Endurance training: the systems behind it and what follows for planning
Endurance is not a single concept but a family of capacities arising through different routes. Understanding which systems work at which intensity makes it possible to plan sessions rather than string them together by feel.
Where the energy comes from
Muscle work requires an energy carrier present in the muscle itself only in very small amounts. It therefore has to be continuously remade, and several routes are available for that, all running at once — only in changing proportions.
For very short, maximal efforts of a few seconds, a store within the muscle supplies the energy. For efforts of roughly half a minute to several minutes, glucose is processed mainly without sufficient oxygen — quickly available but limited. For anything beyond that the body works predominantly with oxygen, using both carbohydrate and fat.
These routes are not switches. Even in a slow steady run the oxygen-independent route runs alongside, and even in a short sprint the oxygen-dependent one works. The widespread idea of a “fat-burning zone” in which only fat is used therefore describes not a state but a proportion.
What training changes
Adaptations to endurance training happen at several points at once, and most of them are not visible in the mirror.
The heart works more economically: the volume it ejects per beat increases, and resting heart rate falls accordingly. In the working muscles the number of fine blood vessels increases, as does the number and size of the cell components where oxygen-dependent energy production takes place. The capacity to take up, transport and use oxygen rises with it.
These adaptations take time — weeks to months — and they proceed at different speeds in different places. Muscle generally adapts more quickly than tendons, ligaments and bone, which is one reason why raising volume too quickly regularly causes problems in endurance sport.
The two thresholds
In performance diagnostics two points are distinguished at which the metabolic situation changes noticeably.
Below the first threshold the effort stays possible for a long time; speaking is easy. Between the first and second thresholds effort rises but remains sustainable for a longer period — here speaking in full sentences becomes laboured. Above the second threshold fatigue rises quickly, and the effort can only be maintained for a limited time.
These points are not fixed heart rates. They sit in different places for different people, shift with training status and even differ between sports within the same person. Knowing them precisely requires measurement; for everyday planning, cues such as the ability to speak usually suffice.
Why most programmes contain a lot of easy training
In endurance training practice a distribution has established itself in which most of the volume is completed well below the first threshold and only a small part at high intensity. Observations of endurance athletes frequently show this distribution.
The justification is twofold. First, long easy sessions produce adaptations in the periphery — blood vessels, enzymes, metabolism — at comparatively low fatigue. Second, it is precisely that low fatigue which allows volume to be kept up over weeks and the few hard sessions to be completed genuinely hard.
The most common error at recreational level is accordingly not too little intensity but too much moderate intensity: sessions too demanding to build volume and not demanding enough to set a clear stimulus. Whether the exact distribution is decisive, or whether several patterns work similarly well, continues to be debated.
How volume can be increased
Progression in endurance training usually addresses duration first, then frequency and only then intensity. That is not a rule but follows from the differing adaptation speeds of the structures involved.
Rules of thumb for weekly increases in volume circulate in practice. They are usable as orientation and rest on experience, not on measured limits — studies are inconsistent, and tolerance differs considerably between people.
More reliable than a percentage is observation across several weeks: does performance stay stable or improve at the same effort? Does resting heart rate stay in its usual range? Are the planned sessions actually completed? If so, the progression is sustainable.
Where this text stops
This text describes training physiology, not suitability. Whether and to what extent endurance work is appropriate for a particular person depends on circumstances not known here.
Anyone starting after a long break, having complaints, taking medication or aware of pre-existing conditions sensibly clarifies that with a doctor beforehand. The same applies to symptoms arising during effort that do not fit the exertion.
The key points
- The energy routes always run at once, only in changing proportions — there is no pure “fat-burning zone”.
- Adaptations affect the heart, blood vessels and muscle cells and take weeks to months.
- Tendons, ligaments and bone adapt more slowly than muscle.
- The two thresholds are not fixed heart rates; they shift with training status.
- The most common recreational error is too much moderate intensity, not too little high intensity.
- Progression usually addresses duration first, then frequency, then intensity.
Last reviewed: 14.08.2026
Created by AI