RPE: the cheapest, most underrated measurement in cycling

Every measurement in cycling costs something. A power meter costs several hundred pounds and needs calibrating. A lactate analyser costs a meter plus strips plus a finger. A metabolic cart costs a lab appointment. Rating of perceived effort costs one question, asked once, about half an hour after you get off the bike.

It is also the measurement most riders throw away. Partly because it feels unscientific next to a number generated by a strain gauge, and partly because most people have never been shown how to use the scale properly, so their ratings really are closer to a mood diary than a measurement.

That is a shame, because perceived effort is not a soft proxy for the real numbers. It is measuring something the real numbers cannot reach.

Two scales, and why there are two

Gunnar Borg published the 6–20 scale in its modern form in 1982. The strange range is deliberate: it was built so that, in a healthy young adult exercising in a steady state, the rating multiplied by ten lands somewhere near the heart rate. Rate a ride 13 and your heart rate is plausibly around 130. That mapping is a rough design convenience rather than a law, and it falls apart with age, with medication and at the extremes, but it explains why the scale does not start at zero.

Borg later produced the CR10 scale — a category-ratio scale running 0 to 10 with the anchors spaced non-linearly, so that the gap between 7 and 10 covers much more physiological ground than the gap between 2 and 5. CR10 behaves better when you want to compare the intensity of sensations rather than just rank them, and it is the one nearly all training-load arithmetic uses.

Both work. What matters far more than which you pick is that you pick one and never switch, because a 7 means something completely different on each.

What you are actually rating

The intuitive model is that effort perception is your body reporting upwards: lactate in the muscle, stretch in the lungs, strain in the heart, all summed into a feeling. That model is probably wrong, or at least badly incomplete.

The better-supported account is that perceived effort tracks central motor command — the size of the signal your brain is sending down to the muscles — read off through a corollary discharge to the sensory cortex, rather than read back from the periphery. Marcora set this out forcefully in 2009, pointing to evidence that blocking afferent feedback from working muscles does not abolish the sense of effort. The position is contested and the debate is not settled, but it explains things the peripheral model cannot.

It explains why effort rises through a long steady ride at constant power: as fibres fatigue you must recruit more of them to produce the same watts, so the command gets bigger even though the external work does not. It explains why a poor night's sleep makes an easy ride feel like work. And it explains why perceived effort and pain come apart — they are different perceptions, and Pageaux's definitional work makes a point of separating effort from pain, from discomfort and from fatigue. Riders who conflate them produce ratings that measure their saddle sores.

This also sets the limits. If RPE tracked blood lactate directly it would be a cheap lactate meter. It does not. Meta-analytic estimates of the correlation between Borg RPE and physiological criteria sit around 0.6 for heart rate, blood lactate and percentage of VO₂max — real, useful, and nowhere near a replacement for measuring the thing itself.

Session-RPE: one number for a whole ride

The idea that made RPE into a load metric is embarrassingly simple. Take your CR10 rating for the whole session, multiply it by the duration in minutes, and call the product training load in arbitrary units. Two hours at a 5 is 600. Ninety minutes of intervals at an 8 is 720. Foster's group published it in 2001 and it has been validated in most sports since.

The obvious objection is that it cannot possibly be as good as a power-based figure. In practice it is closer than you would expect. In competitive road cyclists, session-RPE load tracks power-derived load closely enough that the two rank a training week the same way, and reviews of the method report it holding up across sports and across athletes of different levels. It is not identical to a power-based score, and it should not be — it is weighting the ride by internal cost rather than external work.

Where it diverges is where it earns its place. Two rides, same duration, same normalised power, one done fresh and one done on day four of a block: identical by power, materially different by session-RPE. If you track fitness, fatigue and form from power alone, the second ride is invisible. Running session-RPE alongside a power-based weekly load costs you ten seconds a day and gives you a second opinion from a different instrument.

What RPE catches that power cannot

A power meter measures work done at the crank. That is its virtue: it is external, objective and indifferent to your opinion. It is also its blind spot, because the same external work can cost wildly different amounts internally.

None of these is exotic. Between them they cover most of the weeks in a year. The pattern to watch for is the simplest one available: power flat, RPE rising, across several sessions. That is the signature of a rider heading towards non-functional overreaching, and it is visible weeks before the power numbers admit anything is wrong.

The same logic works in the other direction. If your HRV is jumpy but your endurance rides keep feeling easy at unchanged watts, the HRV reading is probably noise. Two independent instruments disagreeing is more informative than one instrument insisting.

Calibrating your own scale

Most riders' RPE is useless for a boring reason: they have never anchored it. If your hardest-ever effort is an 8 because you are saving 9 and 10 for something worse, your scale is compressed at the top and every number below is meaningless. Five steps fix it.

There is a useful cross-check available once a fortnight. Ride a steady twenty minutes at a fixed power you know well, rate it before you look at anything, and write both down. Over a few months you build a table of RPE against known watts, and drift in that table is information. Indoors is the easiest place to do this, because the wind, the traffic lights and the descents are all removed and the only variable left is you. Moveee Indoor runs a fixed-power block in a browser tab and saves the full file, so the comparison is genuinely like for like. It is free while in alpha.

Be aware of one asymmetry: perceived effort at a given power is usually higher indoors than out, for reasons covered in why your indoor FTP is lower. Compare indoor with indoor.

Where it fails, honestly

RPE is poor at distinguishing intensities close together at the easy end. The gap between 55% and 65% of threshold is real physiologically and mushy perceptually, which is exactly why riders drift out of zone 2 without noticing. For that job use power or heart rate.

It is also sensitive to expectation. Tell a rider the interval is the last one and it gets easier. Tell them there are two more and it does not. Session-RPE inherits this: knowing you rode a "hard session" nudges the rating upward independently of what you did. And group ratings are contaminated by whoever speaks first, so rate before you discuss the ride.

Finally, a single rating carries almost no information. The value is in the series. One ride at 8 tells you nothing you did not already know; eight weeks of ratings against known power tells you when something has changed. Write the number down every day for a month, do nothing with it, and then look back — that is the point at which most riders start believing the cheapest instrument they own.