CSCS Study Guide Chapter 3

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    Chapter Goals

    • Discuss the energy systems that are able to supply ATP while exercising.
    • Describe lactate accumulation, cellular manifestations of fatigue, and metabolic acidosis.
    • Show patterns of substrate depletion and repletion while exercising at different intensities.
    • Explain bioenergetic factors limiting performance during exercise.
    • Make training programs demonstrating metabolic specificity. 
    • Detail metabolic demands and recover from different training types in order to optimize our work to rest ratios.
    • Read and know the basic terminology found in the chapter. 

    Chemical Structure of ATP molecules

    Chemically, ATP molecules have an adenosine triphosphate group, and high energy chemical bonds.

    ATP hydrolysis causes the terminal phosphate bond to break and release energy. This leaves ADP, inorganic phosphate, and one hydrogen ion.

    ADP hydrolysis breaks the remaining terminal phosphate bond and releases energy, leaving AMP, H+ and P.

    Biological Energy Systems

    There are three basic energy systems present n muscle cells that allow ATP to be replenished. 

    The Phosphagen System

    This is used to provide ATP for most short term and high intensity activities. No matter what intensity you work at, this energy system is active at the start of exercise.

    ATP stores

    • There is not enough ATP stored for exercise by the body.
    • ATP is needed for basic cell functions too.
    • Creatine kinase is used by the phosphagen system in order to maintain the proper concentration of ATP. With this system, ATP is replenished very quickly.

    Control of the Phosphagen system

    • The Law of Mass Action states that concentrations of reactants or products in solutions drives the direction of all reactions.


    The basic definition of glycolysis is: Breaking down carbohydrates in the form of glycogen in the muscles or glucose in the blood, in order to resynthesize ATP.

    The final result of glycolysis is pyruvate, which can follow one of two paths.

    • Pyruvate can convert to lactate
    • Pyruvate can be taken to mitochondria

    Glycolysis and the formation of lactate.

    Lactate is formed from pyruvate thanks to the enzyme called lactate hydrogenase. 

    It does not result in lactic acid.

    Lactate does not result from fatigue.

    Glucose + 2Pi + 2ADP → 2Lactate + 2ATP + H2O 

    Lactate is converted into glucose when it is taken to the liver. 

    All of this is known as the Cori Cycle.

    Glycolysis and the Krebs Cycle

    Pyruvate going into the mitochondria converts into a substance called acetyl-CoA

    Acetyl Co-A is the substance that enters the Krebs Cycle.

    NADH molecules enter the electron transport system and are used there to synthesize ATP again.

    Glucose + 2Pi + 2ADP + 2NAD+ → 2Pyruvate + 2ATP + 2NADH + 2H2O 

    The Energy Yields for Glycolysis

    When going through glycolysis, one blood glucose molecule yields a net of two ATP.

    Glycolysis that comes from the glycogen in muscles yields a net three ATP.

    Control of Glycolysis

    Glycolysis is stimulated with high concentration of ADP and P and ammonia. Also, a decrease in pH and AMP.

    Glycolysis can be inhibited by low pH, CP, Citrate, ATP, and free fatty acids.

    The Lactate Threshold and The Onset of Blood Lactate

    A lactate threshold is representative of an increasing reliance on anaerobic mechanisms. 

    LT is also a marker for Anaerobic threshold.

    The lactate threshold is the intensity of exercise where the blood lactate begins to increase well above the baseline concentration. 

    This threshold begins at roughly 50-60% of the max oxygen uptake in people who are not trained.

    For trained athletes this typically begins at around 70-80%.

    The onset of blood lactate is second in the body’s rate of lactate accumulation.

    This occurs at high intensities of exercise. 

    The Oxidative System (Aerobic System)

    The main source of our ATP during longer low intensity exercise.

    The system uses fats and cars as substrates

    Glucose and Glycogen Oxidation

    Metabolizing muscle glycogen and blood glucose begins with glycolysis and ends with the Krebs Cycle. 

    Fat Oxidation

    Triglycerides are broken down by lipases that are hormone sensitive. This releases the free fatty acids into the bloodstream. They now circulate and enter the muscle fibers.

    The free fatty acids enter the mitochondria of the muscle cells. They are then broken down to form acetyl-CoA and hydrogen protons.

    Protein Oxidation

    Protein is not the body’s primary source, because it is not a significant energy source. 

    Protein is broken down to amino acids and then made into glucose, pyruvate and other things needed for the Krebs Cycle in order to end up as ATP.

    Energy Production and Capacity

    The max rate of ATP production and the total ATP able to be produced has an inverse relationship.

    Because of this, the phosphagen system is responsible for energy for high intensity exercise, the glycolytic is responsible for moderate to high, and the oxidative is responsible for low intensity things.

    Energy use based on duration and intensity

    From zero to six seconds we see the phosphagen system solely used.

    From six to thirty seconds, we use a combination of phosphagen and glycolysis systems, it shows a gradual shift to the glycolysis

    From thirty seconds to around two. Minutes we use our glycolysis system solely.

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    From two to three minutes we use glycolysis but also begin to shift to oxidative.

    Finally, activity greater than 3 minutes relies on the oxidative system.

    There is no time where energy is used only by one system, but at times it is the primary source.

    Substrate Depletion and Repletion


    Creatine phosphate has a quick decrease during the first stage of high intensity exercise. In the 5-30 seconds we see a 50-70% decrease. 

    Complete resynthesis of ATP from the Phosphagen system takes three to five minutes, with complete creatine resynthesis taking 8 minutes.


    Glycogen depletion relates to intensity of exercise

    The repletion of glycogen in the muscles is related to carbohydrate ingestion following exercise. 

    Oxygen Uptake and the Contributions of Aerobic and Anaerobic Systems

    EPOC is the excess post exercise consumption of oxygen. This is the uptake of oxygen higher than the resting values that are used to restore preexercise conditions. It can be called oxygen debt.

    • Oxygen is replenished in the muscles and blood
    • ATP and CP are resynthesized
    • The body’s temperature rises along with circulation and ventilation
    • There is an increased rate of triglyceride to fatty acid cycling
    • Protein turnover is raised
    • There are significant changes to the efficiency of energy during recovery times.

    Metabolic Specificity of Training

    Interval Training

    This training method puts a priority on bioenergetic adaptations in order to transfer to efficient energy within the metabolic pathways. It uses predetermined intervals of rest and exercise.

    More training can happen at higher intensities.

    Work to rest ratios are difficult to define

    High-Intensity Interval Training

    This involves brief but repeated bouts of high intensity exercises followed by intermittent recovery times.

    You can manipulate many different variables such as: 

    • Active intensity during work cycles
    • Duration of the work cycles
    • Recovery cycle intensity 
    • Recovery cycle duration
    • Number of cycles of work or rest
    • Rest time for sets
    • The intensity of recovery between sets
    • And the mode of HIIT exercise

    Combination Training

    This adds more aerobic endurance training to athletes training anaerobically. It is used to enhance recovery. 

    This has the ability to reduce anaerobic performance.

    This may reduce muscle gains.

    It is possibly counterproductive in strength and power sports.

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    CSCS Chapter 3: Bioenergetics of Exercise and Training 1
    CSCS Chapter 3: Bioenergetics of Exercise and Training 2
    CSCS Chapter 3: Bioenergetics of Exercise and Training 3

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    2 thoughts on “CSCS Chapter 3: Bioenergetics of Exercise and Training”

      1. This is just a good place to start in regards to studying for the cscs test. I definitely recommend looking over the textbook as well. Trainer Academy also has excellent additional study materials you might want to check out.

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