Muscular and Structural Load
Why one number was not enough
Cardio load (AU) measures what a workout asked of your heart and circulation. It does that well — but it says nothing about what the session did to your muscles, tendons and bones.
Two real examples show why that matters.
An athlete spent a week paddling. Over 1600 minutes on the water, the sessions earned around 2000 AU — a genuinely huge cardiovascular week. Yet the load on the legs was essentially zero. Reported as a single number, it looked like the biggest training week of the year, and any advice based on it would have been wrong.
The same athlete later did a long day in the hills. Six hours of walking earned barely 120 AU, because the heart rate stayed low. Reported as a single number, it looked like an easy day. In reality the legs took a serious beating and needed days to recover.
We were wrong in both directions, and neither error could be fixed by adjusting the other. Cardiac load cannot be derived from mechanical load, or the other way around. So instead of one number, a workout now gets three.
The three numbers
| Metric | Unit | What it measures | Recovers in |
|---|---|---|---|
| Cardio load | AU | Strain on the heart and circulation | ~24–48 h |
| Muscular load | ML | Fatigue in the working muscles | ~24–72 h |
| Structural load | SL | Impact on bones, tendons and connective tissue | days to weeks |
Those three unit labels are what you will see beside the numbers in the app — on a workout, and in the weekly summary panel beside each week in the calendar. They mark which dimension a figure belongs to, and they are a reminder that a value in one unit cannot be compared with a value in another.
They are listed separately because they recover at different speeds. A single number cannot get better at three different rates, which is the real reason they cannot be merged.
Muscular and structural load are calculated automatically for new workouts. Historical workouts are filled in gradually — older sessions may show these values only after they are opened.
How to read them
Muscular load
Muscular load tracks the mechanical work your muscles did — roughly force multiplied by time — scaled by how much the sport in question actually uses the muscles.
It is deliberately not cardio load in disguise. Cardio load climbs very steeply with intensity, because that is how strain on the heart behaves. Muscular work does not: three hours of steady riding is more work for the legs than forty minutes of intervals, even though the intervals cost the heart far more.
Muscular load is set so that an hour at a moderate, familiar effort scores close to its cardio load. It moves away from there in both directions:
- long and easy scores higher than heart rate alone would suggest
- short and hard scores lower
6 km easy jog, 36 min : cardio load 51 | muscular load 77
6 km steady, 32 min : cardio load 82 | muscular load 76
6 km of 6x3 min hard, 30 min : cardio load 167 | muscular load 81
Cardio load more than triples across the three. Muscular load barely moves — the same distance in a similar time asks the legs for a similar amount of work, however hard the heart was working.
Structural load
Structural load is counted in impact units, where 1 unit is about 100 m of flat running. So:
10 km flat run = 100 units
20 km flat run = 200 units
5 km flat run = 50 units
It is based on distance, not time, because what damages connective tissue is the number of footstrikes, not the minutes on the clock. That is exactly why classic "don't increase by more than 10% a week" advice is always stated in kilometres.
Distance is not quite the whole story though, so pace also counts a little. Running fast means fewer footstrikes per kilometre, but each one lands considerably harder, and tendon and bone care more about how hard a landing is than how many there were. So 6 km of intervals scores roughly 15–25% above 6 km jogged at the same distance:
6 km easy jog = 53 units
6 km steady = 63 units
6 km of 6x3 min hard = 76 units
The effect is deliberately gentle. Pace nudges structural load; distance and descent still decide it. Descent is not affected by pace at all — dropping 800 m is the same eccentric work whether you took forty minutes over it or ninety, so a slow mountain day is never discounted for being slow. If you want a number that reacts strongly to how hard you ran, that is cardio load — across those same three sessions it more than triples, while muscular load barely moves.
The pace comparison is made against a fixed reference for each sport rather than against your own fitness, so it means the same thing for every athlete. It also means the spread between your easy runs and your hard ones works out the same whether you race 10 km in 35 minutes or in 75.
Muscular and structural load use different scales. A session showing muscular 300 and structural 100 is not "three times more muscular" — the numbers simply are not in the same currency.
Compare each number against your own history for that same metric. Is today's structural load high for you? That is the useful question.
What different sports do
This is where the split earns its keep. Sports load these systems in completely different proportions:
| Sport | Muscular (legs) | Structural (legs) | Notes |
|---|---|---|---|
| Running | High | Very high | Every step is an impact |
| Trail running | High | Highest | Impact plus braking on descents |
| Walking / hiking | Moderate | Moderate | Low force, but an enormous number of steps |
| Cycling | High | Almost none | Same muscles as running, no impact at all |
| Swimming | Very low | None | Loads the shoulders instead |
| Rowing / kayaking | Very low | None | Almost entirely upper body |
| Strength training | High | Low | Real muscle work, little impact |
| Stair climbing | Moderate | High | High force per step |
| Meditation / sauna / stretching | None | None | Logged, but not training |
This is why a triathlete can ride 15 hours a week but only run 6. The bike is limited by how much the heart and muscles can take; the run is limited by what the tendons and bones can take. One blended number cannot answer that question — three numbers can.
1 hour bike, 30 km : cardio 300 | muscular 118 | structural 15
1 hour run, 10 km : cardio 300 | muscular 157 | structural 100
1 hour swim, 3 km : cardio 300 | muscular 16 | structural 0
Identical cardiovascular cost. Completely different consequences for your legs.
Hills, mountains and going downhill
Going down is what hurts. Climbing is hard on your heart — your heart rate rises, and that already shows up in cardio load and muscular load. Descending is the opposite: it barely raises your heart rate, but every step absorbs your body weight against gravity, and that is what causes the deep soreness two days later.
Structural load therefore counts descent as a cost of its own. As a rule of thumb, 1 metre of descent costs roughly 10 metres of flat travel — somewhat more when running down, somewhat less when walking down, because running gives you far less control over each landing.
10 km flat run, 50 min, no hills : muscular 135 | structural 100
10 km mountain run, 75 min, 1000 m down : muscular 179 | structural 250
The mountain run earned more muscular load despite a lower heart rate, because it took half an hour longer and the legs were working throughout — and two and a half times the structural load. A system watching only heart rate would have called this the easier session.
For mountain athletes, this is the whole point of the metric:
Valley walk, 8 km, flat : 36 units
Alpine day, 8 km, 1400 m descent : 162 units
Big descent, 6 km, 2000 m descent : 207 units
Hut-to-hut, 15 km, 2500 m descent : 292 units
Note that the 6 km day scores higher than the 8 km day. It is shorter, but you descended more — and descent is what your quads pay for.
If you are building towards an Alpine week or a long trail race, structural load is the number to watch. Cardiac fitness usually arrives well before your legs are ready to absorb thousands of metres of descent, and structural load is the metric that shows the gap.
Descending on a bike adds nothing to structural load — you are sitting down and gravity is doing the work, so there is no braking cost to your legs.
When a number is zero or missing
Structural load of 0 means the sport genuinely does no impact loading — swimming, rowing, kayaking. That is a real zero, not missing data. Those sessions still produce cardio load and muscular load, and they still need recovery.
Zero on both numbers means the activity was rest rather than training — meditation, breathwork, sauna, ice baths, stretching and mobility work. These are worth logging, but nothing was asked of your muscles and nothing landed on your bones, so both numbers are zero on purpose. Note the difference from swimming above: a swim scores zero structural load and real muscular load, because it is still training.
A blank value means we did not have enough information — usually a manually added workout with no distance and no duration.
No distance recorded? Sports that never report distance — stair machines, tennis, gym sessions, team sports — are estimated from their duration instead, so they still count. Rest activities are the deliberate exception: they are recognised by name and left at zero, rather than earning impact simply for having lasted an hour.
Data quality
Every value is graded, so you know how much to trust it:
- Measured — calculated from full workout data recorded by your device
- Estimated — calculated from summary values such as total distance and average heart rate
- Reported — derived from your RPE and duration, because no heart rate data was available
An estimated value is still useful. It simply carries less weight than a session your watch recorded in full.
Where your heart rate limits come from
Muscular load is driven by heart rate reserve, so it depends on the athlete's maximum and resting heart rate being right.
If those have not been entered in the athlete's Performance Data, they are now estimated rather than being left out — the session still gets a load value instead of falling back to RPE. The estimate uses the athlete's age, raised to their highest observed heart rate if the recorded data is higher than the age-based figure.
An estimate keeps the metric working, but a coach-entered maximum heart rate always wins over it, and it is worth entering. Because cardio load rises steeply with heart rate reserve, a maximum that is 10 bpm out moves every load figure for that athlete noticeably — and muscular load moves with it. Entering the true value recalculates the athlete's history.
Common questions
Q: My hike shows low cardio load but high structural load. Is that a bug?
A: No — that is the metric working exactly as intended. Walking keeps your heart rate low, so it earns little cardio load. But six hours on your feet, especially descending, is a lot of impact. This combination is precisely what a single number used to hide.
Q: My long ride shows almost no structural load. Should I be worried?
A: Not at all. Cycling really does place almost no impact stress on your legs — there is no footstrike. Your ride still shows high cardio load and high muscular load, which is the honest picture. It is also why you can ride far more hours per week than you could run.
Q: My easy long run scored more muscular load than my interval session. Is that right?
A: Yes, and it is the point of the metric. Intervals cost the heart far more, which is why their cardio load is much higher. But muscular load asks a different question — how much work the legs actually did — and two hours of steady running is simply more work than forty minutes of intervals, even though the intervals felt much harder. If you want the number that reflects how hard a session felt, that is cardio load.
Q: Can I add muscular and structural load together?
A: No. They are different quantities on different scales, so a total would be meaningless. Track each one against its own history.
Q: Which number should I actually watch?
A: It depends on what limits you.
- Runners: structural load — it is what stress fractures and tendon problems come from
- Cyclists: cardio load and muscular load — structural load will always be low
- Swimmers and paddlers: cardio load — legs are not the limiter
- Multi-sport athletes: all three, since your heart is shared across sports but your legs are not
Q: My strength sessions show some structural load. Why?
A: Lifting does load bone and tendon, so it is not zero — but it is far below running, and it is estimated from session duration rather than measured, since a gym session has no distance. Most of a strength session's cost appears as muscular load.
Best practices
DO:
- Compare each metric against your own trend over weeks
- Watch structural load when increasing running volume, or before a mountain trip
- Expect structural load to be near zero for swimming, rowing and cycling
- Use muscular load to judge how much hard work your legs have absorbed recently
- Record elevation on hilly sessions — descent is a large part of structural load
DON'T:
- Add the numbers together or compare one metric against another
- Compare your values with another athlete's
- Assume a low cardio load means an easy day — check the structural number
- Apply the "10% per week" rule across different sports; it belongs to structural load within one sport
- Panic over a single session — trends over weeks are what matter
Summary: the simple version
- Cardio load (AU) tells you what the session cost your heart
- Muscular load tells you what it cost your muscles
- Structural load tells you what it cost your bones and tendons
A kayak session is all heart and no legs. A long hike is all legs and little heart. A downhill run is easy on the heart and brutal on the legs. Now you can see all three, instead of guessing from one.
Technical notes
For coaches and data enthusiasts:
- Muscular load = mean heart-rate reserve × duration × a per-sport muscular coefficient. It scales roughly with mechanical work, so it is far more duration-driven than cardio load, which uses a steep power of heart-rate reserve. The scale is anchored so an hour at 70% heart-rate reserve matches its cardio load.
- On sessions with no heart rate, the intensity term is taken from the RPE rating directly, on the same 0–1 scale, rather than by working backwards from the cardio load. Deriving it from cardio load compressed a fourfold spread in reported effort into a few percent, which made the dimension read as little more than duration.
- The two therefore diverge on purpose. Muscular load is not cardio load rescaled. A long easy session scores above its cardio load; a short intense one scores below it. Earlier versions of this metric were a rescaling of cardio load and under-reported long easy work by roughly an order of magnitude.
- Structural load = (distance × a per-sport impact coefficient + descent × 10 × a per-sport descent coefficient) ÷ 100, expressed in units of 100 m of flat-running equivalent.
- Descent is additive, not a multiplier. The eccentric work of descending depends on how far you dropped, not on how far you travelled while dropping.
- Ascent has no separate structural term. Climbing is concentric work — it raises heart rate and therefore already appears in cardio load and muscular load.
- Where full workout data exists, distance is accumulated only while genuinely moving, so GPS drift during stops does not inflate the result.
- Sessions with no usable distance fall back to a per-sport rate per minute; sessions with neither distance nor duration report no structural value at all.
- The coefficients are informed judgement, not laboratory measurement. They are calibrated so that flat running is the reference point, and they will be refined as real data accumulates.
Both metrics are deliberately conservative: where the data does not support a value, we mark it as estimated or leave it out rather than publish a number that looks more precise than it is.