Calculation of cardio load
This metric used to be called training load. It is now cardio load, because it describes what a session asked of your heart and circulation — and it now has siblings that describe what the same session asked of the rest of your body: muscular and structural load.
Nothing else changed. The calculation, the AU unit and your entire history are exactly as they were — only the name is more precise now that it is one of several load metrics rather than the only one.
Up to now, cardio load has been calculated on the basis of how the athlete has determined the intensity of the workout in workout completion (RPE). A workout could receive between 0 and 10 intensity points. The cardio load was calculated as the product of the training duration in minutes and the intensity.
For example, if a workout was defined as Easy (2) and lasted 60 minutes, its cardio load was 120. This value is expressed in AU (Arbitrary Units) — a unit that comes from the session-RPE method developed by Carl Foster. It is called "arbitrary" because it is not a physical or physiological unit (like watts or beats per minute), but a relative score used to track training trends and compare sessions for the same athlete over time. We report all calculation methods in the same AU unit, so the values remain comparable regardless of the method used.
The majority of running workouts usually fell between 2 and 6. It is important to remember that the intensity of a workout should be determined in a unit of time. That is, training intensity was not affected by whether the workout lasted five minutes or two hours. The duration only affects the cardio load value.
There are three methods for calculating cardio load:
- Accurate calculations
- General calculations
- Simple calculations
Accurate calculations and General calculations are based on an algorithm using the athlete's heart rate during training for the calculation. Simple calculation is an existing method based on the RPE determined by the athlete after training.
The Accurate Calculation and General Calculation methods work from the athlete's Maximal Heart Rate and Resting Heart Rate, set in their Performance Data. The algorithm uses the so-called heart rate reserve, so the resting heart rate matters just as much as the maximum.
These are no longer required for the calculation to run. Where they have not been entered, they are estimated — the maximum from the athlete's age, raised to their highest observed heart rate whenever the recorded data is higher than the age-based figure. This means a heart-rate-based method stays available instead of the session dropping to Simple calculation.
An estimate keeps the metric working, but it is still an estimate, and a coach-entered value always takes priority over it. Because cardio load rises steeply with heart rate reserve, a maximum that is 10 bpm out shifts every load figure for that athlete noticeably. Entering the real numbers is worth doing, and doing it recalculates the athlete's history.
Accurate calculation
This most accurate method of calculating cardio load uses detailed workout data imported from an external service (e.g. Garmin Connect) and containing heart rate data during the workout. If the workout is added manually or the workout has no heart rate record, this method will not be available.
How do the calculations work?
The workout is divided into 60-second segments and for each such segment a partial cardio load is calculated, which can be assigned to that segment based on the average heart rate of that particular segment.
The value of the cardio load does not increase linearly with increasing heart rate. Taking into account the fact that more intensive workouts affect the recovery time considerably more (not only because of the strain on the cardiovascular system, but also because of the fatigue of muscles and other systems in our body), we have adopted a power function for the calculation, so that the load rises non-linearly, at an ever-faster rate. Thus, the closer we are to the maximum heart rate, the higher the rate of increase in intensity.
Athlete minimum heart rate 45, maximum heart rate 190. Training 60 minutes average heart rate 140 - cardio load 92 Training 15 minutes average heart rate 170 - cardio load 105 As you can see the recovery after an hour of easy running will be faster than after 15 minutes of very intense running.
In the "Accurate calculations" and "General calculations" method, we also took into account the duration of the workout. This means that as the length of the training unit increases, for the same intensity of the entire workout, the cardio load increases gently in a non-linear (accelerating) rather than linear manner.
Our athlete above, training 180 minutes heart rate 140 - cardio load 311 With two separate workouts (e.g. morning and evening) of 90 minutes each, at the same heart rate - the cardio load will be 145
Thus, the cardio load of a three-hour workout will not be the simple sum of the cardio loads of two separate one and a half hour workouts at the same intensity.
The Accurate Calculation method is most beneficial and precise for calculating the load of training units where there is high variability in heart rate during training (e.g. intervals). As there are rest periods between intervals where the heart rate drops significantly, this can affect the average heart rate of the entire workout, which despite the high intensity of the intervals will have a low average heart rate value, which again can result in an underestimation of the cardio load.
General calculations
If no training downloaded from an external service is available for a workout completion, it is possible to calculate its cardio load using the General Calculation method. It requires the athlete to specify an average training heart rate when adding the completion; the maximum and resting heart rate are taken from the Performance Data, or estimated where they are not set.
This method makes a simplified calculation based on the average heart rate of the entire workout. It does not divide training into small units. This may underestimate the cardio load of training units where there is a large variation in intensity (e.g. intervals).
This method takes into account the length of the workout, as mentioned when discussing the 'Accurate calculation' method.
Simple calculation
This training method is a way of calculating cardio load based on the athlete's subjective perception of training intensity (RPE) and the length of training time.
Is this the same as TRIMP?
In short — yes, in essence. Our cardio load belongs to the same family of metrics as TRIMP (Training Impulse). Both methods use heart rate reserve (the gap between your resting and maximum heart rate) to measure the intensity of effort and combine it with the duration of the workout.
Our algorithm goes a step further than classic TRIMP:
- Whole-session analysis, segment by segment. When full heart rate data is available (from Garmin Connect, Polar Flow, Strava, etc.), we split the workout into segments and calculate the load for each one. Classic TRIMP usually relies on a single average heart rate for the whole workout, which can underestimate the load of sessions with variable intensity (e.g. intervals).
- Stronger emphasis on intensity. We discriminate more sharply between high- and low-intensity effort, so a hard session is clearly distinguished from an easy one of the same length.
- A duration adjustment that reflects how longer sessions accumulate fatigue.
The one meaningful methodological difference is that classic TRIMP applies a small correction based on sex, which our model does not currently take into account.
Which method is used
The method is chosen automatically, from the best evidence the workout actually carries:
- the workout is imported from an external service and contains heart rate data → Accurate calculation
- otherwise, an average heart rate has been entered on the workout completion → General calculation
- otherwise → Simple calculation, from RPE and duration
Heart rate limits no longer affect which method is chosen — where they have not been entered they are estimated, as described above, so a session with heart rate data always uses a heart-rate-based method.
Earlier versions let you switch a completion to a different method. That option has been removed, and the choice is now made from the data alone.
The reason is that a manual choice was sticky in the wrong direction. Once a completion had been calculated from RPE, it stayed on RPE — so attaching an activity file to it afterwards left that file unread, and the load never improved even though better data had arrived. Deriving the method from the evidence each time means a workout's load now upgrades itself automatically the moment better data is connected.
If the heart rate data is wrong
A watch can misread — a strap dropout, or a chest belt picking up cadence instead of heart rate — and that produces a cardio load that does not reflect the session.
Because the method follows the data, the fix is to correct the data rather than the method. Open the workout, disconnect the activity, and enter an average heart rate on the completion by hand. With no activity attached, the General calculation method applies to your entered value. Leaving the average heart rate blank falls back to Simple calculation from the athlete's RPE, which is the right choice when no heart rate figure can be trusted at all.
Both the coach and the athlete can do this.
Cardio load on graphs
In the Reports section of the browser application, we have the option to select to view the cardio load statistics from the last time, both on a daily, weekly and monthly basis.
You can also select a graph showing the cardio load according to the old RPE method.
Training intensity
From the cardio load values calculated according to the new algorithm, we also calculate the training intensity. This can be viewed in the graphs.
When determining the intensity of a workout using the Accurate calculation method, it is specific that training segments whose heart rate is lower than the average heart rate of the entire workout are not taken into account. This ensures that exercise intensity (but only intensity) is not affected by rest segments in interval training, warm up or cool down.
An athlete completed an interval workout of 6 x 400 metres, the warm-up was a relaxed 20 minutes. The average heart rate of the entire workout was 145. The average heart rate of the warm-up was 138. The average heart rate of the cool-down was 140. The average heart rate of the fast sections was 165. The average heart rate of the slow sections was 144.
In the example above, the intensity of the workout is only affected by the fast episodes because they are the only ones above the average heart rate of the whole workout. This will allow us to see the intensity that the workout had when the athlete was working hard and not resting.
It is important to remember that intensity is a time-independent value (as is RPE).
The intensity of the workouts can be viewed in the Reports section of the web application.
Intensity and cardio load are calculated from heart rate for every sport, cycling included. Power and normalised power are read from the file, stored and shown on the workout, but they do not feed either calculation.
This is deliberate. Load has to describe what a session cost the athlete, and two riders holding the same watts for the same hour pay very different physiological prices. It also keeps a ride, a run and a swim on one comparable scale, which power cannot do — there is no swimming equivalent, and running power differs between devices.
Power still has its place: an intensity zones profile can be built on functional threshold power, which is the right tool for prescribing and pacing rides. If a ride's intensity looks low for how hard it felt, check the heart rate record rather than the power figures.