The short answer
- External load is what you did. Internal load is what it cost you. Adaptation follows the second one.
- A watch measures external load well and estimates internal load from heart rate, which is a model rather than a measurement.
- Session effort multiplied by duration is a validated way to quantify a session, it costs nothing, and it works across every kind of training.
- Nobody has established the dose-response between changes in training load and injury yet. A study of more than 20,000 runners was built to answer it.
- A personal plan is one that changes when your response changes. A plan generated once from a form is a template with your name on it.
Two runners do the same session. Same route, same pace, same 50 minutes. One slept badly, is three weeks into a hard block and has a stressful week at work. The other is rested.
The file on the watch is nearly identical. What the session cost them is not remotely the same, and it is the cost that decides what happens next.
Two things are both called load
Sport science separated these a long time ago, and the distinction is the single most useful thing a runner can take from the field.
- What it is
- External loadThe work you did.
- Internal loadWhat that work cost you.
- Examples
- External loadKilometres, minutes, pace, elevation, number of strides.
- Internal loadEffort, heart rate response, how the session felt, how you slept after it.
- How well a watch captures it
- External loadVery well. This is what GPS and an accelerometer are for.
- Internal loadIndirectly, through an estimate.
- Changes when you are tired
- External loadNo. Ten kilometres is ten kilometres.
- Internal loadYes. That is the whole point of it.
- What it is good for
- External loadPlanning, describing and repeating training.
- Internal loadDeciding whether today's plan is still the right plan.
The reason this matters more than the weekly total is that what your body can take moves too, and it moves more slowly than your training plan does.
What the number on the screen actually is
This is not an argument against wearables. The monitoring consensus statement notes how fast this field has grown, and how much industry has grown with it, precisely because the promise is real2. The caution is about what you do with one number.
The measure that costs nothing
There is a method for quantifying internal load that needs no hardware at all. Rate the session from 1 to 10 for how hard it felt, about half an hour after finishing, and multiply by the number of minutes.
The absolute scores were higher with the perceived exertion method than with heart rate, so the two are not interchangeable in units. As a way of tracking one runner over time, which is what you care about, it holds up.
The honest gap
Runners reasonably expect that somebody knows how much of an increase is too much. The two best-known answers, the 10 percent rule and the acute to chronic workload ratio, both fall apart on inspection, which we have gone through in detail in our article on coming back from injury. The ratio in particular has no evidence supporting its use for managing load or reducing injury risk4.
What is left is a genuinely open question, and the scale of the study built to answer it tells you how open.
You do not build a study that size for a question that is already settled.
What is worth tracking
Load is not one quantity, even inside your own body. Bone responds to the interaction between the number of loading cycles and their magnitude, and there is no single optimal workload, only a range that shifts with the situation6. Tendon, muscle and your cardiovascular system each have their own version of that.
So a short list beats a dashboard.
- Weekly minutes of running, not kilometres, so pace changes do not distort the total.
- A session effort score out of 10 for every run.
- How the easy runs feel, which is the first thing to change when the week is too much.
- Sleep, in hours, written down rather than remembered.
- Anything that hurt, and whether it was still there the next morning.
What "personal" has to mean
A plan that was generated once, from a form, is a template with your name at the top. The running injury literature is clear that risk sits across training characteristics, health and lifestyle factors, and biomechanics all at once7, and none of those hold still for twelve weeks.
So the test of a personal plan is not how detailed it looked on the day you got it. It is what happens to it in week four when you sleep badly for a week, or your knee grumbles, or work eats the Wednesday session.