Cellular Respiration

Let’s talk about cellular respiration, the microscopic machinery that makes this all possible.

1. Glycolysis – The Anaerobic Spark

This is where the energy story begins—deep in the cytoplasm of your muscle cells, where glucose gets torn apart without the help of oxygen.

Glycolysis is fast and anaerobic. A glucose molecule (C₆H₁₂O₆) gets broken down into two molecules of pyruvate, generating a net gain of:

• 2 ATP (adenosine triphosphate)

• 2 NADH (electron carriers)

• 2 H₂O

• And a shift in your acid-base balance via hydrogen ion release

At lower intensities (Zone 2), glycolysis runs calmly in the background. But as you ramp into Zone 3 and 4, especially during sub-threshold intervals, glycolytic flux increases. The muscle is demanding more ATP than fat oxidation can supply, so carbohydrates take center stage.

If oxygen is limited—as it will be during VO₂ bursts—pyruvate can’t enter the mitochondria fast enough. Instead, it’s converted into lactate, which is shuttled to other cells or the liver for recycling. This is the infamous "burn," but it’s not lactate causing the pain—it’s the accumulation of hydrogen ions (H⁺) that lowers pH and impairs muscle contractility.

Training Benefits:

• Sub-threshold intervals improve lactate clearance and buffering capacity.

• Repeated glycolytic engagement increases PFK (phosphofructokinase) activity, enhancing your ability to break down glucose rapidly.

• Pre-VO₂ priming enhances oxidative enzyme activation, giving you better “gas mileage” during the VO₂ sets.

2. Krebs Cycle – The Metabolic Treadmill

Once pyruvate makes it into the mitochondria, it’s converted by pyruvate dehydrogenase (PDH) into acetyl-CoA. This is the entry ticket to the Krebs Cycle (a.k.a. the Citric Acid Cycle or TCA Cycle), the central processing unit for aerobic metabolism.

The Krebs cycle is a cyclical series of enzyme-driven reactions that:

• Strip electrons from acetyl-CoA

• Load up NAD⁺ and FAD to make NADH and FADH₂

• Generate GTP/ATP directly

• Release CO₂ (which you exhale)

For every molecule of acetyl-CoA, you get:

• 3 NADH

• 1 FADH₂

• 1 GTP (ATP equivalent)

• 2 CO₂

These electrons, carried by NADH and FADH₂, are now poised for the final step: the Electron Transport Chain.

During your Friday Smash ride, the sub-threshold efforts will fully engage the Krebs Cycle, especially if you’re adequately fueled. This is where the real aerobic fitness builds: maximizing ATP yield with minimal fatigue by tapping into the deep reserves of oxygen-driven energy production.

Training Benefits:

• Increases in mitochondrial density mean more cycles running in parallel.

• Enhanced fat oxidation capacity spares glycogen for high-intensity efforts.

• Improved PDH activation helps maintain carbohydrate flux into the mitochondria under stress.

3. Electron Transport Chain – The Cellular Jet Engine

This is the last and most powerful stage of cellular respiration—and it takes place entirely inside the mitochondria.

The Electron Transport Chain (ETC) sits along the inner mitochondrial membrane. NADH and FADH₂ donate their electrons to a series of protein complexes (I-IV). These electrons flow down the chain, releasing energy to pump protons (H⁺) across the membrane.

This builds a proton gradient, a charged separation of particles that creates immense potential energy. When the protons rush back through ATP synthase, they generate ATP.

For each molecule of glucose:

• ETC yields up to 34 ATP

• Oxygen serves as the final electron acceptor, combining with H⁺ to make water

This is why your VO₂ max—your maximal oxygen uptake—is the ceiling of your aerobic system. The more oxygen your muscles can utilize, the more ATP you can produce through this high-efficiency route.

During Friday’s VO₂ sets, you’ll push ETC capacity to its max. You’ll feel the strain as the supply-and-demand equation becomes unstable. But with training, that line moves. You’ll produce more ATP, tolerate more lactate, and recover faster between intervals.

Training Benefits:

• Increased cristae density (folds of the mitochondria) = more surface area for ETC.

• Upregulation of cytochrome oxidase and other ETC enzymes.

• Activation of PGC-1α, the master switch for mitochondrial biogenesis.

The Continuum: Aerobic and Anaerobic – Not a Switch, But a Spectrum

One of the biggest myths in endurance training is the idea that you’re either “aerobic” or “anaerobic.” In truth, these systems are on a continuum, with both contributing at all times.

• Zone 1–2: Low glycolysis, high fat oxidation, Krebs and ETC dominate

• Zone 3: Increased carb contribution, faster glycolysis, steady lactate turnover

• Zone 4: Balanced aerobic and anaerobic, lactate production = clearance

• Zone 5: High glycolysis rate, lactate spikes, ETC working at max throughput

Your job as an athlete is to expand the capacity and efficiency of all systems. The best endurance riders are not just “aerobically fit”—they’re metabolically flexible. They can shift between systems effortlessly, depending on the demands of the terrain, the effort, and the moment.

How To Upgrade Your Cellular Engine

1. Zone 2 for Mitochondrial Growth

Easy, long rides increase mitochondrial density, enhance fat oxidation, and improve oxygen delivery.

2. Threshold for Metabolic Efficiency

Sub- and supra-threshold efforts raise your lactate threshold, enhance pyruvate metabolism, and stretch your ability to clear and reuse lactate.

3. VO₂ Max Work for Maximal Oxygen Utilization

Intervals above 110% FTP increase stroke volume, capillary density, and mitochondrial enzyme activity—especially in fast-twitch Type IIa fibers.

4. Fuel For the Work

• Carbohydrates fuel glycolysis and support high-intensity work.

• Fats support low-intensity, long-duration efforts.

• Nutrient periodization allows your body to train multiple fuel pathways without burnout.

5. Rest and Sleep for Adaptation

Recovery is when mitochondrial proteins are rebuilt and multiplied. Sleep drives growth hormone and repair processes, especially in deep stages.

Final Words: The Fire Inside

As you toe the line this Friday, remember: the burn you feel isn’t punishment—it’s adaptation in real time. Every VO₂ burst is a message to your mitochondria: “You need to grow.”

This is the essence of training—riding the razor’s edge between overload and adaptation, between biochemical chaos and perfect control.

So bring the fire.

Because your body listens.

And your cells?

They answer.