Reps in Reserve and Muscle Hypertrophy: Implications for Training

Key Takeaways

Proximity to Failure Matters: Muscle hypertrophy improves as RIR decreases (effort increases), but stopping just short of failure (1–3 RIR) can produce similar growth compared to 0 RIR (failure) while reducing fatigue.
Volume and Load Influence RIR's Effect: Heavy loads (≥80% 1RM) can still produce hypertrophy with slightly higher RIR (e.g., 3–5 RIR), while lighter loads (<60% 1RM) require training closer to failure (0–2 RIR) to recruit high-threshold motor units.
Training to Failure Is Not Required: While going to 0 RIR (failure) can maximize muscle activation, it is not essential for hypertrophy if total volume is sufficient. Periodizing some failure training may benefit advanced lifters but isn’t necessary every session.
Fatigue vs. Stimulus Trade-off: Training at 0 RIR every session can lead to excessive fatigue and impaired recovery, potentially limiting training volume and frequency.
Beyond-Failure Techniques (Negative RIR): Methods like drop sets, rest-pause, and forced reps can extend hypertrophic stimulus but should be used sparingly due to high fatigue costs.
RIR Allows Autoregulation: Adjusting RIR based on daily readiness (e.g., higher RIR on fatigued days, lower RIR on peak days) can optimize training consistency and long-term progress.
Practical Application for Trainers: Most working sets should be programmed between 0–3 RIR, with lower RIR for isolation exercises and slightly higher RIR (1–3) for compound lifts to maintain good form and avoid injury.
Periodization with RIR: Effective training plans can progressively decrease RIR over time (starting at ~3–4 RIR and progressing to 0–1 RIR) before deloading, optimizing hypertrophy while managing recovery.
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The table below from Robinson ZP summarizes the changes in muscle size based on RIR.

Introduction

Repetitions in Reserve (RIR) is a concept used to quantify how close a set of resistance exercise is performed to muscle failure. It represents the number of additional repetitions one could perform before reaching failure, usually defined as the point of technical failure (inability to complete another rep with proper form). For example, if a lifter completes 10 reps of an exercise and could have done 2 more with good form, that set was performed at 2 RIR. RIR has gained popularity as a practical intensity measure and autoregulation tool in resistance training, allowing coaches and athletes to adjust training loads based on daily readiness. Dr. Eric Helms and colleagues formalized RIR-based rating of perceived exertion (RPE) scales, where an RPE 10 means 0 RIR (no reps left) and RPE 9 corresponds to ~1 RIR. This concept is highly relevant to muscle hypertrophy because training intensity (proximity to failure) influences muscle fiber recruitment and the hypertrophic stimulus. In this article, we explore the relationship between RIR and muscle hypertrophy, drawing on current research and expert insights from Dr. Mike Israetel, Dr. Eric Helms, and Zac P. Robinson. Key topics include how different RIR levels affect muscle growth, the merits of training to failure versus stopping short, the idea of “negative RIR” (training beyond failure), and practical applications for optimizing hypertrophy in resistance training programs.

RIR and Muscle Hypertrophy

Training proximity to failure is thought to impact hypertrophy through motor unit recruitment. According to the size principle, as one approaches failure in a set, more high-threshold motor units (including those innervating type II fibers) are recruited to maintain force. Thus, sets taken closer to failure (lower RIR values) are generally presumed to induce a greater hypertrophic stimulus. Recent evidence supports this idea: a meta-regression by Robinson et al. (2023) found that muscle hypertrophy tended to increase as sets were terminated closer to failure, evidenced by a negative relationship between RIR and hypertrophy gains. In other words, lower RIR (fewer reps left in reserve) was associated with greater increases in muscle size across studies. Importantly, this relationship was specific to hypertrophy; the same analysis showed strength gains were not meaningfully affected by RIR in that range. These findings align with the notion that maximizing muscle growth requires a high level of effort in each set.

However, the relationship between RIR and hypertrophy is not simply “failure or nothing.” Research indicates there is a range of effective RIR values. Many studies show that as long as sets are carried sufficiently close to failure (e.g. within ~4-5 reps of failure), robust hypertrophy can occur. For example, a recent study using 80% of 1RM (a relatively heavy load) reported that a group training with an estimated ~5–7 RIR achieved similar muscle growth as a group training to failure, provided the volume (total work) was matched by performing more sets in the higher-RIR group. This suggests that with heavy loads, stopping several reps short of failure can still maximize muscle fiber recruitment per set, especially when additional sets are done to compensate. Notably, the same study found that when using lighter loads, training far from failure was less effective – the group that stayed well shy of failure saw inferior hypertrophy results. This underscores that the required proximity to failure may depend on load: lighter loads generally need to be taken closer to failure to recruit the largest motor units, whereas heavy loads inherently recruit high-threshold fibers earlier, possibly allowing more leeway in RIR. As a whole, literature reviews have concluded that volume (number of hard sets) and proximity to failure are both important variables for hypertrophy biolayne.com.

Training with too high an RIR (stopping too early) might under-stimulate muscle growth, but training at 0 RIR for every set may not be necessary to maximize gains in most cases. The optimal approach appears to be reaching a low RIR (high effort) on most sets, while managing fatigue. In summary, current evidence indicates that hypertrophy improves as RIR decreases (i.e. effort increases), though gains can be similar between, say, 1–3 RIR and 0 RIR if volume and load are appropriate.

Training to Failure vs. RIR-Based Training

A key debate in program design is whether training to failure (0 RIR) is superior for hypertrophy compared to stopping shy of failure (leaving some RIR). Traditional bodybuilding lore often promotes pushing sets to failure to thoroughly exhaust muscle fibers. On the other hand, proponents of RIR-based training (including many strength scientists and coaches) argue that similar hypertrophy can be achieved without going to absolute failure on every set, which may improve recovery and allow more training volume. Scientific evidence on this topic shows nuanced results. A 2021 systematic review and meta-analysis by Grgic et al. compared training to momentary muscle failure versus non-failure training across 15 studies. Overall, they found no significant difference in muscle hypertrophy between training to failure and stopping short of failure when volume was equated. In other words, doing an extra rep or two to reach failure did not produce markedly greater muscle growth on average if the total work was matched. Notably, that meta-analysis did observe a small benefit to training to failure in trained lifters (a slight extra hypertrophy effect), suggesting that very experienced individuals might eke out a bit more growth by occasionally pushing to failure. This aligns with a subgroup analysis reported by other researchers, indicating a trivial but present hypertrophy advantage for failure training in resistance-trained subjects biolayne.com.

One reason training to failure doesn’t always show clear benefits is the cost of added fatigue. Reaching 0 RIR in a set induces high neuromuscular fatigue and muscle damage, which can lengthen recovery time before the muscle can be trained again optimally. Dr. Mike Israetel emphasizes the concept of stimulus-to-fatigue ratio: the last rep to failure might provide only a small additional stimulus for growth but a disproportionately high fatigue cost. Thus, stopping just short of failure (e.g. at 1–2 RIR) can yield most of the hypertrophic stimulus with less impairment to subsequent training. Indeed, coaches like Dr. Eric Helms often recommend leaving a rep in reserve on big compound lifts to maintain form and accumulate more total training volume over time. The trade-off between intensity and volume is critical. Training at 0 RIR on every set might force a lifter to reduce the number of quality sets or require longer rest and recovery, potentially limiting weekly volume. Conversely, maintaining a couple of reps in reserve can allow for additional sets or higher frequency training since each set is slightly less taxing. For example, a lifter might achieve more total reps or sets by terminating sets at 2 RIR instead of grinding to failure each time, ultimately equating or even increasing the total stimulus. The consensus emerging from the evidence is that training to failure is not required for hypertrophy in most cases. It can be used as a tool, particularly for advanced lifters or in lower-risk exercises, but a well-planned program that leverages RIR can maximize growth while better managing fatigue. Practically, this means a combination of approaches: use RIR-based training as the default, and strategically include some sets to failure where beneficial, rather than an all-or-nothing mentality.

It is also important to clarify the definition of “failure” in these contexts. Most research and programming recommendations distinguish technical failure (inability to perform another rep with proper form) from absolute failure (inability to perform another rep at all, even if form degrades). Training to technical failure (0 RIR by that standard) is typically the reference in studies. Good practice is to avoid pushing so far that form breaks down significantly, as that raises injury risk and may shift tension away from target muscles. Thus, an RIR of 0 generally implies the last rep was the final one with acceptable form. Keeping 1–2 RIR effectively provides a small buffer to ensure quality reps. Overall, RIR-based training allows precision in programming intensity: coaches can prescribe, for example, 3 sets of 8 reps at 2 RIR, meaning the load should be such that about 2 reps are left “in the tank” each set. This approach, backed by research, lets athletes train hard enough to stimulate hypertrophy while balancing recovery needs.

Negative RIR: Drop Sets and Beyond

While RIR ordinarily ranges from 0 (failure) upward, some advanced techniques push the boundaries of failure – effectively venturing into “negative RIR.” Negative RIR implies training beyond the point of initial failure by extending a set through methods like drop sets, forced reps, rest-pause, or partial repetitions. In a drop set, for instance, a lifter reaches failure at a given weight (0 RIR), then immediately reduces the load and continues to rep out, effectively performing additional reps past the initial failure point. These approaches create extreme metabolic stress and fatigue, aiming to recruit every possible muscle fiber by exhausting the muscle in extended fashion. Legendary bodybuilders like Dorian Yates championed beyond-failure methods (e.g. forced reps, where a spotter helps you complete a rep past failure) in pursuit of maximal hypertrophy. Dr. Mike Israetel has discussed that techniques such as lengthened partials – continuing to perform partial range reps when full reps can no longer be achieved – can be used to go “super beyond failure,” which he speculates may be “superior for hypertrophy” in certain cases. The idea is that these methods provide an even greater stimulus by training when the muscle is fully fatigued, potentially triggering additional growth in stubborn muscle fibers.

What does the science say about such beyond-failure training? Research on drop sets and similar intensity techniques generally shows they are effective for hypertrophy, but not necessarily magic. In fact, studies indicate that drop sets produce hypertrophy gains comparable to traditional sets, with their main advantage being efficiency. A recent meta-analysis comparing drop set training to traditional resistance training found that both methods led to significant increases in muscle size, with no clear superiority of one over the other in hypertrophy outcomes. In practical terms, a drop set allows a lifter to accumulate a high level of muscle fatigue and volume in a short time. For example, instead of doing three straight sets to failure with rest in between, a drop set might compress that work into one extended set by reducing weight as fatigue builds. The hypertrophic stimulus can be similar, but the drop set might induce more acute stress and require careful recovery. Another study on rest-pause training (a technique involving short intra-set rest to push past failure) likewise found comparable muscle growth to traditional training when volume was equated, but with greater time efficiency. Thus, negative RIR techniques are viable tools to maximize hypertrophy, especially when time is limited or to occasionally shock a plateauing muscle.

That said, training beyond failure should be used judiciously. The excessive fatigue and muscle damage from these methods can quickly accumulate, potentially leading to overtraining or injury if overused. Most experts, including Dr. Israetel, recommend reserving beyond-failure sets for advanced trainees who have a solid base and need an extra stimulus, or for smaller isolation exercises where the systemic stress is lower. Personal trainers should consider the risk-to-reward ratio: a set taken to absolute failure with forced reps might stimulate slightly more growth, but if it compromises the next 2–3 days of training due to soreness and fatigue, the net benefit could be lost. In summary, negative RIR methods like drop sets, rest-pause, and forced reps can enhance hypertrophy by pushing the muscle past its normal point of failure, but their role is supplementary. The foundation of hypertrophy training remains hard sets performed near failure (0–2 RIR) in a sustainable volume. Think of beyond-failure techniques as a potent spice – useful in small doses to flavor a program, but not the main course.

Practical Applications for Personal Trainers

Implementing RIR effectively in hypertrophy-focused programs can help optimize results for clients while managing fatigue. Here are practical guidelines for personal trainers based on the current literature and expert recommendations:

Educate and Calibrate RIR: Start by teaching clients what RIR means and how to rate their effort. Novice lifters may struggle to estimate reps in reserve accurately, often stopping too early or misjudging failure. Simple tests, like having a client perform reps until they feel they have 2 RIR and then seeing how many more reps they actually can do, can calibrate their perception. As Dr. Helms’ work suggests, using an RIR-based RPE chart (e.g. 8 RPE ≈ 2 RIR) can provide a reference for clients to gauge intensity. With experience, lifters become better at assessing their RIR mid-set, which is crucial for auto-regulating load on the fly.
Program within an Effective RIR Range: For muscle hypertrophy, program most working sets in a moderate to high effort range (approximately 0–3 RIR). This range ensures each set is challenging enough to recruit high-threshold fibers without necessarily reaching exhaustive failure every time. Multi-joint compound exercises (squats, presses, rows) often respond well to being kept around 1–3 RIR for the majority of sets. This avoids excessive form breakdown and central fatigue, while still providing a strong stimulus. Smaller isolation exercises (curls, triceps extensions, lateral raises) can be taken closer to 0 RIR more frequently, since the systemic fatigue and injury risk are lower; going to failure on the last set of an isolation movement is generally acceptable. A practical approach is to leave a couple reps in reserve on the first one or two sets of an exercise, then optionally push the final set to 0–1 RIR to ensure maximal fiber fatigue in that target muscle.
Balance Volume and Intensity: Use RIR to modulate training volume. If you prescribe slightly higher RIR (further from failure), you can likely increase the number of sets or training frequency because each set is less taxing. If training at very low RIR (0–1 RIR) on many sets, you may need to compensate with lower volume or more rest days. For example, rather than doing 3 all-out sets to failure, a trainer might program 4 sets at ~2 RIR. The total stimulation could be comparable or better, with improved quality across sets. Monitor the client’s recovery and performance; if they struggle to complete prescribed reps or show signs of excessive fatigue (poor sleep, persistent soreness, performance drops), it may indicate too many sets to failure and a need to pull back to higher RIR on some sets.
Periodize Intensity (RIR) Over Time: Incorporate RIR into periodization. Dr. Mike Israetel advocates an approach where a training cycle might start with higher RIR (e.g. 3–4 RIR in week 1 when volume is also high) and progressively increase intensity by reducing RIR week to week – ending with many sets at 0–1 RIR in the final week before a deload. This progression allows the athlete to accumulate volume and momentum, then achieve an overload stimulus by pushing close to failure toward the end of a cycle, followed by recovery. Similarly, trainers can cycle periods of training farther from failure (to focus on technique, volume, or recovery) with periods of pushing near failure (to spur growth and test limits). Such RIR undulation helps manage fatigue and can keep clients mentally fresh, since training to failure week after week can be draining.
Use Beyond-Failure Techniques Sparingly: When appropriate, integrate advanced techniques like drop sets or rest-pause sets, but do so sparingly and intentionally. For a client who has plateaued or for finishing a muscle group’s workout, you might include one drop set at the end of the last exercise. Ensure the client understands proper form and maintains safety even as they fatigue. Clearly communicate that these techniques are high intensity and require extra recovery. Track the client’s response; if a beyond-failure finisher leaves them overly sore or impacts the next workouts, reserve such methods for occasional “shock” microcycles rather than regular use. Always prioritize consistency and long-term adherence over short bursts of extreme training.

In practice, RIR-based programming means tailoring the difficulty of each set to the individual on that day. Encourage clients to tune into their exertion: for instance, if a weight feels lighter than expected and they have more reps in reserve than prescribed, they can do a couple extra reps or increase the load slightly to hit the target RIR. Conversely, if they are unusually fatigued and hit the target RIR sooner, they should stop the set to avoid unplanned failure. This autoregulatory aspect of RIR is a powerful tool to optimize training stimulus in fluctuating day-to-day conditions. For personal trainers, using RIR in programming fosters a collaborative approach with clients – it demands communication and honesty about effort – but ultimately it leads to more individualized and effective hypertrophy training programs.

Conclusion

Repetitions in Reserve is a valuable framework for understanding and prescribing training intensity in resistance training, especially for hypertrophy. RIR operationalizes the age-old question of “how hard should I push this set?” in a systematic way. Current scientific evidence indicates that muscle hypertrophy is maximized when training is performed with high effort and low RIR, i.e. close to, but not necessarily at, failure on most sets. Training to absolute failure every time is not required for muscle growth – and may even be counterproductive if it compromises volume or recovery – yet occasionally reaching failure can provide a slight edge for advanced lifters or specific exercises. The use of RIR allows athletes to get the benefits of near-maximal effort while intelligently managing fatigue. Techniques that involve “negative RIR,” such as drop sets or forced reps, can further extend the hypertrophic stimulus beyond normal failure, but these should be employed cautiously and strategically. For academic-minded personal trainers, the take-home message is to program smart: aim for an optimal stimulus-to-fatigue balance. This means pushing clients hard enough (usually within about 0–3 RIR) to activate growth, but not so hard that they cannot adequately recover and continue training progressively. By applying RIR-based training principles—grounded in research from experts like Israetel, Helms, and Robinson—trainers can design hypertrophy programs that are both scientifically sound and tailored to the individual. The evidence-based approach of modulating reps in reserve will help maximize muscle growth outcomes while ensuring training remains sustainable and effective in the long run.

References:

Robinson ZP, et al. Sports Medicine (2023) – Meta-regression on proximity to failure (RIR) showing hypertrophy increases as sets near failure.
Grgic J, et al. Journal of Sport Health Science (2022) – Systematic review/meta-analysis finding no significant hypertrophy difference between failure vs non-failure training (volume equated), except a small benefit of failure in trained lifters.
Helms ER, et al. (2016) – Introduction of RIR-based RPE scale for resistance training; RPE 10 = 0 RIR, 9 = 1 RIR, etc..
Data-Driven Strength (Robinson & colleagues, 2023) – Analysis suggesting heavy-load training can achieve hypertrophy with higher RIR if volume is increased, whereas light loads require training closer to failure.
NASM – Guidelines for applying RIR in training phases; recommends ~2–4 RIR for multi-joint lifts and using 0 RIR primarily on final sets of isolation exercises.
Israetel M. (2024) – Discussion on beyond-failure techniques (lengthened partials) as advanced hypertrophy tools; highlights difference between technical vs absolute failure.
Drop sets Meta-Analysis (2021) – Found drop sets and traditional sets yield similar hypertrophy gains, indicating beyond-failure methods are effective but not strictly superior when total work is matched.

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