Lifting to Failure? Reps in Reserve for Endurance Athletes

Most triathletes who start lifting are told two things that pull in opposite directions. One is that strength training only works if it's hard: heavy weights, the last rep a fight. The other is that the gym mustn't wreck the swim, bike and run sessions that matter most. Both are reasonable. The question is how hard each set needs to be to get the benefit.

That's what "proximity to failure" research addresses. This article covers what training to failure and reps in reserve mean, what the meta-analyses show for strength and muscle size, how much extra fatigue failure costs, how accurately you can judge it, and how to use reps in reserve in a winter strength block.

Failure, and reps in reserve

Training to failure usually means continuing a set until you can't complete another repetition with good form. Researchers distinguish momentary muscular failure, where the next rep genuinely can't be completed, from stopping when you think you've reached your limit, which is often a little earlier.

Reps in reserve (RIR) describes how close to failure you stop. If you finish a set of squats and could have done two more good repetitions, that set was at 2 RIR. A set to failure is 0 RIR.

In 2016, Zourdos and colleagues published a resistance-training rating of perceived exertion scale built around reps in reserve, in the Journal of Strength and Conditioning Research. On it, a rating of 10 means no reps left, 9 means one left, 8 means two, and so on. They tested it on 29 lifters (mean age 24), 15 experienced and 14 novice squatters, and found that higher ratings closely tracked slower bar speed (r = -0.88 in experienced and -0.77 in novice lifters). That suggests the scale reflects real changes in effort, and it's now widely used to set and adjust training loads.

The practical appeal for triathletes is that RIR adjusts automatically. On a day when you're tired from a long ride, the same weight will feel closer to failure, and the scale tells you to back off.

Is failure needed for strength?

The 2022 meta-analysis

Grgic, Schoenfeld, Orazem and Sabol compared failure and non-failure training in a systematic review and meta-analysis published in the Journal of Sport and Health Science. They included 15 studies, all in young adults.

• Strength: no significant difference between training to failure and stopping short (effect size -0.09, 95% CI -0.22 to 0.05).
• Muscle size: no significant difference overall (effect size 0.22, 95% CI -0.11 to 0.55).
• In resistance-trained people only, failure training produced a small but significant advantage for muscle size (effect size 0.15).

The authors concluded that training to failure doesn't appear necessary for gains in strength or muscle size, and that stopping short doesn't seem to impair them.

Proximity to failure as a dose

Comparing "failure" with "not failure" treats proximity as a yes-or-no switch. A 2024 analysis by Robinson and colleagues in Sports Medicine instead estimated the reps in reserve used in each study and ran a series of meta-regressions to look for a dose-response relationship.

• Strength gains were similar across a wide range of reps in reserve. The estimated slopes for strength were negligible, with confidence intervals that included zero.
• Muscle size increased as sets were stopped closer to failure. The slopes for hypertrophy were consistently negative, meaning fewer reps in reserve went with larger gains.

The authors described the analysis as exploratory. Reps in reserve had to be estimated from study descriptions, and the models fitted only modestly. But the pattern is clear and consistent with the earlier meta-analysis: proximity to failure matters little for strength and somewhat more for size.

Muscle size, looked at directly

A 2023 meta-analysis by Refalo and colleagues, also in Sports Medicine, focused on hypertrophy across 15 studies.

• Training to set failure had a trivial advantage for muscle growth (effect size 0.19, 95% CI 0.00 to 0.37).
• Training to momentary failure specifically showed no significant advantage (effect size 0.12, 95% CI -0.13 to 0.37).
• Higher velocity-loss thresholds, a proxy for going closer to failure, did not always produce more growth.

The authors suggested the relationship between proximity to failure and growth may not be linear: getting reasonably close may matter, but the last rep or two may add little.

Why muscle size matters less for triathletes

Inference: most triathletes lift to improve strength, rate of force development, tendon stiffness and resistance to injury, not to add muscle mass that has to be carried uphill. TriForward's "Heavy Lifting for Bike Power" and "Two Sessions a Week" cover the evidence that heavy strength training helps endurance performance. If strength is the target and the meta-analyses show strength gains don't depend on going to failure, the small hypertrophy advantage of failure training is mostly beside the point for endurance athletes.

There are exceptions. Masters athletes trying to maintain muscle mass, or anyone returning from injury with visible muscle loss, may value size more. TriForward's "Strength Training After 50" discusses why power and muscle quality matter with age.

The cost: fatigue and recovery

If failure added nothing but discomfort, it would be easy to drop. The bigger issue for triathletes is what it does to the next day.

How long failure takes to recover from

A 2017 study by Morán-Navarro and colleagues in the European Journal of Applied Physiology compared three bench press and squat protocols in 10 resistance-trained men, with measurements from 12 hours before to 72 hours after each session:

• 3 sets of 5 reps with a load they could have lifted 10 times (well short of failure).
• 6 sets of 5 reps with the same load (the same total volume as the failure protocol, still short of failure).
• 3 sets of 10 reps with that load (to failure).

The results:

• Failure caused much larger immediate drops in countermovement jump and in lifting speed, in both exercises, even compared with the equal-volume 6 x 5 protocol.
• Recovery between 24 and 48 hours was significantly faster after both non-failure protocols than after the failure protocol.
• Blood markers, including creatine kinase, followed markedly different recovery courses between protocols.

The authors concluded that training to failure considerably increases the time needed for neuromuscular function to recover. That's the key point: it wasn't just the amount of work, because the 6 x 5 protocol did the same number of repetitions with less fatigue.

What extra fatigue costs endurance sessions

A 2017 review by Doma, Deakin and Bentley in Sports Medicine found that residual fatigue from a single resistance session can impair a following endurance session for several hours to several days if recovery is inadequate. TriForward's "Sore From the Gym, Flat on the Bike" covers that research in detail. Inference: if failure sets lengthen recovery, they lengthen the window in which your next bike or run session is compromised.

Stopping short can improve some qualities

A 2017 trial by Pareja-Blanco and colleagues in the Scandinavian Journal of Medicine and Science in Sports used bar speed to control fatigue. 22 young men did eight weeks of squats, stopping each set when speed had fallen by either 20% (closer to fresh) or 40% (much closer to failure).

• Squat strength improved similarly in both groups.
• Jump height improved more with 20% velocity loss (9.5% versus 3.5%), despite fewer repetitions.
• The 40% group gained more size in one quadriceps muscle and lost some of their fast type IIX fibres. The 20% group preserved them.

This was a small trial in young men, not endurance athletes. But it fits the wider picture: going much closer to failure may add size, while stopping earlier gives similar strength with less fatigue and better explosive qualities.

How accurate is reps in reserve?

RIR only works if you can judge it. A 2022 scoping review and meta-analysis by Halperin and colleagues in Sports Medicine pooled 12 studies with 414 participants.

• People underpredicted how many reps they had left by about one repetition on average (0.95).
• Accuracy was better with heavier loads and sets of 12 reps or fewer than with lighter, longer sets.
• Accuracy improved slightly closer to failure and in later sets.
• Training experience didn't clearly affect accuracy, though variation between studies was very high.

Inference: if you aim for 2 RIR, you're probably stopping closer to 3. That's not a problem for a triathlete, since the margin errs towards less fatigue. It's worth occasionally checking your judgement on a safe exercise, such as a leg press or machine, by continuing a set to see how many you actually had left.

Who was studied

Population gaps matter here, as in most strength research:

• Age: the Grgic meta-analysis included only young adults, and the two mechanistic trials (Morán-Navarro and Pareja-Blanco) were in young men.
• Sex: the recovery and velocity-loss trials included only men. The meta-analyses don't report sex splits in their abstracts, and women are generally underrepresented in this literature.
• Training background: many participants were resistance-trained lifters or untrained students, not endurance athletes fitting lifting around a full week of swimming, cycling and running.

Inference: the strength findings are probably robust, because they come from many studies with consistent results. The fatigue findings are probably more important for triathletes than for the lifters studied, because a triathlete's next session is an endurance session that already depends on fresh legs. That direction of transfer is reasoning, not something a study has tested.

Using reps in reserve in a winter strength block

The following combines the studies above. It's inference for triathletes, not a tested triathlon programme:

1. Stop most sets with 1 to 3 reps in reserve. The meta-analyses suggest this keeps most of the strength benefit, and the recovery data suggest it costs less fatigue.
2. Use the lower end when it matters least. Early in the winter, when endurance sessions are easier, sets at 1 RIR are a reasonable way to drive strength. Closer to key blocks, 3 RIR or more is safer.
3. Prefer more sets short of failure over fewer sets to failure. The Morán-Navarro study showed equal volume with less fatigue when it was split into more, shorter sets.
4. Keep heavy compound lifts further from failure than isolation exercises. Grinding out the last rep of a heavy squat risks technique breakdown and is the most fatiguing. Failure on a calf raise or hamstring curl is lower risk.
5. Let RIR, not a fixed weight, set the load on tired days. If the usual weight feels like 0 RIR after a big ride, drop it.
6. Plan around key sessions. As TriForward's "Lifting and Endurance in the Same Week" covers, placing lifting after hard endurance sessions rather than before them helps protect quality.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 36-year-old triathlete lifts twice a week over the winter. Her programme says 3 sets of 6 squats. She has been picking a weight that leaves her failing on the last rep of every set, and her Thursday threshold run is always flat.

Applying reps in reserve:

• She reduces the load so each set of 6 finishes at about 2 RIR, judged honestly.
• She adds a fourth set on her Monday session, keeping total work similar.
• On weeks after a long weekend ride, she uses 3 RIR or drops a set.
• Every few weeks, she adds load when sets at the planned weight feel like 3 or 4 RIR.

Based on the evidence above, she should expect similar strength progress and less carry-over fatigue into her runs.

Common mistakes

• Equating hard with effective. Strength gains don't depend on reaching failure.
• Grinding out the last rep of heavy squats or deadlifts. It adds fatigue and technical risk for little extra strength.
• Ignoring what you did yesterday. A long ride or run changes how close to failure a given weight takes you.
• Overestimating your reserve. Most people underestimate it, but someone new to lifting may also misjudge it in either direction. Learn your scale on safe exercises.
• Never getting close. Stopping with five or six reps in reserve on every set may be too easy to drive strength. The research supports stopping short, not avoiding effort.
• Copying bodybuilding programmes. They're built around hypertrophy, where failure matters slightly more, not around the next day's swim, bike or run.

What we don't know

• Whether these findings apply to triathletes. No study has tested proximity to failure alongside a full swim, bike and run load.
• Women and masters athletes. Most of the research is in young men, many of them experienced lifters.
• The best RIR for endurance performance itself, as opposed to strength or size.
• Long-term effects. Most trials last 6 to 12 weeks. Whether the fatigue cost of failure accumulates over a winter isn't known.
• How RIR should change for different exercises, such as plyometrics or the Nordic curls covered in TriForward's "Nordic Hamstring Curls", where failure means something different.

References

• Grgic J, Schoenfeld BJ, Orazem J, Sabol F. Effects of resistance training performed to repetition failure or non-failure on muscular strength and hypertrophy: a systematic review and meta-analysis. Journal of Sport and Health Science, 2022;11(2):202–211. doi:10.1016/j.jshs.2021.01.007
• Robinson ZP, Pelland JC, Remmert JF, Refalo MC, Jukic I, Steele J, Zourdos MC. Exploring the dose–response relationship between estimated resistance training proximity to failure, strength gain, and muscle hypertrophy: a series of meta-regressions. Sports Medicine, 2024;54(9):2209–2231. doi:10.1007/s40279-024-02069-2
• Refalo MC, Helms ER, Trexler ET, Hamilton DL, Fyfe JJ. Influence of resistance training proximity-to-failure on skeletal muscle hypertrophy: a systematic review with meta-analysis. Sports Medicine, 2023;53(3):649–665. doi:10.1007/s40279-022-01784-y
• Morán-Navarro R, Pérez CE, Mora-Rodríguez R, de la Cruz-Sánchez E, González-Badillo JJ, Sánchez-Medina L, Pallarés JG. Time course of recovery following resistance training leading or not to failure. European Journal of Applied Physiology, 2017;117(12):2387–2399. doi:10.1007/s00421-017-3725-7
• Pareja-Blanco F, Rodríguez-Rosell D, Sánchez-Medina L, Sanchis-Moysi J, Dorado C, Mora-Custodio R, et al. Effects of velocity loss during resistance training on athletic performance, strength gains and muscle adaptations. Scandinavian Journal of Medicine and Science in Sports, 2017;27(7):724–735. doi:10.1111/sms.12678
• Halperin I, Malleron T, Har-Nir I, Androulakis-Korakakis P, Wolf M, Fisher J, Steele J. Accuracy in predicting repetitions to task failure in resistance exercise: a scoping review and exploratory meta-analysis. Sports Medicine, 2022;52(2):377–390. doi:10.1007/s40279-021-01559-x
• Zourdos MC, Klemp A, Dolan C, Quiles JM, Schau KA, Jo E, et al. Novel resistance training-specific rating of perceived exertion scale measuring repetitions in reserve. Journal of Strength and Conditioning Research, 2016;30(1):267–275. doi:10.1519/JSC.0000000000001049
• Doma K, Deakin GB, Bentley DJ. Implications of impaired endurance performance following single bouts of resistance training: an alternate concurrent training perspective. Sports Medicine, 2017;47(11):2187–2200. doi:10.1007/s40279-017-0758-3