Ask most endurance platforms what intensity you can hold and you get one number. An FTP, or a threshold pace. One number, set on one day, treated as the truth until you test again. The body doesn't work like that. Your sustainable intensity depends entirely on how long you intend to sustain it, and a single number throws that whole dimension away.
Critical power is the concept that puts the time dimension back. It comes out of a relationship physiologists have understood for a century: the harder the effort, the less time you can hold it (Hill, 1925). Plot your best efforts across durations and you get a curve, not a point. Critical power is a specific, meaningful feature of that curve, and it tells you something FTP cannot.
This is the profiling layer Athletica is built on. Get the profile right and everything downstream, your zones, your phenotype, your prescription, follows from real physiology instead of a guess.
What critical power and critical pace actually are
Picture your best sustainable power plotted against time. It falls steeply at first, the maximal sprint you can hold for seconds, then flattens as the duration grows. The flattening is the part that matters. Critical power is the asymptote the curve approaches, the boundary above which you cannot reach a metabolic steady state (Poole et al., 2016; Jones et al., 2019). Below it your physiology settles and you can hold on. Above it there is no settling. You are drawing down a finite reserve, and the clock to exhaustion is already running.
In the four-domain model of exercise intensity, critical power marks the line between the heavy and severe domains (Burnley & Jones, 2018). For runners and swimmers the same idea goes by critical speed or critical pace, read off the pace-duration curve rather than power. The physiology is identical. Only the unit changes.
This is a real threshold, not an arbitrary one. The transitions between intensity domains are smooth inflections rather than hard lines (Leo et al., 2022), but critical power sits at a genuine physiological boundary, which is exactly why it is more useful than thresholds chosen for convenience.
The problem with a single threshold number
Functional Threshold Power, the largest power you can hold for roughly an hour, became the default because it is easy to test and easy to grasp. The trouble is that it was only ever assumed to approximate a steady-state threshold, and several studies have shown it drifts from that mark, differently for different athletes (Borszcz et al., 2018; Valenzuela et al., 2018). More to the point, it is one number. It says nothing about your sprint, nothing about your durability, nothing about where your fatigue mechanisms change.
We are not purists about this. Athletica still uses a 20-minute FTP test as one input, and a near-maximal 20-minute effort is genuinely useful, close to necessary, for anchoring a valid critical power estimate. What we don't do is let that single number stand in for the whole athlete. FTP is one point. Your physiology is the curve.
One curve, several models
No single equation describes the entire power-duration curve well. Each captures part of it.The classical critical power models handle the middle, the durations from a few minutes to an hour where the steady-state boundary lives. They were never built to predict sprinting, and they don't. A two-parameter model implies an infinite sprint power at one end and no fatigue beyond about forty minutes at the other, neither of which is true (Morton, 1996).
The sprint end needs a different tool. The anaerobic speed and power reserve (ASR/APR) describes short efforts, under roughly three minutes, as a fraction of the gap between your maximal aerobic and maximal sprint capacities (Weyand et al., 2006; Sandford, Laursen & Buchheit, 2021). It holds in the severe and extreme domains where the critical power models fail, and it lets us estimate your maximal aerobic power from a short maximal effort without a lab (Sanders et al., 2017).
For the long, sub-threshold end, omni-domain models such as OmPD extend coverage across the full range from a handful of all-out efforts (Puchowicz et al., 2020).
Athletica runs these together. The critical power model for the steady-state boundary, the APR model for the sprint domain, the omni-domain model across the rest. You get the whole curve, modelled, rather than one number standing in for it.
Building your profile without the lab coat
Here is the part that separates this from a textbook. You do not have to visit a lab, and for the most part you do not have to stop training to test.
Athletica reads the maximal efforts already buried in your normal training and uses machine learning to estimate the thresholds that used to need a metabolic cart: first ventilatory threshold, critical power or speed, maximal aerobic power or speed, and maximal sprint power or speed. The honest efforts you produce in races and hard sessions populate the curve on their own. We call this invisible monitoring, and it draws on the same logic the literature is now formalising (Leduc & Weaving, 2025).
Real training never touches every duration, though. The gaps are real, and a curve with holes in it gives soft estimates. That is what the Test Week is for: a short, sport-specific battery of maximal efforts that fills the thin zones, a modified version of the Quod et al. (2010) cycling protocol with running, swimming, and rowing equivalents. For cycling that might be efforts at six seconds, thirty seconds, one minute, five minutes, and ten minutes. For running, a 400m, a 1k, and a 5k. When those points land, Athletica recalculates your critical power and pace, refreshes your seven zones, and rescales every upcoming session in seconds. The plan you see the next morning matches your physiology today, not your form from January.
From profile to prescription
A profile that just sits there is a dashboard. The point is what Athletica does with it.
First, it tells you what kind of athlete you are. Set your thresholds against population benchmarks and a phenotype emerges. A diesel-engine distance type with a high critical power relative to their aerobic ceiling, or a twitchy sprint type with a large anaerobic reserve. The shape of your curve points at the training that will move you, and at where the gaps are.
Second, it sets your zones. Athletica's seven-zone model is anchored to these thresholds rather than to a percentage of one number. Critical power marks the boundary between Zone 3 and Zone 4. The upper zones, where the sprint work lives, are calibrated from the anaerobic power reserve. Your zones are yours, built from your curve.
Third, the zones and thresholds feed the load model. The profiling layer hands off to the Athletica-modified impulse-response model (Banister, 1975) that prescribes how hard each session should be and projects where your fitness is heading. This is the division of labour worth being precise about. Machine learning finds your profile. Deterministic math prescribes your training against it. One is pattern recognition over your own data; the other is an auditable calculation a coach can interrogate.
Where critical power sits in the Athletica methodology
Critical power and pace are the profiling foundation. Athlete phenotyping reads the profile to classify you. Workout Reserve extends it, folding in durability so the curve reflects not just what you can do fresh but what you can do deep into a session, which the classic short-effort models miss. And the load model turns all of it into a plan. Critical power is where the chain starts.
Explore the full Athletica Methodology Hub Frequently Asked Questions
What is critical power?
Critical power is the highest intensity at which you can still reach a metabolic steady state, the boundary between the heavy and severe exercise domains. Below it your physiology stabilises and you can sustain the effort; above it you draw down a finite reserve and fatigue to exhaustion. It is derived from the power-duration relationship, not from a single test.
What is the difference between critical power and FTP?
FTP is a single number, the power you can hold for about an hour, assumed to sit near a steady-state threshold. Critical power is a feature of your whole power-duration curve and reflects a genuine physiological boundary. FTP describes one point; critical power describes where your fatigue mechanisms actually change. Athletica uses FTP tests as one input but does not treat that single number as your full profile.
What is critical pace or critical speed?
It is the running and swimming equivalent of critical power, derived from the pace-duration curve instead of power. The physiology is the same; only the unit differs. For swimming it is often expressed as critical swim speed.
Does Athletica need a lab test to find my critical power?
No. Athletica estimates your thresholds from the maximal efforts in your normal training using machine learning, an approach called invisible monitoring. Where your training has not covered certain durations, a short sport-specific Test Week fills the gaps. No metabolic cart required.
How does Athletica use critical power to set my training zones?
Critical power anchors the boundary between Zone 3 and Zone 4 in Athletica's seven-zone model, and the upper zones are calibrated from your anaerobic power reserve. Because the zones are built from your own thresholds rather than a percentage of a single number, they reflect your individual physiology.
How is Workout Reserve related to critical power?
Workout Reserve builds on the power-duration profile but accounts for durability, how your usable reserve changes as fatigue accumulates within a session. Classic short-effort models such as W' ignore that. Critical power is the foundation; Workout Reserve is the durability-aware extension.
Updated: 06/19/2026
