Skeletal systemSkeletal MusclesThere are well over 600 skeletal muscles in the human body. Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg.
USE IT OR LOSE IT In exercises such as weight lifting, skeletal muscle contracts against a resisting force . Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it. 
Muscles, Bones, and Movement
Skeletal muscles are attached to the skeleton by tendons. A tendon is a tough band of connective tissue that connects a muscle to a bone. Tendons are similar to ligaments, except that ligaments join bones to each other.
Muscles move the body by contracting against the skeleton. When muscles contract, they get shorter. By contracting, muscles pull on bones and allow the body to move. Muscles can only contract. They cannot actively extend, though they can move back into the non-contracted neutral position. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. Each muscle in the pair works against the other to move bones at the joints of the body. The muscle that contracts to cause a joint to bend is called the flexor. The muscle that contracts to cause the joint to straighten is called theextensor. When one muscle is contracted, the other muscle from the pair is always elongated. For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. When you want to bend your elbow, your biceps muscle contracts, and, at the same time, the triceps muscle relaxes. The biceps is the flexor, and the triceps is the extensor of your elbow joint. |
Locomotion
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