Fatigue, Part 2 – The Anaerobic Alactic Energy System

Posted by Scott Christensen

Information presented in the first section on fatigue during exercise (Part 1 Fatigue-Energetics) pointed out the importance of beginning every discussion on energy system metabolism with knowledge of adenosine triphosphate (ATP) and its role in muscle contraction.  Humans have evolved using three metabolic energy systems for their daily activities which may range from lifting a heavy rock, to walking 20 miles in search of food.  All of the activities that require a muscle to contract require ATP.  The only variable being how quickly the ATP can be re-synthesized in muscle to begin the next contraction cycle.

Phosphocreatine1.2Phosphocreatine (PCr) is an energy molecule native to most animal cells.  Like ATP, PCr has the ability to transfer energy effectively if the proper enzymes are present and in abundance.  Basically, a PCr molecule is cleaved and the energy that is lost in the process is used to re-synthesize ATP from ADP and loose phosphates that are present in the muscle due to a previous muscle contraction.  While the transfer is quick, there is not much of an endurance component. 

Most of the PCr present in a muscle cell is depleted after 7-9 seconds with almost all of it gone by 30 seconds.  Because the energy transfer is so quick and does not require oxygen, the force production in the muscle is very high using this energy system.  Also, because the PCr molecules are not a carbohydrate, then no lactate or loose hydrogen ions are produced that lead to acidosis.

Physiologists define maximal speed as that covering 60-80 meters or less in distance.  After such an effort, everybody in the world begins to decelerate because PCr has reached a critical storage value and force production declines.  In reality, the first 30-80 meters of any race distance is tied into the PCr energy system to some degree.  However, maximal speed is tied to the efficiency of the PCr energy system.  Even the ability to maintain sub-maximal speed is tied into the PCr energy system.  The system is trainable in its efficiency; it appears to be non-trainable regarding extending its duration.  In other words, training in this system of 30-80 meters improves only the velocity which is a result of muscle force production

A decline in muscle anaerobic ATP production and/or the corresponding increase in ADP accumulation caused by a depletion of PCr have been implicated in fatigue development during maximal exercise, particularly in Type II muscle fibers.

Related Article: Muscle Development, Fatigue, and Regeneration

The PCr concentration in mixed fiber skeletal muscle is three to fourfold greater than the ATP reservoir, amounting to about 6-8 seconds versus 2 seconds of potential energy for muscular activity.  There is little cellular storage of ATP because the conversion from PCr is so effective.  At the immediate onset of contraction there is a momentary rise in muscle ADP concentration which triggers PCr hydrolysis via the creatine kinase reaction in order to rapidly re-phosphorylate ADP and thus re-synthesize ATP.  

For each unit (mole) of PCr degraded, one unit (mole) of ATP is re-synthesized via creatine kinase (enzyme), with the rate of PCr hydrolysis being greater in Type II muscle fibers.  The importance of this immediate onset of PCr hydrolysis lies in the extremely rapid rates at which it can re-synthesize ATP, especially during maximal short duration exercise.  However, the muscle PCr store is finite and can only maintain ATP re-synthesis during maximal exercise before the onset of fatigue in this alactic anaerobic energy system.

Scientific studies show mixed results on how maximal velocity training improves the anaerobic alactic energy system and lessens fatigue in the system.  It has shown to be unlikely that frequent sessions of maximal velocity training can extend the time before alactic fatigue leads to exhaustion.  Thus, training of this type does not appear to increase the quantity of PCr in the cell.  Some studies have shown that dietary supplements containing creatine can increase the quantity of cellular PCr by up to 20%, which might extend the time to fatigue for an additional second or two before exhaustion.

athletes practiceThere is no doubt an athlete will improve their maximum speed with frequent training sessions of 30-60 meters on the track when set up in a type of repletion and set style of workout.  Perhaps two sets of five repetitions of “flying” 40 meter distance where there is a short acceleration zone and an extended deceleration zone outside the 40 meter fly zone. 

As in any fast workout, the key to the training effect will be in the recovery between each bout of work.  It is suggest that the athlete recover for 3-4 minutes between each 30-60 meter repeat.  During the recovery time the PCr will replenish itself and because the rest is long enough, the effects of acidosis will be minimal. 

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Some scientists attribute the increase in maximal speed to lie in nervous system improvements and better coordination, while others contend it is greater efficiency in the alactic energy system. 

As coaches we can probably conclude that as scientists fight it out in the research, it is no doubt a combination of improvements in both areas, along with a psychological boost to the confidence of the athlete as they get faster.

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See Also:

Fatigue, Part 1 – Energetics




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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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