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If you've ever tried to guess how many more reps you can do during a workout, you're not alone and knowing this number is the cornerstone of a method of measuring intensity known as Repetitions in Reserve (RIR). It tends to be popular for a variety of reasons, but one is that it gives you a relatively simple way to know how hard you are working and getting within a certain threshold of failure can be a good target intensity for basic strength and hypertrophy from resistance training. The caveat is that you need to be relatively accurate in your assessment of how close to failure you are in a given set. A new study “Accuracy in predicting repetitions in reserve during resistance training: Differences across load intensities, distance from muscular failure, sexes, and exercises” looked at whether people can accurately predict when their muscles are about to reach failure—and the results might surprise you. The study is linked below in the references.
Researchers Casanova, et al (2025) wanted to know how good people are at estimating their "repetitions in reserve". They tested 34 healthy adults who had been lifting weights for at least six months to see how accurate their guesses were.
The study looked at several factors that might affect accuracy:
Here's the most important finding: people were much more accurate at predicting when they had 1 rep left compared to when they had 3 reps left. In other words, the closer you get to the point where your muscles give out, the better you can sense it coming.
When participants thought they had 3 reps left, they were often wrong by several repetitions. But when they thought they had only 1 rep left, their guesses were much closer to reality. This suggests that your body gives you clearer signals when you're really close to hitting your limit.
Participants were significantly more accurate when lifting heavier weights (75% of their max) compared to lighter weights (50% of their max). When doing more repetitions with lighter weights, people consistently underestimated how many more reps they could actually do.
Why does this happen? When you're doing a lot of reps with lighter weight, the discomfort and fatigue build up differently than with heavy weights. You might feel uncomfortable and want to stop, even though your muscles could technically keep going. With heavier weights, your muscles reach their mechanical limit more clearly, making it easier to sense when failure is truly approaching.
Good news: sex doesn't seem to make much difference in how accurately you can judge your effort. Both men and women struggled with the same things and succeeded in similar ways. There was only one minor exception during one specific exercise at lighter weight, but overall, everyone's in the same boat.
This means that whether you're male or female, you'll benefit from the same strategies to improve your accuracy.
The researchers tested three different exercises to see if some movements made it easier to judge effort. While there were small differences (the lying leg curl was slightly easier to judge), overall the specific exercise didn't make a huge difference in accuracy.
This is actually helpful information because it means the challenges of judging your effort level are pretty universal across different exercises, so the lessons learned apply broadly to your entire workout routine.
Understanding how close you are to failure is crucial if you want to use Repetitions in Reserve properly in your training. Training close to failure (but not necessarily all the way to failure every time) is one of the key factors for building muscle and getting stronger.
However, if you consistently misjudge your effort by thinking you're at your limit when you still have 3-5 reps left in the tank, you might not be pushing yourself hard enough to see the results you want. On the flip side, if you're constantly training to complete failure when you didn't mean to, you might be overdoing it and risking burnout or injury through constant exposure to an unrecoverable training stress.
The study found that when people thought they were leaving 3 reps in reserve, they often actually had 4-6 reps left. This means many people might be cutting their sets short without realizing it, which could slow down their progress.
Before we talk about how to apply these findings, it's important to understand that training close to or at muscular failure comes with real risks if you're not prepared. When your muscles are about to give out, you can end up trapped under a heavy barbell or stuck in a dangerous position—and that's not something to take take chances with.
Set up safety equipment before you start. If you're squatting in a power rack or squat rack, always adjust the safety bars (sometimes called spotter arms or safety pins) to just below the lowest point of your squat. This way, if you can't complete a rep, you can safely lower the bar onto the pins instead of having it come down on you. For bench press, set the safety pins at chest height so the bar can't pin you down if you fail a rep. These simple adjustments take five seconds but can prevent serious injury.
Choose exercises that allow safe failure. When you're learning to train close to failure, machines are your friend. Exercises like the chest press machine, leg press, or cable exercises allow you to safely stop mid-rep without risk of being trapped. You can simply stop pushing and guide the weight back to its start position. This makes them ideal for beginners who are still learning to gauge their limits. Free weight exercises like barbell squats and bench press require more caution and planning.
Use a spotter for barbell exercises. A spotter is someone who stands by to assist you if you can't complete a repetition. For exercises like the bench press or squat, having a spotter isn't just helpful—it's essential when you're pushing close to failure. Don't be shy about asking someone; most gym-goers are happy to help because they know they might need the same favor someday.
Communicate clearly with your spotter. Before you start your set, tell your spotter exactly what you need: "I'm going for 8 reps. Please don't touch the bar unless I ask for help or I'm clearly stuck." Let them know if you want them to help you get the bar back on the rack after your last rep, or if you'll rack it yourself. For squats, explain whether you want them to grab the bar or help lift you from under your arms if you get stuck in the bottom position. Clear communication prevents confusion when you're in the middle of a tough set and can't talk easily.
Ask a gym trainer to spot you. While any gym member can technically spot you, trainers are your best option. They know proper spotting techniques, understand how to assist without taking over the lift, and can recognize when you're truly at failure versus just uncomfortable. Many trainers are happy to spot you between clients, and this is a great way to ensure your safety while you're still building confidence. A well-intentioned stranger might mean well, but they might grab the bar too early, lift too much of the weight, or not know how to safely assist with certain exercises.
Training close to failure is an effective tool, but only when done safely. Taking these precautions seriously will protect you from injury and give you the confidence to truly push your limits when it matters.
Get comfortable training closer to failure. Since people are much more accurate at judging 1 rep in reserve versus 3 reps in reserve, consider structuring your sets to finish when you feel you have just 1-2 reps left. This gives you a better chance of actually hitting your target intensity. As a beginner, this means pushing yourself a bit more than you might naturally want to—that uncomfortable feeling doesn't always mean you're done. Again, if you are doing this with a barbell exercise, it’s probably a good idea to get a spotter.
Use heavier weights when accuracy matters most. The study showed that people were much better at judging their effort with weights around 75% of their max (roughly 8-12 reps per set) compared to lighter weights that allowed 20+ reps. If your goal is to train at a specific intensity level, stick with moderate to heavy weights where you can more reliably gauge how close you are to failure. Save the super-light, high-rep work for when precise effort matters less.
Learn what true failure feels like. To get better at estimating how many reps you have left, you need to occasionally experience what it actually feels like when your muscles completely give out. Dedicate some training sessions (not every workout) to taking at least a few sets all the way to failure. This teaches your brain and body what the warning signs really mean. Think of it like learning the edges of the road—you need to know where the line is to stay close to it without crossing it.
Track your accuracy over time. For a few weeks, make notes about when you think you're at 1-2 reps left, then push yourself to see if you can actually do more. Write down: "Thought I had 1 rep left, actually had 3." Over time, you'll start recognizing the difference between discomfort (which happens during any hard set) and true muscular failure. This self-awareness will make your training more effective because you'll know whether you're actually hitting your target intensity.
Be extra cautious with lighter weight, high-rep sets. When you're doing sets of 15+ reps with lighter weights, remember that these are the hardest conditions for judging your effort accurately. You'll probably feel like stopping sooner than you actually need to. Push yourself to do 2-3 more reps than you initially want to when working in this rep range—chances are, you've still got more in the tank than you think.
1RM (One-Repetition Maximum) — The maximum weight you can lift for a single repetition of an exercise. This is used as a benchmark to calculate what percentage of your max you're lifting. For example, if your 1RM on the bench press is 200 pounds, 75% of your 1RM would be 150 pounds.
Intensity of Effort — How hard you're working relative to your maximum capacity. This isn't just about how heavy the weight is, but how close you are to complete muscle failure. A lighter weight taken very close to failure can have a high intensity of effort.
Muscular Concentric Failure — The point during an exercise where you can no longer lift the weight through the full range of motion, no matter how hard you try. "Concentric" refers to the lifting phase of an exercise (like pressing the bar up during a bench press). This is when your muscles are completely exhausted and can't generate enough force to complete another rep.
RIR (Repetitions in Reserve) — A way of measuring how close you are to failure by counting how many more reps you could do before your muscles give out. For example, if you stop a set at "3 RIR," it means you believe you could have done 3 more reps before reaching failure. "1 RIR" means you stopped with only 1 rep left in the tank.
RIR Scale — A numbered scale used to rate how many repetitions you have left before failure. A rating of 0 means you reached complete failure, while 3 means you stopped with 3 reps still possible. It's a tool to help you regulate training intensity without always having to train to complete failure.
Resistance Training — Any form of exercise where your muscles work against an external resistance to get stronger. This includes lifting weights (dumbbells, barbells), using resistance bands, or even bodyweight exercises like push-ups. Also called strength training or weight training.
Safety Bars/Safety Pins/Spotter Arms — Adjustable horizontal bars or pins inside a squat rack or power rack that catch the barbell if you can't complete a lift. They act as a mechanical spotter, preventing you from being trapped under heavy weight.
Set — A group of consecutive repetitions of an exercise. For example, if you do 10 squats, rest, then do 10 more squats, you've completed 2 sets of 10 reps.
Skeletal Muscle Hypertrophy — The scientific term for muscle growth. When you lift weights and stress your muscles, they adapt by getting bigger and stronger. "Hypertrophy" specifically refers to the increase in the size of your muscle fibers.
Spotter — A person who stands by during an exercise (especially with free weights) to assist you if you can't complete a repetition or need help racking the weight. A good spotter helps keep you safe without interfering with your set unless needed.
Training Intensity — In resistance training, this can refer to two things: (1) the weight you're lifting as a percentage of your maximum (external load), or (2) how hard you're pushing yourself relative to failure (intensity of effort). Both are important for making progress.
Volitional Failure — The point where you voluntarily stop an exercise because you genuinely cannot complete another repetition with proper form, no matter how hard you try. It's reaching your true limit, not just stopping because it feels uncomfortable.
Casanova, N., Gomes, D., Russo, F., Mateus, J., Vieira, A., Malcata, G., Tenfen, P., Soares, D., & Flôres, F. (2025). Accuracy in predicting repetitions in reserve during resistance training: Differences across load intensities, distance from muscular failure, sexes, and exercises. Journal of Physical Education and Sport, 25(11), 2373-2381. https://doi.org/10.7752/jpes.2025.11262
