The skeletal muscles are those tissues that are attached to the bones of the body beneath the skin. As the muscles on examination appear to be constructed of varying lengths of strips, due to the manner in which the muscle fibers are situated, these muscles are also known as striated muscle.
Skeletal muscle is a distinct type of specialized muscles found within the body. Cardiac muscle (heart muscle) is used only to power the contractions of the heart. Cardiac muscles are controlled through the function of the autonomic nervous system, the aspect of human function regulated by the hypothalamus region of the brain. Smooth muscles are located within every hollow organ in the body, with the exception of the heart. Smooth muscles are also controlled involuntarily, performing such functions as the pushing of blood within the arteries of the cardiovascular system and the movements of ingested foods within the digestive system.
All skeletal muscles are positioned relative to the bone in a similar fashion no matter where in the body they may be positioned, irrespective of the muscle function. The prime place of attachment between a skeletal muscle and the adjacent bone is the point of origin for the muscle. The muscle will taper at its opposite end into a more slender connective tissue, the muscle tendon, to the connection with the bone, the point of insertion. Imbalances between the strength of the skeletal muscle, the laxity or otherwise in the tendon, and the connection to the bone surface are common causes of muscle injury in athletes.
Skeletal muscle can only exert its desired force on the skeleton to produce movement when the muscle is contracted. Almost all joints in the body are comprised of muscles that operate in pairs: one muscle acts as an extensor, to extend or straighten the joint, and the other muscle in the pair acts as a flexor, to facilitate the bending of the joint. The biceps and triceps muscles of the upper arm are an extensor/flexor pair for the elbow joint, as are the quadriceps (extensor) and the hamstrings (flexor) in the movements of the knee.
The muscle fibers that are the substance of each muscle are of similar construction throughout all skeletal muscles. The fibers are generally long, slender cylinders that extend from the point of origin to the tendon that connects at the point of insertion. The fibers are bundled, in quantities ranging from a few fibers to several hundred. The contraction of each muscle fiber bundle is controlled through the nerve impulses directed into the fiber bundle by a neuron, a type of electrical relay that is connected to the larger nervous system. The speed with which the neurons communicate impulses to the muscle fiber group determine whether the fibers will be a fast-twitch fiber (useful in sports that require, power, strength, and reaction time), or a slow-twitch fiber (best suited to endurance sports). In fine motor control muscles, such as the eyelid, the neuron may only control a group of 10 muscle fibers or fewer. In a large muscle such as the quadriceps or the gastrocnemius, each neuron may be connected to as many as 2,000 fibers. The fibers are made up of myofibrils, filaments that run the length of the muscle fiber.
The operation of the nervous system and its relation to the skeletal muscular system is sometimes referred to as the neuromuscular system. When nerve impulses are communicated to the muscle, a complex series of electrochemical reactions convert the impulse into a muscle contraction. Central to the reaction is the balance between sodium and potassium in the muscle membrane fluid. Sodium floods the membrane at the time the impulse is registered, replaced by potassium to return the membrane to a rest state. The reactions occur very quickly, and a muscle can be restored to its rest position after the activity generated by an impulse in approximately one millisecond.
Muscle fibers require resistance to grow stronger; an inactive muscle cannot ever become stronger. The act of applying resistance to the muscle, such as is achieved through weight training, is not itself an immediately strengthening act; the muscle repairs itself during rest between resistance training sessions. As the body rests, the muscle fibers attract cells known as myoblasts, which fuse with the existing fiber, causing the muscle fibers to become denser and stronger. Muscle size is not limitless, and the fibers will not attract unlimited numbers of myoblasts for repair, due to the presence of myostatin in the muscle cells. Myostatin is the hormone produced by the body that regulates muscle size, a natural limit on how large muscles can grow.
The actual muscle contraction generate within the muscle is fueled by the chemical reaction that occurs involving the compound adenosine triphosphate (ATP), which participates in a series of energy-producing reactions that involve creatine phosphate, present in the muscle cell, and gylcogen, transported to the cell through the blood as glucose.