Give two athletes with identical thresholds the same interval session and they can respond in opposite directions. One nails it. The other is buried by rep four. Same numbers on paper, completely different days. The reason is phenotype, and most platforms have no way to see it.
This is the part of the profile that decides not how fit you are but what kind of athlete you are, and therefore what training will actually work on you. Get it right and you stop handing the average session to everyone and start giving each athlete the session that fits. Athletica reads it from your training data, with no lab and no muscle biopsy.
What the anaerobic speed and power reserve is
Every athlete has a top aerobic speed, the pace at which they reach VO2max, and a top sprint speed, flat out for a second or two. The gap between the two is the anaerobic speed reserve, or the anaerobic power reserve when you measure it in watts on the bike (Weyand et al., 2006; Bundle & Weyand, 2012). For running and swimming it is a speed reserve, the span from maximal aerobic speed (MAS) to maximal sprinting speed (MSS). For cycling it is a power reserve, from maximal aerobic power (MAP) to maximal peak power (MPP).
Everyone has a reserve. What differs is its size, and the size tells you a great deal about how an athlete should train.
Why "anaerobic" is the wrong word
A quick piece of honesty about the name, because it changes how the reserve is used. The efforts that sit in this range are fuelled in large part by anaerobic energy, but they are not limited by it (Bundle & Weyand, 2012). The ceiling is neuromuscular and mechanical, set by how much force your fast-twitch fibres can produce and sustain, not by how fast glycolysis can run.
The cleanest evidence is this. Express performance as a percentage of the reserve and plot it against the duration of the effort, and the rate of decline is identical for running and for cycling, for an Olympic sprinter and for a television reporter (Bundle & Weyand, 2012). Same curve, every time. A universal decline like that does not come from metabolism, which varies enormously between people. It comes from a shared mechanical constraint. The label stuck for historical reasons, but the physiology says the reserve is mechanically bounded, which is exactly why it works as an axis for individualising training.
What your phenotype is, and how Athletica reads it
Put athletes on a line from fatigue-resistant diesel to explosive sprinter and you have the speed-endurance continuum. A slow-twitch-dominant diesel holds a high fraction of MAS for a long time and carries a modest reserve. A fast-twitch-dominant sprinter has a large reserve and fades quickly at sustained intensities. The amplitude of the speed or power reserve is a non-invasive proxy for that fibre-type dominance, which otherwise needs a needle in your quad to measure.
Athletica estimates the two anchors, MAS or MAP and MSS or MPP, from the maximal efforts already in your training, fitting the same models that build your power-duration profile. The mechanics of that fitting live on the critical power and pace page; what matters here is the output. From your own files Athletica places you on the continuum and works out the kind of athlete you are.
Why phenotype rewrites the prescription
Here is the worked example that makes the case. Two runners, both with a maximal aerobic speed of 18 km/h, roughly the pace of a 35-minute 10k runner. Athlete A tops out at a sprint of 22 km/h, athlete B at 24. Athlete A's reserve is 4 km/h wide; athlete B's is 6. Put them both through 400m reps at 20 km/h, a touch above 110 percent of their shared aerobic ceiling. For athlete A that pace sits halfway into his reserve. For athlete B, with the wider reserve, the same pace is only a third of the way in (Sandford, Laursen & Buchheit, 2021). Same absolute pace, same percentage of threshold, and yet A is reaching far deeper into what he has than B. Prescribe that session by percentage of maximal aerobic speed alone and you have quietly handed the two athletes different doses.
That is why phenotype is the blueprint for load individualisation, not a profiling curiosity. Respecting the reserve is what lets you match the severity of a session to the athlete, not just its intensity.
It also points at the right session types. Fast-twitch-dominant athletes are explosive and fatigue fast, so they thrive on short intervals, 30/30s and short hill sprints, and struggle when held at threshold for long stretches. Slow-twitch-dominant athletes are the opposite, built for long intervals of four to eight minutes near VO2max and for sustained tempo work, and they gain comparatively little from short bursts. The phenotype even predicts who breaks. Athletes with more fast-twitch fibre are measurably more likely to overreach under intensified load (Bellinger et al., 2020), which means phenotype should shape the volume and the recovery around a session, not only the session itself.
From phenotype to plan
In Athletica the profile does real work. It suggests the session types you stand to gain most from, VO2, threshold, short or long HIIT, sprints, and flags where your data is too thin to trust. It informs the interval structure the plan reaches for. And it ties to your event, since a time triallist living near threshold and a road racer surging over it are loading different parts of the same reserve.
All of it runs on the profiling-to-prescription loop: the locomotor profile classifies you, the prescription logic builds sessions to fit, and the Athletica-modified load model underneath governs how hard and how often. Phenotype is the part that makes the plan yours rather than the population's.
The full treatment, including how to read your own profile, is in our free course at athleteprofilingprimer.com.
Where phenotyping sits in the Athletica methodology
Phenotyping reads the critical power and pace profile and turns it into a classification: what kind of engine you are running. Workout Reserve extends the profile with durability, how the picture shifts as fatigue builds within a session. The load model turns the whole thing into a plan. Phenotyping is the step that decides which training actually fits you.
Explore the full Athletica Methodology HubFrequently Asked Questions
What is the anaerobic speed reserve (ASR)?
The anaerobic speed reserve is the gap between an athlete's maximal aerobic speed (the pace at VO2max) and their maximal sprinting speed. On the bike the equivalent is the anaerobic power reserve, from maximal aerobic power to maximal peak power. Every athlete has a reserve; its size shapes how they should train.
What is the difference between ASR and APR?
They are the same construct in different units. ASR (anaerobic speed reserve) applies to running and swimming and is measured in speed; APR (anaerobic power reserve) applies to cycling and is measured in power. Both describe the span between maximal aerobic and maximal sprint capacity.
Is the anaerobic speed reserve actually anaerobic?
Not in the limiting sense. Efforts in this range draw heavily on anaerobic energy, but they are bounded by neuromuscular and mechanical factors, how much force fast-twitch fibres can produce and sustain, rather than by anaerobic energy supply. The name is historical. The decline in performance across the reserve is identical for running and cycling and across athletes of very different ability, which points to a shared mechanical constraint rather than a metabolic one.
What is athlete phenotyping in endurance sport?
Athlete phenotyping places an athlete on the speed-endurance continuum, from fatigue-resistant slow-twitch types to explosive fast-twitch types. The amplitude of the anaerobic speed or power reserve serves as a non-invasive proxy for fibre-type dominance, allowing training to be matched to the athlete's underlying physiology rather than prescribed one-size-fits-all.
How does Athletica determine my phenotype without a lab or muscle biopsy?
Athletica estimates your maximal aerobic and maximal sprint capacities from the maximal efforts already present in your training, using the same models that build your power-duration profile. The size and shape of your reserve then place you on the speed-endurance continuum. No lab test or biopsy is needed.
Why do two athletes with the same threshold respond differently to the same intervals?
Because the same absolute intensity represents a different fraction of each athlete's reserve. Two runners with an identical maximal aerobic speed but different sprint speeds will be working at different percentages of their reserve during the same interval, so one is far closer to their limit than the other. Threshold alone does not capture this; the reserve does.
How does phenotype change my HIIT prescription?
Fast-twitch-dominant athletes generally respond best to short, high-intensity intervals and tolerate sustained threshold work poorly; slow-twitch-dominant athletes thrive on longer intervals and tempo efforts and gain less from short bursts. Phenotype also predicts overreaching risk, with fast-twitch athletes more prone to it under heavy load, so it informs volume and recovery as well as session type.
Updated: 06/19/2026
