Training load

Internal and external load: what training load really means

External load is the work you do: the watts, the pace, the distance, the time. Internal load is what that work costs your body, shown by heart rate and how hard it felt. The same session can cost two athletes very different amounts, and it can cost you more on a bad day than a good one.

You plan with external load. Your body adapts to internal load. Watching both, and noticing when they drift apart, tells you more than either one alone.

By , Team GBR age-group triathlete Reviewed 7 min read

At a glance

External loadWhat you did: power, pace, distance, duration, elevation, and scores built from them such as TSS
Internal loadWhat it cost you: heart rate, perceived effort, session RPE, blood lactate
Why bothThe same work costs different amounts depending on fitness, fatigue, heat, sleep, stress and fuelling
The warning signInternal load rising for the same external load, especially for several days in a row
The simplest internal measureSession RPE: how hard the whole session felt, from 0 to 10, multiplied by its length in minutes
How to use itPlan the work in external terms, check the cost in internal terms, and adjust when they disagree

One long ride, two loads

One long ride, two loadsShape of a three-hour ride. Power stays flat through two and a half hours of steady riding while heart rate slowly climbs, so the same work costs more as the ride goes on. In a tempo block in the last half hour, both rise, and heart rate ends far higher than at the same effort early on. 0 1 h 2 h 3 h Power (external load): flat Heart rate (internal load) Same work, rising cost TEMPO One long ride, two loadsShape of a three-hour ride. Power stays flat through two and a half hours of steady riding while heart rate slowly climbs, so the same work costs more as the ride goes on. In a tempo block in the last half hour, both rise, and heart rate ends far higher than at the same effort early on. 0 1 h 2 h 3 h Same work, rising cost TEMPO
Power: the work doneHeart rate: what it cost
The shape of a three-hour aerobic ride with tempo in the last half hour. Power holds flat for over two hours, but heart rate creeps up, so the same work costs more as the ride goes on. The plan only sees the first line; your body responds to the second.Illustrative shape, no values.

What is external load?

External load is everything your device can measure about the work itself. On the bike that's power, duration and the kilojoules you produced. On the run it's pace, distance and elevation. In the pool it's distance, pace and the number of hard repeats.

Scores such as TSS combine intensity and duration into one number. TSS scales each session against your FTP, so it accounts for your fitness on the day you set that FTP. It still describes the work rather than its cost to you: a 60-minute ride at the same power gets the same score whether you slept well or not.

External load is what you plan with. "3 x 10 minutes at threshold" or "a 2-hour ride at 65% of FTP" are external prescriptions. See why FTP isn't the whole story for the limits of building every session on percentages of one number.

What is internal load?

Internal load is how your body responds to that work. Sport scientists measure it with heart rate, blood lactate, heart-rate-based scores, and the athlete's own rating of how hard it felt.

The distinction matters because adaptation follows internal load. Two athletes doing the same session receive different training stimuli if one finds it easy and the other is near their limit. Franco Impellizzeri, who helped popularise the terms, argued in a 2019 review that external load is what you prescribe, but internal load is what drives the outcome.

That's also why the same session feels different from week to week. Heat, poor sleep, a hard week at work, a cold coming on, or not eating enough before training all raise the internal cost of the same external work.

Session RPE: the simplest way to measure internal load

In 2001, Carl Foster and colleagues found that a single number, how hard the whole session felt, tracked heart-rate-based measures of training load closely in cyclists and basketball players. It's now widely used in sport science. It works for any sport, needs no equipment, and takes ten seconds.

  1. About 30 minutes after the session, rate how hard it was overall, from 0 (rest) to 10 (maximal). Rate the whole session, not the hardest interval.
  2. Multiply that rating by the session's length in minutes.
  3. The result is your session load in arbitrary units. A 60-minute run rated 5 gives 300. A 90-minute ride rated 3 gives 270.
  4. Add up a week's sessions to see your weekly internal load, across swim, bike, run and strength on one scale.

Waiting half an hour matters. Rate it straight away and the last effort tends to dominate your answer.

Reading the two together

A review of fatigue monitoring by sport scientist Shona Halson made the key point: a gap between external and internal load can show how fatigued an athlete is. No single marker tells you everything, so the comparison is what's useful.

  • Same work, lower cost: the session you rated 6 last month now feels like a 4, or your heart rate is lower at the same power. You're getting fitter.
  • Same work, higher cost for a day: one hard-feeling session after a short night or in the heat. Usually nothing to act on.
  • Same work, higher cost for several days: sessions that should be easy feel hard, and heart rate runs high or won't rise in efforts. That's the pattern worth acting on, with an easier day or two, more sleep, or more food.
  • Less work, same cost: power or pace drops while effort stays high. Often fatigue, sometimes illness, sometimes heat.

People respond to the same training in very different ways, so compare yourself with yourself, not with someone else's numbers. The Fitness, Fatigue and Form chart is usually built from external scores such as TSS, which is one reason it can't see a bad week at work or a cold coming on.

The limits of each

  • Perceived effort is personal and shifts with mood and motivation. That's partly the point, because those things affect how training lands, but compare your ratings with your own history rather than anyone else's.
  • Heart rate lags behind short efforts and moves with heat, caffeine and hydration. It's most useful for steady sessions.
  • Power and pace are precise but blind to cost. A perfect power file can come from a day you shouldn't have trained.

Used together, each covers the others' blind spots. For a morning view of how you're recovering, see the HRV guide.

How to use this in your training

  • Log how every session felt. One number, 30 minutes after. It costs nothing, and over a season it shows you how you respond to training.
  • Plan in external terms, review in internal terms. Set the session by power or pace, then check what it cost.
  • Watch for drift over days, not single sessions. One hard session is noise. Three in a row is a message.
  • Adjust the next session, not the whole plan. An easier day after a run of high-cost sessions usually does more than rewriting the week.

Common questions

What is the difference between internal and external training load?

External load is the work you do, such as power, pace, distance and time. Internal load is what that work costs your body, shown by heart rate and how hard it felt. Adaptation follows internal load.

How do I calculate session RPE?

About 30 minutes after a session, rate how hard it was overall from 0 to 10, then multiply by the session's length in minutes. A 60-minute session rated 5 gives a session load of 300.

Is perceived effort accurate enough to use?

Yes, for tracking your own load. In the original study, session RPE tracked heart-rate-based load closely, and it's now widely used across sports. Compare your ratings with your own history rather than with other people's.

Is TSS internal or external load?

Mostly external. TSS combines power and duration and scales them against your FTP, but the same ride scores the same whether you slept well or not.

What does it mean if a normal session suddenly feels much harder?

For one day, usually not much: heat, a short night or a stressful day can do it. If it happens for several days in a row, take an easier day or two and look at sleep, food and stress.

Should I plan training by heart rate or by power and pace?

Plan the work by power or pace, which respond instantly, and use heart rate and how it felt to check what the session cost.

Related

Sources

  1. Impellizzeri FM, Marcora SM, Coutts AJ. Internal and external training load: 15 years on. International Journal of Sports Physiology and Performance. 2019;14(2):270–273.
  2. Foster C, Florhaug JA, Franklin J, Gottschall L, Hrovatin LA, Parker S, Doleshal P, Dodge C. A new approach to monitoring exercise training. Journal of Strength and Conditioning Research. 2001;15(1):109–115.
  3. Halson SL. Monitoring training load to understand fatigue in athletes. Sports Medicine. 2014;44(Suppl 2):S139–S147.
  4. Mann TN, Lamberts RP, Lambert MI. High responders and low responders: factors associated with individual variation in response to standardized training. Sports Medicine. 2014;44(8):1113–1124.
  5. Filliol J. It wasn't talent: lessons from 4 world champions. Real Coaching (Substack), 9 October 2026.