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What is a joint?
Joints occur wherever two or more bones meet. They are, unfortunately, the weakest part of the skeleton, but we’d be in real trouble without them: they hold our bones together and they permit us to move in various ways.
How do joints stay together?
Joints are held together by three types of connective tissue:
- Tendons usually connect muscle to bone. They are mostly composed of parallel groups of collagen fibres closely packed together.
- Cartilage, stiff and inflexible, is very dense, has poor blood supply, and has no nerves. It is not as hard and rigid as bone. There are three categories: fibrocartilage (creates a shock absorber between bones), costal cartilage (connects ribs to the sternum), and articular cartilage (reduces friction during movement at the end of articulating bones).
- There are actually three types of ligaments — articular, peritoneal, and fetal remnant. When people use the word ligament by itself, they are usually referring to articular ligaments which stabilize the bones of a joint and connect bone to bone.
Classification of Joints
The structure of a joint can be one of three types:
- Fibrous joints are held together by fibrous connective tissue and do not have a joint cavity.
The bones of the skull are fibrous.
- Cartilagenous joints are held together by — surprise! — cartilage. They do not have a joint cavity.
Intervertebral joints of the spine and cartilage which joins ribs to the sternum are both cartilagenous joints.
- Synovial joints have long, articulating bones which are separated by a cavity filled with fluid.
The knee joint is a synovial joint.
The function of a joint will be:
- slightly movable; or,
- freely moveable.
Movement Patterns of Joints
Joints can move in various ways. Some of the most common types are:
- hinged joint: The rounded portion of one bone fits into the depression of another.
The knee is a hinge joint.
- Pivot joint: The rounded end of one bone extends into a sleeve which is formed by either a bone or ligament. This type of joint can only rotate around its axis.
The C1 and C2 vertebrae of the spine when the head is rotated are pivot joints.
- Condyloidal (or elliposoidal) joint: Both surfaces are oval in shape: the surface of one bone fits into the hollow or depression of the other.
The wrist is a condyloidal joint.
- Saddle joint: Each of the articulating bones come together to look like a saddle, thus giving it its name. Both bones have elevated and depressed aspects.
When you twiddle your thumbs, you are using the saddle joint at the base of the thumb.
- Ball-and-socket joint: The most mobile joint because the round head of one bone fits into the depression of the other.
The shoulder and hip joints are both ball-and-socket joints.
Here are drawings of some of the different types of joints. 2
Can exercise hurt your joints?
Exercise is widely promoted in the media and in fitness journals as having many benefits, including weight control, prevention of cardiovascular disease and diabetes, as well as improving psychological well-being.
In contrast, unfortunately, the community perception tends to be that exercise is potentially harmful to one's joints.
Which viewpoint is right? Here are two:
Canadian Online Fitness Education (a Fitness Theory course for fitness instructors) says this: “Articular cartilage does not have its own blood supply and must rely upon repetitive weight bearing movements to bring in nutrition and remove waste. When the joint is compressed, nutrition is forced into the cartilage and when weight is removed the metabolic wastes flow out and can be removed by the surrounding blood vessels. When synovial fluid is warmed up during exercise, it becomes thinner and absorbed more easily by the cartilage. This increased absorption leads to swelling of the articular cartilage and increased cushioning ability.” 3
Two men — Dr. David J. Hunter (from New England Baptist Hospital in Boston, Boston University School of Medicine) and Felix Eckstein (from The Institute of Anatomy and Musculoskeletal Research Medical Lab) — combined research data to determine if exercise could assist the joints. Hunter and Eckstein particularly focused on those who suffered from osteoarthritis but exercised, and they published their results in October 2008. The conclusion of the researchers was: “Despite the common misconception that exercise is deleterious to one's joints, in the absence of joint injury there is no evidence to support this notion. Rather it would appear that exercise has positive benefits for joint tissues in addition to its other health benefits.”
And my own view: Moderation in all things! We know that marathoners have trouble with their knees. If you want to exercise for general health and fitness, there are easier ways to do it than running 26 miles.
- Dem Bones, Dem Bones: The Skeleton
- Planes of Movement
- The Shoulder Joint
- Exercise and Arthritis
- The Hip Joint
- Hip Replacement
- The Hip Flexors
- Is the old wives' tale true: Does your arthritis forecast tomorrow's weather?
- The Knee Joint
- Knee Replacement: The Basics
- The Shoulder Joint
- The Wrist and the Ankle
I am a BCRPA-certified fitness instructor in Vancouver, BC. I teach four classes at the West End Community Centre in Vancouver, BC, mostly designed for the older adult. The Inevitable Disclaimer: Everything published here expresses only my opinion, based on my training and research. What you do with the information is entirely your own responsibility. I am not liable for any injury you suffer that seems to be related to anything you read here. Always consult your doctor before beginning an exercise program. For other articles, return to the table of contents.