Bioenergetics is the study of how energy moves through biological systems. In exercise and training, it explains how your body converts food into energy to power muscle contractions and sustain physical activity. Understanding energy systems, ATP production, and how they relate to exercise intensity and duration is essential for designing effective training programs.

This chapter covers the three major energy systems - phosphagen, glycolytic, and oxidative - along with the metabolic processes involved in energy production, substrate use, and recovery. Keep in mind that these energy systems never work in isolation. Instead, they operate on a continuum, and their relative contributions shift based on how hard and how long you’re exercising.

Bioenergetics

Bioenergetics refers to the conversion of macronutrients - carbohydrates, proteins, and fats - into usable energy for cellular functions. It includes:

• Catabolism: The breakdown of larger molecules into smaller ones to release energy (e.g., glycogen to glucose).
• Anabolism: The synthesis of larger molecules from smaller ones, which requires energy (e.g., protein synthesis).
• Exergonic reactions: Energy-releasing reactions, such as ATP hydrolysis.
• Endergonic reactions: Energy-consuming reactions, such as muscle contraction.

Adenosine triphosphate (ATP)

ATP is the body’s primary energy currency. It’s made of an adenosine molecule bound to three phosphate groups. When ATP is hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate (Pᵒ), energy is released. That released energy powers muscle contraction and many other cellular processes.

• $\text{ATP}\to \text{ADP}+\text{P}+\text{Energy}$

During exercise, your body relies on three primary energy systems to continually resynthesize ATP. Which system contributes most depends on the intensity and duration of the activity. Because stored ATP in muscle lasts only a few seconds, ATP must be constantly replenished to sustain movement.

Phosphagen system

The phosphagen system supplies ATP for very short, high-intensity efforts such as sprinting or heavy lifting. It depends on:

• Creatine phosphate (CP): A molecule stored in muscle that donates a phosphate group to ADP to regenerate ATP.
• Enzyme: Creatine kinase catalyzes the reaction.

Key characteristics:

• Operates anaerobically (without oxygen).
• Produces ATP at a very high rate but has low capacity.
• Primarily supports activities lasting up to 10 seconds.

Equation:

$\text{ADP}+\text{CP}\to \text{ATP}+\text{Creatine}$

ATP stores:

• Muscle ATP stores are limited and are depleted quickly.
• Full resynthesis of ATP occurs within 3 to 5 minutes during recovery.

Glycolytic system

The glycolytic system produces ATP by breaking down carbohydrates. Glycolysis can proceed through two pathways:

Fast glycolysis

• Pyruvate is converted to lactate in the sarcoplasm.
• Predominates during high-intensity activities lasting 15 seconds to 2 minutes.

Slow glycolysis

• Pyruvate is transported into the mitochondria for further oxidation via the Krebs cycle.
• Supports moderate-intensity activities lasting several minutes.

Equation:

$\text{Glucose}+2\text{P}+2\text{ADP}\to 2\text{Lactate}+2\text{ATP}+2{\text{H}}_2\text{O}$

Key enzymes: Phosphofructokinase (PFK) regulates the rate of glycolysis.

Byproducts:

• Hydrogen ions (${\text{H}}^{+}$) contribute to metabolic acidosis.
• Lactate is a usable energy source in the Cori cycle, not a cause of fatigue.

Oxidative (aerobic) system

The oxidative system is the primary energy source for prolonged, lower-intensity activity. It can generate ATP from:

• Carbohydrates: Oxidized through glycolysis, the Krebs cycle, and the electron transport chain (ETC).
• Fats: Broken down via beta-oxidation to produce acetyl-CoA, which enters the Krebs cycle.
• Proteins: Contribute minimally, except during prolonged exercise.

Key features:

• High capacity for ATP production, but a slower rate of ATP delivery.
• Requires oxygen and is the most efficient system for sustained activity.

Krebs cycle and electron transport chain (ETC)

The Krebs cycle (also called the citric acid cycle) is a series of enzyme-catalyzed reactions in the mitochondria. Its main role in cellular respiration is to oxidize acetyl-CoA and capture energy in high-energy electron carriers (NADH and FADH${}_2$). These carriers then deliver electrons to the electron transport chain, where ATP is produced through oxidative phosphorylation.

Key steps of the Krebs cycle:

1. Acetyl-CoA combines with oxaloacetate to form citrate.
2. Citrate is transformed through a series of reactions, releasing two molecules of CO${}_2$.
3. NADH and FADH${}_2$ are generated through redox reactions.
4. ATP (or GTP) is produced through substrate-level phosphorylation.
5. The cycle regenerates oxaloacetate, allowing the process to continue.

ATP yield:

• 1 glucose molecule produces ~38 ATP in the oxidative system.
• Each turn of the Krebs cycle yields 1 ATP, 3 NADH, and 1 FADH${}_2$.
• 1 glucose molecule produces ~38 ATP in the oxidative system.

Energy systems and exercise intensity

Energy systems contribute along a continuum that depends on exercise intensity and duration:

Intensity	Duration	Primary energy system
Very high	0-6 seconds	Phosphagen
High	6-30 seconds	Phosphagen and glycolysis
Moderate	30 sec - 2 min	Fast glycolysis
Low to moderate	2+ minutes	Oxidative system

Even when one system is dominant for a given activity, all three energy systems contribute at the same time - just in different proportions.