Anatomically speaking, joints are where two or more bones touch, and they can be fixed or mobile. There are three categories of joints in the human body, according to the National Library of Medicine (NLM): fibrous, cartilaginous and synovial. There are six types of synovial joints: ball-and-socket, condyloid, gliding, hinge, pivot, and saddle joints.
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Synovial joints are freely mobile, and are therefore the main functional joints of the body, per the NLM. The six types of synovial joints get their names from either their appearance or the type of motion they facilitate, and they contain synovial fluid that reduces friction between the bony articulations that touch each other. This allows for smoother movement and increased range of motion.
It's easy to see that synovial joints are an essential part of exercise, as they not only enable movement but provide lubrication and cushioning to bones and tissues as you work out or play a sport. In return, exercising has the potential to maintain synovial joint health: It's a "use it or lose it" kind of situation, as the Cleveland Clinic explains.
"It's a circular approach in some ways," they acknowledge. "By keeping your joints healthy with an active lifestyle and continuing to stay active throughout your life, it sets you up to continue these activities that you love later on — whether it's walking, running or other sports."
Read on to learn more about the six types of synovial joints, synovial fluid, and how they are a critical part of any exercise regimen.
The 6 Types of Synovial Joints
1. Ball-and-Socket Joints
According to Anatomy & Physiology, a book published by Oregon State University (OSU), ball-and-socket joints have the greatest range of motion. As their name implies, they occur where the ball of one bone end fits into the socket of another. Ball-and-socket joints allow for stable movement in several directions without slipping, creating a highly stable, strong joint. Your hip and shoulder joints are the only ball-and-socket synovial joints in your body, and you use them whenever you hug, walk, or high-five.
2. Condyloid Joints
Condyloid joints have an irregular surface where the bones move past one another. This type of joint is like two bowls nested together. Per OSU, your knuckles are examples of condyloid synovial joints. Since they have an oval shape, they are sometimes called ellipsoid joints. Whenever you flip someone the bird or do jazz hands, you are using your condyloid joints.
3. Gliding Joints
Also known as plane joints, gliding joints allow for smooth movement in several directions along a plane or other smooth surface. Their articulation is like two plates sliding across each other. As OSU explains, not all gliding joints move alike; they "may exhibit movement in a single plane or in multiple planes," depending on where they're located. Some of the joints in your wrists are examples of the gliding type of synovial joints, and you use them when waving hello or goodbye.
4. Hinge Joints
Hinge joints, as their name implies, look like door hinges between two bones. They allow for stable flexion and extension without sliding or deviation, a la opening the door. Knees and elbows are examples of hinge joints. You use them whenever you kick something or serve a tennis ball.
5. Pivot Joints
Pivot joints rotate around a single axis, according to OSU. Because of this, they enable turning motions without sideways displacement or bending. The joint between your first and second cervical vertebrae is a pivot synovial joint and allows for most of your head's range of motion while maintaining the stability of the head on the neck. Whenever you do a double take, you're using a pivot joint.
6. Saddle Joints
Saddle joints are characterized by two bones that fit together in a manner similar to a rider in a saddle, per OSU. This sort of articulation allows for certain bending motions in several directions without sliding. The joint at the base of your thumb is an example of a saddle synovial joint. As OSU points out, it's what gives humans the opposable thumbs that separate us from most other mammals. You use a saddle joint whenever you give a thumbs-up.
Common forms of exercise — walking, running, biking, swimming — use a variety of different synovial joints, often simultaneously. Your knee plays a major role in bipedal motion; it is necessary for the flexion and extension of your leg. Your feet contain condyloid and hinge joints that absorb impact and transfer motion. During a tennis match, your shoulder blade and collarbone glide together while your elbow hinges to power your serve and ground strokes. Your hips are in almost constant motion, with the ball and socket joints rotating to keep you upright or help you sit, twist or move laterally.
Synovial Fluid and Your Joints
Synovial joints are composed of a cavity surrounded by a membrane that is filled with with fluid, as described by the Hospital for Special Surgery in New York City. This synovial fluid nourishes and lubricates the cartilage that cushions your bones; it contains substances that increase cartilage density and elasticity.
Synovial fluid also helps protect your body from infection and inflammation. According to the NLM, this thick liquid can therefore be analyzed to diagnose problems such as gout, osteoarthritis and rheumatoid arthritis.
Exercising regularly can help keep your synovial joints in fine working order. It may also help increase the protective power of synovial fluid, and ultimately help mitigate the symptoms of diseases such as rheumatoid arthritis and osteoarthritis, which are characterized by a breakdown of joint cartilage.
Synovial fluid "acts like oil in an engine," explains the Arthritis Foundation's Living with Arthritis Blog. "Physical activity encourages circulation of the fluid."
If you have a joint-related disease such as gout, rheumatoid arthritis or osteoarthritis, talk to your doctor for guidance in coming up with an exercise plan for synovial joint health.
- U.S. National Library of Medicine: "Anatomy, Joints"
- Cleveland Clinic: "The Best Exercises To Keep Your Joints Healthy"
- Oregon State University: "Anatomy & Physiology — Synovial Joints"
- U.S. National Library of Medicine: "Synovial Fluid Analysis"
- Hospital for Special Surgery: "Synovitis"
- Arthritis Foundation Living with Arthritis Blog: "How Exercise Helps Your Joints"