What “strain” measures that calorie burn doesn’t

Four hundred calories, four workouts

Take four sessions that each burn roughly 400 kcal: a 90-minute walk, a 50-minute easy ride, a 40-minute steady run, a 25-minute interval run. A calorie counter scores them as identical. Your body does not. The intervals leave your heart rate elevated long after you stop, your glycogen drawn down and your nervous system taxed in a way the walk never touches. Cardio load — strain — is the number built to see that difference.

What a calorie count can say

Calorie burn estimates energy: how much mass moved, for how long, adjusted for body weight. It answers one question — how much fuel did this cost? That makes it the right number for energy balance, appetite and refuelling. It is also an estimate. When Stanford researchers tested seven wrist devices in 2017, the most accurate still missed energy expenditure by a median of 27%; heart rate was fine, calories were not.

What it cannot say: how hard the work was for you. 400 kcal at a conversational pace and 400 kcal gasping between intervals are identical entries in a calorie ledger. The ledger has no column for intensity, none for how long your heart stayed near its ceiling, and none for what tomorrow will cost.

What load sees instead

Cardio load is computed from heart rate, not movement. Heala uses a TRIMP-style model: every minute is weighted by how close your heart rate sits to your personal maximum, and the weighting is steep — a minute near threshold counts several times more than a minute of strolling. The result lands on a 0–21 scale. It measures internal cost: what the session demanded of your cardiovascular system, scaled to your capacity and nobody else's. One plain caveat: heat, illness and caffeine push heart rate up, so they push load up too — which is honest, since they raise the real cost.

That is why the four equal-calorie workouts spread from 4.6 to 13.8 — a three-fold difference calorie burn cannot see. Load also accumulates all day, not just inside workouts: a hilly commute, three flights of stairs, an afternoon spent moving boxes are small deposits in the same account, which is why Heala counts strain across the whole day rather than starting the meter when you press start.

Two numbers, two questions

Neither number replaces the other. Calories answer "how much fuel?" — use them for energy balance and for eating enough on hard days. Load answers "how big a dose?" — use it to ramp training gradually, to explain why an easy-feeling 700-kcal long run still wants a quiet day after it, and to spot weeks where every session was medium-hard. Endurance research since Seiler's 80/20 work points the same way: mostly easy plus occasionally hard tends to beat always-moderate.

A calorie count prices the work; load prices what the work took out of you. Read together, a training week stops being a pile of burned energy and becomes a dose — one you can plan, repeat, and raise on purpose rather than by accident. That is the difference a second number buys.