Sourced review
Lactate is not what makes an AMRAP hurt
Lactate is a fuel, not a waste product. Your body makes it continuously, including at rest and with plenty of oxygen available, and then burns it. It does not cause the burn during a workout and it definitely does not cause the soreness two days later — that was ruled out in 1983 and the idea has outlived its evidence by four decades.
By Justin Fassio · Sources last checked
Lactate is fuel
The single most useful correction: lactate forms continuously under aerobic conditions, not only when you run out of oxygen. It is shuttled between the cells that produce it and the cells that consume it, where it serves as a major energy source, as the main precursor for making new glucose, and as a signalling molecule.1
So when your blood lactate is high at the end of a hard AMRAP, that is not a measure of damage or of a system failing. It is a measure of how much fuel is in transit. Your muscles, heart and brain are consuming it while it is there.
The old story — anaerobic effort produces lactic acid, lactic acid is waste, waste makes you hurt — is tidy, memorable, and wrong in every clause. It has survived because it explains a real sensation, and because nothing equally memorable replaced it.
So what does cause the burn? Here the science is still arguing
This is where most articles on this subject overstate their case, so it is worth being careful. The burning sensation in hard work is associated with acidity — a fall in muscle pH — plus other metabolic byproducts and the nerve endings that respond to them. What is genuinely contested is the biochemistry of where those hydrogen ions come from.
An influential 2004 review argued there is no biochemical support for lactate production causing acidosis, and that lactate production actually consumes a hydrogen ion, with the protons coming from elsewhere — principally ATP hydrolysis.2 That review has been cited thousands of times.
It has also been argued with. In 2008 the Journal of Applied Physiology ran a formal Point:Counterpoint on whether lactic acid is or is not the only physicochemical contributor to exercise acidosis, and specialists took opposing positions on the accounting.3
What survives both sides of that argument, and is not in dispute: the simple "lactic acid builds up and burns you" model is not how it works. Anyone telling you the exact mechanism with total confidence is ahead of the literature.
It is not why you are sore on Thursday
This part is not contested at all. In 1983 researchers ran the obvious experiment: the same subjects ran on the level, which raises blood lactate, and downhill, which does not. Level running raised lactate and produced no significant soreness. Downhill running never raised lactate and produced significant delayed-onset soreness.4
The two dissociate completely. Delayed-onset soreness follows mechanical stress — especially eccentric loading, the lowering half of a movement — and the inflammatory response to it. Lactate has cleared long before the soreness arrives.
Which means the practical advice built on the myth is aimed at the wrong target. There is nothing to "flush out". If a cool-down helps you, it is not because it is removing lactate.
What lactate does tell you about an AMRAP
Discarding the myth does not make the measurement useless. Lactate is a reasonable marker of which metabolic route a workout took, and in AMRAP training it separates time domains that heart rate cannot.
| Workout | Blood lactate | Source |
|---|---|---|
| Short CrossFit session (~4 min) | 15.9 ± 2.2 mmol/L | Tibana et al.5 |
| Long CrossFit session (~17 min) | 12.6 ± 2.6 mmol/L | Tibana et al.5 |
| "Isabel" (~117 s), glycolytic share of energy | 45% of total | Rios et al.6 |
The pattern is the one you would predict once you stop treating lactate as damage: the shorter, denser effort routes more energy through glycolysis and leaves more lactate in circulation. Heart rate in those same two sessions was statistically indistinguishable.5 Same apparent effort, different metabolic route — which is covered in full onenergy systems and the time cap.
What this changes about training
- Stop treating the burn as a warning light.It is information about intensity, not a signal that something harmful is accumulating. In a5-minute AMRAP you should expect it and work through it; in a20-minute one its early arrival means you went out too fast.
- Do not program a "lactate flush". There is no substance to remove, and the session you spend on it is a session.
- Expect soreness from the eccentric work, not from the hardest-breathing workout. The two are not the same day.
- Judge pacing on round times. They are free, they need no blood sample, and they respond to the thing you can actually change. Seewhat the research supports about pacing.
What the research does not show
- A settled account of exercise acidosis. The direction of travel is clear; the accounting is not.3 Any page giving you a clean mechanism is choosing a side and not telling you there is one.
- A target lactate value for an AMRAP. The numbers above come from studies of nine and fourteen participants.56 They show a direction, not a benchmark to train toward.
- That any particular AMRAP dose improves lactate handling by a specific amount.Trained athletes clear lactate better than untrained ones, but the claim that a given time domain produces a given transporter adaptation is not something the AMRAP literature supports yet.
- Much about women. The workout studies here are on trained men, or on samples too small to separate.7
Related questions
Does lactic acid cause muscle burn?
No. Lactate is a fuel your body produces continuously, including at rest and in the presence of plenty of oxygen. The burning sensation during hard work is associated with acidity and other metabolic byproducts, and the exact biochemistry is still argued over — but lactate is not the culprit.
Does lactic acid cause soreness two days later?
No, and this has been settled since 1983. In one experiment, level running raised blood lactate but produced no soreness, while downhill running produced significant soreness without raising lactate at all. Delayed soreness comes from mechanical damage and inflammation, not lactate.
Should you try to flush lactate out after a workout?
There is nothing to flush. Lactate clears on its own within roughly an hour, and while it is present your body is using it as fuel. A cool-down is worth doing for other reasons; removing a harmful substance is not one of them.
Go and feel it for yourself
The difference between a five-minute and a twenty-minute AMRAP is not how hard your heart works. It is which route the energy takes to get there, and you can feel that difference in the same week.
References
- Brooks GA (2018). The science and translation of lactate shuttle theory. Cell Metabolism, 27(4), 757–785. doi.org/10.1016/j.cmet.2018.03.008Comprehensive review by the researcher who proposed the shuttle. The standard reference.
- Robergs RA, Ghiasvand F, Parker D (2004). Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology — Regulatory, Integrative and Comparative Physiology, 287(3), R502–R516. doi.org/10.1152/ajpregu.00114.2004Argues lactate production does not cause acidosis. Influential and heavily cited — and contested; see the Point:Counterpoint below.
- Journal of Applied Physiology Point:Counterpoint (2008). Point:Counterpoint: Lactic acid is/is not the only physicochemical contributor to the acidosis of exercise. Journal of Applied Physiology, 105(1), 357–362. doi.org/10.1152/japplphysiol.00162.2008The formal published disagreement. Cited here because the biochemistry is genuinely unsettled, not to pick a side.
- Schwane JA, Watrous BG, Johnson SR, Armstrong RB (1983). Is lactic acid related to delayed-onset muscle soreness?. The Physician and Sportsmedicine, 11(3), 124–131. doi.org/10.1080/00913847.1983.11708485The dissociation experiment: level running raised lactate without soreness, downhill running caused soreness without raising it.
- Tibana RA, de Sousa NMF, Prestes J, Voltarelli FA (2018). Lactate, heart rate and rating of perceived exertion responses to shorter and longer duration CrossFit training sessions. Journal of Functional Morphology and Kinesiology, 3(4), 60. doi.org/10.3390/jfmk3040060Nine trained men, two sessions each. Small, but it is the direct duration comparison.
- Rios M, Becker KM, Cardoso F, Pyne DB, Reis VM, Moreira-Gonçalves D, Fernandes RJ (2024). Assessment of cardiorespiratory and metabolic contributions in an extreme intensity CrossFit benchmark workout. Sensors, 24(2), 513. doi.org/10.3390/s2402051314 highly trained male CrossFitters. Measures the three pathways directly, in a real workout.
- Kliszczewicz B, Snarr RL, Esco M (2014). Metabolic and cardiovascular response to the CrossFit workout 'Cindy': a pilot study. Journal of Sport and Human Performance, 2(2), 1–9. jhp-ojs-tamucc.tdl.org/jhp/article/view/jshp.0038.2014Pilot study, nine participants. The authors call it a pilot; treat the numbers as indicative.
This page is a practitioner's synthesis of published research, not original research and not peer reviewed. Every claim above is sourced; where the evidence is thin, a study is small, or specialists disagree, it says so. It is not medical advice — seeour terms.