RPE (rating of perceived exertion) and RIR (reps in reserve) are two ways a training plan can instruct how difficult a certain exercise should be. Both are self-estimations of proximity to failure within a certain rep and load combination. For example, if a program lists 3 sets of 5 reps at a 9 RPE you should choose a load that you can at least do 6 reps with. This way of instructing loads for an exercise can be incredibly valuable because many exercises do not work with traditional methods such as percentage of 1RM. I suppose if you were to test (max out) every exercise in a training plan to find your 1RM you could then theoretically use percentages but no one has time for that and even if you did you wouldn’t garner an adaptive stimulus from most exercises. Let’s face it, you aren’t going to gain much muscle or strength by performing a 1RM on your DB curls, machine rows, and other accessory movements.
RPE and RIR are useful because they provide a scalable means to instruct load. As useful as each method might be, many individuals still find it challenging to select the perfect weight for the RPE or RIR being used. There are a few ways we can make this process easier. The first of which is to take the ambiguity out of RPE and use RIR. RPE uses an inverted scale. For example, many people new to RPE might assume a 9 RPE means to keep 9 reps in the tank, not 1 rep in the tank. I propose you eliminate the RPE altogether and use RIR. Unlike RPE, RIR is not an inverted scale. 1 RIR means leave 1 in the tank. It might seem kind of like semantics to make that big of a fuss about the whole rating system but I think it makes a difference in simplicity for training programs that utilize other methods along with RPE/RIR to instruct load.
The next way we can make RPE/RIR easier and more effective is to use ranges. Instead of saying leave 1 rep in the tank, why not 1-2 reps? The former might be more precise, but does that level of precision really matter in a situation where you can’t even quantify how much biological stress someone is carrying? I say no, it probably does not matter. Not only are ranges more flexible but, they are also easier to select proper loads from. In my experience, it helps athletes stay away from the overshooting or sandbagging their range while providing them with more flexibility.
The final way we can make RPE/RIR easier and more effective is by using historical training data for load targets. Anytime I write a training program for an athlete they always include load targets with their RIR number. If the exercise does not have a load target it usually means they have never performed that specific exercise, with that specific rep and RIR number. By basing load targets off prior training and with three conditions (exercise, rep #, and RIR range) you can establish very accurate training targets for each RIR range. Now you have a goal target to shoot for but can dial in your range by using the RIR for that day. I prefer to use a high and low range that is about 2.5% in either direction from the previous best average loads used. That might seem like a ton of work but with a few simple formulas, you can make Excel or Google Sheets do all the work for you.