The bonk is not a mental failure. It is a biochemical event — entirely predictable and entirely preventable. Here is exactly what happens in your body when you go out too hard, and how the program is built to stop it.
There is a specific face that endurance athletes make when they hit the wall. If you’ve seen it, you know it immediately — the glassy stare, the shoulders dropping, the shuffle that used to be a stride, the expression of someone who has stopped caring about the outcome and started caring only about whether they can remain upright.
Marathon runners call it the bonk. Cyclists call it the crack. Whatever you call it, it is the same event: the body’s glycogen stores have been depleted faster than they can be replenished, and the organism has downshifted to the only fuel source it has left — fat metabolism alone — which is nowhere near fast enough to sustain the pace that got the athlete into this position.
The bonk is not a mental failure. It is not a lack of toughness. It is a biochemical event, and it is entirely predictable and entirely preventable — if you understand what causes it and train accordingly.
Every athlete attempting Event 3 (800m + 30 Deadlifts + 30 Burpees) and Event 6 (400m carry / hold / 100m carry / hold / 50 front squats) is at serious risk of bonking on the opening effort if they don’t understand the pacing principles that govern these events.
The body stores energy in three forms: phosphocreatine (in muscle tissue), glycogen (in muscle and liver), and fat (in adipose tissue and intramuscular lipid stores). These fuels are not interchangeable at arbitrary rates. They have fixed delivery speeds, fixed availability, and fixed rates of ATP production per gram of fuel.
Phosphocreatine is the fastest fuel. It can regenerate ATP at roughly 36–40 mmol of ATP per second per kilogram of muscle — an extraordinary rate that powers the brief, explosive efforts the alactic system handles. But the supply is tiny. There is approximately enough phosphocreatine in a well-trained muscle to power about 6–10 seconds of all-out effort. After that, it takes 2 to 5 minutes of rest to fully replenish via aerobic metabolism.
Glycogen is the workhorse fuel at moderate to high intensities. The glycolytic system can produce ATP from glycogen at approximately 16 mmol per second per kilogram — fast enough to power a hard 400-meter run or an aggressive set of barbell deadlifts, but at a steep metabolic cost. Here is the number that changes how you think about pacing: glycolytic metabolism captures only about 8–9% of the available energy per gram of glucose. The other 91–92% is wasted as heat and hydrogen ions. Compare this to aerobic metabolism’s 95%+ efficiency, and the implication is stark — at high glycolytic intensities, you burn through glycogen approximately ten times faster per unit of useful energy produced than you would at aerobic intensities.
Fat is the virtually unlimited fuel, but it is slow. The aerobic system can oxidize fat to produce ATP at a maximum rate of about 6–10 mmol per second per kilogram — enough to power a sustained jog, a Zone 2 run, or a moderate carry. The moment you need more power than the aerobic fat-burning system can supply — which happens in any meaningful sprint, hard barbell movement, or loaded conditioning piece — the glycolytic system must supplement it.
When you go too hard in the opening effort of a multi-phase event, you are not just spending energy faster. You are spending your most expensive fuel at its least efficient rate, creating metabolic byproducts that actively degrade subsequent performance, and doing so in a way that the aerobic system cannot compensate for fast enough.
The hydrogen ion accumulation that accompanies intense glycolytic metabolism — the “burn” that forces you to slow down during a hard set — doesn’t clear instantly when you stop. It requires blood flow, oxygen delivery, and lactate shuttling to the liver to normalize. If your aerobic base is underdeveloped, this clearance process is slow, which means the hydrogen ions hang around longer and continue interfering with muscle contraction even as you transition to the next phase of the event.
“The inexperienced athlete looks to push to threshold on more ‘pure cardiovascular’ components of a given workout, such as a rowing or running component, without realizing that this is absolutely destroying their ability to perform the skill portions with any degree of competence.” — Viada
The 800m is the pure cardiovascular component of Event 3. The deadlifts and burpees are the skill and strength portions. The athlete who treats the 800m as a race will arrive at the barbell compromised. The athlete who treats the 800m as a controlled aerobic effort — hard enough to be competitive, conservative enough to protect the glycolytic reserves needed for the barbell work — will perform the deadlifts and burpees at a far higher quality.
The body stores approximately 400–500 grams of glycogen in muscle tissue and an additional 80–100 grams in the liver. At moderate glycolytic intensities sustained over time, these stores can be exhausted in 60 to 90 minutes. At high glycolytic intensities — sustained sprint intervals, aggressive conditioning pieces, heavy barbell work — they can be exhausted in 30 to 45 minutes.
When muscle glycogen drops below critical levels, the muscle cells lose their ability to maintain anaerobic output. The cells don’t die. They simply can’t produce ATP fast enough to sustain the intensity demanded. The body hasn’t run out of fuel — there is plenty of fat available. But fat oxidation alone can power only a slow walk or easy jog. The gap between what fat can supply and what the exercise demands is the bonk.
In Events 3 and 6, the bonk arrives at the most inopportune moment: in the middle of a barbell or loaded movement that requires meaningful force production. A bonked athlete doing front squats isn’t slowing down — they’re genuinely unable to produce enough power to move the load efficiently. Their form collapses. Their time balloons. Their score suffers not because they lack the strength to do 50 front squats when fresh, but because they’ve depleted the fast fuel that the nervous system needs to fire those muscles forcefully.
The aerobic base is not primarily a “running fitness” quality. It is a metabolic infrastructure quality — the sum of adaptations that determine how efficiently the body produces ATP from fat and how effectively it clears lactate and hydrogen ions from working muscle at any given intensity.
A well-developed aerobic base produces three specific outcomes that directly prevent the bonk and improve performance in Events 3 and 6:
1. Higher fat oxidation rate at any given intensity. A trained aerobic system can burn fat at higher absolute exercise intensities than an untrained one. This means that at any given pace, a well-trained athlete is meeting a larger proportion of the energy demand from fat and a smaller proportion from glycogen. They arrive at the deadlifts with more glycogen in reserve.
2. Faster lactate clearance. A well-developed aerobic system, with its dense capillary networks and high mitochondrial density, can clear lactate faster during submaximal efforts and during brief recovery periods between effort phases. The trained athlete’s hydrogen ion accumulation drops faster between the 800m and the deadlifts.
3. Improved glycogen sparing at moderate intensities. Mitochondrial density — which increases with Zone 2 training — means each muscle fiber can produce more ATP aerobically per unit of time. This reduces the glycolytic contribution needed at any given submaximal intensity. The aerobic base shifts the entire metabolic demand curve, so more work happens aerobically and less glycolytically at any given output level.
The fastest time over a multi-component event comes from even pacing across the components, not aggressive early pacing followed by survival. This is a mathematical reality, not a training philosophy preference.
At intensities above the lactate threshold, performance degrades non-linearly with time. An athlete who runs the 800m in Event 3 at 85% effort doesn’t arrive at the deadlifts 85% depleted — they may arrive 50% depleted because the aerobic contribution was sufficient to keep the glycolytic cost manageable. An athlete who runs the 800m at 95% effort arrives depleted disproportionately more than the 10% effort difference would suggest.
The practical prescription: operate just under the fatigue threshold consistently, not above it early and try to hold on. In Events 3 and 6, this means:
The Day 7 conditioning prescription across all three waves is specifically engineered to train even pacing and metabolic management under fatigue. Three rounds with a 15–20% increase in difficulty from round 1 to round 3 is not accidental — it is designed to force pacing discipline. An athlete who redlines round 1 will fail round 3 badly. An athlete who paces intelligently will complete all three rounds with close to equal splits.
The threshold intervals on Day 4 — 2×6 minutes at 75–80% max HR — train the specific metabolic zone that governs Event 3 pacing. Six minutes at threshold is approximately the same intensity and duration as the 800m portion of Event 3 for a well-trained athlete. The athlete who can sustain threshold for 6 minutes without glycolytic spiraling is the athlete who can run the 800m at a controlled pace that protects the deadlift and burpee portions.
Go out at a pace you know you can sustain through the last movement of the last phase, not at a pace that represents your maximum capacity for the first movement. The bonk is not a mystery. It is a predictable consequence of exceeding your glycolytic budget in the early phases of an event that requires sustained output across multiple phases. The aerobic base is what expands that budget. The program is what builds that base.