The pHastr Effect, Measured: What an FTP Test Looks Like With and Without Buffer
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Most performance claims are written in adjectives. "Faster." "Stronger." "Longer."
This one is written in numbers.
In March, we ran a paired FTP test on a single athlete — Emily, cyclist, full gas-exchange measurement — once without Phastr, once with. Same protocol. Same legs. Same lab. Two days. The bicarbonate was the only variable that moved.
Here's what came back.
The mean values across the full test
| Metric | No Phastr | With Phastr | Δ |
|---|---|---|---|
| V̇O₂ (ml/min) | 1,469.9 | 1,583.7 | +7.7% |
| V̇CO₂ (ml/min) | 1,415.0 | 1,582.1 | +11.8% |
| Heart rate (bpm) | 134.1 | 144.9 | +10.9 bpm |
| Calorie burn rate | 7.36 | 7.99 | +8.6% |
| RER | 0.928 | 0.953 | +2.7% |
These are averages across the entire test. Not the peak. Not a single moment. Mean.
Now the peaks (99th percentile):
| Metric | p99 No Phastr | p99 With Phastr | Δ |
|---|---|---|---|
| V̇O₂ (ml/min) | 2,968.9 | 3,411.9 | +14.9% |
| V̇CO₂ (ml/min) | 3,399.6 | 4,044.4 | +19.0% |
| Calorie burn rate | 15.5 | 17.9 | +15.8% |
| Relative V̇O₂ (ml/kg/min) | 56.4 | 64.8 | +14.9% |
A nearly 15% jump in peak V̇O₂. A 19% jump in peak CO₂ output. A relative V̇O₂ that moved from 56 to 65 ml/kg/min on the same body, doing the same test, two days apart.
Let's read what those numbers actually say.
The disproportionate V̇CO₂ jump is the bicarbonate fingerprint
If you only look at one number on this table, look at the gap between the V̇O₂ rise and the V̇CO₂ rise.
V̇O₂ went up 7.7% on average. V̇CO₂ went up 11.8%. At peak, V̇O₂ rose 14.9% and V̇CO₂ rose 19.0%.
That gap isn't noise. It's chemistry. The buffering reaction at the heart of how bicarbonate works is:
H⁺ + HCO₃⁻ → H₂O + CO₂
Every hydrogen ion that bicarbonate neutralizes generates a CO₂ molecule. That CO₂ is "non-metabolic" — it didn't come from oxidizing fat or carbohydrate, it came from the buffer doing its job. It still has to be exhaled. So your CO₂ output rises faster than your O₂ uptake.
This shows up in two places: the inflated V̇CO₂ itself, and an inflated RER. Emily's mean RER rose from 0.928 to 0.953, and her peak RER from 1.205 to 1.269. Some of that is real — she was working harder, burning more carbohydrate. But part of the RER bump is the buffering chemistry pushing on the denominator. In a bicarbonate trial, RER is no longer a clean substrate-utilization signal. That's a useful caveat for anyone reading bicarb data going forward.
A higher heart rate, a similar peak — she spent more time at the top
Mean heart rate went up by 10.9 bpm. That's a lot.
Peak heart rate barely moved — 185 vs. 188. So she didn't reach a higher ceiling. She spent more of the test pressed against it.
Read that the way an exercise physiologist reads it: with bicarbonate, the acidic inhibition that normally caps the working power output earlier was pushed back. She held a higher absolute intensity for longer. The heart rate average rises because she's been at higher zones throughout, not because she's been at maximum.
That's the "Phastr effect" said in cardiovascular language: more time at the intensity that produces the result.
What barely moved, and why that matters
Ventilation rose only 2.7%. Tidal volume only 1.7%. Respiratory rate only 2.1%.
Take that as a useful negative finding. The lungs were not the limit in either condition. Breathing mechanics weren't being recruited harder to compensate for the higher intensity — they had headroom. The bottleneck Phastr lifted was at the muscle and the cardiovascular system, not the airway.
This is the cleanest possible read of where bicarbonate is doing work in a real human. It's not making her breathe harder. It's letting her muscles produce force longer at a higher pace, with the cardiovascular system tracking that increased demand.
The bottom line, in plain language
Phastr did not make the test feel easier. We're not making that claim, and the data wouldn't support it if we did.
What it did was let Emily produce more work, at a higher intensity, for longer, before her body said no. Same person, same legs, same protocol — different ceiling.
If you want to know what bicarbonate is actually doing inside an athlete during a maximal effort, this is one of the cleaner pictures of it we've seen:
- Aerobic output climbed 7.7% on average, 14.9% at peak.
- CO₂ production climbed faster than oxygen uptake — the chemical signature of the buffer doing its job.
- Cardiovascular load rose by 11 bpm on average, with the same peak — meaning she lived in the higher zones longer.
- Energy expenditure rose by 8.6% on average and 15.8% at peak — direct downstream consequence of the higher sustained intensity.
- Ventilation barely moved. The bottleneck wasn't the lungs, and Phastr wasn't operating there.
That's the test. That's the data. That's what changed when we put the buffer in.
Test details: FTP (Functional Threshold Power) protocol, March 2026. Data processed with 30-second centered rolling average and IQR-based outlier handling (factor 2.5). Summary statistics computed on outlier-handled raw series. Notes: order effects (fatigue, learning) cannot be fully excluded without randomization across multiple subjects, and individual response to bicarbonate varies — Emily showed a clear positive response, but bicarbonate is not universal.
References: Siegler et al. (2016) Sports Medicine Open; Hadzic et al. (2019) J Sports Sci Med; Grgic et al. (2021) ISSN Position Stand, JISSN; Gurton et al. (2024) Eur J Appl Physiol.
