane, synovial fluid, the joint capsule, ligaments, tendons, muscles, movement and loading, why stiffness can occur after inactivity, and the difference between joint structures and the surrounding soft tissue.

Bones and joint types

A joint, in the strict anatomical sense, is any point where two bones come together. Not all joints move the same way. The joints most people think of when they hear the word — knees, shoulders, hips, elbows, knuckles — are synovial joints, which are built for a wide range of motion. Other joints, like the sutures of the skull or the joints between vertebrae, allow very little or no movement. This guide focuses on synovial joints, because they are the ones most involved in daily mobility and the ones most often discussed in the context of joint health products.

Synovial joints come in several shapes — hinge joints like the knee and elbow, ball-and-socket joints like the hip and shoulder, and gliding joints like those in the wrist — but they all share the same basic set of components described below.

Articular cartilage

Where two bones meet inside a synovial joint, the bone ends are capped with a smooth, pale, resilient tissue called articular cartilage. Unlike most tissues in the body, cartilage has no direct blood supply. It receives nutrients by absorbing them from synovial fluid, which is one reason cartilage health and synovial fluid quality are often discussed together, as in our guide to synovial fluid and joint lubrication.

Cartilage's job is to let bones glide past each other with very low friction and to help spread out mechanical load so it is not concentrated on a single point of bone. It is firm enough to bear weight but flexible enough to compress slightly under pressure and rebound afterward.

The synovial membrane

Surrounding the joint space is a thin lining called the synovial membrane (or synovium). This membrane is the manufacturing site for synovial fluid. It is made up of cells that secrete fluid components into the joint space and also help filter and reabsorb fluid, keeping the internal joint environment in balance. The synovial membrane is sensitive to inflammation, which is part of why joint conditions that involve inflammation can affect fluid quality and joint comfort.

Synovial fluid

Inside the joint capsule sits a small volume of thick, slightly viscous fluid known as synovial fluid. It performs three main jobs: lubricating the cartilage surfaces so they can slide with minimal friction, distributing nutrients to cartilage, and acting as a shock-absorbing cushion during movement and impact. Synovial fluid's viscosity changes depending on how fast the joint is moving — a property discussed in more detail in our guide on the joint "gel" phenomenon and in our dedicated page on synovial fluid.

The joint capsule

The entire synovial joint is enclosed by a tough, fibrous sleeve called the joint capsule. It seals in the synovial fluid, helps keep the joint surfaces aligned, and contains nerve endings that provide feedback about joint position and movement — part of what is sometimes called proprioception.

Ligaments

Ligaments are dense bands of connective tissue that connect bone to bone across a joint. Their primary role is stability: they limit how far a joint can move in a given direction and help prevent excessive or abnormal motion. Ligaments have relatively limited blood supply and a limited capacity to stretch, which is why sprains (ligament injuries) can take a long time to heal fully.

Tendons

Tendons are related but distinct from ligaments: instead of connecting bone to bone, tendons connect muscle to bone. When a muscle contracts, the tendon transmits that force to the bone, producing movement at the joint. Tendons are built to handle repeated tensile loading, but they can become irritated or inflamed (tendinitis) with overuse, which is a soft-tissue issue distinct from a joint problem itself.

Muscles

Muscles are the engines that move joints. They cross joints and attach via tendons, and their coordinated contraction and relaxation is what produces controlled movement rather than random flailing. Muscles surrounding a joint also contribute meaningfully to joint stability — strong, well-conditioned muscles can reduce the load that ligaments and cartilage have to absorb during activity.

Movement and loading

Every joint structure described above responds to movement and load in some way. Cartilage relies on the gentle pumping action of movement to help draw in nutrients from synovial fluid. Synovial fluid itself becomes more effective at lubricating a joint once the joint is in motion. Muscles and tendons need regular use to maintain strength and elasticity. In short, joints are tissues that are built to move — and many of the everyday sensations people notice, including stiffness, relate to how much and how recently a joint has been used.

Why stiffness can occur after inactivity

A joint that has been still for a while — sitting through a long meeting, sleeping overnight, or a long car ride — can feel stiffer than a joint that has been moving regularly, even in people with no diagnosed joint condition. Part of this relates to how synovial fluid behaves when a joint is at rest versus in motion, and part of it relates to muscles and soft tissue around the joint temporarily shortening or cooling. This general sensation is explored in depth in our guides on stiff knees after sitting and the joint "gel" phenomenon. Gentle, gradual movement typically helps this kind of stiffness ease within a few minutes.

Joint structures vs. surrounding soft tissue

It's useful to separate "the joint" — bone ends, cartilage, synovial membrane, synovial fluid, and the capsule — from the soft tissues that surround and operate it, namely ligaments, tendons, and muscles. Problems can arise in either category, and they are not always the same thing: a strained muscle or an irritated tendon is a soft-tissue issue, while cartilage wear or synovial membrane inflammation is a joint-structure issue. Understanding this distinction is a useful foundation before exploring specific support strategies such as oral joint supplements, topical relief options, or physical mobility support.

This overview is educational and does not replace individualized medical advice. If you have persistent joint pain, swelling, or stiffness that doesn't ease with movement, a clinician can evaluate the specific structures involved.