Muscle defined is a tissue capable of changing shape and length to cause movement.

Although most of us think of muscle as a tissue that allows us to move, push, pull or lift weight, there are several other roles for different types of muscle that are critical in human function. Skeletal ddmuscles primarily serve the body as contractile tissue to allow bones to move around joints. Smooth muscle helps move blood through our blood vessels and food through our digestive tract. Perhaps one of the most important muscles is cardiac, which is your heart.

Muscles typically work in pairs to support and balance the body. The front thigh (quadriceps) muscle is the opposite to the back thigh (hamstrings) muscle. The chest (protractor) muscles are the opposite to the back (retractor) muscles. These are also called antagonistic muscle groups. Each contracts to move joints in a certain direction or rotation to the angle of force being created. When one contracts, the other must relax. For example, your bicep must relax and lengthen when your triceps contract and shorten. If this relationship did not exist and if both muscle groups were to contract at the same time, your arm would lock and there would be no movement at the joint.

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.

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 fasciculi (fascicle), and each one contains a bundle of up to 150 muscle fibers. Endomysium envelopes all the muscle fibers in a fascicle. Perimysium wraps all of the fasciculi and the epimysium surrounds the entire muscle. Thus, when a muscle fiber contracts, it pulls on the endomysium which pulls on the perimysium which pulls on the epimysium which in turn pulls on connective tissue fascia which finally pulls on the tendon, and this causes the bone to move. The bigger the muscle, the more force it can generate on the bone.

Each muscle fiber shares a nerve ending with other nearby fibers, making up a group of fibers known as a motor unit. A motor neuron must fire from the spinal cord to produce a signal telling the muscle to contract. 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. When the fibers in a motor unit contract in unison, the result is a muscle contraction. Whatever form of exercise you're doing, from running to swimming to bicycling, your movements depend on the repeated, coordinated firing of the appropriate motor units. The more you develop an athletic skill the more efficient your motor units become at firing for the desired result.