skeletal muscle moves the skeleton and is responsible for all our voluntary movements, as well as for the automatic movements required, for example, to stand, to hold up our head, and to breathe. (Other involuntary functions involve smooth muscle and cardiac muscle.)

As well as being the ‘motors’ of the body, muscles are also the brakes and shock absorbers. They can be used as heaters (when shivering) and also function as a store of protein if we should face malnutrition.

Individual muscles, such as the biceps in the arm, are made up of large numbers (about 100 000 in biceps) of giant cells, known as muscle fibres. Each fibre is formed from fusion of many precursor cells and therefore has many nuclei. The fibres are each as thick as a fine hair (50 μm in diameter) and 10–100 mm long. They are arranged in bundles, separated by sheets of connective tissue containing collagen. These bundles rarely run straight along the axis of the muscle, more usually at an angle, called the angle of pennation because many muscles show a pennate (featherlike) pattern of fibre bundles.

Each muscle fibre is surrounded by a cell membrane, which allows the contents of the fibres to be quite different from that of the body fluids outside them. Inside the fibre are the myofibrils, which constitute the contractile apparatus, and a system for controlling the myofibrils through changes in calcium concentration. This system, the sarcoplasmic reticulum (SR), is a closed set of tubes containing a high concentration of calcium. Each myofibril runs the whole length of the muscle fibre with a variable number of segments, the sarcomeres; it is only one or two micrometres in diameter, and is surrounded by the SR network. The myofibril consists of many much thinner and shorter protein rods, which are the myofilaments. These are of two kinds: thick filaments, which are made predominantly from a single protein, myosin, and thin filaments, which contain the protein actin. The actual contraction takes place by an interaction of the actin with projections on the myosin molecules (crossbridges). Each of the crossbridges can develop force (about 5 × 10^-12 Newtons) and can pull the thin filament along past the thick filament by about 10 × 10^-9 metres (10 millionths of a mm). The net effect of many of these small movements and small forces is to shorten the myofibrils, and thus the whole muscle; hence some part of the skeleton is moved, by means of the attachment of the muscle at each end to bone, directly or via tendons.

When a person initiates a movement, events in the brain and the spinal cord generate action potentials in the axons of the motor neurons. Each of these axons branches to send action potentials to many muscle fibres. (A motor unit is this collection of perhaps several hundred muscle fibres controlled by one axon.) At the nerve terminals of each axon branch (neuromuscular junction) acetylcholine is liberated by the arriving action potential, and this combines with receptors on the membrane of the muscle fibre, causing it, in turn, to generate an action potential. This action potential spreads over the whole surface of the fibre and also down an extensive network of fine tubes (T-tubules), which conduct it into the interior. Here a message, the nature of which is uncertain, passes from the T-tubule to the sarcoplasmic reticulum, causing it to allow some of the calcium it contains to leak out into the interior of the muscle fibre. The thin filaments in the myofibrils contain, as well as actin, two proteins, troponin and tropomyosin; the calcium which leaks from the SR is able, for a brief period, to interact with the troponin molecule of the thin filament; this, through movements of the tropomyosin molecules, alters the thin filament so that the actin molecules are available to be joined by the crossbridges, starting the process of contraction. As soon as calcium escapes from the SR the process starts of mopping it up again. There are calcium pumps in the membranes of the SR, which are able to move the calcium back inside, thus bringing to an end the short period of muscle activity (a muscle twitch). More sustained periods of activity are the norm in the movements we make; they require a sequence of action potentials to be sent to the muscle, at perhaps 30 per second. The contractions produced in this way are stronger than a twitch.

Muscle contraction requires energy to drive the crossbridges through their cyclic interactions with actin: in each cycle the myosin molecule does work in moving the thin filament. Also, energy is used for the process of calcium pumping by the SR. Energy consumption is highest when muscles are used to do external work — for example in climbing stairs, when the body weight has to be lifted. However energy is also used when a weight is held up without doing work on it (isometric contraction). Least energy is used when muscles are used to lower weight, as when descending stairs.

The energy for muscle contraction comes from the splitting of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate. The muscle contains enough ATP to power it at maximum output for only a couple of seconds. ATP can be regenerated in muscle rapidly from phosphocreatine (PCr), and there is enough of this substance in the muscle to last perhaps 10 to 20 seconds of maximum activity. The fact that we can sustain strenuous activity beyond 10 seconds is due to the utilization of carbohydrate in the muscles, where it is stored as glycogen. This can be used to regenerate the ATP supply in two ways. If oxygen is available, glucose can be oxidized to water and carbon dioxide, with two-thirds of the energy released used to rebuild the ATP supply. If oxygen is not available, the process stops with glucose converted to lactic acid and only about 6% of the energy used for building ATP. The lactic acid leaves the muscle cells and can accumulate in the blood. In addition to carbohydrate, muscles use fat, in the form of fatty acids taken up from the blood, as a substrate for oxidation; this is important for prolonged activity, since the body's energy stored as fat is much greater than that stored as carbohydrate. The availability of oxygen depends on its delivery by the blood; when muscle becomes active, the products of its metabolism cause the vessels to dilate, and this enables a rapid increase in the blood flow.

Muscle fatigue is the effect of a set of mechanisms which ensure that muscle is not made active when there is not enough energy available for the activity. If that were to happen, theoretically the muscle could go into rigor mortis, and could fail to retain the large amount of potassium it contains, with dire consequences for the body as a whole.

The body contains several different varieties of skeletal muscle fibre, which can be seen as specialized for different purposes. The ‘slower’ muscles are more economical at holding up loads, such as maintaining posture of the body itself, and probably also more efficient at producing external work. Related to their lower energy use they are less easily fatigued. Faster muscle fibres, however, can produce faster movements and higher power outputs, and are essential for such tasks as jumping or throwing. The way different muscles are constructed also allows for specialization of function: muscles with shorter fibres hold forces more economically, muscle with longer fibres can produce faster movements. A pennate arrangement allows muscles to be built with many short fibres, increasing the force they can exert, whereas long fibres, running almost parallel to the axis of the muscle, give the fastest movements.

Some people have more muscular strength than others; they can exert larger forces, do external work more rapidly, or move faster. To a large extent this is because the stronger individuals have larger muscles, but there seem to be other factors at work as well. Training can change the properties of muscle. Strength training consists in using the muscles to make just a few very strong contractions each day. Over months and years this leads to an increase in the force that can be exerted and in increase in the size of the muscles. Force increase often precedes size increase. Endurance training consists of using the muscles less intensely but for longer periods. Again, over months of training the ability of the muscles to get energy through the oxidation of carbohydrate and fat is raised. The supply of blood to the muscle is also increased through changes in the blood vessels and also in the heart. Training can also lead to changes in the fatigue resistance of muscle fibres, and perhaps cause them to change into a slower type of fibre.

Roger Woledge

Skeletal Muscle

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.

see also Bone, ligaments, tendons; Muscle fibers: Fast and slow twitch; Musculoskeletal injuries; Skeletal muscle function and energy metabolism.

skeletal muscle (skel-i-t'l) n. see striated muscle.

skeletal muscle See voluntary muscle

skeletal muscle See voluntary muscle.