Fatigue, Part 3 – The Anaerobic Glycolytic Energy System

Posted by Scott Christensen

Information presented in the first two sections on fatigue during exercise (Part 1 Fatigue-Energetics and Part 2 Fatigue-Anaerobic Alactic Energy System) pointed out the importance of beginning every discussion on energy system metabolism with basic knowledge of adenosine triphosphate (ATP) and its role in muscle contraction. 

Humans have advanced through natural selection using three metabolic energy systems for their daily activities which may range from moving a heavy object, to walking great distances in search of food.  All of the activities that require a muscle to contract or an organ to function require ATP.  The only two variables being how quickly the ATP can be re-synthesized in muscle to begin the next contraction cycle, and the waste products produced in the process.

Glycolysis is an anaerobic sequence reaction of 11 enzyme-catalyzed reactions in which glucose or glycogen stored in the muscle is converted to lactate.  This cellular process requires no special cell organelle, as it takes place throughout the cell cytoplasm in a very rapid reduction process. 

Related Article: The Curious Case of Lactate

At the same time, ATP is re-synthesized from ADP using the energy captured from reducing the glucose molecule.  One molecule of glucose ultimately produces 2 ATP molecules during glycolysis which then proceed to the contraction protein myosin, where they are used to contract a muscle.  The by-product of the glycolysis reaction, beyond lactate, is free hydrogen ions (H+) which chemically are acidic in nature.  Like any acid, from acid rain (from pollutants), to battery acid, the reaction against tissue is caustic and causes damage to living things. 


Chemists measure the strength of an acid on a logarithmic scale known as the pH scale.  The scale is graded from 1-14 with water neutral at 7.  Living tissue in humans is measured at about 7.2 pH. 

As hydrogen ions begin to accumulate in the cell the pH of the cell begins to drop.  It could drop so low (6.5) that the enzymes present in the cell that facilitate the thousands of bio-chemical reactions that take place there begin to be ineffective and eventually stop their action.  We call this anaerobic glycolytic fatigue in the muscle leading to anaerobic glycolytic energy system exhaustion.  The muscle can no longer muster force production due to the high acidity in the cell.  This condition is termed acidosis.

Myosin protein and actin protein constitute the structural components of the skeletal muscles’ “sliding filament theory”.  Millions of myosin filaments slide against and contract with millions of actin filaments in each muscle fiber contraction-relaxation cycle. 

Each myosin “head”, which is the grabbing and holding unit of the system needs about 30 ATP molecules to function.  Because there are millions of myosin heads in each muscle fiber, a tremendous amount of ATP needs to be re-synthesized prior to each new contraction, because each contraction robs the energy from ATP and reduces them to ADP and loose P molecules.

The critical zone of a mid-distance race is defined as the part of the race in which the race is won or lost.  In these races it is usually about the last 400 meters of the race that constitute the critical zone.  Much force production is needed to run fast during this portion of the race.  The energy required to increase muscle cell force production falls to the anaerobic glycolytic system in order to step up the rate and volume of the extra ATP molecules that are required for the needed muscle contractions.

As the anaerobic glycolytic energy system kicks out more energy for ATP re-synthesis it also produces more hydrogen ions.  As the ATP demand gets higher and higher in the contraction process the hydrogen ion accumulation raises proportionately.  The pH of the muscle cell begins to drop and soon it is low enough that the enzymes begin to fatigue and slow down. 

The loss of enzyme effectiveness slows glycolysis which leads to exhaustion.  The time frame for this process beginning to end is about 30-90 seconds of near maximum effort.  Certainly the 400 meter race falls in this time frame. So does the critical zone of the mid-distance races.  The training goal of the athlete is to be as anaerobically “fresh” as possible before entering the exhausting critical zone of the race.

Anaerobic-VS-Aerobic-ExercisesAnaerobic training has been shown to improve the efficiency of the anerobic glycolytic energy system.  Repeated training sessions, using intervals and repetitions at between 92-100% of maximum 400 meter effort, has been shown to be effective.

Varying rest between bouts of work within the session controls the intensity of the work.  Some sessions work best on short rest and produce one type of training stimulus, while extending to a longer recovery during other sessions stimulates different training responses. 

The anaerobic glycolytic system becomes more efficient by tolerating higher and higher levels of acidosis as the season progresses.  The chief means for doing so is by building hydrogen buffering molecules and storing them due to the training stimulus. 

The main buffering molecule is sodium bicarbonate.  The time needed to build these stores of buffering agents is 9-12 weeks of two anaerobic training sessions per week  If these sessions are done at the frequency and duration described, then the athlete can delay the onset of anaerobic fatigue and improve the quality of their critical zone in mid-distance races.

Coaching Resource: How to Develop the Modern High School Middle Distance Runner (800m – 1600m)

Middle-distance runners need highly developed anaerobic glycolytic and aerobic energy systems to achieve their genetic ceiling in performance. 

These races are “combined zone” races that have both a strong aerobic and a somewhat lesser anaerobic component that combine to produce the ATP molecules necessary to run fast.  Do not neglect development in either energy system.

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See Also:

Fatigue, Part 1 – Energetics

Fatigue, Part 2 – Anaerobic Alactic Energy System



Scott Christensen - Scott Christensen’s teams have been ranked in the national top 10 eight times. He won the 1997 High School National Championship and his squads have captured multiple Minnesota State Championships. Scott has coached 13 Minnesota State Championship-winning teams and 27 individual Minnesota State Champions. He was the USTFCCCA Endurance Specialist School junior team leader for the World Cross Country Team in 2003 and the senior team leader in 2008. Scott is a 14-year USATF Level II endurance lead instructor.

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