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Study Up For The CSCS! Chapter 2

Chapter 2: Bioenergetics of Exercise and Training

Bioenergetics is the transfer of energy in the body from one form to another.  To understand how to effectively program exercise, we need to have a basic understanding of energy production and breakdown in the body.  We also need to know how the different pathways and systems relate to different types of training.

Essential Terminology

  • Energy: the ability or capacity to do work.  In terms of bioenergetics, we get this energy by breaking down macronutrients (fat, protein, carbohydrates) and creating smaller molecules, more on that in a bit.
  • Catabolism: the breakdown of large molecules into smaller molecules, this is also associated with the release of energy.
  • Anabolism: just the opposite, now we are creating larger molecules from smaller ones, we are building things, like muscles!  This requires energy be put into the system in order to achieve.
  • Exergonic Reactions: these are reactions that release energy, think of the exorcist, remember how the priest gets the demon out, the energy came out, and it was exergonic.
  • Endergonic Reactions: these reactions require energy to be used to make something happen, such as the contraction of muscles.
  • Metabolism: this is the sum of all catabolic or exergonic reactions and anabolic or endergonic reactions that take place in our body, aka, a biological system.
  • Adenosine Triphosphate (ATP): ATP is an intermediary high energy compound that allows us to take energy from other exergonic and endergonic reactions and move this energy from one system to another.  It is the universal currency of energy in our bodies, kind of like bitcoin.



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  • Hydrolysis: is the breakdown of something, in this case ATP, using one molecule of water hence the prefix hydro.  In this process a phosphate is lysed from ATP turning it into ADP and releasing energy.  We can also do this a second time and turn ADP into AMP and release more energy.
  • ATPase: An enzyme that must be present for hydrolysis to occur.
  • Myosin ATPase: ATPase that is in the myosin head, breaks down ATP in the myosin head in order for muscles to contract.
  • Calcium ATPase: used for pumping calcium into the sarcolemma of the muscle fiber.



Biological Energy Systems

We have three basic energy systems that supply our body.

  1. Phosphagen system
  2. Glycolysis
  3. Oxidative system

These systems are used all the time but the amount that each one is contributing to our energy demand is based on exercise intensity, and duration.  When we are metabolizing energy we can do so aerobically or anaerobically.  The phosphagen system and the first part of glycolysis are anaerobic and the oxidative system is aerobic, meaning it requires an oxygen molecule to be the final electron receptor (more on that later).  The Krebs cycle, electron transport and the rest of the oxidative system are aerobic.

The phosphagen system is used for short high intensity activities like sprinting and maximal effort lifts.  This system uses ATP already in the muscle and creatine phosphate (CP) to quickly refill ATP.

ADP + CP ß Creatine Kinase à ATP + Creatine

Our body only has about 3 ounces of ATP in it at any time, but we have 6 times that in CP, which means we can quickly refill our ATP using this reaction, but this supply will run out in about 10 seconds (see graph below).



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The adenylate kinase reaction can also rapidly replace lost ATP, also called the myokinase reaction.  This reaction creates more AMP which is a strong driving force to up regulate glycolysis.  So the more we use this system and deplete our ATP, we will stimulate the next energy system, glycolysis.  The phosphagen system is controlled by the law of mass action, meaning the concentrations of reactants and products as well as the presence of enzymes is what drives the direction of the reactions.

Glycolysis is the breakdown of carbohydrates either in the form of glycogen in the muscle, or glucose (sugar) in the blood, to resynthesize ATP.  This process is much more complicated and involves many more enzymatic reactions and for this reason this system is slower to produce energy.  The end product of glycolysis is pyruvate which can be used to create lactate if energy demand is high or be shuttled to the mitochondria and used in the krebs cycle if energy demand is lower.  The krebs cycle will net more ATP using pyruvate as opposed to converting it to lactate, but it takes longer.  The more pyruvate we have building up in the muscle cell, the more acidic the cell becomes, known as metabolic acidosis.  This is what slows down glycolysis and makes us unable to continue at high intensity.  We must slowdown in order for the pyruvate to clear, one way this happens is through the Cori cycle, seen below.


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We can also clear lactate by oxidizing it in the muscle fiber or even shuttling it off to another muscle fiber to do the work.  One thing we can do to increase the rate of lactate clearing is to do low intensity work following our high intensity training, this is where the idea of active recovery has some good scientific merit.  Trained individuals whether aerobically trained or anaerobically trained will both have faster clearing rates than untrained individuals.


Glycolysis Leading to the Krebs Cycle

If there is enough oxygen in the mitochondria, and exercise intensity is low, then pyruvate can be shuttled into the mitochondria to undergo different reactions and produce energy through the Krebs Cycle by transforming pyruvate into acetyl-CoA.

Energy Yield of Glycolysis

We need to phosphorylate, which is to add a phosphate to another molecule, in order to create ATP.  Adding Pi to ADP and getting ATP is phosphorylation.  We can do this at the substrate level or through oxidative means in the electron transport chain.  When we bring a blood glucose into our cell, we have to phosphorylate it before we can start hacking it apart and getting ATP out, but putting on a phosphate takes energy.  But glycogen already has a phosphate on it, and for this reason it is more energy efficient to use glycogen rather than blood glucose.  If we use blood glucose we net 2 ATP per reaction, if we use glycogen, we net 3 ATP.

So how does glycolysis regulate itself?  It happens through allosteric inhibition and allosteric activation.  This means that the end product binds to the catalyzing enzyme and either slows the reaction down or speeds it up, kind of like positive and negative feedback systems.  Hexokinase is one enzyme important to this process, which phosphorylates glucose to glucose-6-phosphate, which is needed to further the process.  However, the presence of glucose-6-phosphate allosterically inhibits hexokinase and therefore the production of glucose-6-phosphate.  This is an example of a negative feedback loop.

One other effect of exercise intensity, is the accumulation of blood lactate.  The point at which blood lactate is starting to accumulate is called the lactate threshold.  Beyond this we can see steeper increases in blood lactate when we approach near maximal intensities and activate large muscle groups.  This point is called OBLA or onset of blood lactate accumulation.  Some studies suggest that if we train at or above these thresholds we can in effect delay them and increase our performance by being able to perform at higher intensity for longer, interesting implications for sports performance there.

The Oxidative (Aerobic) System

This system is primarily used at rest, and uses fats and carbohydrates as fuel.  At rest about 70% comes from fats and 30% from carbs.  When doing exercise for long periods of time (greater than 90 minutes) protein can also start to be metabolized, so keep your workouts under an hour and a half, ideally.  The Krebs cycle produces only two ATP per glucose, yet it yields 6 molecules of NADH and two of FADH, these will be used in the electron transport chain to create a bunch of ATP.  From one molecule of glucose we get 40 ATP in total from our aerobic pathways, very efficient, but slow.  We can also oxidize fat and proteins, and use these aerobically.  We use something called hormone sensitive lipase to do beta oxidation and turn fats into free fatty acids and then into acetyl-CoA.  Protein is not the best source of energy, but through a process called gluconeogenesis, we can turn protein into amino acids and then glucose and create ATP this way.  The main amino acids that are oxidized in muscle are the branched chain amino acids (isoleucine, leucine, and valine).


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Substrate Depletion and Repletion

When we exercise we burn phosphates and glycogen, with more intense activities we burn mostly phosphates.  Some studies have shown that resistance training increases phosphates stores, probably due to hypertrophy of type II fibers.  Glycogen is used during intense exercise through fast glycolysis but seems to be more important during low level activity.  High intensity intermittent exercise like weight training also causes glycogen substrate depletion.  But this glycogen can be completely recovered within 24 hours if enough carbohydrates have been eaten.

Interval training can be used to target specific energy pathways, and target work to rest ratios can serve to maximize the potential adaptation.  See table below.


Photo Source: Textbook (see reference)


So given this information, we can direct our efforts towards training the energy system most necessary in our given sport.  With multi faceted sports such as mixed martial arts, this becomes more complicated.  Research has shown that extended aerobic training can hinder anaerobic performance, but the opposite is not true.  It seems that that aerobic system can be trained by exercising anaerobically but not necessarily the other way around.  This has implications for the way both power and endurance athletes train for their chosen sport.








Baechle, Thomas R. Roger, Earle W. (2008). 3rd Edition. ESSENTIALS of STRENGTH

TRAINING and CONDITIONING. National Strength And Conditioning Association.


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