You finish a hard session, upload the ride, and admire the numbers like a proud parent at prize-giving. 🏆
A few more watts. A few more seconds at a seriously uncomfortable intensity. Maybe you even survived the final interval without opening negotiations with your smart trainer. (“Just one more minute and I’ll never ask anything of you again.”)
It feels like progress — and it might be. But exercise science likes to ask a slightly annoying follow-up question: does a better workout today actually lead to more fitness over time? Turns out those are two very different things.
Performance and adaptation run on different clocks ⏱️
Immediate performance is about what you can do today, in this session. Researchers measure things like power, speed, time to exhaustion, and perceived exertion.
Training adaptation is about what changes in your body after weeks of repeated sessions and recovery — a higher VO₂max, better endurance, or a faster result in the same standardized test.
Something can help you push more power today, without anyone knowing yet whether it will make you fitter in six weeks. Kind of like how a good playlist makes you ride harder, but nobody’s claiming Spotify raised your FTP. 🎧
A recent cycling study shows the gap perfectly
In a study published in the European Journal of Sport Science, 24 well-trained cyclists did demanding interval sessions — once after taking sodium bicarbonate and once after a placebo.
With bicarbonate, average interval power was about 2% higher, and riders spent roughly 24 extra seconds above 90% of their VO₂max. Their repeated sprint power improved too.
Those are genuinely useful findings about that day’s performance. But here’s the fine print: the study didn’t compare fitness gains after several weeks of training with and without bicarbonate. The researchers said so themselves — whether it leads to better long-term adaptation is still an open question.
Why “time near VO₂max” gets so much attention
VO₂max is the maximum rate at which your body can take in and use oxygen during intense exercise — basically the size of your engine’s air intake.
Coaches and researchers love “time spent close to VO₂max” because it describes how physiologically demanding an interval session is. More time up there may mean a stronger training stimulus.
But — and say it with me — a measure of how hard the session was is not the same as a measure of how much fitter you became. To demonstrate a long-term benefit, researchers must follow athletes through a proper training block and measure the resulting changes. A single-day experiment can’t answer that on its own, no matter how sweaty it was.
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Want to spot patterns across your training weeks? Keep your sessions, notes and weekly reflections together in the Weekly Training Log & Goal Tracker. Now includes the Compare Two Sessions bonus worksheet: a printable A4 PDF and editable local HTML companion to help you compare similar sessions and reflect on the conditions.
Read the latest HIIT headlines carefully, too
An early-October research update from Stanford described a 12-week comparison between high-intensity interval training and moderate continuous exercise in sedentary adults. Early measurements hinted at bigger VO₂max gains with the higher-intensity work.
But the researchers were still analyzing the results, so this isn’t a finished, peer-reviewed verdict yet. It’s an interesting signal to watch — not proof that trained cyclists should bin their long endurance rides and live on short intervals. And it certainly doesn’t tell us what will happen to you on Zwift next month. (I know. I wanted the shortcut too. 🙃)
What does this mean for your training diary?
- Celebrate the good session. You performed better under those conditions. That counts. 🎉
- Keep the claim in proportion. One strong workout shows what you managed that day.
- Look for patterns. Repeated improvement across comparable sessions and tests is what tells the real story.
- Apply the same filter to adverts. “Improved interval power” and “produces greater fitness gains” sound like twins. They’re not — the second one needs a lot more evidence.
So enjoy the good numbers. Screenshot them. Send them to the team WhatsApp group if you must. Just remember that fitness is a story told across a whole training block — not a verdict delivered by one particularly flattering ride.
One great ride is a highlight. Twelve consistent weeks is a story.
Sources: Bicarbonate interval study (European Journal of Sport Science) · Stanford’s preliminary research update
🩷The Endurance Diaries🩷
