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AMRAP energy systems: what the research shows
Aerobic and anaerobic metabolism both run from the first second of an AMRAP. They are not sequential systems that hand off to each other. Equal contribution arrives at roughly 75 to 80 seconds of maximal effort, which means every AMRAP long enough to be worth programming is already aerobically dominant — including the five-minute ones.
By Justin Fassio · Sources last checked
The three systems all start together
The model most of us were taught — phosphagens for ten seconds, then glycolysis, then the aerobic system takes over — is wrong in an important way. All three pathways are active from the onset of exercise; what changes over time is their relative contribution, not whether they are switched on.1
You can see it measured inside a real workout rather than on a cycle ergometer. When researchers instrumented the CrossFit benchmark "Isabel" — a workout that took the participants about 117 seconds — the energy came from oxidative phosphorylation (40%), glycolysis (45%) and the phosphagen system (15%).3 Two minutes of all-out work, and aerobic metabolism was already supplying two fifths of it.
Where the energy came from in a ~117-second workout
"Isabel" — 30 snatches for time, 245 ± 25 kJ total. Percentages of total energy turnover.
The crossover is around 75 seconds
The duration at which anaerobic and aerobic metabolism contribute equally during a single bout of maximal exercise is around 75 seconds — considerably earlier than the older textbooks suggested.1 A 2026 systematic review covering 102 studies and 311 data points put the equal-contribution point at approximately 75 to 80 seconds.2
Worth being precise about what that figure is, because it gets quoted loosely: it describes a single continuous maximal bout. An AMRAP is not that. It is a self-paced effort with movement transitions, changing muscle groups and self-selected micro-rests. The crossover tells you which system is doing the bulk of the work in the window you are training; it does not model the workout.
Heart rate will not tell you which time domain you are in
This is the finding most likely to change how you program. Nine trained men completed both a short CrossFit session (about 4 minutes) and a long one (about 17 minutes). Heart rate was statistically indistinguishable between them. Blood lactate was not.4
Heart rate did not separate the two sessions. Lactate did.
Nine trained men, each completing a short and a long session. Bars are means; the whiskers are one standard deviation.
| Shorter (~4 min) | Longer (~17 min) | |
|---|---|---|
| Heart rate | 176 ± 6 bpm (91 ± 4% max) | 174 ± 3 bpm (90 ± 3% max) |
| Difference | Not significant (p = 0.387) | |
| Blood lactate in recovery | 15.9 ± 2.2 mmol/L | 12.6 ± 2.6 mmol/L |
| Difference | Significant (p = 0.019) | |
Nine participants is a small study and one comparison is not a literature. But the direction is worth taking seriously, because it matches the mechanism: near-maximal heart rate is reached in almost any hard AMRAP and then has nowhere left to go. It saturates. Whatever distinguishes a five-minute effort from a twenty-minute one, your heart rate monitor is not going to show it to you.
What a longer cap actually changes
It changes which qualities decide your score. Twenty-two experienced athletes performed the same circuit — rowing, barbell thrusters, box jumps — for as many repetitions as possible in either 5 or 15 minutes. Performance measures, rowing in particular, related more closely to the 5-minute result; training experience and body composition related more to how athletes handled the 15-minute version.6
That is a useful way to think about a time cap: it is not a dial for how hard the workout is, it is a selector for what the workout is testing. A five-minute AMRAP asks what you can do. A twenty-minute AMRAP asks what you can keep doing, and rewards the athlete who has done this before.
It is also why the same movements at5 minutes and20 minutes are not the same workout at two lengths. In our own library the programming reflects that: a five-minute cap averages 2.1 movements and 27 reps a round, a twenty-minute cap averages 4.0 movements and 60 reps. See the library data.
A longer cap does not just mean more rounds
Mean round composition across the 203 AMRAP workouts in this library, by time cap.
What has actually been measured on the benchmarks
Very little of the AMRAP literature is about AMRAPs in the abstract. Nearly all of it is a handful of named benchmark workouts, measured in small groups of trained people. Here is essentially all of it, in one table, so you can see how thin the foundation is.
| Workout | n | What was measured | Source |
|---|---|---|---|
| "Cindy" — 20 min AMRAP, 5 pull-ups / 10 push-ups / 15 air squats | 9 | HR 170.8 ± 13.5 bpm · VO₂ 33.3 ± 5.5 ml/kg/min (63.8 ± 12.3% of max) · 9.5 ± 1.5 METs · 13 ± 2.9 kcal/min | Kliszczewicz et al.5 |
| "Isabel" — ~117 s, 30 snatches | 14 | Energy split: oxidative 40% · glycolytic 45% · phosphagen 15%, of 245 ± 25 kJ | Rios et al.3 |
| Short (~4 min) vs long (~17 min) session | 9 | HR indistinguishable (91 ± 4% vs 90 ± 3% max) · lactate higher in the short one (15.9 ± 2.2 vs 12.6 ± 2.6 mmol/L) | Tibana et al.4 |
| Cindy vs interval vs weightlifting session | 34 | Jump performance depressed after Cindy; recovered by the end of the interval session | Maté-Muñoz et al.7 |
| Same circuit at 5 vs 15 min | 22 | Different athlete traits predicted performance at each duration | Mangine et al.6 |
The most instructive single row is Cindy. Heart rate sat in the vigorous range while oxygen uptake sat around two thirds of maximum.5 A twenty-minute AMRAP is not a maximal aerobic test — the limit is local muscular fatigue and the transitions between movements, not your ability to deliver oxygen. That is why it feels harder than the oxygen cost suggests, and why a strong runner is not automatically good at it.
Add up the participants in that table and you get well under a hundred people, most of them trained men, across five studies using different equipment and protocols. That is the entire evidence base for physiological claims about AMRAP training. Treat any confident number you read about it — including the ones above — as indicative rather than established.
What the research does not show
Being straight about the gaps, because this field is younger and smaller than the confidence of most articles about it suggests:
- The sample sizes are small. Nine, fourteen, twenty-two participants. These are pilot and exploratory studies, and effects that look clean in a group of nine often shrink in a group of ninety.
- Participants are mostly trained men. Where women are included the numbers are usually too small to analyse separately, so almost nothing here should be assumed to transfer without qualification.
- There is no per-minute energy system breakdown for a long AMRAP.The direct pathway measurement we have is on a two-minute workout.3Anyone giving you an exact aerobic percentage for minute fourteen of a twenty-minute AMRAP is extrapolating, and should say so.
- HIFT is not HIIT. They are distinct modalities — HIFT uses varied functional movements and may not include prescribed rest8 — so borrowing HIIT interval research to describe an AMRAP is a substitution worth flagging rather than making quietly.
What to do with this
Three things follow that you can act on this week.
- Stop programming by heart rate zone. In an AMRAP it saturates near maximum and stops discriminating. Round times tell you far more — which is what pace variance measures.
- Pick the cap for the quality you want to test.Short caps reward what you can currently do; long caps reward what you can repeat.
- Do not treat five and twenty minutes as the same workout scaled. They select different traits, and programming them identically wastes one of them.
Related questions
Is an AMRAP aerobic or anaerobic?
Both, from the first second. Reviews of maximal exercise show all three energy pathways engage immediately rather than switching on in sequence, and equal contribution from anaerobic and aerobic metabolism arrives at roughly 75 to 80 seconds. Every AMRAP long enough to be called an AMRAP is already aerobically dominant.
Does a longer AMRAP mean lower intensity?
Not by heart rate. In one direct comparison, a roughly 4-minute session and a roughly 17-minute session produced statistically indistinguishable heart rates — about 90 to 91% of maximum in both — while blood lactate was significantly higher after the shorter one. Duration changes the metabolic route more than it changes the effort.
What does the time cap actually change?
Which qualities decide your score. When the same circuit was run for 5 and for 15 minutes, performance measures predicted the 5-minute result while training experience and body composition related more to how athletes managed the 15-minute version. The cap selects the trait being tested.
References
- Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine, 31(10), 725–741. doi.org/10.2165/00007256-200131100-00003Narrative review. The paper the ~75-second crossover figure comes from.
- Gastin PB, Suppiah HT (2026). Anaerobic and aerobic energy system contribution during maximal exercise: a systematic review. Sports Medicine. doi.org/10.1007/s40279-026-02414-7Systematic review of 102 studies and 311 data points. Revisits the 2001 estimate.
- 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.
- 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.
- 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.
- Mangine GT, McGeehan KC, King W, Hines A, Henley JW, Grazer JL, Esmat TA, McLester JR (2025). Workout duration alters the importance of predictive traits on high-intensity functional training workout performance. Sports, 13(6), 156. doi.org/10.3390/sports1306015622 trained participants, the same circuit run for 5 and for 15 minutes.
- Maté-Muñoz JL, et al. (2017). Muscular fatigue in response to different modalities of CrossFit sessions. PLOS ONE, 12(7), e0181855. doi.org/10.1371/journal.pone.018185534 subjects, three workout types including a 20-minute AMRAP. Jump testing before, during and after.
- Feito Y, Heinrich KM, Butcher SJ, Poston WSC (2018). High-intensity functional training (HIFT): definition and research implications for improved fitness. Sports, 6(3), 76. doi.org/10.3390/sports6030076The definitional paper separating HIFT from HIIT.
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 or a study is small, it says so. It is not medical advice — see our terms.