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How Bones Help Us Move : Muscles, Tendons, Ligaments
Bone must do far more than provide shape and support for the body. It must also provide us a means for moving about. Thus, the bones of the arms, wrists, hands, legs, ankles, feet, chest, and back all fit together to form many different kins of connections or joints. These joints permit motion where it is necessary and prevent it where it would be dangerous.
Muscles Moves Bone
It is the muscles attached to the bone across these joints which permit us to move.
In most cases, the joint muscles work in opposite pairs. When one muscle of such a pair tightens or contracts and pulls on a bone, the opposite muscle relaxes and allows the bone to move.
Then, when the opposite muscle contracts and pulls the bone back again, the first muscle relaxes.
Example 1: How Your Forearm Moves
To see how this works, hold your arm out straight from your shoulder with your palm up. Now raise your forearm up to a right angle at your elbow. What makes the forearm lift up? The answer is, the heavy muscle called the biceps on the inner surface of the upper arm. This muscle has contracted across the elbow joint and lifted the bones of the forearm.
You can feel that the biceps muscle is bulging and much shorter than it was when the arm was extended out straight. Now straighten the arm out again and feel the biceps muscle. You’ll see that it is much longer and feels flabby – but now the triceps muscle on the back of the upper arm (opposite the biceps) feels bulging and tight. One muscle tightens or contracts while the other relaxes and vice versa. It is the two muscles working as a team in this manner that allows you to bend and straighten your elbow.
Example 2: How Your Fingers Move
Each of your finger bones is connected by means of a pair of long, stringlike tendons to muscles in your forearm. (A tendon is a tough, strong tissue, much like a narrow leather shoestring, which binds muscle to bone.) When the muscles on the back of your hand contract, their tendons pull your fingers out straight or extend them; these muscles are know as finger extenders. Their opposite muscles on the inner surface of the forearm are called finger flexors because when they contract, their tendons bend or flex the fingers in toward your palm.