Every muscle is actually a wrapped package, containing other smaller wrapped packages of long, slender cells known as muscle fibers. The outer wrapping, made of connective tissue, is called the muscle fascia. The smaller packages are called muscle fascicles, and each one contains a bundle of up to 150 muscle fibers. At both ends of every muscle, the fascia covering the muscle tapers to form a strong, rope-like length of connective tissue called a tendon, which is connected directly to one of your bones. One end, which connects to a relatively unmoving skeletal part, is the origin of the muscle. The point where it's attached to a moving bone is the insertion of the muscle. For example, the biceps muscle originates at the shoulder, and its insertion is in the forearm, near the elbow, which allows the forearm to flex during muscle contraction.
When the muscle contracts, it pulls on the tendon, and this causes the bone to move. The bigger the muscle, the more force it can generate on the bone. During contraction, the muscle pulls its origin and insertion closer together. Often a muscle is attached to either side of a joint, allowing motion of the joint during muscle contraction. For example, the biceps pulls the forearm up toward the body across the elbow joint.
Each muscle fiber shares a nerve ending with other nearby fibers, making up a group of fibers known as a motor unit. Every time the master motor nerve fires (sends an impulse to a muscle), this motor unit contracts simultaneously. This effect is called the "all-or-nothing" principle of muscle contraction.
So how many fibers are in a motor unit? It depends on whether the muscles are used for large, powerful movements, which require less nerve control, or for intricate activities, which call for more nervous system input. A typical finger muscle contains 40,000 muscle fibers divided into 120 motor units — a ratio of 340 fibers per nerve ending. The eye muscles are even more finely controlled, with 10 fibers per nerve. On the other hand, each of the 580 motor units in the large muscle of the calf is much bigger — averaging about 2,000 muscle fibers per nerve ending.
Every time a nerve ending fires, a burst of energy is released in each individual muscle fiber, causing tiny filaments to slide toward each other. The result is a significant shortening of the muscle fiber. When the fibers in a motor unit contract in unison, the result is a muscle contraction. Whatever form of exercise you're doing, from swimming to bicycling, your movements depend on the repeated, coordinated firing of the appropriate motor units. Improved coordination of this firing sequence is a major reason you get more skilled at any physical activity with practice.
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