RPE

The RPE scale was created in the 1950s by Gunnar Borg to measure exertion in cardiovascular activity, but in the lifting world it has been adapted to indicate proximity to muscular failure. In practice it works like this: RPE 6 means you could have done 4 more reps, RPE 7 three more, RPE 8 two more, RPE 9 one more, RPE 10 you've reached total failure.

The advantage of RPE is that it adjusts automatically to your day. If you slept badly, you're stressed, or you haven't eaten enough, the load that corresponds to an RPE 8 will be lower than usual. You don't have to force a fixed weight written on your sheet when your body is telling you it can't deliver today. By the same logic, on good days you can push more without being anchored to conservative percentages.

RPE has one important limitation: it requires experience to be used well. Beginners tend to underestimate their effort, stopping at RPE 6-7 when they think they're at RPE 9. The fix is to practice taking sets close to failure under controlled conditions, to calibrate your perception. Over time, it becomes an extremely reliable tool that lets you dose effort set by set.