Two athletes walk into a testing session. Both hold the same power for 20 minutes. Both produce similar numbers at their second threshold. By every conventional measure, they are equal.
Give them the same training programme and one will improve. The other will stagnate or go backwards.
This is not bad luck. It is the predictable consequence of using a single number to describe something that is not a single number.
The Single-Number Problem
Most training programmes anchor everything to one threshold number. You ride hard, take a percentage, and set your zones. Every easy ride, every interval session, every race plan is built on that single result.
The problem is that your threshold tells you where your engine currently redlines. It says nothing about how that engine is built.
Two athletes at the same threshold can have completely different relationships between their aerobic and anaerobic energy systems. One might produce that power primarily through aerobic metabolism, with an efficient oxidative system that clears metabolic byproducts steadily. The other might reach the same number by leaning harder on anaerobic glycolysis, producing the same mechanical output but accumulating hydrogen ions faster and relying on a less developed aerobic base.
From the outside, they look identical. Inside, they are different engines running on different fuel mixes. Coaching them the same way is like servicing a diesel and a petrol engine with the same manual.
This is not a theoretical distinction. At every level of endurance sport, coaches working with groups of similarly talented athletes have found that identical programmes produce wildly uneven results. The Norwegian triathlon programme identified this early: athletes training side by side under the same philosophy still required meaningfully different emphasis within that framework, because their individual physiological profiles were different. Some athletes lost top-end capacity quickly during base phases and needed regular ceiling work to maintain it. Others held their peak easily but lost sub-threshold fitness within weeks if the volume dropped. Same squad, same coaching system, different physiological priorities.
What the Curve Reveals
A single threshold number is a point. Your fitness is a curve.
When you test across multiple durations, from short, explosive efforts through to long, sustained outputs, the shape of that curve tells a story no single number can. The relationship between your 3-minute power and your 60-minute power. The ratio between your short-duration capacity and your ability to sustain work for hours. The gap between your first threshold and your second threshold.
These ratios are where the real information lives. Two athletes can share the same point on the curve at 20 minutes and have completely different shapes around it. One curve might drop steeply from a high 3-minute peak. The other might hold remarkably flat from a modest peak through to sustained efforts. The point is the same. The picture is entirely different. And the training that each athlete needs flows directly from that picture.
The Steep Curve
An athlete whose power drops off steeply from short to long durations typically has strong glycolytic capacity but underdeveloped aerobic infrastructure. Their anaerobic system produces impressive numbers over short efforts. But the metabolic cost of sustained work is disproportionately high, because the aerobic system cannot clear byproducts quickly enough to keep pace with what the glycolytic system generates.
These athletes often present with a compressed lower zone structure. Their first threshold sits low, meaning they transition into glycolytically demanding intensities at relatively modest power outputs. The gap between their first and second threshold is wide. On race day, this profile tends to produce fast opening kilometres followed by a steep and painful decline. The engine has a high rev ceiling but the foundation that supports it is narrow.
The priority for these athletes is not more high-intensity work. Their top end is already strong relative to their base. The priority is growing the aerobic infrastructure underneath it. More time in Zones 1 and 2. Structured threshold work in Zone 3, but with a structure that prevents metabolic byproducts from accumulating past the point of productive stimulus. The goal is not to suppress the glycolytic system. It is to build the aerobic system around it so that the byproducts of glycolysis are processed more efficiently at higher workloads. Aerobic development at this level is a years-long project, and no amount of top-end intensity can shortcut it.
The Flat Curve
An athlete whose curve holds its shape from short to long durations has a well-developed aerobic system but a limited ceiling. Their oxidative capacity is strong. They clear metabolic byproducts efficiently and sustain threshold intensities without excessive strain. But their peak power is constrained. The first threshold sits high. The gap between their first and second threshold is narrow. There is not much headroom above the second threshold.
On race day, these athletes pace well and hold form deep into the event. But their ceiling limits how fast "well paced" can be. They rarely blow up. They also rarely surprise themselves with a breakthrough, because the top of the curve is holding the rest of their performance hostage.
These athletes do not need more base training. Their aerobic engine is already doing its job. They need ceiling work. More time in Zones 4 and 5 to raise the peak of the curve, expanding the range that the aerobic system can fill. Zone 2 alone will not move the needle for an athlete whose limiter is the height of the ceiling, not the width of the base.
Why Generic Plans Fail
This is the fundamental limitation of any programme built on a single number. A plan that prescribes the same session structure and the same intensity distribution to both of these athletes is optimised for neither.
The steep-curve athlete following a programme heavy on VO2max work is doubling down on their strongest quality while neglecting the aerobic development they need most. The flat-curve athlete following a programme heavy on easy volume is reinforcing an existing strength while starving the capacity that limits their racing.
The shape of the curve dictates the training priorities. Without knowing the shape, you are training blind.
The Shape Is Not Fixed
Your metabolic profile changes as your training shifts the balance between your energy systems. An athlete who starts with a steep curve and commits to consistent aerobic development will see the lower durations of their curve rise over months and years. The gap between their first and second threshold narrows. Their zone structure expands where it was previously compressed. The engine becomes more balanced, and the race performances that follow reflect it.
This is why testing once and building a programme from the result is not enough. A profile built six months ago describes an athlete who no longer exists. Multi-duration testing every six to eight weeks builds a current picture of where the curve sits and how it has moved. The ratios between durations tell you whether the programme is delivering what it should and where the emphasis needs to shift next. The shape of your fitness evolves with the training that shapes it, and the training should evolve in response.
What This Means for You
If your training has been built around a single threshold number, you have been training a point on the curve. The rest of the curve, the part that determines how you actually perform over hours of racing, has been left to chance.
Understanding your metabolic profile is the difference between training that happens to work and training designed around what your physiology actually needs. The number is the starting point. The shape is where the answers live.
Your threshold tells you what your engine can do. The shape of your curve tells you what it needs.