Chapter 6: The Interplay of Energy Systems - Notes

Chapter 6: The Interplay of Energy Systems

Key Concepts

• Resynthesizing ATP:

  • Via two anaerobic energy systems: The ATP-PC system and the anaerobic glycolysis system.
  • Via the aerobic energy system.

• Energy Systems Overview:

  • Two anaerobic energy systems: The ATP-PC system and the anaerobic glycolysis system.
  • One aerobic energy system: The aerobic system.

Fuels for Energy Systems

Anaerobic Systems

• ATP-CP System:
  • Fuel: PC (Phosphocreatine) or CP (Creatine Phosphate).
• Anaerobic Glycolysis:
  • Fuel: Glycogen.

Aerobic System

• Fuels:
  • CHO (Carbohydrates): Moderate rate of ATP production.
  • Fat: Slower rate of ATP production.

Energy System Activation

• All 3 energy systems are activated at the start of exercise (INTERPLAY).
• The contribution of each system depends on:
  • Intensity of exercise.
  • Duration of exercise.
  • Amount of oxygen available to be used by muscles.
  • Fuel availability: Depletion of chemical (ATP & PC) and food fuels (CHO, fats, & protein) during exercise.

Cell Respiration in Mitochondria

• Formula for cellular respiration (in the presence of O2):
  • Oxygen + Glucose \rightarrow Energy + CO2 + H2O
• Source of glucose:
  • Glycogen (CHO carbohydrates).
  • FFAs or triglycerides (Fats).
  • Amino Acids (Protein).

The Role of ATP

• Every muscular contraction is due to ATP (Adenosine Triphosphate) being split apart and releasing energy.
• After being split, we are left with ADP (Adenosine Diphosphate) and Pi (inorganic phosphate) - these are metabolic byproducts.
• ADP & Pi must resynthesize (recharge) back to ATP to continue exercising.
• ATP resynthesis occurs through the 3 energy systems working together (interplay) to provide the energy that is required.
• ATP \rightarrow ADP + Pi + Energy

ATP Breakdown and Energy Release

• The three energy systems break down fuel stores releasing energy for the resynthesis of ATP.
• Adenosine - P - P - P \rightarrow Adenosine - P - P + Energy

Chemical Fuels

• ATP (Adenosine Triphosphate):

  • The major source for muscular contraction (no ATP = no contractions).
  • Consists of one adenosine molecule with three phosphates joined together.
  • The human body only has a small amount stored in the muscles for quick access, roughly enough for 2-3 seconds of muscular work.
  • Must continually be resynthesized from energy substrates (PC, Glycogen, Triglycerides, & Amino Acids).

• PC (Phosphocreatine) also can be referred to as CP:

  • Broken down to resynthesize ATP as part of the ATP-PC energy system.
  • Approximately 10 seconds of PC is stored in the muscle.
  • ATP \rightarrow ADP + Pi + Energy

• The breaking of ATP Gives Energy for movement

  • ATP \rightarrow ADP + Pi + Energy

• PC or CP = Phosphocreatine is used to create more ATP. we only have a small amount stored at muscles (10-15 seconds)

  • ADP + PC \rightarrow ATP + C

Food Fuels

• The food we eat refuels the three energy systems.
  • Carbohydrates (CHO): The preferred source of energy during exercise as they require less O2 to be broken down.
  • Fats: The body’s main source of fuel at rest and during prolonged submaximal exercise. Require more O2 than carbohydrates to be broken down.
  • Protein: Used mainly for growth and repair. ‘last resort’ fuel source.

Food Types, Fuel Conversions, and Storage

Nutrient Breakdown

Food as Energy Sources

• ATP stored in muscle by using
  • Chemical fuels = phosphocreatine (PC) stored in muscle or
  • Food fuels = carbohydrates, fats and proteins stored around the body
    • 1 gram = 4 calories
    • 1 gram = 4 calories
    • 1 gram = 9 calories

Capacity of Chemical and Food Fuels

Food Fuel Sources During Physical Activity

• The body has a preference for fats at rest.
• CHOs are the only fuel source utilized at max intensities.
• As CHO storage is limited (60-90mins), extended endurance events see an increased contribution from fats for ATP production, hence performance slows due to fats having a slower rate of ATP production compared with CHO.
• Fats take a lot of oxygen away from working muscles in order to rebuild ATP and they require many more chemical reactions than carbohydrates to be broken down in order to “recharge/rebuild” ATP.

PC Restoration Rates

Summary of Food Fuels (CHO, Fats, Protein)

• CHO carbohydrates are the bodies preferred fuel source rather than fats to release energy.
• CHO loading only applicable for athletes competing > 1-2 hours up to 10 days prior
• CHO loading helps to spare glycogen for higher intensity efforts
• CHO are needed to use fats for energy
• Fats can produce more ATP than carbohydrates but they require more oxygen to produce an equivalent amount of ATP.
• Fats also transport fat-soluble vitamins A, D, E and K.
• In prolonged exercise, fats becomes increasingly important energy source as glycogen becomes depleted.
• Protein forms the building blocks of tissue for growth and repair.
• All enzymes which speed up chemical reactions are proteins.
• Large amounts of oxygen is required for breaking down protein (used only in extreme extended duration exercise)
• 1 glucose molecule = 2 ATP (anaerobically)
• 1 glucose molecule = 36 ATP (aerobically)
• 1 fat molecule = 3x147 = 441 ATP
• CHO is preferred to fats because fats require more oxygen to produce the same amount of energy and the rate of ATP produced using fats is slower than that of CHO

Summary of the ATP & PC Chemical Fuels

• A limited amount of PC is stored at the muscles (about 10 seconds’ worth at maximal intensity), with larger muscles capable of storing slightly more PC than this (12 to 14 seconds at maximal intensity).
• ATP and PC are stored at the muscles and available for immediate energy release. Stores are limited – the more intense the activity, the quicker the chemical fuels are utilised to produce ATP.
• After approximately five seconds of maximal activity, the PC stores are 40 to 50 per cent depleted
• There is approximately four times as much PC stored at muscles as there is ATP.
• Once PC has been depleted, it can only be replenished when there is sufficient energy in the body, and this usually occurs through the aerobic pathway or during recovery once the activity has stopped.
• Passive recovery is the most appropriate form of recovery to maximize replenishment of PC stores
• Time to fully replenish PC stores is approximately 2 minutes.
• Once phosphocreatine has been depleted at the muscle, ATP must be resynthesised from another substance − typically glycogen, which is stored at the muscles and the liver

Characteristics of the 3 Energy Systems

• Rate
• Yield
• Fuel
• Time

Key Terms

• Rate: Refers to how quickly ATP is resynthesized
  • ATP-PC = fastest
  • Anaerobic glycolysis
  • Aerobic = slowest
• Yield: The total amount of ATP that is resynthesized
  • ATP-PC = lowest
  • Anaerobic glycolysis
  • Aerobic = highest

Characteristics of the 3 Energy Systems

Methods of Generating ATP During Muscle Activity

ATP-CP Energy System

• Anaerobic (no Oxygen required)
• Most rapidly available source of ATP as it’s stored in the muscles and simple reactions (fastest rate)
• Breaks down phosphocreatine (PC) to resynthesize ATP anaerobically.
• PC splits releasing energy and leaving Pi and C.
• Energy released is used to resynthesize ATP stores (ADP + P)
• ATP stores last max 3 secs
• PC stores last for 10 secs @max intensity activity
• After 5 seconds @max, Anerobic glycolysis ES will become dominant
• Once PC stores have depleted, can be replenished via 3 minutes of passive recovery, or an intensity low enough not to call upon PC (Oxygen required for P + C to be returned to PC)
• Intensity usually maximal, >95% HR max.
• Fastest Rate, lowest yield (0.7-1 ATP)
• Duration Around 10 seconds
• Fuel PC (Phosphate Creatine)
• Typical events – jumps, throws, short sprints, diving
• Equation: PC + ADP \rightarrow ATP + Creatine

Anaerobic Glycolysis Energy System

• How the system works:
  • Glycogen is broken down in the absence of oxygen (Anaerobic glycolysis).
  • This produces pyruvic acid which is converted to Lactic Acid.
  • A further byproduct of Lactate are hydrogen ions (H+) which make the muscle pH decrease (More acidic), reducing glycolysis and causing muscular discomfort and an inability for the to contract maximally.
  • H+ cause this by effecting the actions of enzymes needed for glycolysis to occur
  • This is a safety mechanism that prevents the cells being destroyed under extremely acidic conditions
  • In recovery, (when sufficient oxygen is available), H+ combines with pyruvate to form lactate which is reconverted to glycogen in the liver
• Supplies ATP at a slower rate than the ATP-PC system as it requires longer and more complex chemical reactions (12) however still “fast” rate
• The yield of ATP production is twice that of the ATP-PC system (2-3 ATP)
• Intensity is usually >85% HR max.
• Duration Around 30-40 seconds
• Fuel Glucose/CHO/Glycogen
• Typical events – 200-400m run, 50m swim, repeated sprints in team sport

Summary of the Anaerobic Glycolysis Energy System

• The anaerobic glycolysis system produces ATP without oxygen
• Involves more complicated and longer chemical reactions than the ATP−PC system to release energy.
• It also supplies energy from the start of intense exercise, and peak power from this system is usually reached between five and fifteen seconds and will continue to contribute to ATP production until it fatigues (two to three minutes).
• During maximal exercise, the rate of glycolysis may increase to 100 times the rate at rest.
• It produces lactic acid, which breaks down into lactate and H+ (hydrogen irons) and lactate (in the presence of O2) can be broken down to glycogen to provide further energy.
• About 12 chemical reactions take place to make ATP under this process, so it supplies ATP at a slower rate than the ATP-PC system.
• It provides energy for longer during submaximal activities when PC is depleted and lactic acid accumulation is slower. This provides a stop-gap until sufficient oxygen is transported to working muscles for the aerobic system to become the major energy contributor.
• It provides twice as much energy for ATP resynthesis as the ATP−PC system.
• It increases it’s ATP contribution if performance intensity exceeds the lactate inflection point

Aerobic Energy System

• How the system works:
  • Requires Oxygen
  • CHOs (preferred during exercise) & FFAs (preferred during rest) are broken down to release energy.
  • When using CHOs pyruvic acid is produced and further broken down producing CO2, H2O & ATP (via Kreb’s cycle)
  • Further breakdown via the electron transport chain. It requires hydrogen ions and oxygen, producing water and heat.
• Slowest rate of ATP resynthesis & requires most chemical reactions
• The yield of ATP
  • 36-38 ATP (when using CHO)
  • 147*3=441 ATP (when using FAT).
• Intensity 70-85% HR max (sub-maximal)
• Duration Anything over 2 mins is dominant “aerobic” overall
• Fuel CHO and fat
• No fatiguing by-products
• Typical events – archery, marathon, road cycling

Summary of the Aerobic System

• The aerobic system is the slowest system to contribute towards ATP resynthesis due to the complex nature of its chemical reactions.
• It is capable of producing the most energy in comparison to the other two energy systems ~ between 30- 40 times
• It requires oxygen, which can be provided (90 per cent of VO2 maximum) within 60 seconds.
• It involves many more complex chemical reactions than the anaerobic energy systems to release energy.
• It preferentially breaks down carbohydrates rather than fats to release energy.
• Fats can produce more ATP than carbohydrates but they require more oxygen to produce an equivalent amount of ATP.
• It releases no toxic/fatiguing by-products and can be used indefinitely.
• It provides 50 times as much ATP as the anaerobic systems combined.
• It contributes significant amounts of energy during high-intensity/maximal activities lasting one to two minutes.
• The aerobic system is also activated at the start of intense exercise, and peak power from this system is usually reached between one and two minutes and will continue to be the major ATP contributor as the anaerobic glycolysis system decreases its contribution.
• Any activity that lasts over 2mins will mean the aerobic system is dominant.

Key Characteristics of the Energy Systems

Determining Energy System Contribution

• Intensity
• Duration
• Availability of Oxygen
• Availability of Fuel

Energy System Interplay

• All the energy systems are contributing towards ATP production simultaneously throughout any exercise bout, but the proportional contribution of ATP from each system to the metabolic demand will shift according to exercise intensity and duration.
• The longer the activity lasts, the more likely it is that the ATP-PC system will contribute less, unless given the opportunity to recharge.

Oxygen Uptake

• At rest: The body’s need for ATP is relatively small, requiring minimal oxygen consumption.
• At Max: Oxygen uptake at Max
• When exercise begins, oxygen uptake increases as the body attempts to meet the increased oxygen demand of the working muscles that results from their need to produce more energy for ATP resynthesis
• Oxygen Deficit: A discrepancy (shortfall) between supply and demand of Oxygen to the working muscles. Occurs during the transition from rest to exercise, particularly high-intensity exercise, and at any time during exercise performance when exercise intensity increases. During these times, anaerobic sources must be involved in providing energy. In a graph, this will appear as an incline, or hill, in the line. The oxygen deficit occurs because the respiratory and circulatory systems take some time to adjust to the new oxygen demand (even at low exercise intensities)
• Steady State: When Oxygen supply meets oxygen demand. Depending on the intensity of the exercise, this may take anywhere between a few seconds or 1 minute or more to achieve. This steady state in oxygen uptake also coincides with a plateau in heart rate and ventilation. If exercise intensity is increased again after reaching a steady state, the athletes anaerobic pathways will need to supplement the gap until a steady state is again reached. A steady state can only be held up to and including the lactate inflection point. It should be noted that in trained endurance (aerobic) athletes, the oxygen deficit is reduced due to these athletes attaining steady state sooner than untrained individuals. In a graph, this will appear as plateau, or flat line.
• Excess post-exercise oxygen consumption (EPOC): The amount of oxygen consumed during the recovery period after the cessation of an exercise bout that is over and above the amount usually required during rest. This is where we ‘repay’ the oxygen needed during exercise that we were unable to provide. The higher the intensity and duration of activities, the larger the oxygen debt and the longer it takes to repay it

Factors Influencing Recovery Rates

• Energy system contribution.
• Intensity of exercise
• Duration of exercise
• Amount of oxygen available to be used by muscles
• Fuel availability
  • Depletion of chemical (ATP & PC) and food fuels (CHO, Fats & Protein) during exercise