Sets to Failure vs Reps in Reserve: What Actually Drives Muscle Growth

By Olivia Bennett

Two lifters can follow the exact same program, hit the same sets and reps, and walk away with completely different results depending on one variable most programs barely mention: how close each set actually came to failure. This single training decision, more than almost any other besides total volume, appears to separate solid muscle-building programs from mediocre ones.

For years, “train to failure” was treated as a badge of honor. Grind out that last rep until the bar stalls, until your training partner has to peel it off your chest, until your legs simply refuse to stand. More recently, a rival approach has taken hold in serious strength and physique circles: reps in reserve (RIR), a method of deliberately stopping a set while you could still perform one, two, or three more reps.

Which one actually builds more muscle? Which one builds more strength? And does the answer change depending on your goals, your recovery capacity, or how long you’ve been training? Recent peer-reviewed research, including a large 2024 dose-response analysis out of Florida Atlantic University and several controlled training studies, gives a fairly clear (if nuanced) answer. This guide breaks down what the science says, how to apply it in your own training, and where the two approaches genuinely differ versus where they don’t.

What “Training to Failure” and “Reps in Reserve” Actually Mean

Before comparing outcomes, it helps to define the terms precisely, since a lot of gym-floor confusion comes from people using them loosely.

Training to Failure

Training to failure means performing a set until you physically cannot complete another repetition with proper form, sometimes called momentary muscular failure. If you’re benching and rep eight stalls halfway up no matter how hard you push, that’s failure. It’s an objective endpoint: the set ends because your muscles, in that moment, cannot generate enough force to move the load through the full range of motion.

Reps in Reserve (RIR)

Reps in reserve is a self-rated estimate of how many more reps you could have done before hitting failure. A set performed at “2 RIR” means you stopped with an estimated two good reps still left in the tank. This concept is closely tied to the Rating of Perceived Exertion (RPE) scale adapted for resistance training by sports scientist Mike Zourdos and colleagues, first published in the Journal of Strength and Conditioning Research in 2016. On that 10-point scale, an RPE of 10 corresponds to 0 RIR (true failure), RPE 9 corresponds to roughly 1 RIR, RPE 8 to about 2 RIR, and so on.

The Core Question: Does Proximity to Failure Drive Muscle Growth?

This is where the research has moved fastest in the past two to three years. The most influential recent contribution is a 2024 paper by Zac Robinson and Michael Zourdos at Florida Atlantic University, published in Sports Medicine, which pooled dose-response data from 55 resistance training studies to model how proximity to failure relates to strength and hypertrophy outcomes separately.

The pattern that emerged was consistent: as sets moved closer to failure, muscle growth outcomes improved in a graded, dose-dependent way. Robinson and Zourdos suggested a practical working range of roughly 0 to 5 reps in reserve for people training primarily for hypertrophy, noting that training closer to failure also seems to make lifters better at accurately judging how many reps they truly have left. Strength gains told a different story: the analysis found stopping sets 3 to 5 reps short of failure produced strength gains statistically indistinguishable from training to failure, while allowing for more manageable fatigue and load selection focused on heavier weights.

A separate meta-regression posted to SportRxiv reached a similar split conclusion using its own modeling of estimated RIR across the training literature: the confidence intervals for the RIR-to-hypertrophy relationship did not cross zero, meaning the closer-to-failure effect on muscle size was statistically real, if modest. For strength, the RIR relationship’s confidence interval did include zero, meaning no reliable strength advantage could be attributed to training closer to failure once other variables were accounted for.

What a Head-to-Head Trial Found

Meta-analyses combine many imperfect studies, so it’s worth looking at a clean, controlled trial too. A 2024 study published in the Journal of Sports Sciences assigned 18 resistance-trained lifters to train one leg using leg press and leg extension to momentary failure, and the other leg using a matched RIR protocol (2 RIR on leg press, 1 RIR on leg extension), twice weekly for eight weeks. Quadriceps thickness increased by a nearly identical amount in both conditions (about 0.18 cm in each group), while the failure condition produced noticeably more velocity loss and rep-to-rep fatigue during sessions. In other words, both legs grew about the same amount of muscle, but the leg trained with 1 to 2 reps in reserve accumulated less acute fatigue getting there.

Put these threads together and a coherent picture appears: pushing sets closer to failure tends to nudge hypertrophy upward across a population, but the effect isn’t so large that stopping a rep or two short sabotages your gains. Strength training shows essentially no advantage from grinding to true failure, and often shows a cost, since fatigue accumulates faster than benefit.

Sets to Failure vs Reps in Reserve: Side-by-Side Comparison

Why Proximity to Failure Matters Physiologically

Muscle growth is driven largely by mechanical tension applied to muscle fibers over time, along with a smaller contribution from metabolic stress and muscle damage. The reason training closer to failure tends to help hypertrophy slightly is mechanical: as a set progresses and the easier motor units fatigue, your nervous system recruits higher-threshold motor units, the ones that control the largest, fastest-growing fibers. Those units get meaningfully activated mostly in the final few reps of a hard set.

That’s also exactly why reps in reserve still works reasonably well up to a point. If you stop a 10-rep set at 2 RIR, you’ve already recruited most of those higher-threshold fibers during reps 7 through 8. The last two reps you skip are the ones with the steepest fatigue cost and the smallest additional stimulus. Diminishing returns kick in earlier than most lifters assume.

Strength adaptation runs on a different mechanism: it’s driven substantially by neural efficiency, coordination, and the ability to produce maximal force under load, which responds mainly to how heavy the weight is relative to your maximum, not how close the set gets to failure. That’s the physiological reason the strength data comes out flat between the two approaches once load is held constant.

How Accurately Can You Actually Judge Reps in Reserve?

A fair criticism of RIR-based training is that it depends on self-perception, and self-perception can be wrong. The data on this is more encouraging than skeptics expect, but it comes with real caveats.

Research reviewed by strength scientist Greg Nuckols and colleagues found that lifters, on average, are off by roughly one repetition when estimating how many reps remain before failure, regardless of training experience or sex. Accuracy is not constant throughout a set, though. When lifters guess early in a set (around the one-third mark), the average error balloons to nearly five reps. By the two-thirds mark, that error drops to roughly two reps. By 90% of the way through a set, most lifters are within about one rep of their true failure point. There’s also a consistent bias: people tend to underestimate how many reps they have left, meaning they usually have slightly more in the tank than they think.

The practical takeaway is straightforward: don’t try to predict RIR from the first rep of a set. Wait until you’re most of the way through before deciding whether to stop, add a rep, or push on. This alone meaningfully improves how well RIR-based training works in practice.

A Practical Framework for Choosing Your Approach

Rather than treating this as an either-or debate, most evidence-based coaches now use both tools deliberately, matched to the exercise and the training goal.

Choosing by Training Age

Newer lifters generally don’t need to approach failure often. Novices see substantial muscle and strength gains from moderate effort simply because their bodies are unaccustomed to any structured overload; chasing failure adds fatigue without adding much extra benefit and can compromise the technique they’re still learning. More advanced lifters, who have already captured the easy gains from consistent training, tend to need to push closer to the edge periodically to keep progressing, which is part of why RIR and RPE tracking becomes more valuable as training experience increases.

There’s also a practical coaching reason RIR tends to outperform blanket failure training over a full training cycle: it’s programmable in advance. A coach or a self-written plan can specify “3 sets of 8 at 2 RIR” and adjust that target week to week as a mesocycle progresses, typically trimming the RIR target down as a block builds toward a peak week, then backing off again during a deload. Training to failure on every set removes that dial. You either grind every set as hard as physically possible from week one, which leaves nowhere to progress toward except more failure, or you have to layer in an inconsistent, harder-to-track version of effort management anyway. RIR gives you a built-in progression variable that complements load and volume rather than competing with them.

Reading Your Own Recovery Signals

Proximity to failure doesn’t exist in isolation from the rest of your week. Sleep quality, stress, nutrition, and the volume already accumulated in prior sessions all shift how much a given RIR target actually costs you in fatigue. Two sets performed at 1 RIR on a well-rested Monday and a poorly slept, high-stress Friday are not equivalent stimuli, even though the label on paper looks identical. Lifters who track simple markers, resting heart rate, bar speed on a known warm-up weight, or even a basic daily readiness rating, tend to get more consistent long-term results from RIR-based training because they can nudge the target up by a rep or two on rougher days rather than forcing the same number regardless of readiness.

Fatigue, Recovery, and the Case for Managing Effort

One of the most consistent findings across recent hypertrophy research is that training to failure carries a fatigue cost disproportionate to its growth benefit. The 2024 head-to-head leg training study found the failure condition produced significantly more velocity loss within sessions and greater rep-to-rep breakdown, despite ending up with essentially the same quadriceps growth as the RIR-trained leg. That extra fatigue has to go somewhere: it either reduces the quality of subsequent sets and sessions, or it extends the recovery window needed before the same muscle group can be trained hard again.

This matters most for people training a muscle group multiple times per week, which is increasingly common in higher-frequency hypertrophy programs. If every set for a muscle group is pushed to failure, weekly volume capacity shrinks because recovery between sessions takes longer. Stopping most sets a rep or two short allows more total volume across the week, and total weekly volume is one of the most reliably supported drivers of muscle growth in the literature, arguably a bigger lever than proximity to failure on any single set.

Safety Considerations

Training to failure is not inherently dangerous, but it does raise the stakes on certain lifts. A few points worth taking seriously:

• Avoid taking free-weight barbell exercises like the back squat, bench press, or overhead press to true failure without a spotter or safety bars set at an appropriate height.
• Failure on a deadlift usually shows up as form breakdown (rounding, hitching) rather than a clean stall, which raises lower-back injury risk more than other exercises. Most coaches recommend keeping deadlifts several reps short of failure regardless of goal.
• Machines and cable exercises are generally safer to take closer to failure since the movement path is fixed and there’s rarely a need to “catch” a heavy free weight.
• If you have any cardiovascular condition, uncontrolled blood pressure, or a recent injury, check with a physician or physical therapist before incorporating regular failure training, since near-maximal effort briefly raises blood pressure and intrathoracic pressure.
• Older adults and anyone newly returning to training after a long break should build a technique and conditioning base at moderate effort (3-plus RIR) before introducing sets closer to failure.

Putting It Together: What the Evidence Actually Supports

Across a large pooled analysis, a focused meta-regression, and a controlled head-to-head trial, the same basic pattern keeps showing up. Muscle growth responds, in a modest and graded way, to training closer to failure, with a practical working zone of about 0 to 5 RIR depending on the exercise and how fresh you are. Strength gains don’t reliably respond to proximity to failure at all once load is accounted for, which means grinding every set to failure buys you little to nothing extra if strength is the priority, while costing you real recovery capacity.

For most lifters training for size, strength, or general fitness, keeping the bulk of weekly sets in the 1-4 RIR range, reserving occasional true failure sets for isolation exercises or a session’s final set, captures nearly all of the benefit failure training offers while avoiding most of its downside. That’s not a compromise position, it’s what the current data actually points to.

Frequently Asked Questions

Is training to failure necessary to build muscle?

No. Multiple controlled studies, including the 2024 leg-training trial described above, found comparable muscle growth between sets taken to failure and sets stopped 1-2 reps short. Consistent proximity to failure across many sets and weeks matters more than reaching absolute failure on any given set.

What’s a good RIR target for hypertrophy versus strength?

For hypertrophy-focused training, most working sets performed in the 0-5 RIR range appear effective, with 1-3 RIR being a practical, sustainable target for most sets across a week. For strength-focused training, stopping 3-5 reps short of failure on heavy compound lifts appears to produce strength gains equivalent to training closer to failure, without the added fatigue.

How do I get better at estimating reps in reserve?

Wait until at least two-thirds of the way through a set before estimating how many reps remain, since early-set predictions carry much larger error. Periodically take a set to true failure on a safe exercise (like a machine or isolation movement) to recalibrate your sense of what “0 RIR” actually feels like, then compare that to what you guessed a few reps earlier in the set.

Does training to failure cause more muscle soreness or overtraining risk?

Failure training does tend to produce more acute fatigue and muscle damage markers than reps-in-reserve training performed with matched volume, based on velocity-loss data from recent trials. It doesn’t automatically cause overtraining, but doing it on every set, every session, without adjusting weekly volume downward increases that risk over time.

Should beginners train to failure?

It’s generally not necessary. Beginners tend to gain strength and muscle from moderate effort because almost any structured, progressive resistance training is a meaningful stimulus for an untrained body. Spending the first months of training on technique and consistency at 3+ RIR, then gradually introducing closer-to-failure sets as fatigue tolerance and skill improve, is a lower-risk path to the same long-term results.

Can I mix both approaches in the same program?

Yes, and most evidence-based programs already do. A common structure keeps heavy compound lifts in the 2-4 RIR range throughout, while allowing the final set of an isolation exercise, or a machine-based accessory movement, to go closer to or all the way to failure. This captures the hypertrophy benefit of occasional high-effort sets without adding unnecessary fatigue to the lifts most likely to be affected by it.

References

• Robinson, Z.P., et al. “Exploring the Dose-Response Relationship Between Estimated Resistance Training Proximity to Failure, Strength Gain, and Muscle Hypertrophy.” Sports Medicine, 2024. pubmed.ncbi.nlm.nih.gov/38393985
• Florida Atlantic University. “For Bigger Muscles Push Close to Failure, For Strength, Maybe Not.” FAU Newsdesk / EurekAlert, 2024. eurekalert.org/news-releases/1053119
• Refalo, M.C., et al. “Similar muscle hypertrophy following eight weeks of resistance training to momentary muscular failure or with repetitions-in-reserve in resistance-trained individuals.” Journal of Sports Sciences, 2024. tandfonline.com/doi/full/10.1080/02640414.2024.2321021
• SportRxiv preprint. “Exploring the Dose-Response Relationship Between Estimated Resistance Training Proximity to Failure, Strength Gain, and Muscle Hypertrophy: A Series of Meta-Regressions.” sportrxiv.org/index.php/server/preprint/view/295
• Zourdos, M.C., et al. “Novel Resistance Training-Specific Rating of Perceived Exertion Scale Measuring Repetitions in Reserve.” Journal of Strength and Conditioning Research, 2016. pubmed.ncbi.nlm.nih.gov/26049792
• Nuckols, G. “Overshooting, Undershooting, Or Just Right? How To Perfect Your Ability To Predict Repetitions In Reserve.” Stronger By Science. strongerbyscience.com/reps-in-reserve