Energy Systems and ATP Production

The Three Energy Systems

• The three energy systems are ATP-CP, anaerobic glycolysis, and aerobic.

Fuels, Rate, and Yield

• Each system utilizes different fuels (chemical and food), has a specific rate and yield of ATP production, and contributes differently at rest and varying intensities.

• Recovery rates also vary, with active and passive recovery strategies influencing them.

Nutritional and Hydration Strategies

• Nutritional and hydration strategies enhance performance, delay fatigue, and improve recovery.

• These strategies include carbohydrate ingestion, protein intake, and water consumption.

ATP Production

• ATP production can be anaerobic (without oxygen) or aerobic (with oxygen).

• The ATP-CP and anaerobic glycolysis systems are anaerobic.

• The aerobic system is aerobic.

Rate vs. Yield

• Rate: How quickly ATP is resynthesized.

• Yield: How much ATP is resynthesized.

• There is a trade-off: higher yield (more ATP) means a slower rate of resynthesis.

Factors Determining ATP Demand

The two factors that determine ATP demand of an activity are:

1. Exercise intensity: Determines the rate of ATP use and, therefore, the speed of resynthesis.

2. Exercise duration: Determines the total amount (yield) of ATP that needs to be resynthesized throughout the activity.

ATP-CP Energy System

• An anaerobic energy system that breaks down creatine phosphate (CP) to produce energy rapidly.

• Fuel: Stored ATP + creatine phosphate.

• Intensity: Maximum efforts, >95% max HR.

• Duration: Short duration, fuel depleted after approximately 10 seconds.

• Rate of resynthesis: Very fast.

• Yield: Low.

• Example: Usain Bolt's 100m sprint.

ATP-CP System Details

• May provide enough energy for maximal effort contractions for up to 6-10 seconds.

• ATP lasts about 1-2 seconds, and PC lasts about 7-8 seconds.

• Used for power sports like sprinting, jumping, throwing, and weightlifting.

• Higher intensity leads to more rapid depletion of PC stores.

• PC is replenished within 3 minutes of the activity ceasing with a passive recovery.

• Advantage: Immediate supply of ATP.

• Disadvantage: Limited by the amount of PC stored in the muscles.

Phosphocreatine (PC) Molecule

• Energy is released upon splitting of the PC bond.

• Creatine + Phosphate + Energy -> ADP + Pi -> ATP

Fatigue/Limiting Factors

• Fuel depletion (ATP, PC).

• Accumulation of metabolic by-products (Pi).

  • Accumulation of Pi leads to decreased contractile force production and reduces the amount of calcium that can be released, slowing muscle contractions.

• Passive Recovery

  • 30 seconds: 50% of PC replenished.

  • 3 minutes: 80-90% of PC replenished.

• Once PC stores are depleted, they can only be replenished when there is sufficient energy in the body, typically through the aerobic pathway or during recovery.

PC Replenishment

• After 3-4 minutes, approximately 80-90% of PC supplies have been recovered.

• The use of supplements and aerobic training will improve the rate of recovery.

Alternate Names

• Phosphocreatine system (PC).

• Creatine Phosphate system (CP).

• Phosphagen system.

• Anaerobic Alactic System.

System Characteristics

• Fuel Source: Creatine Phosphate (CP).

• Intensity: High intensity, >95% max HR.

• Rate: Very fast.

• Yield: Smallest (< 1 ATP).

• By Products: ADP and inorganic phosphates (Pi).

• Duration: 0-10 seconds.

• Dominant Duration: 1-5 seconds, peaking at 2-4 seconds.

• Recovery Type: Passive.

• Fatigue factor: Depletion of Creatine Phosphate stores.

• Example: Athletics field events (discus, javelin, long jump), sprinter.

• Muscle Fibre Type: Type IIB Muscle fibres.

• Disadvantages:

  • Resynthesizes a very limited amount of ATP.

  • Limited stores of ATP and PC in muscles.

• Advantages:

  • Resynthesizes ATP explosively/immediately.

  • Doesn’t need long chemical reactions.

  • Used for high-intensity activities.

Anaerobic Glycolysis

• Relies on the incomplete breakdown of glycogen, in the absence of O_2, to produce energy.

• Fuel: Carbohydrate.

• Intensity: 80-95% max HR.

• Duration: Predominant when the ATP-CP system fatigues (PC stores depleted at around 5-10 seconds); predominant for 10-60 second events; Peak power is usually reached between 5-15 seconds.

• Rate of Resynthesis: Fast.

• Yield: Low-Medium – 2ATP.

System Utilization

• Provides ATP for longer during submaximal activities (when PC is depleted) and thus provides a ‘stop gap’ until enough oxygen is transported to working muscles for the Aerobic system to become the major contributor.

• Used for sustained sprint or muscular endurance activities usually lasting between 45-60 seconds.

• Examples: 400m sprint, 200m swim, repeated high-intensity efforts during a continuous game.

Glycolysis Process

• Takes place in the cytoplasm, where the enzymes required are present.

• In a series of steps, one glucose molecule (C6H{12}O6) is broken down to form 2 molecules of pyruvic acid (C3H4O3). This process also forms 2 ATP molecules.

• It does NOT require energy as a larger molecule is being broken down (catabolism).

• ‘Lysis’ means to destroy, so the term Glycolysis is to destroy or break down Glucose.

System Characteristics

• It’s not as quick off the mark as the ATP–CP System but produces twice as much ATP.

• Because oxygen is not present, the glycogen is not totally broken down, and by-products including lactic acid (lactate + hydrogen ions) are formed.

• Although 80% of lactate diffuses from the muscles & is transported back to the liver for conversion to glucose or glycogen (gluconeogenesis), some Hydrogen ions (H^+), accumulate in muscle tissue.

• H^+ causes the muscle pH to fall (become more acidic), which decreases the activity of glycolytic enzymes (enzymes that break down glycogen) & hence the rate of ATP resynthesis, contributing to fatigue.

Fatigue/Limiting Factors

• Accumulation of metabolic by-products:

  • Lactic acid (lactate + hydrogen ions).

  • Accumulation of H^+ decreases activity of glycolytic enzymes & hence the rate of ATP resynthesis, contributing to fatigue.

• Active Recovery

  • E.g. slow walking included in the recovery process Leads to faster removal of lactic acid than passive recovery

  • Requires approximately 60–90 min between events for optimal recovery

Lactic Acid

• Use of the anaerobic glycolysis system results in the production of lactic acid. Lactate purely serves as an indicator that the body is no longer working aerobically. It also represents the accumulation of H^+ ions.

• An increase in lactate levels means pH levels are dropping (a pH level of 7 is neutral – less than 7 means muscles are becoming acidic!).

• Enzymes that are responsible for creating muscle contractions don’t like acid, so when pH levels drop, they stop working as a feedback mechanism to prevent injury.

• As a result, the Anaerobic glycolysis pathway is compromised, and exercise intensity must be reduced!

Active vs. Passive Recovery

• Active recovery results in faster removal of lactic acid compared to passive recovery following high intensity exercise.

Systen Summary

• Fuel Source: Glycogen.

• Intensity: High intensity (> 85% max HR).

• Rate: Fast.

• Yield: Small (2 – 3 ATP).

• By Products: Lactic acid, H^+ ions, ADP.

• Duration: 10 – 75 seconds.

• Dominant Duration: 5 – 60 seconds (Becomes dominant provider from the time the ATP–CP system starts rapidly depleting (approx. 5 sec) to when the Aerobic system takes over as the dominant provider (approx. 30 – 60 sec depending on intensity)). Peaks between 5 –15 sec.

• Type of recovery: Active.

• Fatigue limiting factor: Accumulation of H^+ ions. This causes muscles to become more acidic affecting muscular contractions.

• Example: 400 m runner, series of high-intensity repeat efforts in any team sport, 100 m swim.

• Muscle Fibre Type: Type IIA fibres

  • Disadvantages

    • Produces H^+ ions that cause fatigue in large amounts

    • Slow recovery

    • Only capable of producing small amounts of ATP

  • Advantages:

    • Resynthesises ATP quickly allowing for high intensity efforts

Aerobic System

• Aerobic glycolysis: The breakdown of glycogen in the presence of oxygen to produce energy, carbon dioxide, water and heat.

• The aerobic energy system will be the major energy contributor for events of more than 75 secs up to several hours in duration & are below 80% max HR

• FUEL: Carbohydrate + Fat (depending on intensity, duration).

• INTENSITY: During rest and at sub-maximal intensities < 80% HR max.

• DURATION: Gradual increase in contribution as O_2 becomes available. Predominant energy system after 30 – 60 secs (depending on intensity).

• RATE OF RESYNTHESIS: Slow.

• YIELD: Very High (36 – 38 ATP).

System Activation

• Activated at the start of intense exercise, it contributes significant amounts of ATP during high- intensity activities lasting 1 – 2 minutes and continues to be the major contributor as the Anaerobic Glycolysis system decreases its contribution.

• Once O2 becomes available to the muscle cell, a different chemical reaction known as Aerobic Glycolysis takes place: Lungs work harder to bring in more O2, the heart pumps harder to transport O_2-rich blood to the muscles and arteries expand to increase blood flow

• The Aerobic system takes a while to get going because several processes need to occur:

  • Lungs work harder to bring in more O_2

  • The heart pumps harder to transport O_2-rich blood to the muscles

  • Arteries expand to increase blood flow

• This system has the greatest capacity to produce ATP but is the slowest to do so (high yield but low rate)

Fuel Source

• The fuel source used by the Aerobic system (carbohydrates and fats) depends on the intensity & duration of the activity:

  • Carbohydrates for the first 90 mins, then fats up to 4 hours

• CHO is the preferred energy source during high-intensity exercise as fats can produce more ATP than CHO, but they require more oxygen to produce an equivalent amount of ATP

• Athletes competing in the Tour De France rely heavily on their Aerobic energy system to supply ATP to their working muscles.

Lactic Acid

• When using the Aerobic system predominately, any accumulated lactic acid can be oxidised/removed or converted back into glycogen to be used again as an energy source.

• Called Gluconeogenesis and involves using non-carbohydrate sources (such as lactate) to create glucose/glycogen in the liver. This can only occur when there is sufficient oxygen to do so.

Pyruvate

• Pyruvate can be shuttled off into one of two pathways depending on whether there is sufficient O_2

• If the cell has adequate O_2 for aerobic metabolism, then Pyruvate (formed from glucose) is converted to Acetyl CoA & enters the citric acid cycle. In this reaction, the by-products are:

  • CO_2

  • H_2O

  • Heat

    • These are NOT fatiguing.

Glycolysis

• As long as you continue to provide your body with energy supplies, this energy system could last forever. Stored glycogen in the muscles is broken down into glucose. When the glycogen in the muscles is depleted, glycogen in the liver is used.

Fats

• Fats can produce more ATP than CHO, but they require more O2 to do so. As a result of the increased O2 cost with the transition from CHO to fats as the main fuel source, less O_2 becomes available to the working muscles & the risk of working anaerobically increases. This possibly explains why athletes are forced to ‘slow down’ when fats are used.

Fatigue/Limiting Factors

• Fuel depletion

  • Using fats when glycogen is depleted leads to fatigue as fats take longer to break down and require more O2 than CHO, which means less O2 is available for muscles

• Active Recovery

  • E.g. slow walking / jogging included in the cool-down process or on a ‘rest day’

• Heat (elevated body temperature)

  • As core temperature ↑ , sweat rates ↑ and blood is redistributed to the skin’s surface, less blood, O2 and fuels for working muscles, so aerobic exercise may become increasingly anaerobic. HR and Cardiac Output also ↑ to continue supplying O2 to working muscles

Summary

• Fuel Source: CHO (Glycogen) – during moderate to high intensity, Fats (FFA’s) – during low to moderate intensity, Protein – during extreme situations such as ultra endurance events when CHO and Fats have run out

• Intensity: Resting & Submaximal intensity < 80% HR max

• Rate: Slow, Fats slower to resynthesis ATP than CHO

• Yield: Largest 36 – 38 ATP per mole of glucose Over 100 ATP per mole of fat

• By Products: CO2, H2O & Heat

• Duration: >75 seconds

• Dominant Duration: 75+ seconds

• Type of recovery: Active

• Fatigue limiting factor: Depletion of glycogen stores, elevated body temperature

• Example: Endurance athletes (triathlete, marathon runner, Tour De France rider) Mid field player in AFL, soccer, hockey

• Muscle Fibre Type: Type I fibres

  • Disadvantages

    • Resynthesises ATP slowly (particularly fats)

    • Fats have a high oxygen cost resulting in reduced intensity

  • Advantages:

    • Resynthesises large amounts of ATP & produces non-fatiguing by-products