J. K. Johnson, K. J. Farnand, J. Derivan, D. Facey

(presented by Saint Michael's College Department of Biology)

Muscles make and restrain motion, supplying force for movement as well as holding bodies in position. Muscles not only act in conjunction with the skeletal system to produce gross body movement, but they also act upon the viscera to produce movement of organs, blood vessels and glands. Muscles are an integral part of both respiratory and circulatory functions. They play a role in heat production.

Skeletal muscle is composed of numerous long, cylindrical cells called muscle fibers. These fibers lie parallel with one another in length and are are subdivided into cylindrical subunits called myofibrils. Muscles are attached to the bones of the skeleton by tendons, which integrate into the muscle. Myofibrils are composed of many sub-units in series called sarcomeres. Each sarcomere contains thick filaments of myosin, and thin filaments of actin. The sarcomere is the functional unit of the muscle during contraction.


What are the structural and functional differences between skeletal, smooth and cardiac muscles and where are they found?

Skeletal muscle is the type normally thought of when one thinks of the word muscle, but do not be mislead, this is only one of three different types of muscle. The three different muscles, classified by their general microscopic appearance, are skeletal, cardiac and smooth. Let's start first with skeletal.

Skeletal muscle is characterized by elongated, multi-nucleated fibers, or cells, with striations due to light, I band, and dark, A band, alternations. The nuclei of the fibers are not centered, but rather located towards the outer edge of the cells. These muscles are under somatic or voluntary control. This means they are under immediate conscious control by the nervous system. Skeletal muscle has a name related to its function since it is usually associated with attachment to and movement of bones and cartilage. Skeletal muscle requires either extrinsic nerve or hormonal stimulation to create action potentials, which trigger movement.

Cardiac, like skeletal muscle, is also striated. Though unlike skeletal, cardiac muscle cells are short and branched with a single, centered nucleus. They are also involuntary or not under immediate conscious control. Rather than Z-disks, which join skeletal muscle cells, intercalated disks join cardiac muscle fibers. Cardiac muscles are located only in the heart. Unlike skeletal, cardiac muscle can contract without extrinsic nerve or hormonal stimulation. It contracts via its own specialized conducting network within the heart, with nerve stimulation causing only an increase or decrease in rate of conducting discharge. The heart also has some very beneficial features such as an increased number and larger mitochondria, which allow it to produce more ATP. This is very important since the heart is constantly contracting and relaxing. Cardiac muscle can also convert lactic acid produced by skeletal muscle to ATP. This is quite ingenious since lactic acid is a by-product of muscle when in a deoxygenated state, a state that would be detrimental to cardiac muscle. This muscle also remains contracted 10 to 15 times longer than skeletal muscle due to a prolonged delivery of calcium (see discussion of cardiac action potential in Circulation section). Likewise, it also has a relatively long refractory period, lasting several tenths of a second, allowing heart to relax between beats. This also allows heart rate to increase significantly without causing it to go into tetanus, which would be fatal since it would cause blood flow to cease.

Lastly there is smooth muscle in which ellipsoidal cells have a single centered nucleus. Like cardiac, smooth muscle is also involuntary. Smooth muscle is not striated like cardiac and skeletal due to a non-orderly arrangement of myosin and actin filaments. Its muscle also lacks Z-disks, instead microfilaments are attached to each other by dense bodies. Sliding of the filaments generates tension that is transmitted to intermediate filaments, in turn they pull on dense bodies attached to sarcolemma causing lengthwise shortening of muscle fibers. Contractions are corkscrew-like, start more slowly and last much longer in comparison to striated muscle. This is because smooth muscle lacks T-tubules, therefore it takes longer for calcium to reach the filaments as well as be reabsorbed by the terminal cisternae. Smooth muscle can also stretch to a greater extent than striated. The bladder, for example, contains calmodulin instead of troponin, which striated muscle contains. In smooth muscle, the myosin binding sites on actin are blocked by caldesmon. When the calcium-calmodulin complex binds to the caldesmon, the caldesmon moves away from the myosin binding sites on the actin molecule and cross-bridges can form. Smooth muscles also is sensitive to mechanical stimulation, and may contract when stretched. Depoalariztion of smooth muscle often involves the inward flow of calcium across the cell membrane, rather than sodium. There are two different types of smooth muscle, single-unit and multi-unit. Single-unit cells, usually small and spindle shaped, are connected to one another by gap junctions which allows excitation to spread throughout the muscle without neuronal input. Internal pacemaker cells usually regulate contraction in this type of muscle. Single unit cells form the walls of vertebrate viceral, hollow organs such as the urinary bladder, ureters, uterus, stomach, intestines, small arteries and veins. Multi-unit cells each act independently and only when they receive neuronal input. This type of muscle forms the iris of the eye, large arteries, bronchioles and arrector pili muscle (for elevating hairs - the "goose bump" muscles).


What is a T-tubule?

T-tubules are small infoldings of the sarcolemma which has terminal cisternae on both sides (all of which is referred to as a triad). The t-tubules aid in the conduction of action potentials, and trigger the release of calcium from the terminal cisternae of the sarcoplasmic reticulum, resulting in the contraction of the muscle.


What composes a thin filament?

Thin filaments are composed of actin, tropomyosin, and troponin, and are found in sarcomeres. Actin is the major component of thin filaments, which are composed of several actin units in a chain series that are twisted into a helical form. Each actin strand consists of about 200 actin units and each unit contains a cross bridge binding site. Next, tropomyosin wraps around the helical actin and covers the cross bridge binding sites in an unstimulated actin chain. Finally, troponin attaches to the tropomyosin in evenly spaced increments to complete the thin filament.


What is a sarcomere, and how does it function?

Sarcomeres are the functional unit of striated muscle (skeletal & cardiac). It is a series of sub-units, divided by Z lines, and is found in the myofibrils of muscle. The sarcomere is composed mainly of thick filaments (myosin), thin filaments (actin), and elastic filaments (titin). All of these filaments have a specific function in the sarcomere and are arranged in specific patterns that are distinguishable by color. The darker region of the sarcomere is the A (anisotropic) band which consists of overlapping actin and myosin filaments. Next, the lighter, less dense area called the I (isotropic) band consists of actin only. A light narrow zone in the middle of the sarcomere defines the H zone which contains only myosin and is divided in half by the M line which connects the thick filaments. Finally, the Z line defines the end of each sarcomere and divides each I band in the middle. The actin filaments are attached directly to the Z lines, and the myosin filaments are attached to the Z lines via the elastic (titin) filaments. These attachments ensure proper placement in order for optimal cross bridge attachment during contraction.

When the muscle contracts the myosin heads (cross bridges) attach to the actin binding sites and the myosin head produces a "power stroke", pulling the actin filaments towards one another, and thereby causing movement of the bones by the muscle. After the "power stroke" has occurred, ATP breaks the myosin head bond with actin, and the myosin head returns to its resting position until another contraction is needed. Note that there are many cross bridges along the myosin filament and that they work in series with one another. At no time during contraction are all of the cross bridges attached or not attached. This lack of synchrony in the formation of cross bridges allows the muscle to contract smoothly.


What is the role of calcium in muscle function?

Calcium is the so called "trigger" for muscle contraction. Calcium aids in the formation of action potential in the motor end plate, and is later released from the terminal cisternae of the sarcoplasmic reticulum into the cytosol of a striated (cardiac and skeletal) muscle cells. Next the calcium ions bind to troponin which causes a change in the conformation of the troponin-tropomyosin complex that exposes the myosin binding sites on the actin filament. The myosin heads then attach to the actin filament and a muscle contraction occurs. In smooth muscle, the influx of calcium leads to depolarization. The calcium binds to calmodulin, causing the calmodulin-caldesmon complex to change its confuguration and pull the caldesmon away from the myosin binding sites on the actin strand. Muscle contraction follows because the myosin heads bind to the actin.


How does an action potential initiate a muscle contraction?

The nerve generates an action potential and it arrives at the axon terminal where it causes a change in the voltage of the axon terminal membrane, thereby causing the calcium channels to open. This allows calcium ions to enter the axon terminal which causes the release of ACh (acetylcholine). As the ACh is released it attaches to the motor end plate and the calcium ions are pumped out of the axon terminal. Next, the chemically regulated ion channels on the motor end plate open and an influx of sodium ions, and efflux of potassium ions occurs. This causes a depolarization of the motor end plate. The ACh diffuses from the motor end plate and the ion channels close. The depolarization of the motor end plate then moves along the sarcolemma and down theT-tubules. This depolarization of the T-tubules causes the terminal cisternae to release calcium ions into the cytosol of the muscle cell. The calcium binds with troponin and causes the tropomyosin complex to shift, thereby exposing the myosin head binding sites and allowing the myosin heads to attach and perform a "power stroke", or contraction.


How is chemical energy transformed into physical energy?

Chemical energy in muscle fibers is in the form of ATP. In order to produce physical energy, ATP is hydrolyzed to become ADP and Pi (adenosine diphosphate and inorganic phosphate). When ATP is broken down into ADP and Pi the cross bridges are energized which allows for the "power stroke", or force production of a muscle contraction


Why don’t muscles produce maximum contractions all the time?

It is unnecessary to expend the amount of energy needed to carry out this operation when muscles can still function at fractions of the energy. Depending upon the task, muscles need only contract the necessary amount of fibers, i.e. when lifting a sheet of paper muscles utilize less fibers than when lifting a bowling ball. Contraction is the activation of muscles and the resultant generation of force; not as much force is needed to lift the paper as is needed to lift the ball, therefore not as many components of a muscle need to be activated to when generating this smaller force. The same muscle can deliver a large force, as well as a small force, to the same bone. This is called graded motion. One way in which it is generated is by rate modulation. Nerve impulses must be at or above a threshold level for muscle twitch to occur. Up to a certain point force increases as rate of nerve impulses increase, this is rate modulation. After a certain point force peaks regardless of increase in rate of impulses.

Most vertebrate striated muscle fibers respond to impulses from a single motor unit with all-or-none twitches. Although the muscle fiber stimulus is all-or-none, entire muscles can function upon graded contractions. It is groups of muscle cells that compose muscles, therefore single cells do not act alone to produce movement. Increasing the force of movement many times means stimulating more cells, a phenomenon known as recruitment. This increases the total output force a muscle generates.

This question can also be seen in light of a different perspective. Muscles are not in a constant state of contraction because this would allow for no movement. In order for movement to occur relaxation of muscles must also occur. Muscles would be in tetanus (tetani), or rigidity if in a constant state of contraction. In tetanus the calcium pumps do not have enough time to recollect all the calcium released into the myoplasm after the first stimulus and before successive action potentials occur. This buildup of calcium causes saturation of troponin, which remains in this state until action potentials cease. A rapid succession of impulses to muscle cells does not always produce a state of rigidity and can actually be beneficial. The cumulative effect of successive action potentials causes stronger contractions that can produce more force, an event known as summation. (Please note that this use of the term "summation" is different than the temporal and spatial summation concept discussed in the section on nerve function.)


What determines the strength or force of a muscle?

Although maximum force for individual muscle fibers occurs when the fiber is within a small range a specific lengths, due to optimal cross bridge attachment, muscle force is proportional to cross-sectional area. The cross section of a muscle represents all of the muscle fibers perpendicular to their longitudinal axes or all of the muscle fibers present. The more (parallel) fibers present, the greater the tension and the greater the force produced. Therefore a long muscle and a short muscle with physiological cross sections of the same girth generate equal forces or tension.


How does the length of a muscle affect the strength of a contraction?

If the muscle fibers are at a short length while they are relaxed they will not produce maximum tension when they contract. This is due to an overlapping of the thin filaments in the sarcomere which inhibits cross bridge binding, and results in less tension. If a muscle fiber is at an extended length prior to contraction the thin filaments are pulled nearly to the ends of the thick filaments which results in a lack of cross bridge attachment and poor production of force. Optimal muscle contraction is usually achieved when a muscle has been repeatedly stimulated (warmed up) prior to it’s intended use and then allowed to rest for a short period of time before it is used for the intended purpose. This "warm up" allows for the thick and thin filaments to position themselves in such a way that there will be optimal cross bridge attachment during the contraction.


How does exercise affect muscle formation and growth?

The number of fibers in any given muscle is fixed at birth for any given person. Exercise stimulates production of greater amounts of the contractile proteins, actin and myosin, making more cross bridges available to accomplish more work. An increase in protein causes the myofibrils to thicken and expand. This can be seen in muscles that cut through the skin. Unlike most machines, muscles work better the more they are used. Muscle enlargement, or hypertrophy, is usually not as great in women as in men partially because it is regulated by the male sex hormone testosterone.

Exercise not only increases the size of skeletal, but also cardiac muscle. Endurance exercises enlarge the cavity of the left ventricle, allowing it to hold and pump out more blood. Spurt exercises enlarge the wall of the left ventricle, allowing it to pump with more strength and pressure.

Just as muscles can undergo hypertrophy they can also undergo atrophy or shrink. This can be caused by degenerative diseases, such as muscular dystrophy or simply by aging. Muscle strength in humans peaks at approximately age thirty, as people age myofibrils degenerate (the number and size of muscle fibers dwindle). Connective tissue replaces lost fibers, making muscle more rigid causing them to react more slowly. Destruction of nerves can also cause atrophy since muscle will cease to contract. This causes the actin and myosin content in the fibers to fall and thus the fibers grow smaller. Muscle atrophy can also occur without muscle use, such is the case with a broken and casted bone, like the leg. This loss of strength due to atrophy can be a problem for some elderly people, who often become convinced that they should "take it easy" because of their age. If they become too inactive, muscles will atrophy, leading to greater loss of strength and greater risk of injury. Therefore, activity to maintain muscle tone is important at any age.


What determines a muscle’s resistance to fatigue?

The functional energy source of a muscle cell is ATP (adenosine triphosphate). However, when multiple contractions of a muscle are performed ATP sources are depleted and must be replenished by one of three methods.

These methods are:

  1. Phosphagen System
  2. Glycolysis
  3. Kreb's Cycle and Oxidative Phosphorylation

The phosphagen system is usually only used when the muscle is in a relaxed state and transfers a high energy phosphate group to ADP (adenosine triphosphate) which produces one ATP. Glycolysis, is the breakdown of muscle glycogen into glucose and the metabolism of glucose which produces 2 ATP and 1 molecule of pyruvic acid. No oxygen is required for glycolysis to take place. The third method of ATP production does require the presence of oxygen. The Kreb’s Cycle and Oxidative Phosphorylation involves the breakdown of pyruvic acid which produces 36 molecules of ATP. This is obviously the largest contributor of ATP to muscles, however, it also takes the longest time and also requires the presence of oxygen.


What causes rigor mortis?

After death muscles begin to deteriorate. Increading leakage of calcium from the sarcoplasmic reticulum, combined with the inability to actively transport it back leads to increased intracelular calcium levels. This calcium binds to troponin and triggers the sliding of the filaments. ATP production has ceased so myosin cross bridges do not have the required energy to detach from actin and relax. Muscles are therefore stuck in a state of rigidity, they can neither contract nor stretch. Rigor mortis usually lasts 24-48 hours after death until the muscle tissue starts to disintegrate.


What are the differences between red and white muscle fibers?

The skeletal muscles of vertebrates are made up of a mixture of tonic fibers (slow contraction), and twitch fibers (faster contraction) of which there are three different types. They are distinguished by their electrical properties of the membrane, maximal rate of contraction, and resistance to fatigue from lack of ATP production.

Tonic fibers are found in the postural muscles, of amphibians, reptiles, and birds, and also in extraocular muscles of mammals. These fibers contract very slowly and often do not produce action potentials as one is not required for them to become excited. The reason that they are very slow in the contraction phase is because of the slow attaching, and detaching of the myosin cross-bridges. Tonic muscles typically do not have a single motor endplate with a typical chemical synapse. Rather, nerves run along the muscles and release neurtotransmitter at multiple synapses.

Twitch fibers are often characterized by their color - "red" and "white". Red fibers include slow twitch and fast twitch oxidative fibers, whereas white fibers are fast twich glycolytic fibers. These categories are somewhat general, however, and there are intermediate fibers.

Slow twitch muscle fibers also contract slowly, however, they produce all or none action potentials. They produce slower movements because the calcium-pumps in the cells do not lower the amount of Ca2+ in the myoplasm, which does not allow the cross-bridges to release. These fibers are mainly used for maintaining posture and for somewhat fast, repetitive movements. They also fatigue slowly because they have a rich supply of blood (carry oxygen), and many mitochondria. Their reddish color is due to the large concentraction of myoglobin in the muscle cells, which aid in oxygen storage. Some examples of this are in the dark meats of many fish and fowl.

Fast twitch oxidative fibers have very fast movements, and are activated quickly. They are also abundant with mitochondria, and can produce ATP quickly by oxidative phosphorylation. This method of ATP production is efficient and is able to provide the amount of energy being used by the rapid movements of the muscle and thus, these fibers become fatigued slowly. These muscles are prominent in such areas as the flight muscles of migratory birds, as they are designed for rapid repetitive movements over long periods of time.

Fast twitch glycolytic fibers contract very fast also, but they fatigue quickly. This is due to the relatively low number of mitochondria in the muscle cells, which forces ATP to be produced by means of glycolysis. As we have seen earlier, glycolysis does not provide a lot of ATP and therefore these muscle fibers become fatigued very quickly. These muscle fibers are generally recruited for extremely quick movements, that are not performed over an extended period of time. An example of such muscle would be in the white breast muscle of domestic fowl.

It should be realized though that the slow twitch muscle fibers of one animal may be faster in velocity than those of another animal’s fast twitch muscle fibers. These muscle fibers may also combine with one another to form a large muscle. Mammals, for example, often have different fiber types mixed within a mass of muscle, so there are no clear "light" or "dark" muscle areas. In many fishes, however, the white and red muscles fibers are often concentrated in distinct sections of muscle (take a look at a swordfish or shark steak the next time you're in a market with fresh fish).


What is a tendon and what is its composition?

Tendons are fibrous, connective tissues that attach muscles to bones. They are dense and composed of collagen and spindle-shaped cells called fibroblasts.

Collagen is the protein component of connective tissue. It is made of a triple helix of polypeptides, that, when lined up side by side and end to end form collagen fibrils. The fibrils, when bound together, make up collagen fibers that can be several hundred microns wide. Finally, these fibers make up the primary components of a tendon: the fascicles and membranes.

Fibroblasts are the principal cells of connective tissue. They are large and spindle-shaped, with a nucleus that is flat and oval-shaped, and found in the rows between collagen fibers. Fibroblasts produce the extracellular substance tropocollagen (a precursor to collagen) and ground substance.

Some tendons have a synovial lining called a tendon sheath. This sheath not only prevents the tendon from slipping out of place, but also allows the tendon to easily slide during muscle contraction. However, after some sort of trauma or strain on the tendon, a condition called tendonitis (or tenosynovitis) may occur. Tendonitis is an inflammation of the tendon sheath accompanied by swelling due to fluid accumulation. Those tendons associated with the shoulders, elbows, finger joints, and ankles are the most susceptible to this condition.


What is creatine phosphate and how is it an energy source?

Creatine phosphate (PCr), also known as phosphocreatine, belongs to a class of compounds called phosphagens. Phosphagens contain a phosphoryl group that can be transferred to adenosine di-phosphate (ADP) to make ATP. Although there are different kinds of phosphagens, only creatine phosphate is found in vertebrates.

At rest, the vertebrate muscle only has enough ATP to power a maximal muscle workout for less than a second. However, during a rigorous workout such as weight lifting, the ATP concentration of the muscle remains constant. This is because the reaction between creatine phosphate and ADP, catalyzed by the enzyme creatine phosphokinase, occurs so quickly that the ATP content of the muscle remains constant. Because of the speed of this reaction, the amount of creatine phosphate present in the muscle is about ten times that of ATP.

Creatine phosphate is primarily found in skeletal muscle, as opposed to cardiac or smooth muscle. This is because of the different activities that each muscle performs. Skeletal muscle is used for bursts of activity and therefore need large amounts of ATP quickly; a perfect environment for the use of creatine phosphate. Cardiac and smooth muscle, on the other hand, contract continuously and regularly, and use ATP at a slower rate. Thus, they depend on reactions that produce ATP more slowly.


Do creatine supplements boost creatine phosphate levels and provide more energy?

Research by Casey et al. (Am. J. Physiol. 271:E31-E37, 1996) showed that creatine ingestion favorably affects performance and muscle metabolism during maximal exercise in humans. The study showed that ingesting creatine monohydrate (Cr) increased the muscle total creatine (TCr) concentrations, the resting creatine phosphate (PCr) concentrations, and total work out production; while the cumulative loss of ATP decreased. Subsequently, PCr degradation during exercise decreased despite increased work production. These shifts imply that advancements in performance were caused by improved ATP resynthesis due to the rise in PCr concentrations in the muscle.

Despite these positive findings, there is little consensus as to how much a person should ingest per day. The subjects observed by Casey et al. ingested 20 grams over a five-day period, but obviously a "standard" amount of creatine consumption will not suffice since people have different weights and muscle-to-mass percentages. In fact, research has shown that while ingesting ~30 grams a day, elimination of the supplemental creatine steadily increases to near 70%. Hultman et al. (J. Appl. Physiol. 81:232, 1996) found that ingesting 0.3 grams of creatine monohydrate for every 2.2 pounds of bodyweight is sufficient to attain increased muscle performance.

Although it appears from these studies that increased creatine may enhance short-term muscle performance in some individuals, it is not recommended as a dietary supplement. The papers cited above do not provide information on whether short-term enhancement will lead to long-term benefits, or whether there may be detrimental health effects associated with the use of creatine supplements.


Special Topic Report: Creatine Phosphate (by Jake Derivan)

Adenosine triphosphate (ATP) is the indispensable chemical energy source on which much life depends. By the hydrolysis of one of its phosphate groups to produce adenosine diphosphate (ADP) and a free inorganic phosphate, energy is released that can be used to power cellular biological processes. In animals, ATP may be produced by way of oxidative phosphorylation (36 molecules of ATP) or glycolysis (2 molecules of ATP), and each species has evolved to accomplish this in the most efficient way possible according to its needs and resources. For the smaller, microscopic organisms, a simplified cycle of producing and using ATP as needed is sufficient to satisfy their energy requirements. However, the larger and more complex an organism is, the larger and more complex its immediate needs for ATP become. This is especially apparent in the muscles of animals, where ATP plays a key role in muscle contraction. When used continuously, though, the ATP concentration of the muscle begins to drop faster than the aforementioned processes can replenish it. Creatine phosphate counters this loss by partaking in the phosphorylation of ADP to reestablish ATP.

Creatine phosphate, also known as phosphocreatine, belongs to a class of compounds known as the phosphagens. These phosphagens are high-energy molecules that contain a phosphoryl group. When ATP concentrations in the muscle fibers begin to fall due to muscle contraction, an enzyme called creatine phosphokinase catalyzes the transfer of the phosphoryl group from the phosphagen to ADP, regenerating ATP. Although there are different phosphogens in nature, only creatine phosphate is found in vertebrate muscle tissue.

While resting, vertebrate muscle only has enough ATP to maintain continual maximal muscle contractions for less than a second. However, during periods of intense muscle workouts, the concentration of muscular ATP remains constant. This is because the affinity of creatine phosphokinase for its substrates creatine phosphate and ADP is so high that the phosphorylation of ADP occurs fast enough to keep the concentration of ATP steady. This reaction occurs at such a great speed because the concentration of creatine phosphate present in muscle tissue is about ten times that of ATP. Thus, the phosphorylation reaction is favored to a much greater extent. As a result, only creatine phosphate concentrations fall while ATP concentrations remain constant.

Of the three kinds of muscle found in vertebrates – skeletal, cardiac, and smooth – creatine phosphate is primarily found in skeletal. The reason for this can be found in the primary activities of each. Cardiac and smooth muscle contractions are continuous and regular. Thus, they use ATP at a slower, more consistent rate than oxidative phosphorylation and glycolysis can accommodate. Skeletal muscle contractions, though, are primarily used for quick bursts of activity that require greater amounts of ATP than cardiac or smooth muscle, creating the ideal conditions for the use of the catalyst creatine phosphokinase, and subsequently creatine phosphate.

The average individual in the United States ingests about one gram of creatine phosphate per day through his or her diet and synthesizes approximately another gram by way of the liver, kidneys, and pancreas. Nevertheless, there has recently been a popular trend to take creatine phosphate as a dietary supplement with the hopes of enhancing muscle performance. The theory follows the logic that by increasing creatine phosphate intake, there will be more creatine available in the muscles to replenish the existing ATP supply longer, and consequently, muscle workouts may be sustained longer. Research by Casey et al. (1996) supported this logic, demonstrating that ingesting higher levels of creatine phosphate on a daily basis led to increased performance during maximal muscle workouts. Their study showed that ingesting creatine phosphate increased total muscle creatine levels, and during strenuous workouts, cumulative losses of ATP decreased. This shift implies that the advances in performance were caused by increased ATP resynthesis that were a direct result of the higher creatine phosphate levels in the muscle tissue.

Despite these conclusions, there is much disagreement as to how much to ingest per day. Commercial creatine producers suggest an intake of five grams of creatine a day to enhance muscle performance. However, this hardly seems rational since different individuals have different weights and body muscle content. Logically, it seems that those who weigh more should ingest more than those who weigh less. However, Casey et al. displayed that there is a ceiling restricting how much creatine phosphate a body can retain. Their study concluded that as creatine ingestion increased passed 30 grams per day for any individual, creatine elimination was found at levels up to seventy percent. Hultman et al. (1996), though, has developed a formula to maximize creatine phosphate retention and performance enhancement. Their study found that ingesting 0.3 grams of pure creatine supplement for every 2.2 pounds of bodyweight was enough to increase muscle performance.3

The phosphagen compound creatine phosphate is an integral part of the Phosphagen System that replenishes ATP levels in the skeletal muscle of vertebrates. In addition to being naturally present in the body because of diet and metabolic production, studies have shown that increasing intake levels of creatine phosphate lead to an increase in muscle performance. Despite these positive conclusions, though, the FDA has not endorsed the intake of creatine phosphate for performance enhancement during workouts, primarily because the long-term effects of its use as a supplement have not yet been determined.

1 David Randall, Warren Burggren, and Kathleen French, Animal Physiology: Mechanisms and Adaptations (New York: Freeman, 1997) 378.

2 Darrell Ebbing, General Chemistry, 5th Ed. (Boston: Houghton, 1996) 500.

3 Hultman et al., "Muscle creatine loading in men," Journal of Applied Physiology, 81 (1996) 236.