Training Load Explained: What It Measures and What It Can't

Training load is a way of putting a number on how much stress your training is putting on your body. It combines how long you trained with how hard it was, usually through heart rate or perceived effort, then adds that up across days. Most systems then split the total in two: acute load, which is roughly your last 7 days, and chronic load, which is a rolling average over the last 3 to 6 weeks. The part that actually predicts trouble is not either number on its own. It is how far the recent one has climbed away from the long-term one.

If you have ever come back from a break, felt fantastic for ten days, then picked up a niggle out of nowhere, you have already met this idea from the wrong side. You were not undertrained. You were under-prepared for the specific amount you suddenly asked of yourself.

Internal load and external load are not the same thing

This is the distinction that makes everything else make sense, and it is the one most training apps skip. Impellizzeri, Marcora and Coutts drew the line in 2003 and sharpened it in a 2019 paper: external load is the work you did, measured without reference to you. Kilometres covered, watts pushed, kilos lifted, minutes elapsed. Internal load is what that work cost you, the physiological and psychological response your body actually had to produce.

Ten kilometres is ten kilometres. It is not the same ten kilometres for a marathoner on a cool morning and for you three days into a head cold, in August, on four hours of sleep. The external number is identical. The internal cost is nowhere close.

Your watch mostly measures the second one, and that is a strength rather than a limitation. Heart rate during a session is a proxy for internal load, which is why a training load figure built from heart rate reacts to how you turned up that day. Halson’s 2014 review in Sports Medicine makes the practical point: the interesting information often lives in the gap between the two. Holding the same pace at a noticeably higher heart rate, week after week, says something that neither number says alone.

There is one honest caveat. Heart rate is a poor proxy in exactly two situations: very short, very intense efforts, where your heart rate has not caught up with the work before the interval ends, and heavy strength work, where the mechanical cost is enormous and the cardiovascular cost is modest. A hard set of squats can be the most demanding thing you do all week and will barely register. If lifting is a large part of your training, read your load figure as covering only part of what you did.

What is acute load?

Acute load is your recent work, typically the last 7 days. It answers one question: how much have I just done? It moves fast, and it is meant to. One long Sunday ride, one race, one week where you squeezed in four sessions instead of two, and acute load jumps.

What is chronic load?

Chronic load is a rolling average of the last 3 to 6 weeks. It answers a different question: how much is my body used to? This is your base, the accumulated tolerance you have built through repetition. It moves slowly by design, because adaptation is slow. Tendons, bone and connective tissue remodel over weeks and months, not over one good weekend.

Two runners both cover 40 km this week. One has averaged 45 km a week for two months, so this is a slightly easy week. The other has averaged 12 km, so this is more than triple anything her body has rehearsed. Same number on the screen, completely different meaning. That gap is the entire reason chronic load exists as a concept.

Why does the ratio matter more than either number?

Divide acute by chronic and you get what sports science calls the acute:chronic workload ratio. Around 1.0 means this week looks like your recent normal. Meaningfully below that and you are doing less than usual, which is fine for a week and a problem across a season. Well above it and you are asking for considerably more than your body has been prepared for.

The idea spread through Tim Gabbett’s 2016 paper in the British Journal of Sports Medicine, which described what he called the training-injury prevention paradox: athletes who had built high chronic loads gradually tended to be more resilient, not more fragile, while sharp week-to-week jumps were where injuries clustered. The practical takeaway that came out of that work was to keep weekly increases modest instead of dramatic.

Is the acute:chronic ratio actually proven?

No, and it would be dishonest to pretend otherwise. The ratio has taken serious methodological criticism over the last several years. Critics have pointed to mathematical coupling, since acute load sits on both the top and the bottom of the division and can manufacture a relationship that is not really there. They have also flagged inconsistent thresholds between studies, different sports producing different answers, and results that occasionally point the opposite way entirely. Reviews in the sports medicine literature, including a recent systematic review and meta-analysis, have argued that a causal link between the ratio and injury has not been established and that it should not be used as a standalone decision rule.

So treat it as a lens, not a verdict. The durable part of the idea is older and simpler than the ratio itself: your body adapts to what you repeat, adaptation takes weeks, and sudden jumps outrun it. You do not need to believe in any specific cutoff number to act on that.

What does ramping too fast actually do?

Different systems in your body adapt at different speeds, and that mismatch is the real hazard. Your cardiovascular fitness improves relatively quickly. Your Achilles, your plantar fascia, your shins and your knees do not. So a few good weeks of aerobic progress can leave you feeling capable of a session your connective tissue has not caught up to. The aerobic system writes the check and the tendons pay it.

There is a second cost that shows up before any injury does. Load lands on your nervous system and your recovery, not only on tissue. Sleep gets shallower, resting heart rate drifts upward, motivation quietly goes flat, and easy sessions start feeling strangely hard. That grey zone between normal training tiredness and doing too much is worth learning to read, and we covered it separately in overreaching versus just being tired.