Field note
Cardio load is a heart-rate read of how much work your heart has done, held against the fitness base you have built over recent weeks.
Today's number says how hard you pushed right now. The longer trend says how much base you have earned. Read together, they say where you stand. They do not say what you must do next.
There is no distance here, no power meter, and no log of how hard a set felt. The whole read comes from heart rate. A harder, faster heart counts for much more than a gentle one, because climbing higher costs your body far more than ticking along. That is why a short, hard effort can weigh more than a long, easy one.
Because it reads your heart, cardio load is an internal effort signal. It is not a measure of pounding, lifting volume, or joint stress. Banister and Calvert (1991) and Morton and colleagues (1990) shaped the idea of modeling training effect from how hard the system worked over time. Foster and colleagues (2001) offered a practical session-load approach many coaches still use.
| Status word (typical) | Honest meaning |
|---|---|
| Below base | Recent load sits under the modeled base |
| Holding / Building | Near or moderately above that base |
| Above range | Well above an established base; load magnitude only |
| Load spike | Large jump on a still-light base; unstable ratio territory |
Two timescales run side by side. Roughly the last week tracks how tired you are right now. Roughly the last month tracks the base you earned. Each new day nudges both. Where today's effort sits against that base is what gives you a status word.
A big, steadily built base is protective, not dangerous. People who build gradually handle hard weeks better than people who jump in cold. The thing to watch is rarely a high number you reached slowly and kept. It is the sudden jump: a week that leaps far past what you usually do.
Comparing recent effort to the base you built is a long-standing sports-science idea, often framed as the acute-to-chronic workload ratio. Gabbett (2016) popularized training smarter around that lens. Impellizzeri and colleagues (2020) laid out conceptual pitfalls: no exact ratio reliably predicts injury. Read the band as your own usual range, not a safe zone.
It sits next to What is training strain? and What are heart rate zones?: load against base, and the shape of effort inside a session, answer related but separate questions.
The size of the load is the sturdier part when it is built from heart rate. The status word is softer. It only knows where heart-rate load sits against a modeled base. It cannot see sleep, symptoms, nutrition, psychological load, mechanical impact, or performance decline.
Meeusen and colleagues (2013) described overtraining syndrome as needing a much broader clinical picture than a load ratio. Above range and Load spike therefore describe magnitude only. Different apps may disagree on the same day because they weight recovery, sleep, and heart signals differently. When words diverge, look at your own HRV, resting heart rate, and how you feel rather than trusting any one label. See also What does a readiness score mean? for another morning call that is not a health grade.
A single heart-rate average can undercount short, very hard bursts like sprints or intervals, since one average smooths over the spikes. The number is a clean read on steady effort, a rough one on stop-and-go work. The base also needs roughly a month or two of steady use before the trend settles.
A soft corridor around your base is a picture of where recent load sits against your own weeks. Treat that band as your usual range, not an optimal zone and not a safe zone. Sitting outside it is information, not a warning. Injury is many things at once, and a heart-rate load cannot see most of them.
Use the status as context. Recovery, sleep, symptoms, the type of training, and how you feel all matter before deciding what to do next.
Watch the sudden jump more than the high number you reached slowly. That is the one line worth carrying.
I write these because I build Aera, an iPhone app that reads Apple Health against your own baseline and puts the research and the error bars underneath every number it shows you, including this one. You do not need it to use anything above.