There are many types of joints in the body including fibrous, cartilaginous and synovial joints. The synovial joints are characterized by the presence of synovial fluid within a space that encapsulates the articulating surfaces (surfaces that touch each other) of the joint. The synovial capsule reduces the friction between the bones allowing more smooth movement. There are six types of synovial joints: gliding, condoloid, saddle, hinge, ball and socket, and pivot joints.
Gliding joints allow for smooth movement in several directions along a plane or other smooth surface. The articulation is like two plates sliding across each other. An example would be the carpal bones of the wrist, which form a gliding synovial joint.
Similar to gliding joints, condyloid joints are somewhat different in that they have an irregular surface where the bones move past one another. This type of joint is like two bowls nested together. The radio-carpal joint of the wrist is an example of a condyloid synovial joint.
Saddle joints are characterized by two bones that fit together in a manner similar to a rider in a saddle. This sort of articulation allows bending motion in several directions without sliding. The carpal-metacarpal joint of the thumb is an example of a saddle synovial joint.
Hinge joints, as the name implies, are hinged joints formed between two bones. A hinge joint allows for stable flexion and extension without sliding or deviation. The elbow joint between the humerus and ulna is a hinge synovial joint.
Ball and Socket Joint
Ball and socket joints allow for stable movement in several directions without slippage. Like a saddle joint, the ball and socket joint allows bending in several directions without slipping, creating a highly stable, strong joint. The hip joint (femur-acetabulum) is an example of a ball and socket synovial joint.
A pivot joint is a joint in which rotational motion occurs without gliding movement. This type of joint allows for turning motions without sideways displacement or bending. The joint between the first and second cervical vertebrae (atlas-axis) is a pivot synovial joint and allows for most of the head's range of motion while maintaining the stability of the head on the neck.
Synovial joints offer the widest range of motion for your body, making them an essential part of exercise. These joints provide lubrication and cushioning to bones and tissues as you exercise. In return, exercising has the potential to maintain synovial joint health.
Synovial Joint Features
Synovial joints feature a synovial cavity surrounded by a synovial membrane that is filled with with fluid, which nourishes the articular cartilage. Bones encased in this type of joint are covered in hyalin cartilage, a substance that provides cushioning between the bones. Synovial membrane cells secrete substances called hyaluronan and lubricin into the synovial fluid to increase hyalin cartilage density and elasticity. Synovial fluid also helps protect your body from infection and inflammation.
Movement of Synovial Joints
Exercise requires the use of almost all six types of synovial joints. Ball and socket joints -- like the hip and shoulder -- provide the greatest range of motion while your fingers, toes and elbows are hinge joints and move in two directions. Pivot joints are found between bones like the radius and ulna in the forearm, with one bone rotating around the other. Your wrist contains condyloid or ellipsoidal joints -- where the contact surface of one bone is concave and the other is convex, allowing the joint to move in a circle. Saddle joints -- found in your thumb -- have similar range of motion to condyloid joints, but are shaped like a saddle. Your acromioclavicular joint, which is part of your shoulder, is an example of a gliding joint.
Joints During Exercise
Common forms of exercise like walking, running, biking and swimming use a variety of synovial joints. 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 joint rotating to keep you upright or help you sit, twist or move laterally. Exercising regularly can help keep your joints in working order.
Exercise and Synovial Fluid
Exercising may help increase the protective power of synovial fluid, and may potentially help manage arthritis and osteoarthritis. These diseases are characterized by a breakdown of joint cartilage as well as decreases in synovial fluid and increases in inflammatory substances. An article in "Arthritis Research & Therapy" in July 2010 examined the effects of exercise on women with osteoarthritis of the knee. The study found that women who exercised experienced increases in anti-inflammatory substances in their synovial fluid. In comparison, the non-exercising group did not experience these effects. If you have arthritis or osteoarthritis, talk to your doctor before beginning an exercise program.