In many respects the human body is like an internal combustion engine. Both need fuel to supply energy and both give off waste products. Both need constant maintenance and both eventually wear out – much more quickly if not properly cared for. The following discussion of muscle anatomy and physiology may bring back memories of high school biology. In fact you may still be in high school. That’s fine, as there’s nothing like practical application to help you remember something.

Muscle

The human skeleton is covered by approximately 650 muscles, which create the distinct contours and shape of the human body. Anatomists disagree on the exact number of muscles. The main area of controversy concerns whether certain muscles are one unit or two units working together. (Clearly we did not evolve to please anatomists.) It is generally accepted,however, that there are at least 630 muscles in the human body. By contracting and relaxing,these muscles produce movement.

Although bodybuilding helps strengthen bones, ligaments and tendons, it is the muscles that receive the greatest benefit from this sport. The words bodybuilding and muscles go hand in hand. Muscles show the results of your labor. You don’t ask a person to show you his Achilles’ tendon – you ask to see his biceps. Muscles are the symbol of strength, even though strength relies on tendon power and leverage as much as muscle power. The associated ligaments and tendons are forgotten in the excitement – until you pull one.

There are three types of muscle tissue: cardiac, smooth and skeletal. From a bodybuilding standpoint we are concerned with the third type, so we will be brief when describing the first two. Cardiac muscle is found only in the heart. Its main function is to force blood through the body's arteries, veins and capillaries. Smooth muscle makes up the walls of such internal hollow organs as the small and large intestines, the respiratory tract, and most of the reproductive system.

Skeletal muscle acquired its name because, for the most part, it is attached to the body's skeleton. This type of muscle is often said to be striated because of the alternating dark and light areas. (Smooth muscle is non striated, while cardiac muscle is finely striated.) Unlike smooth and cardiac muscle, which contracts involuntarily (i.e. you have no direct control over it), skeletal muscle contracts voluntarily in most cases. This means you can contract skeletal muscle whenever you want. Skeletal muscles are covered and held together by fibrous connective tissue called fascia.

Fascia can be divided into two types – superficial and deep. Deep fascia is composed of layers of dense connective tissue and is found between individual muscles and groups of muscles. As does superficial fascia, deep fascia contains an assortment of nerves, blood vessels and lymph vessels. 

Bodybuilding

Function

Skeletal muscle has three main functions – movement, heat production and posture. In normal day-to-day activities (standing, walking, sitting, etc.) the contracting and relaxing of muscles produces much of the heat responsible for maintaining the body’s internal temperature (average of 98.6 degrees Fahrenheit, 37 degrees Celsius). Even holding normal posture is a function of skeletal muscle. This is most obvious in people with lower-back injuries. The muscles of the lower back, called the spinal erectors, help keep the body in an upright position. If they become injured or even just tired, the person will start to slouch,having great difficulty sitting or standing upright.

Structure

Skeletal muscle is composed of long muscle cells called muscle fibers. These fibers give skeletal muscle its striated look. Muscle fibers are composed of smaller fibers called myofibrils,which in turn can be broken down into myofilaments. The muscle fibers are supplied with blood by a network of capillaries, small blood vessels that branch off from larger arteries. 

Capillaries serve two main functions: They bring oxygen and nutrient-rich blood to the muscles, and at the same time they remove the waste products of metabolism. Muscle contraction is under the control of the central nervous system and follows an all-or-none principle. As this book is not a biophysiology text, I will not go into great detail here. Suffice it to say that for a given muscle fiber to contract, it needs a certain amount of stimulus. When this stimulus is received, the muscle fiber in question fires (neurophysiological jargonfor contracts) to its full extent. A smaller stimulus does not produce a smaller contraction of the muscle fiber. 

Once the minimum stimulus needed is received, the particular muscle fibercontracts fully. Keep in mind, however, that one muscle is made up of perhaps half a million individual muscle fibers. If the stimulus is spread over a large area (i.e. the whole muscle), all the necessary muscle fibers will fully contract. The strength of the muscle contraction is based on the number offibers contracting, not the intensity of each fiber. Lifting a 20-pound dumbell will cause more fibers to contract than lifting a 10-pound dumbell. The individual muscle fibers, however, are contracting with the same intensity.

Muscle Contraction

Muscles become shorter when contracted and lengthen when relaxed. This action enables us to lift and hold objects, walk, breathe and perform many other activities that involve movement. While you are likely aware of this concept, you may not understand the actual mechanisms involved.

The theory states that muscle fibers contain two major proteins called actin and myosin, which are arranged in cylindrical bundles. When a muscle contracts, the ends of the myosin attach to the actin molecules of adjacent fibers. The myosin then undergoes a conformational change,resulting in the end of the fiber bending. This pulls on the actin fiber. Once the myosin filament completes this change, it detaches itself from the actin fiber and straightens out.

The whole process then repeats in a kind of “leap frog” manner, with adjacent fibers sliding past one another. In order for this process to work most effectively, sufficient quantities of calcium,sodium and tropomyosin must be present. Most of these are supplied in the diet, so it's obvious why nutrition plays such an integral part of muscle contraction and growth.

Fast-Twitch Muscle Fiber

Fast-twitch muscle fibers are adapted for rapid short-duration contraction. For example, the small muscles controlling the eyes and fingers are adapted for very quick movements, but they have a very short duration. Try blinking your eyes very fast for a sustained period of time. You’ll find that after a short time they tire to the point that you can hardly move them.

The reason? These muscles have a reduced amount of myoglobin, an oxygen-binding protein that speeds up the rate of oxygen movement into a muscle fiber. Fast-twitch muscle also has fewer capillaries than slow-twitch muscle. This means a slower rate of nutrient replenishment. The shortage of capillaries and myoglobin means a reduced number of red blood cells, giving the muscle a pale color. Such muscles are called white muscle.

Slow-Twich Muscle Fiber

Areas of the body that need prolonged, steady contractions are controlled by slow-twitch muscle fibers. Slow-twitch muscle fibers don’t tire as easily as fast-twitch. For example, the spinal erectors of the back, which keep us upright all day, are primarily composed of slow-twitch muscle fibers. Unlike fast-twitch, slow-twitch muscle fibers have a large capillary network and increased amounts of myoglobin. The increased myoglobin gives the muscle are ddish color.

The most easily visible examples of red and white muscle tissue are in poultry (chicken, turkey, duck, etc.). What we refer to as white meat and dark meat are in reality fast-twitch and slow-twitch muscle fibers. So what does this distinction mean for bodybuilding? For starters, most of the body’s main muscle groups are predominantly fast-or slow-twitch. This means that each muscle group will respond differently to varied rep ranges. In general, bodybuilders find that calves and forearms respond best to high reps (20 plus), whereas other muscles such as those of the chest and back seem to require lower reps (8 to 10).

Of course, some bodybuilders find heavy weight and low reps (6 to 8) to be the most effective combination for the calves, and others find high reps optimal for chest. An individual with a predominance of slow-twitch muscle fibers would need to include a high number of sets and reps to adequately stimulate his or her muscles. Conversely, someone with a proportionately high number of fast-twitch muscle fibers would probably respond better to heavier weights and lower reps. The latest research suggests that fast-twitch muscle fibers contract twice as quickly (intakes about one millisecond for a muscle fiber to contract) and are 10 times stronger than slow-twitch muscle fibers.

They respond best to workouts involving 2 to 6 sets of 4 to 6 reps. The sets should be performed with the maximum amount of weight that can be handled for the given number of reps, the sixth rep being the last that could be completed. Slow-twitch muscle fibers were found to respond most effectively to a high-set (8 to 10), high-rep (12 to 20) routine. The average untrained person has muscles with a 50:50 fast-twitch to slow-twitch ratio. In top athletes, however, we find a different ratio. This is partly because athletes excel at sports their bodies have an aptitude for, and partly because their muscles respond to the specialized demands placed on them. In simple terms, the body adapts to the type of stress endured (e.g. long-distance running versus weight training).

The ratio of the two contributes to success in the athlete’s chosen sport. Muscle biopsies on marathon runners reveal muscles with as highas 90-percent slow-twitch muscle fiber. For bodybuilders the ratio is reversed – red muscle fibers account for 80 percent of the muscle. The high percentage of red fibers can be explained by the type of workouts performed. Slow and continuous exercises with moderate to heavyweights (average rep range of 8 to 12) stimulate the production of red muscle fibers.

As for the white muscle fiber, recent studies suggest that it can be converted into red muscle fiber. This change results from slow and steady contractions over time. It’s believed that the type of nerve impulse determines the ratio of fast-twitch to slow-twitch (white to red)muscle fibers. However, even top bodybuilders display much variation in their muscle composition. Some have ratios in favor of fast-twitch, others in favor of slow-twitch.

Through trial and error each individual has determined which type of training works best for him. Unless your ratio is extreme – a rare phenomenon for the average person – you can make great progress no matter which type of muscle fiber predominates. To take full advantage of both slow-and fast-twitch muscle fibers, bodybuilders generally alternate rep ranges (6 to 8 versus12 to 15). This way, no matter what the individual fiber ratio, you can be sure your muscles are being fully stimulated.

Muscles – How They Work

Skeletal muscles produce movement by pulling on tendons, which in turn pull on the connecting bone. Most muscles pass across a joint and are attached to both bones that form the joint. When a muscle contracts, one of the attached bones remains stationary while the other moves along with the contraction. For convenience we say that the muscle section attached to the stationary bone is called the origin, and the point of muscle attachment on the moving bone is called the insertion.

Keep in mind that origin and insertion are relative terms,and can be reversed depending on the action involved. Contrary to popular belief, muscles can only pull, not push. Even though many exercises are considered “pushing” movements, the muscle being worked is actually pulling the associated bone. When you’re performing triceps extensions, the triceps muscle is contracting and pulling the forearm to an extended position, yet most bodybuilders refer to triceps extensions (as well as any other triceps exercise for that matter) as a pushing movement.

Similarly the bench press, while commonly called a pushing exercise, causes the chest muscles to contract, pulling the arms toward the center of the body. The human body evolved such that many of the major muscle groups work in opposing pairs. Triceps and biceps provide a classic example. Contracting the biceps draws the forearm toward the upper arm, whereas contracting the triceps extends the forearm away from the upper arm and back to its original extended position. To further enhance your bodybuilding biology vocabulary, the muscle that contracts to produce a desired movement is called the agonist.

The muscle that produces the opposite movement is called the antagonist. English majors will recognize the terms from studying novels, the protagonist being the central character, while his or her opponent is the antagonist. When the agonist (e.g. biceps) is contracting, the antagonist (triceps) is relaxed. As with insertions and origins, an agonist in one movement may be an antagonist in another. Bodybuilders often replace the words agonist and antagonist with flexor and extensor. Contraction of the biceps bends, or flexes, the elbow bringing the lower and upper arm bones closer together.

Conversely, contracting the triceps extends the elbow joint, drawing the bones away from one another, increasing the angle. Bodybuilders use the terms “flexing” and“extending” to describe the function of the muscle rather than the joint, so therefore you “flex”the biceps and “extend” the triceps. Most complex exercise movements are the result of many muscles working together. Forexample, the bench press is considered a chest exercise, but the shoulders and triceps also receive a great deal of stimulation. In fact, some trainers find the bench press a poor exercise for the chest.

Instead of great pecs, they end up with well-developed front deltoids or triceps.There is nothing wrong with this. It just means you have to start doing other exercises to workyour chest, and make sure to work your rear delts to keep up with the front. In the bench press the chest muscles (pectorals) are called the primary movers, and thedeltoids and triceps are referred to as secondary movers. Aside from secondary movers, other muscles also assist the primary movers. The back muscles (latissimus dorsi) help stabilize the body. Though they are not directly involved in moving the weight, they nevertheless assist the primary movers.

Source: Robert Kennedy: Encyclopedia of Bodybuilding, The Complete A-Z Book On Muscle Building. 2008
Nick Evans: Bodybuilding Anatomy. 2012
Arnold Schwarzenegger: The New Encyclopedia of Modern Bodybuilding, 2013