Not All Kilojoules Are Equal: Why Riding Above Threshold Costs You Twice

Picture two riders who finish a three-hour ride having done the exact same total kilojoules. One rode it steady, just under threshold the whole way. The other surged over threshold on every rise and coasted the flats to average out the same number. Ask which one is more “fatigued” by kilojoule count, and the answer is: identical. Ask which one has a lower critical power at the finish, and the answer is not close.

We've already covered how power variability costs you time on the run split in “Smooth Beats Surgy.” This article is strictly about what's happening underneath that: the physical parameter that riding above threshold actually damages, and why the total kilojoule number on your head unit hides that damage completely.

Why this matters mid-ride, not just at the finish

Age-group racing is full of moments that invite an above-threshold surge: a short climb, closing a gap, powering through a headwind section, catching a draft-legal pack. None of those feel expensive in the moment, and your kilojoule total barely notices. But if the mechanism below is right, that's exactly the problem — the damage isn't showing up in the number you're watching.

It's worth being precise about who this evidence describes, because the transfer isn't automatic. Almost everything below comes from cyclists tested on a bike alone, in a lab or on a closed course — nobody in this evidence rode 90 kilometres and then ran a marathon off it. If you're a triathlete, treat this as a mechanism that plausibly applies to your bike leg, not as a finding that has been tested on you specifically.

What the evidence says

The core finding: it's not the kilojoules, it's where they came from

The cleanest evidence for this comes from 17 male professional cyclists tested for critical power, then given three different doses of prior work specifically calibrated to sit above critical power — 2.5, 5.0 and 7.5 kJ per kilogram of body mass — versus the identical kilojoule doses delivered below critical power. Above critical power, critical power itself fell 2.2%, 6.1% and 16.2% at the three doses respectively (all p < 0.001). Below critical power, work-matched to the same totals, critical power did not change at any dose (p > 0.05) Mateo-March, Leo, Muriel, Javaloyes, Mujika, Barranco-Gil, Pallárés, Lucia & Valenzuela, Journal of Science and Medicine in Sport, 2024. The authors' own conclusion is worth quoting because it undercuts a habit most riders and most training software share: this “raises concerns on the use of mechanical work per se as a single fatigue/stress indicator.” Two rides with the same kilojoule total are not the same physiological event.

This is one field study, in 17 male professional cyclists. It has not been replicated in amateurs, and it says nothing directly about what happens to an age-grouper's critical power specifically. Grade this evidence moderate: real, specific, statistically clean — and not yet generalised beyond a small, elite, all-male sample.

It also drags your top-end power down, from 30 seconds to 20 minutes

The same study measured maximal mean power across a range of durations after each dose. After work above critical power, 30-second maximal power fell 4.0%, 5-minute power fell 1.7%, 10-minute power fell 1.8% and 20-minute power fell 3.2% (all p < 0.001). None of those changes appeared after the equivalent below-critical-power work (all p > 0.05) Mateo-March et al., 2024. In other words, the cost of riding above threshold isn't confined to a slow bleed in your sustainable power — it shows up in your sprint, your minute power and your twenty-minute effort, all from the same above-critical-power dose.

Bigger samples point the same direction

A systematic review pooling 21 studies and 585 participants — overwhelmingly male, ranging from juniors to WorldTour professionals — reached the same conclusion from a wider evidence base: the intensity of prior work, not the total kilojoules alone, is what drives acute durability loss, and kilojoules on their own are a poor stand-alone fatigue metric Sánchez-Jiménez et al., European Journal of Applied Physiology, 2025. That's a review, not a new experiment, so it's best read as directional support rather than independent proof — but it means the Mateo-March finding isn't an isolated result.

A separate dataset of 90 male U23 riders gives a sense of scale at a lower dose: just 6 kJ/kg of accumulated work above critical power was associated with a 13.6% fall in 20-minute power, while a much larger 60 kJ/kg dose was associated with a 53.8% fall in 5-second power Mateo-March et al., Scandinavian Journal of Medicine & Science in Sports, 2026. Read together with the professional data, the pattern holds across two separate cohorts: above-threshold work is disproportionately expensive, and short, high-intensity power fades fastest and furthest.

The second cost: your matchbook doesn't refill on the flat

Critical power decay is only half of why riding above threshold costs you twice. The other half is W′ — the finite amount of work you can do above critical power before you're forced to slow down, sometimes described as your matchbook. In 11 male participants completing repeated exhaustive efforts separated by active recovery, W′ recovered only 46.0 ± 2.7% after 2 minutes of recovery, 51.2 ± 3.3% after 4 minutes and 59.4 ± 4.1% after 6 minutes. Recovering at a genuinely easy intensity (33% of critical power) restored 9.4 percentage points more W′ than recovering at 66% of critical power (56.9 ± 3.9% vs 47.5 ± 3.2%, p = 0.019), and efforts that depleted W′ more slowly also refilled more slowly Caen, Bourgois, Bourgois, Van der Stede, Vermeire & Boone, Medicine & Science in Sports & Exercise, 2019.

A separate model, using a different formulation of the same underlying idea, found the reconstitution estimate correlated with how much oxygen uptake was still elevated at the start of the next effort (r = 0.79) — the higher your oxygen cost is still running from the last surge, the less of your matchbook you actually have back Skiba, Jackman, Clarke, Vanhatalo & Jones, Medicine & Science in Sports & Exercise, 2014. The two studies actually disagree on how accurate the standard W′-balance models are — Caen's data suggest the common model under-estimates real reconstitution by up to nearly 30% after short recoveries — which is a live, unresolved methodological dispute worth knowing about if you use an app that reports a live W′ balance number.

The magnitude of what's at stake in that bank is large. After 15 kJ/kg of either moderate- or heavy-intensity preload, W′ fell from roughly 7.8 kJ to 3.6–4.0 kJ — roughly half — while VO2max and lactate threshold were unchanged, in 12 trained but non-elite riders (9 men, 3 women) Evans, Chorley & Highton, European Journal of Sport Science, 2025. That's a small, mixed-sex sample — grade this limited-to-moderate — but it shows the surging cost lands specifically on your ability to attack or respond, while your “fitness” on paper looks untouched.

One honest gap: every one of the W′-reconstitution numbers above comes from fresh-state testing. Nobody has measured how W′ refills after three or four hours of prior riding — which is exactly the state that matters for a long-course race. That is an open question, not a finding, and should be treated as one.

What this means for how you should actually pace

• Treat above-critical-power efforts as a different currency from below-critical-power efforts, not a bigger version of the same thing. A short push over threshold on a climb isn't “cancelled out” by an easy patch later at the same total kilojoules — the below-threshold kilojoules didn't cost you critical power in the first place, so there's nothing to cancel.
• If you have a genuine choice about when to spend an above-threshold effort — a surge to close a gap, a hard pull, a steep pinch — spend it where you have to, and actively protect the recovery afterward; recovery at a properly easy intensity refills more of the bank than recovery at a moderate one.
• Don't read your ride's average power or total kilojoules and conclude you paced well. The distribution of how those watts were produced, not the total, is what your critical power responds to.
• If you're pacing to a target for a long triathlon bike leg, the risk of an unplanned surge isn't just the extra kilojoules; it's a compounding, semi-permanent knock to the ceiling you're pacing against for the rest of the ride.
• In a draft-legal, intermittent race format, some above-threshold work is unavoidable and tactical; the point isn't to eliminate it, it's to recognise that every one of those efforts draws down a slower-refilling account than the flat kilojoule total suggests, so the surges you choose should be the ones that actually matter.

Common mistakes

• Assuming a smooth average power number means you rode smart. Two rides at an identical normalised power can carry very different amounts of above-critical-power work.
• Treating total kilojoules burned as your fatigue dashboard. The evidence above says explicitly that mechanical work alone is a poor stand-alone fatigue indicator Mateo-March et al., 2024.
• Assuming a W′-balance number on your bike computer is precisely accurate. The two model-validation studies here disagree with each other, and neither was tested in a fatigued, multi-hour state.
• Believing you'll “make it up on the flat” after a hard climb. The below-threshold recovery riding afterward doesn't undo the critical-power hit from the climb; it just doesn't add a second one.
• Extrapolating the 16.2% critical-power loss figure directly to yourself. It's one real number from one study, in 17 male professional cyclists, at a specific dose — not a promise about what will happen to your critical power at a different dose, in a different body, on a different day.

How to apply this week

• Look back at one recent hard ride's power file and count how many minutes you spent clearly above your critical power or FTP, not just your normalised power. That number, not the total kilojoules, is the one worth tracking.
• On your next ride with genuine terrain, plan where an above-threshold effort is actually necessary — a steep pinch, closing a real gap — versus optional, like matching someone else's pace on a flat section for no tactical reason, and consciously skip the optional ones.
• After any effort that pushes you clearly above threshold, give yourself a genuinely easy patch rather than a moderate one; the data above suggests easy recovery refills more of your capacity than riding brisk-but-not-hard.
• If you race with a target average power, add a second number to watch in training: minutes above threshold. Two rides can hit the same average and be completely different physiological events.

References

• Mateo-March, Leo, Muriel, Javaloyes, Mujika, Barranco-Gil, Pallárés, Lucia & Valenzuela, Journal of Science and Medicine in Sport, 2024
• Sánchez-Jiménez et al., “Is intensity the most important factor in determining the amount of prior work accumulated that affects cyclists' acute durability? A systematic review,” European Journal of Applied Physiology, 2025
• Mateo-March et al., Scandinavian Journal of Medicine & Science in Sports, 2026
• Caen, Bourgois, Bourgois, Van der Stede, Vermeire & Boone, Medicine & Science in Sports & Exercise, 2019
• Skiba, Jackman, Clarke, Vanhatalo & Jones, Medicine & Science in Sports & Exercise, 2014
• Evans, Chorley & Highton, European Journal of Sport Science, 2025