There is a curve that tells the complete story of an endurance athlete. Not a power curve, not a speed curve - a metabolic curve. It plots one simple thing: how your body responds to increasing intensity. What energy systems engage. Which muscle fibers get recruited. How your physiology manages - or fails to manage - the relationship between fuel production and fuel clearance as the work gets harder.
Understanding this curve is not an academic exercise. It is the most practical diagnostic tool in endurance coaching, because it tells you exactly what kind of athlete you are right now - and exactly what kind of training will move you toward the athlete you want to become.
What the Curve Is Actually Showing You
Before we talk about archetypes, let's be precise about what this curve represents.
The y-axis is glycolytic demand - specifically, the net rate at which lactate accumulates in your system as intensity rises. This is not simply lactate concentration. It is the balance between lactate production and lactate clearance. When your aerobic system is managing the demand, production and clearance are roughly in equilibrium and the curve stays flat. When that balance tips - when production begins to outpace clearance - the curve rises. When clearance is completely overwhelmed, the curve goes near-vertical.
The x-axis is intensity. Wattage. Pace. Effort. Moving left to right, you are moving from easy to hard.
The curve itself is not a straight line. It has a characteristic shape: a flat or gently rising section at low intensities, a clear inflection point, an escalating rise through the middle, and a near-vertical explosion at the top end. The exact shape of that curve - where it inflects, how steeply it rises, how long it stays flat - is the physiological fingerprint of that athlete.
Three thresholds define the most important landmarks on the curve.
LT1 - Lactate Threshold 1 is the first meaningful inflection point. Below LT1, the aerobic system has the demand fully under control. Above it, production begins to accelerate. LT1 marks the top of QT2's Zone 1 (Z1) - the ceiling of pure aerobic work.
OGC - Oxidative-Glycolytic Crossover is where the curve bends sharply upward. This is the point at which Type IIA muscle fibers - which can operate either aerobically or glycolytically depending on the demand placed on them - tip predominantly into glycolytic expression. Lactate accumulation is now outpacing clearance in earnest. OGC marks the top of QT2's Zone 2 (Z2) and the beginning of QT2's Zone 3 (Z3).
CP - Critical Power is where sustainable effort ends. Below CP, an athlete can theoretically hold an effort indefinitely, given sufficient fuel. Above CP, the body is drawing down a finite anaerobic reserve called W'. Once that reserve is depleted, the effort cannot continue at that intensity. CP marks the beginning of the W' zone - the territory where the only clock that matters is how much anaerobic capacity remains.
Between LT1 and OGC sits what we call the aerobic window - Z2. This is the metabolic real estate that defines long-course racing. The wider it is in absolute terms, the more intensity range an athlete can sustain at a cost the aerobic system can manage.
The Fiber Type Story
The curve does not exist in isolation. It is a direct expression of which muscle fibers are being recruited and how they are behaving.
At the lowest intensities - ZR and the lower reaches of Z1 - the work is done almost exclusively by Type I fibers. These are slow-twitch, oxidative fibers. They are extraordinarily efficient at aerobic metabolism, generate very little lactate, and can sustain output for extremely long durations. Fat is the dominant fuel. The curve is flat because these fibers are essentially lactate-neutral.
As intensity rises through Z1 and into Z2, Type IIA fibers begin to be recruited alongside Type I. Type IIA fibers are metabolically flexible - they can express either oxidative (aerobic) or glycolytic (anaerobic) metabolism depending on the demand. At lower intensities, they run aerobically. This is the IIA-oxidative expression zone. As intensity increases, they begin to cross over into glycolytic expression - this crossover is the OGC, and it is the boundary between Z2 and Z3.
Above OGC, Type IIA fibers are predominantly glycolytic, and Type IIX fibers - fast-twitch, anaerobic, powerful but metabolically expensive - begin to be recruited. This is Z3 and above. Lactate accumulation accelerates rapidly. Above CP, it is essentially unconstrained.
This fiber recruitment story is the physiological mechanism underneath the curve. The curve is not abstract - it is literally showing you which muscle fibers are working, how they are working, and what metabolic cost they are extracting from your system at each intensity level.
The Four Archetypes
Not all athletes sit at the same point on the developmental spectrum. Based on the ratio of Critical Power to predicted VO2 max - CP:pVO2max - athletes fall into one of four physiological archetypes. Each archetype has a characteristic curve shape, a characteristic set of limiters, and a specific training focus that addresses those limiters most efficiently.
Very Low Aerobic Sustainability Relative to Capacity (CP:pVO2max < 78%)
This is the athlete whose curve offers almost no flat section. From the very first moments of effort, the curve is already rising. LT1 arrives early on the intensity axis (from left to right), OGC is not far behind, and by the time the athlete reaches what most people would consider moderate effort, they are already deep into glycolytic territory.
What does this mean in practice? It means that even at relatively easy paces, this athlete is paying a meaningful metabolic tax. Their aerobic system - the machinery of mitochondria, capillaries, and fat-oxidizing enzymes - is not developed to the point where it can contain lactate accumulation at low and moderate intensities. The Type I fibers and oxidative Type IIA fibers have not been trained to their aerobic potential.
The word "structurally" is important here. This is not primarily a threshold problem. The limiter is not where CP sits relative to pVO2max - the limiter is that the aerobic foundation has not been built. The entire curve is shifted left and steep throughout, which means no matter how much threshold or high-intensity work this athlete does, they will be running an expensive engine on a weak chassis.
This is also the archetype where the aerobic window is narrowest. The gap between LT1 and OGC is compressed, which means the range of intensity the athlete can sustain at a manageable metabolic cost is very small.
Low Aerobic Sustainability Relative to Capacity (CP:pVO2max 78–83%)
We have already seen this curve, above. The athlete with moderate aerobic sustainability relative to capacity has an aerobic base - the curve shows a recognizable flat section through the lower intensities. There is a genuine Z1 where the aerobic system is in control. But the thresholds arrive in the middle of the intensity axis, the aerobic window is moderate, and CP sits meaningfully below pVO2 Max, leaving significant aerobic ceiling untapped.
This is the archetype that describes most trained age-group endurance athletes. They have done enough aerobic work to build a foundation, but that foundation has not yet been pushed to its potential. The limiter has shifted from structural to threshold-specific: the system can manage low-intensity work well, but LT1 and OGC are not as far right as they could be, and the gap between CP and pVO2 Max reflects genuine upside in aerobic power expression.
The curve is a recognizable S-shape: a short flat section, a clear inflection at LT1, a steepening through Z2, and an exponential rise above OGC. The aerobic window exists and is trainable.
Moderate Aerobic Sustainability Relative to Capacity (CP:pVO2max 83–87%)
The athlete with moderate aerobic sustainability relative to capacity has a meaningfully developed aerobic base. The curve stays flat or nearly flat for a longer stretch of the intensity axis before inflecting. LT1 and OGC are pushed further right - the athlete sustains aerobic metabolism at higher absolute intensities before the glycolytic cascade begins.
The "moderate" labeling reflects the physiological reality: the aerobic and anaerobic systems are well matched. Neither is dramatically limiting the other. The athlete has a solid foundation and a developed threshold, and is approaching the range where further improvement requires both expanding the aerobic window and pushing the pVO2 Max ceiling higher.
The aerobic window is wider in absolute terms. The post-OGC rise is steeper - which is a good thing, because it means the flat section is genuinely flat, and the contrast between the two is becoming visible. This athlete is on the right side of the developmental spectrum, but still has meaningful physiological leverage available.
High Aerobic Sustainability Relative to Capacity (CP:pVO2max > 87%)
This is the hockey stick. The curve is nearly horizontal across most of the intensity axis - the athlete sustains aerobic metabolism at intensities that would be deeply glycolytic for a less-developed athlete. LT1, OGC, and CP are all compressed into a high-intensity band near pVO2 Max, and the transition from flat to vertical is rapid and dramatic.
What this means in physiological terms is that the aerobic system has been developed to such a degree that it dominates energy production across the vast majority of the usable intensity spectrum. Type I and Type IIA-oxidative fibers are highly trained, mitochondrial density is high, fat oxidation is efficient at intensities most athletes cannot reach aerobically, and lactate clearance capacity is exceptional.
This is the profile of elite and professional endurance athletes. It is also the profile that a well-periodized training approach is always building toward - not just for elites, but for any athlete committed to long-term aerobic development.
The 75–80% Rule: Why Sub-LT1 Work Is the Foundation for Every Archetype
Here is one of the most important principles in endurance training, and one of the most consistently misunderstood: regardless of physiological archetype, regardless of whether the athlete has very low aerobic sustainability relative to capacity or high aerobic sustainability relative to capacity, the vast majority of their training stress - 75 to 80% of it - should come from sub-LT1 work.
Within QT2.0's block periodization model, this is not a compromise. It is not a concession to recovery or convenience. It is the most leveraged training decision we can make, and the physiology explains exactly why.
Sub-LT1 work is the primary and essentially exclusive driver of the adaptations that build the flat left side of the curve. Every hour spent training aerobically below LT1 is an hour investing in mitochondrial biogenesis - the creation of new mitochondria within muscle cells, increasing the engine's aerobic capacity. It is an hour building capillary density, improving oxygen and substrate delivery to working muscle. It is an hour up-regulating the fat oxidation enzymes that allow the aerobic system to sustain work on less glycogen, going further before the metabolic cost rises.
None of these adaptations are driven meaningfully by high-intensity work. Threshold intervals do not build mitochondria the way aerobic volume does. VO2 Max efforts do not expand capillary beds the way long aerobic sessions do. The flat section of the curve - the handle of the hockey stick - is built exclusively through the accumulation of sub-LT1 work over time.
This means that the 75–80% floor is load-bearing for every archetype. The athlete with very low aerobic sustainability relative to capacity needs it most urgently. The athlete with a high aerobic sustainability relative to capacity needs it to maintain the foundation beneath their elite-level ceiling. Strip away the aerobic base, and within weeks the curve begins to steepen on the left side - LT1 drifts left, the aerobic window narrows, and the metabolic cost of every training session increases.
There is a second reason this principle matters: the aerobic base determines how much high-intensity work an athlete can absorb and adapt from. High-intensity training creates stress. Recovery and adaptation require an aerobic substrate. An athlete without a deep aerobic base accumulates fatigue from intensity faster, recovers slower, and derives less adaptation from the quality work they do. The 75–80% floor is not just building the curve - it is building the system's capacity to benefit from everything above that floor.
So when we talk about training focus - what to do with the remaining 20–25% of training stress - we are always talking about what to layer on top of a non-negotiable 75-80% of training stress, specifically aimed at building a solid aerobic foundation.
The Intensity Focus: What the Remaining 20–25% Is Doing
The term "intensity focus" can be misleading, because it does not necessarily mean high-intensity work. It means intentional, targeted training stress applied to the specific physiological limiter of each archetype. Sometimes that is gentle work just above LT1. Sometimes it is maximal aerobic intervals. The intensity is defined by the physiology, not by convention.
Very Low Aerobic Sustainability Relative to Capacity: Aerobic Expansive Work (LT1 to OGC)
For the athlete with very low aerobic sustainability relative to capacity, the intensity focus is work applied across the span between LT1 and OGC - Z2, the aerobic window itself.
Why here, and not higher?
Because the limiter is structural. The aerobic window is narrow and compressed, which means the system has very little capacity to operate in the crossover zone where Type IIA fibers can still express aerobic metabolism. The priority is to develop that zone - to train the Type IIA fibers at their oxidative expression limit, just below and just above the crossover point.
Work just below OGC trains the oxidative ceiling of the Type IIA fibers. It creates specific demand on the crossover zone without pushing the system into glycolytic dominance. Work just above OGC - briefly - challenges the lactate clearance system at the boundary, training the body to buffer and clear lactate at the specific intensity where the crossover occurs.
The goal of this work is to move the OGC rightward. Not by hammering threshold intervals, but by systematically developing the physiological machinery that governs the crossover. Done correctly, over time, this work widens the aerobic window and begins to shift the entire curve right.
Critically, this athlete is not yet ready for heavy threshold or CP work. Their aerobic foundation is insufficient to absorb it without compressing the very window they are trying to build. Applying "blade" work - high-intensity training above OGC - before the structural aerobic base is established would drive CP upward without commensurately moving LT1 and OGC. The curve steepens throughout. The athlete gets faster at the top end for a short period and then hits a ceiling - a metabolically expensive one with a narrow aerobic window and no room to build further.
Low Aerobic Sustainability Relative to Capacity: Build/TH Work (Sub-CP)
For the athlete with low aerobic sustainability relative to capacity, the aerobic foundation exists. LT1 is established. The aerobic window is present and trainable. The limiter has shifted from structural to threshold-specific: CP is not sitting as close to pVO2 Max as it should, and OGC could be pushed further right.
The intensity focus here is threshold work - efforts between the Oxidative-Glycolytic Crossover and Critical Power.
Work just above OGC places stress onto the crossover zone, driving OGC rightward by training the physiological boundary itself. These are not maximum-intensity efforts - they are precise, targeted applications of stress at the intensity just above where the Type IIA crossover occurs.
Work just below CP - Classic threshold intervals, CP efforts, race-specific intensities - serves a complementary purpose. CP is both a ceiling and a target. By training just below CP, the athlete forces the physiological adaptations that raise that ceiling: increased time to exhaustion at CP, improved lactate tolerance in Z3, higher VO2 kinetics, and - critically - a shift in the CP:pVO2max ratio itself.
The reason this is now appropriate, and was not appropriate for the athlete with very low aerobic sustainability relative to capacity, is that the aerobic foundation is sufficient to absorb the stress. The 75–80% sub-LT1 volume provides the recovery substrate and the base adaptations that allow the threshold work to express itself as genuine physiological improvement rather than just fatigue.
This is also the athlete for whom periodization sequence matters most. Base/Durability work first, establishing the handle. Then Aerobic Expansion to widen the window. Then Build/TH to begin "blade" work in earnest. The order is not arbitrary - each phase creates the physiological conditions for the next one to work.
Moderate Aerobic Sustainability Relative to Capacity: Hybrid Approach (Sub-CP and VO2 Max)
The athlete with moderate aerobic sustainability relative to capacity presents a different challenge. The aerobic base is solid. The aerobic window is wide. CP is meaningfully aerobically expressed. The curve has the right general shape. At this point, the physiological levers are more distributed - no single limiter is as obvious as it is in the lower archetypes - and the intensity focus reflects that.
The hybrid approach combines two types of targeted stress: CP boundary work and VO2 Max work.
CP boundary work continues to push the aerobic ceiling outward. Even at this level of development, this intensity can be trained and moved. The physiological ceiling is not fixed - it responds to specific training stimulus throughout an athlete's career.
VO2 Max work is now appropriate and productive in a way it is not for less-developed athletes. VO2 Max intervals - short, high-quality efforts at maximal aerobic power - create a specific roof-raising stimulus. They drive upward adaptation in cardiac output, oxygen delivery, and the maximal rate at which the aerobic system can produce ATP. For athlete with moderate aerobic sustainability relative to capacity, the space between CP and pVO2 Max represents accessible upside. VO2 work targets that space directly.
The Hybrid label reflects the reality that this athlete is simultaneously maintaining and extending the handle while beginning to sharpen the blade. Both levers produce meaningful returns at this stage. The art of coaching an athlete with moderate aerobic sustainability relative to capacity is sequencing and dosing both stresses within the training structure so that neither undermines the other.
High Aerobic Sustainability Relative to Capacity: VO2 Max Work
For the athlete with high aerobic sustainability relative to capacity, the intensity focus is VO2 Max work, and the question shifts fundamentally. This athlete is not building the handle - it is built. They are not widening the aerobic window - it is wide. They are not learning to tolerate threshold stress - they live there.
The question for the athlete with a high aerobic sustainability relative to capacity is: how high can the ceiling go?
VO2 Max work targets the absolute ceiling of aerobic power output. It drives adaptations in cardiac stroke volume, maximal oxygen uptake, and the efficiency of oxygen utilization at the highest aerobic intensities. For an athlete whose CP:pVO2 Max is already above 87%, the most available physiological lever is pushing pVO2 Max itself upward - which, if CP tracks proportionally, shifts the entire hockey stick rightward without changing its shape.
There is a nuance worth naming: VO2 Max work for an athlete with high aerobic sustainability relative to capacity is only productive because of everything underneath it. The flat handle section - years of aerobic base work - is what allows the body to absorb VO2 Max training stress, recover from it quickly, and express it as adaptation rather than breakdown. Without the handle, VO2 Max work is just expensive. With it, VO2 Max work is the sharpest tool available.
This is also the archetype where the relationship between the intensity focus and race performance is most direct. The limiting factor for the athlete with high aerobic sustainability relative to capacity, in long-course racing, is typically not the aerobic system - it is the combination of neuromuscular durability, pacing precision, and nutritional execution. Aerobically, they can sustain intensities that would shatter a less-developed athlete. The VO2 Max work maintains and extends that aerobic ceiling while race-specific training manages the other performance variables.
The Practical Blocks: Managing Context, Not Just Physiology
Everything discussed so far has been about changing an athlete's physiological state - moving the curve, shifting archetypes, manipulating the metabolic fingerprint through targeted training stress. But there is a second category of training blocks in the QT2.0 framework that serves a different and equally important purpose. These blocks are not primarily about changing where an athlete sits on the physiological spectrum. They are about managing the realities of an athlete's season, history, and situation.
Understanding the difference between these two categories is essential. The physiologically-driven blocks - Aerobic Expansion, Build/TH, Hybrid, VO2 Max - ask the question: what does this athlete's curve need? The practical blocks ask a different question: what does this athlete's situation need?
Base/Durability: When the Situation Demands a Foundation
The Base/Durability block is the right prescription when an athlete needs to rebuild or protect their foundation before physiological development can resume in earnest. It is not primarily about curve manipulation - it is about establishing the structural and metabolic conditions that make all other training possible.
Several situations call for a Base/Durability block. An athlete returning to training after a significant break - an off-season, an illness, an extended period of low volume - has lost the aerobic base that supports higher training stress. Loading intensity back in before that base is re-established is not just ineffective; it is how injuries happen and how athletes stall. The Base/Durability block rebuilds the foundation first.
It is also the right choice when race-specific durability is a concern independent of physiological archetype. An athlete can have moderate, or even high aerobic sustainability relative to capacity, and still lack the structural durability to sustain race-specific effort for the duration of their target event. Long-course racing places mechanical and muscular demands on the body that require specific preparation - time on feet, time in the saddle, accumulated load at low intensity. The Base/Durability block addresses that.
Injury management is a third trigger. When an athlete is nursing a soft tissue issue, returning from a stress fracture, or managing a recurring problem, the Base/Durability block allows training to continue in a form the body can tolerate while protecting the injured area and maintaining as much aerobic capacity as possible. Low-intensity, high-volume work is often the most that can be asked of a compromised body - and it is enough to maintain the curve while the athlete heals.
In all of these cases, the Base/Durability block is not a step backward. It is the responsible choice that protects the athlete's long-term trajectory.
Specificity: When the Race Is Close and Execution Is the Priority
The Specificity block is triggered by proximity to a key event and a specific concern about race-day execution. When an athlete is within approximately six weeks of their target race, the training question shifts from physiological development to race-specific preparation.
At this stage, the goal is no longer to move the curve. It is to rehearse the demands of the event so thoroughly that race day feels familiar. Specificity work mirrors the intensity, duration, and pacing demands of the target race distance. For an IRONMAN athlete, that means extended efforts at race-specific intensity. For a 70.3 athlete, it means sharper efforts at a higher specific intensity. The sessions are calibrated to the archetype, within the context of the event.
The Specificity block also addresses race execution concerns that are separate from physiology. Pacing strategy, nutritional execution, transitions, and the psychological demands of sustained effort at race pace are all rehearsed in training. An athlete who is physiologically capable of their goal pace but has never trained at that pace for race-specific durations is not race-ready - and the Specificity block aims to close that gap.
It is worth noting that the Specificity block does not attempt further physiological development. By six weeks out, the curve is what it is. The training adaptation window for meaningful physiological change has largely closed. The focus is on translating existing physiology into race-day performance with precision.
Build/TH (TTE): When Threshold Durability Matters More Than Threshold Elevation
The Build/TH (Time to Exhaustion) block is the most nuanced of the three practical blocks, and it is frequently misunderstood. Its name sounds like the Build/TH physiological block - the one aimed at raising CP and pushing OGC rightward - but the goal is fundamentally different.
Build/TH (TTE) is not trying to move the curve. It is trying to extend how long an athlete can sustain effort near their existing threshold before the curve catches up with them.
This distinction matters enormously. Raising threshold means moving the physiological boundary - getting OGC and CP to occur at higher absolute intensities. Extending time to exhaustion at threshold means building the durability, muscular resilience, and metabolic tolerance to sustain an effort near that boundary for longer, even when the boundary itself does not move.
Several situations make Build/TH (TTE) the right choice. For the athlete with high aerobic sustainability relative to capacity - one with a CP:pVO2max above 87% - VO2 Max work is the prescribed physiological block. But there are times when VO2 Max work is not appropriate: immediately following a VO2 Max block, when the athlete needs a different stimulus before returning to maximal aerobic efforts; when an athlete's training history or current fatigue state suggests that high-end work would generate more breakdown than adaptation; or simply when the coaching judgment is that the athlete would benefit more from extended threshold durability than from roof work at that moment.
In these cases, Build/TH (TTE) provides a productive, meaningful training block that complements the athlete's high aerobic sustainability without pushing into territory that is not currently appropriate. The athlete sustains more time at threshold-adjacent intensities - not to raise the threshold, but to become more durable at it. The result is an athlete who can hold race-specific intensity for longer, with less metabolic cost and better execution, without the recovery demands of VO2 Max work.
Think of it this way: VO2 Max work raises the roof. Build/TH (TTE) makes the roof wider. Both matter. The question is which one the athlete needs right now.
There is a principle embedded in everything above that deserves to be stated explicitly: physiological development follows a sequence, and that sequence cannot be safely reversed.
The handle must come before the blade. The aerobic base must be established before threshold work can be productively applied. OGC boundary work must precede CP and VO2 work. Every layer of the development pyramid must be genuinely established before stress is applied to the layer above it.
This is not just coaching philosophy. It is physiology. The aerobic adaptations that build the flat left side of the curve are the same adaptations that determine how much high-intensity stress the athlete can absorb and recover from. Skip them, and you are not just missing an opportunity - you are actively compromising the effectiveness of everything above.
The four archetypes are not permanent categories. They are developmental stages. With correct training over sufficient time, an athlete with very low aerobic sustainability relative to capacity shifts to low aerobic sustainability. An athlete with low aerobic sustainability relative to capacity transforms their aerobic sustainability to moderate. And, an athlete with moderate aerobic sustainability relative to capacity develops high aerobic sustainability. Each transition is driven by specific, targeted training applied in the right sequence, on top of a non-negotiable aerobic foundation.
The metabolic curve is not just a diagnostic. It is a roadmap. It tells you where you are, why you are there, and exactly what to do next.
Feel free to play with our physiological profile visualization tool - HERE.
QT2 Systems - Data-driven coaching for endurance athletes.
