Skeletal muscle is a classic example of a biological structure-function relationship. Muscle cells and tissue are exquisitely tailored for force generation and movement. Muscle cells are roughly cylindrical, and up to a few centimeters long. Each cell is embedded in a basal lamina of collagen and large glycoproteins. Between the muscle fiber and the basal lamina are large numbers of other cells, that are important in the growth and repair of the fiber.
The muscle fiber itself contains specialized structures for excitation-contraction coupling to ensure that a contractile stimulus (received from a nerve) is rapidly and evenly communicated to the whole fiber. Muscles can receive and respond to a stimulus, usually by contracting, they can shorten forcibly, no other tissue type has this extensibility. The shortening is due to special proteins found in the muscle cells. These proteins slide in relation to each other, causing the entire muscle to shorten. Muscles can only generate force while shortening, they can not push, only pull.
For now, we will look at only one type of muscle; Skeletal. Skeletal muscle is voluntary, striated, and attached to the skeleton. Each skeletal muscle is an organ of 100s or 1000s of fibers, as well as nerves and connective tissue.
When muscle cells contract, they pull on the connective tissue (tendons), which transfers the force elsewhere. The connective tissue also gives the muscle elasticity. Skeletal muscle, unlike other muscles, requires nerves to contract. Because of the large amounts of energy used in contraction and the large quantity of wastes generated, a rich blood supply needed by skeletal muscle.
Skeletal muscles are attached to bones. The word "biceps" means "two-headed"(see the red and purple on the left). The word "brachii" means "of the arm". So the Biceps Brachii is the "two-headed muscle of the arm". At one end, it's attached to the radius, which is one of the two bones in your forearm. At the other end it is twice attached, (it has two heads, just as the name says), one head is attached to the top of your humerus, which is the bone in your upper arm. The other head is attached to the front of your scapula, which is your shoulder blade
Muscle Fibers - Each muscle fiber (aka myofiber) contains an array of myofibrils that are stacked lengthwise and run the entire length of the fiber, an extensive sarcoplasmic reticulum and many nuclei. The properties of a whole muscle depend not only on the properties of the fibers, but also on the organization of those fibers: the muscle architecture. Peak force production (strength) is related to the cross sectional area of all the fibers and the nature of the proteins with the muscle cells.
Although each muscle fiber is innervated by a single axon (nerve cell ending) , a motor neuron may have a hundred or more axons. A single motor neuron, along with all the fibers it controls, is called a motor unit. As the brain's signal for the muscle to contract increases, it both recruits more motor units and increases the "firing frequency'' of those units already recruited. Even during a "maximal voluntary contraction'', it is unlikely that all the motor units (and hence muscle fibers) are activated.
All skeletal muscle fibers possess a neuromuscular junction by which signals are transmitted from the nervous system via neurotransmitter, acetylcholine. The binding of acetylcholine to receptors on the membrane of the muscle causes a change in the voltage of the muscle membrane. This voltage change opens "gates" and causes the release of Ca2+ from the sarcoplasmic reticulum via a set of channel proteins. The released Ca2+ binds to troponin (another muscle protein). The binding of Ca2+ to troponin allows crossbridge formation between actin and myosin (an energy dependent process). The crossbridge formation leads to muscle fiber shortening and the generation of force.
When muscles shorten, they move bones at the joints. Joints are set up as lever systems: the fulcrum is where the two bones meet, one force is produced by the muscle, and the other by a loadon the bone. Muscles can not push bones, they can only pull. Each bone is usually controled by at least two muscles. One muscle pulls the bone one direction, the other pulls it back to its original position.
Strength is not just muscle force, but muscle force as modified by the mechanical advantage of the joint. The strength of a muscle can (obviously) be increased with exercise. Usually strength training increases the number of fibers within a muscle cell, not the overall number of muscle cells.