Joints Have Two Duties
The joint is where two bones meet and can be a fairly vulnerable place in the body. This is because it serves dual purpose: it must allow for maximum movement, flexion, and extension, but it also must remain stacked to maintain support of limbs and load bearing.
Think of those little push puppets that collapse when you press on the button. These are a great example of the two duties of the joint:
- They must be stiff and strong to keep the animal upright.
- When the situation calls for it, they must also be loose and flexible to allow for movement and bending.
Each function is vitally important to the animal's movement, and one without the other leaves the animal either unable to move, or unable support its own weight.
Movement is a beautiful orchestration of nerves signaling some muscle groups to contract and others to relax. This collaboration between various muscle groups allows for the flexion or extension of the joint. The cartilage on the bone endings (coated in thick, viscous joint fluid) keeps the movement smooth when bending and straightening limbs.
As various joints move in concert, the body is able to allow movement forward in one joint while keeping the opposing joint rigid and strong to support the weight of the body.
Elements of a Joint:
- Two bone endings: like at the elbow, where the humerus bone meets the radius & ulna bones, for example
- Joint fluid: thick, viscous fluid (molasses-like) that both lubricates the movement of the bone endings, and supplies nutrients to the surrounding cartilage
- A joint capsule: a membrane that keeps the joint fluid in place around the bone endings
- Cartilage: a smooth, porcelain-like surface that allows for gliding and protects the bone endings
Ligaments Are Like Rubber Bands
Looking at the above illustration of a joint, you can see the outer orange and red "strings" that form around the bone endings. These are an array of tendons and ligaments designed protect the joint from harm. While these ligaments are allowing movement of the joint, they are also restricting too much movement.
There's a good kind of joint movement, which is the motion that is intended by the brain (ie - walking, lifting an arm, bending a finger). Then there's a bad kind of joint movement, which is movement that shouldn't be happening (ie - one bone ending sliding off the opposing bone ending, dislocation).
Weak muscle tissue cannot properly limit the amount of movement in the joint, which results in the joint gliding out of place (and vulnerable to even greater injury). On the other hand, muscles that are too strong can keep the joint from being flexible, leading to muscle strains when the joint wants to move. Much like a rubber band! Too loose, and it holds nothing in place. Too tight, and there's no wiggle room.