Fatigue: Part 1 – Energetics

Posted by Scott Christensen

The onset of fatigue in certain systems of the body is what causes running performance to deteriorate and degrade.  Too much fatigue and exhaustion occurs, with the inability to continue an activity.  Fatigue can be acute, as in a single race or training session, or chronic, as in inadequate day to day regeneration of the physiological systems due to training or other factors.

atp moleculeThere have been multiple controlled scientific studies done on fatigue in performance over the past 40 years.  The results of these studies have led scientists to broadly characterize fatigue into central and peripheral categories.  Central fatigue refers to poor motivation, altered central nervous system (CNS) transmission, or recruitment.  Peripheral fatigue involves impaired functional transmission, muscle electrical activity and activation, and the limitation of molecules used to re-synthesize adenosine triphosphate (ATP).  The latter being a muscle fuel issue.

Skeletal muscles are the organs that move the human skeleton through movement.  Movement is a repeated sequence of contractions followed by relaxation of the muscle filaments that make up the muscle itself.  Since the muscles are attached to the bones with tendons, as a muscle contracts along its length it eventually pulls on the bone itself.  The speed by which the muscle filaments contract ultimately characterizes the muscles as a slow twitch or a fast twitch category of muscle by physiologists.  The contraction speed or energetics of muscular contraction, like all bio-chemical reactions in the body, is controlled by the presence and abundance of various enzymes.

The presence of ATP is what separates living organisms from non-living objects.  It makes no difference if it is the simplest bacteria or the most complex animal; the molecule of life is ATP.  This is the molecule of usable energy and energy reserve for all organisms.  In humans, a problem exists when ATP usage is examined.  Humans can store only a small amount in their cells before it has to be re-synthesized.  Fortunately, humans have three mechanisms for re-synthesizing ATP and that is through the process known as respiration.  One of the respiration processes is known to be aerobic and thus requires oxygen to cycle, while the other two are known to be anaerobic so oxygen is not required.

Coaching Resource: The Training Model for High School Cross Country

The ATP molecule has a high energy chemical bond holding the third phosphate to the rest of the molecule.  When the third phosphate separates from the molecule, energy is released.  When the third phosphate attaches itself again, energy is used or what can also be thought of as being stored.

Skeletal muscles consist of long slender filaments that overlap and slide against one another during lengthening and shortening of the muscle fiber.  These protein filaments are of various types with myosin and actin being the predominate varieties.  Mysosin is a thicker strand, so think of a shape like a pencil.  Actin is thinner so think of a toothpick.  muscle contractionActin is very smooth on the surface, but myosin has little tennis racquet shaped stalks that protrude and have the ability to “grab” (actually turn slightly and hold) onto the actin filament as a muscle “contracts” following shortening. The slight turning action of the myosin stalk causes a cross-bridge attachment known as a muscle contraction because the shortened filaments are actually being held by the myosin heads.  The filaments do not release and return to a relaxed lengthened position until the action of an opposing muscle “pulls” the mysoin away from the actin.  The speed by which all of this occurs characterizes the muscle as a fast twitch (contractions), or a slow twitch muscle.

The molecule ATP becomes important because in order for the myosin head to turn slightly and grab, energy must be used.  The energy is ATP and there must be one molecule of ATP on each of the billions of myosin stalks present in skeletal muscle.  In order for rapid contractions to continue occurring, then ATP must be re-synthesized and reloaded onto the myosin stalk as quickly as possible.

In humans the energy source used to re-synthesize ATP molecules can be traced to the foods that are consumed.  Anything eaten that contains measurable calories is a potential energy source for ATP re-synthesis in respiration.  Some foods contain more energy than others.  Proteins and carbohydrates contain about 4 calories per gram, while fats and oils contain 9 calories per gram.  Some foods are more easily worked with by the systems of the body.  Carbohydrates are water soluble which makes them a preferred fuel because the human body is roughly 60% water anyway.  Fats and oils are not water soluble and go into storage unless the exercise demand (demand for ATP) is high.

Respiration is the process of reducing carbohydrates, fats, oils, and proteins to ATP molecules.  The most efficient respiration is aerobic.  One molecule of carbohydrate yields 36 ATP molecules that can then be placed on the myosin stalks.  Aerobic respiration occurs in the mitochondria of muscle cell (also called fibers) and requires the presence of oxygen and the necessary enzymes to make it all work.  Anaerobic respiration only yields 2 molecules of ATP per molecule of carbohydrate.  It does not occur in the mitochondria of the cell, but throughout the interior of the cell if the proper enzymes are present.  Aerobic respiration also produces carbon dioxide as a byproduct while anaerobic respiration produces lactic acid.

Related: The Curious Case of Lactate

There is a third type of respiration in humans called alactic anaerobic respiration.  In this process ATP molecules are re-synthesized by breaking down creatine phosphate molecules and harvesting the energy.  Creatine phosphate is present in small quantities in the cells of animals.  ATP molecules from aerobic respiration are then used to re-synthesize the creatine phosphate molecules that are native to the cell during a time of less demand.   No lactic acid is produced in this process.

Time needed to yield re-synthesized ATP molecules is the reason humans have three forms of respiration.  The anaerobic alactic system is the immediate source of re-synthesis.  This system lasts up to 6-7 seconds before fatigue drains its capacity.  So the limitation is the amount of creatine phosphate native to a muscle fiber.  The anaerobic lactate respiration process will last from about 5 seconds to 90 seconds in most humans.  It is not that fatigue occurs due to carbohydrate limitations, but with lactic acid complications causing acidosis to the pH of the extra-cellular fluids.  The aerobic respiration process lasts from about one minute to hours or days depending on the demand.  Carbohydrate and fat storage depletion is the cause of fatigue in this system.

Coaching Resource: The Training Model for High School Middle Distance

Why three forms of respiration if the aerobic system appears to produce the highest yield of ATP molecules and is the most resistant to fatigue?  It is about force production and speed of the muscle contraction.  The higher the force demand, the greater the dependence on the anaerobic energy systems and the quicker fatigue sets in.


Fatigue is a 5-part article series published by Coach Scott Christensen.

Look for Fatigue: Part 2  next month.

Scott Christensen is the head track coach at Stillwater Area High School in Oak Park Heights, MN.

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