6.7 - aerobic energy system

What is the primary energy system used during submaximal physical activity

• The aerobic energy system

Why is the aerobic system preferred during submaximal activity

• Because there is enough time to use oxygen to produce ATP

What is the main disadvantage of the aerobic energy system

• It is the slowest energy system due to complex chemical reactions

What is the main advantage of the aerobic energy system

• It produces the greatest yield of ATP

What fuels can the aerobic energy system use

• carbohydrates (glycogen)

• fats (triglycerides, FFAs)

• under extreme conditions, proteins

What fuel does the aerobic system prefer at rest

• Free fatty acids (FFAs)

What fuel does the aerobic system prefer during submaximal to maximal intensity

• 1st: Carbohydrates (CHO)

• 2nd: Triglycerides

• 3rd: Protein

What is the rate and yield of ATP for the aerobic system

• Slow to slowest rate, due to complex chemical reactions and oxygen delays, fats slower than CHO

• large to largest yield (38 ATP from glucose, 441 ATP from triglycerides)

What is the duration and intensity range of the aerobic system

• Duration: Long (over 120 seconds in continuous examples).

• Intensity: Low to submaximal (<80% max HR).

  resting or submaximal intensity

What are the by-products of the aerobic energy system

• Carbon dioxide (CO₂)

• water (H₂O)

• heat

What is an example activity relying on the aerobic system

• Marathon

• basketball game

what does aerobic depend on

• oxygen being transported to working muscles

Why does the aerobic system prefer carbohydrates over fats

Carbs:

• Can be metabolised aerobically or anaerobically

• Produce energy faster than fats

• Require less oxygen to completely oxidise than fats

• Are more accessible than fats

  (this is true because triglycerides have to be reduced to glycerol and free fatty acids before they can be used to generate cellular energy)

Why are fats used more during low-intensity aerobic activities

• Because oxygen is more readily available, and fats can be broken down slowly over time

What limits the aerobic system

Which all end up…

• Fuel depletion

• elevated core temperatures

• dehydration

• reduced CNS firing

all end up reducing muscular contraction and therefore intensity over time

What are the three processes of aerobic metabolism of carbohydrates

• Aerobic glycolysis → Krebs cycle → Electron transport chain

What happens to pyruvic acid in aerobic conditions

• It is converted into acetyl coenzyme A

  

what happens in the Krebs cycle

• Acetyl coenzyme A is oxidised, producing hydrogen ions

  

What happens in the electron transport chain

• Hydrogen ions combine with oxygen to form ATP, CO₂, and H₂O

  

Aerobic system vs anaerobic system diagram of process comparison

---

When is the aerobic system primarily used

• At rest and during low- to moderate-intensity exercise, including recovery

What fuel does the body primarily use at rest

• Fats (about 2/3 of ATP is supplied aerobically from fats due to abundant oxygen availability)

Why is fat the preferred fuel source at rest

• Because there is an abundant supply of oxygen, and the body has time to use the slower aerobic lipolysis system (don’t refer to this system in answers)

What fuel does the body prefer during low- to moderate-intensity exercise (especially steady-state)

• Carbohydrates (CHO)

Why does the body shift to carbohydrates as exercise intensity increases

• They can be metabolised aerobically and anaerobically

• They produce energy faster than fats

• They require less oxygen to oxidise completely

• They are more readily accessible than fats

Why are carbohydrates more readily accessible than fats

• Because triglycerides must first be broken down into glycerol and free fatty acids before being used, while carbohydrate stores are easier to access

How does intensity affect fuel use in aerobic conditions

• As intensity increases, the body shifts from fats to carbohydrates for faster energy production

What is an active recovery

• Low-intensity activity (50–60% MHR) for 5–10 minutes after exercise, using the same muscles

Why can H⁺ ions accumulate during endurance events, even though the aerobic system is the main ATP supplier

• During high-intensity bursts, the athlete exceeds aerobic capacity and relies more on anaerobic glycolysis, which produces H⁺ ions as by-products.

• These can accumulate and contribute to fatigue

What are the benefits of active recovery (summary)

• Oxidises H+ ions and other by-products

• Maintains elevated heart rate and blood flow

• Uses the muscle pump to enhance venous return

• Aids thermoregulation

• Reduces venous pooling

What are the benefits of active recovery (depth)

• accelerates the oxidation of metabolic by-products such as H+ ions that have accumulated in the muscle

• maintains an elevated heart rate to increase blood flow to the working muscles

• provides a skeletal muscle pump to increase blood flow back to the heart

• assists with thermoregulation by slowly reducing core temperature

• decreases venous pooling, where blood tends to remain or “pool” around muscles for longer periods during passive recovery because the muscles are not promoting blood flow returning to the heart.

Why might aerobic recovery also require thermoregulation or nutrition

• Because environmental conditions and exercise duration can affect recovery needs