Major Muscles and muslce power

Since muscle fibers exert pull when they contract, they use energy. The energy comes from food supplied to them through the blood. A muscle works by converting chemical energy into mechanical energy. Actually, only about one fourth of the chemical energy is converted properly into mechanical energy; the remaining three fourths is lost as heat, raising the temperature of anyone doing strenuous work. This efficiency of 25 percent is similar to that of an automobile engine, which also loses much of its energy as heat. There have been calculations which suggest that the maximum energy output for man is about 6 horsepower and that as much as 0.5 horse- power output can be sustained almost indefinitely.

SOME MAJOR MUSCLES Skeletal muscles are of many shapes and sizes suited to their particular jobs-and they have many jobs. The sternomastoid muscles, which are on either side of the head, serve two purposes. When you nod your head, it is because both of the sternomastoid muscles contract simultaneously. You turn your head to one side or the other depending upon which of the two you contract. Among muscles at the shoulder are the trapezius, which shrugs the shoulder when it contracts, and the pectoralis major, which spreads over the chest and attaches to the humerus and helps sweep the arm across the chest. On the forearm are muscles that divide into tendons extending down to the fingertips; they help move the fingers.

 Among the big muscles of the lower extremity are the gluteal muscles on the buttocks, which, with their contraction, move you from sitting to standing position and are involved in walking; the sartorius, in the thigh, the longest muscle in the body, which pulls the thigh into cross-legged position; the quadriceps in the thigh used [or balance during standing and for kicking; and the gastrocnemius and solt'us in the calf which enable you to stand on tiptoe and provide from the ground for walking, running, dancing. Along each side of the spinal column maintain an erect posture, to bend the body, and to help turn it to one side or the other. Inside the body is the major muscle for breathing, the diaphragm, which is attached through tendons to the spinal column, ribs, and lower tip of the breastbone. The contraction of the diaphragm helps fill the lungs with air. And the diaphragm, incidentally, is also used in laughing, sneezing, and coughing. Its spasmodic contractions occasionally produce hiccupping.