What Counts as a Hard Set? Reps in Reserve and Junk Sets

When people say “10 to 20 sets per muscle per week,” they mean ten hard sets. A hard set, for building muscle, is one taken close enough to failure that the last few reps are difficult: roughly within 0 to 3 reps in reserve, or about RPE 7–10. Sets that end well short of that (the ones where you rack the weight with five clean reps still in the tank) recruit far less muscle and add little growth, which is why they shouldn’t count toward the totals you plug into the Training Volume Calculator. Getting this definition right is what makes the whole volume framework work; count easy sets and your “15 sets” might be five.

This is educational content, not coaching or medical advice.

What “hard” means: reps in reserve

The cleanest way to define set difficulty is reps in reserve (RIR): how many more reps you could have done before hitting failure. It maps directly onto the resistance-training RPE scale popularized by Zourdos and colleagues: RPE 10 is true failure (0 RIR), RPE 9 is one rep left, RPE 8 is two, RPE 7 is three.

Reps in reserve	RPE	Counts as a hard set?
0 (failure)	10	Yes (maximal effort, higher fatigue)
1–2	8–9	Yes, the productive sweet spot for most sets
3	7	Yes, at the edge; effective for many trained lifters
4+	≤6	Not really; too far from failure to count as a growth set

The validity of this scale is reasonably established: in the original work, bar velocity tracked RIR-based ratings closely, meaning the internal sense of “reps left” corresponds to a real, measurable slowing as you approach failure. That’s why RIR, despite being subjective, is a usable currency for effort.

Why sets far from failure barely count

Muscle growth is driven mostly by mechanical tension on fibers taken close to their limit. As a set approaches failure, the nervous system recruits progressively more (and higher-threshold) motor units to keep moving the weight. Those late, grinding reps are where the most fibers are working hardest. Stop with five reps in reserve and many of those fibers were never meaningfully challenged.

This is also the principle behind the volume literature itself. Reviews on quantifying training volume note that counting “total sets” only works as a proxy for stimulus when those sets are taken to or near failure: the number of sets is only meaningful if each set is hard. A page full of easy sets is barely volume at all.

But you don’t need to hit failure on everything

A 2023 Sports Medicine meta-analysis by Refalo and colleagues found only a trivial advantage for training to momentary failure versus stopping short for hypertrophy. Sets taken to near failure grow muscle about as well as sets taken to failure, while generating less fatigue and less joint and nervous-system stress.

That’s why the practical target for most sets is 1–3 reps in reserve, not zero. You get nearly the full growth stimulus without the recovery tax of constant failure. Saving true failure for the occasional last set of an exercise, or for machines where failure is safe, is a reasonable way to calibrate your RIR sense without living there. This is the same logic that underpins autoregulating load with RPE: effort, not the number on the bar, is what you’re managing.

The catch: judging RIR is a trainable skill

The whole system depends on your ability to know how close to failure you are, and that judgment is learned, not innate. Research on predicting proximity to failure shows people get more accurate with experience, and that predictions tighten as the set gets closer to failure. Beginners systematically stop too early: they feel like they’re at RPE 8 when they had four or five reps left, quietly turning intended hard sets into junk sets.

Practical ways to calibrate:

• Occasionally take a set to true failure on a safe exercise to recalibrate what “0 reps left” actually feels like.
• Watch bar speed. When reps slow sharply despite full effort, you’re within a rep or two of failure.
• Assume you’re further from failure than you feel, especially early in your training career. Most people leave more reps in the tank than they think.

If your effort ratings are off, everything downstream breaks: your volume totals, your placement against the MEV–MAV–MRV landmarks, and your read on whether you’re doing enough to hit the 10–20 sets that drive growth.

The takeaway

A hard set is one taken close enough to failure (about 0 to 3 reps in reserve) that the final reps are tough. Those are the sets that recruit the most muscle and the only ones that should count toward your weekly volume. You don’t need to reach absolute failure; near-failure work grows muscle about as well with less fatigue. The one skill that makes it all reliable is honest RIR judgment, which sharpens with practice. Count only your hard sets, and your volume numbers will mean what you think they mean.

Once you’re confident which sets qualify, tally them per muscle in the Training Volume Calculator to see where you land against the growth range. RIR targets are general guidelines, not personal prescriptions. If you’re training around an injury or health condition, work with a qualified coach.

Sources

• Refalo MC, et al. Influence of Resistance Training Proximity-to-Failure on Skeletal Muscle Hypertrophy: A Systematic Review with Meta-analysis. Sports Med. 2023;53(3):649–665.
• Zourdos MC, et al. Application of the Repetitions in Reserve-Based Rating of Perceived Exertion Scale for Resistance Training. Strength Cond J. 2016;38(4):42–49.
• Halperin I, et al. “Just One More Rep!” Ability to Predict Proximity to Task Failure in Resistance Trained Persons. Front Psychol. 2020;11:565416.
• Baz-Valle E, et al. Total Number of Sets as a Training Volume Quantification Method for Muscle Hypertrophy: A Systematic Review. J Strength Cond Res. 2021;35(3):870–878.