What the athlete does is not what the athlete assimilates
Two athletes complete exactly the same session. The GPS says they did the same thing. The physiology says they didn't.
In recent years, the availability of data has transformed how training is planned and monitored. GPS, power meters, accelerometers and analytics platforms make it possible to quantify what an athlete does in a session with enormous detail.
But knowing what the athlete does is not the same as knowing how they assimilate it. Two athletes can complete exactly the same session and be going through very different physiological processes.
NoteExternal load describes the task. Internal load describes the response to that task.
What external load is
External load is the objective description of the work performed. It is what can be measured from outside the athlete, regardless of how they are living it.
- Distance covered and speed.
- Mechanical power output.
- Accelerations, decelerations and changes of direction.
- Volume, sets, reps and weights lifted.
- Total time and session density.
It's essential for planning training, but on its own it doesn't explain how the body has responded.
What internal load is
Internal load is the body's response to the stimulus. It's the response that determines how the body assimilates training. It includes physiological variables and perceived ones.
- Heart rate and its kinetics.
- Heart rate variability (HRV).
- Lactate and other metabolic markers.
- Rate of perceived exertion (RPE) and sensations.
- Cardiovascular cost relative to external intensity.
Two athletes run 10 km at the same pace. One does it at 78% of max HR. The other at 91%. External load is identical. Internal load — and therefore the real stimulus — is not.
How the two connect: where the decision is made
The interesting part isn't picking one or the other. It's reading the relationship between the two over time.
When external load stays the same and internal load rises, something is going on: accumulated fatigue, heat, poor sleep, stress, dehydration, illness. When external load rises and the physiological response stays stable, the body is probably becoming more efficient.
NoteThe relationship between external and internal load is one of the most useful signals for reading the athlete's true state.
Common mistakes when reading load
- Using GPS only and assuming everyone in the group is living the same session.
- Reducing internal load to a single daily number, with no context or trend.
- Comparing athletes against each other instead of each athlete against their own baseline.
- Reading an isolated spike as a problem without cross-checking sleep, travel, competition or phase of the cycle.
- Assuming that the same external load always produces the same internal load.
How to integrate it into the team's day-to-day
Integrating internal and external load is not about adding more metrics. It's about organising what's already collected so it speaks the same language as the coaching staff's decisions.
- Select few variables, but ones that truly help you make decisions.
- Establish individual baselines before interpreting change.
- Read the weekly trend, not the isolated data point.
- Close the loop: every metric should connect to a possible decision.
It's to understand better. Data that doesn't change a decision is noise, even when it's perfectly recorded.
The core idea
External load tells you what was done. Internal load tells you what actually reached the body. Only by reading both together can you tell whether training is producing adaptation, fatigue, or both.
NoteExternal load describes the training. Internal load explains how the body lived it. Interpretation appears when both are analysed together.
