How do joints work? A joint is any place where two bones meet. In movable joints like your knee, slippery cartilage covers the ends of the bones and synovial fluid acts as a lubricant, so the bones move smoothly without rubbing. Ligaments hold the bones together, and tendons connect your muscles to those bones.
This is general information, not medical advice. Talk to your doctor or pharmacist about your own situation.
Cleveland Clinic defines a joint as any place in your body where two bones meet, and says adults have about 350 of them, though experts disagree on the exact count. A movable joint, like your knee or shoulder, is a small team of tissues:
Cartilage. Slippery hyaline cartilage covers the ends of the bones, giving them a smooth surface to move on.
Synovial membrane. A fluid-filled sac that lines the joint, lubricating and protecting it.
Synovial fluid. Cleveland Clinic calls it a liquid lubricant. It reduces friction where bones move past each other, cushions the space between them and moves nutrients to the cartilage. Your body makes it from blood plasma, and it contains a lot of hyaluronic acid, which helps make it slick.
Ligaments. MedlinePlus describes ligaments as tissues that connect bones at a joint. They keep the joint stable.
Tendons. MedlinePlus describes tendons as tissues that connect muscle to bone.
Not every joint is built to move. Cleveland Clinic groups joints by how much they move:
Immovable joints (synarthroses) provide structure. Fibrous joints such as the sutures in your skull and the joints that hold your teeth in your jaw are examples.
Slightly movable joints (amphiarthroses) mix stability with a little motion. Cartilaginous joints, such as where your ribs meet your breastbone, have limited movement.
Freely movable joints (diarthroses) allow the most movement. Synovial joints have the most freedom to move.
Ball-and-socket: shoulders and hips. They can move in many directions.
Hinge: knees and elbows. They bend and straighten like a door.
Pivot: the joint in your neck that turns your head side to side.
Planar: the small wrist bones and the joints between the vertebrae in your spine.
Saddle: the base of your thumb.
Condyloid: your wrist and the joints where your toes meet your foot.
Try it: bend your elbow and it only moves one way, like a hinge. Circle your arm from the shoulder and it moves in almost every direction. That is the ball-and-socket design at work.
Cleveland Clinic lists arthritis, osteoarthritis, bursitis and tendinitis among common joint conditions, along with autoimmune conditions such as rheumatoid arthritis, psoriatic arthritis and lupus. Injuries include sprains, fractures and dislocations.
A sprain is a stretched or torn ligament.
A strain is a stretched or torn muscle or tendon.
With arthritis, Cleveland Clinic explains, there can be more friction than your synovial fluid can cushion, and your body may make extra fluid, which causes swelling.
MedlinePlus says early treatment for sprains and strains usually involves rest, ice, a compression bandage and medicines, and later may include exercise and physical therapy.
It can feel safer to rest a sore joint, but movement is part of joint care. The CDC says physical activity can help people with arthritis reduce joint pain and improve function and mood. Cleveland Clinic suggests staying active with options like cycling, strength training, swimming, low-impact cardio or water aerobics, chosen with your provider's input.
The CDC recommends that adults with arthritis aim for at least 150 minutes a week of moderate aerobic activity and muscle-strengthening activity on at least 2 days a week. It lists joint-friendly choices such as brisk walking, cycling, swimming, tai chi, dancing and water exercise.
Example only: a joint-friendly week might be 30 minutes of brisk walking, cycling or swimming on 5 days, plus 2 short strength sessions with moves like sit-to-stands or bodyweight squats. Our guide on how to squat correctly covers form.
The CDC advises starting slowly and paying attention to how your body feels. Some pain, stiffness or swelling can be normal at first and should improve over time. If it does not, contact your health care provider. The NHS adds that you should rest a painful joint without stopping moving it completely, and that regular exercise and not smoking help.
Based on NHS guidance, see your doctor if:
Pain stops you doing normal activities or disturbs your sleep.
Pain or swelling is getting worse or keeps coming back.
Pain has not improved after 2 weeks of home treatment.
Your joints stay stiff for more than 30 minutes after you wake up.
Your child has joint problems.
Get an urgent appointment the same day if the skin around the joint is swollen and hot, or if you feel generally unwell, have a high temperature, or feel hot, cold or shivery along with the joint pain.
Go to the emergency room or call emergency services if you have severe joint pain after a fall or injury, cannot walk or put weight on the joint, the joint looks out of place, or you have tingling or numbness around it after an injury.
Synovial fluid. It reduces friction, cushions the space between bones and carries nutrients to cartilage.
Ligaments connect bone to bone at a joint. Tendons connect muscle to bone.
Adults have about 350 joints, according to Cleveland Clinic. The exact number depends on how a joint is defined.
For many people, yes. The CDC says activity can reduce arthritis pain and improve function. Start slowly and check with your provider if pain does not settle.
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Joints in the Human Body: Anatomy, Types and Function (Cleveland Clinic)
Synovial Fluid: What It Is, Composition and Function (Cleveland Clinic)
Sprains and Strains (MedlinePlus, National Library of Medicine)