RPE vs Percentage of 1RM: Strength and Muscle Growth

Introduction

Prescribing resistance training load traditionally relies on fixed percentages of an individual's one-repetition maximum (%1RM). While %1RM provides an objective baseline, it assumes consistent daily physical readiness and uniform fatigue responses across individuals. In contrast, subjective autoregulation strategies—primarily rating of perceived exertion (RPE) based on repetitions in reserve (RIR)—adjust daily loads according to acute readiness and perceived proximity to failure [4, 20].

Understanding whether subjective load autoregulation outperforms or matches traditional %1RM loading is essential for optimizing maximal strength development and skeletal muscle hypertrophy.

Chronic Adaptations: Strength and Hypertrophy

Direct meta-analytic evidence indicates that autoregulated load prescription and standardized percentage-based training result in similar adaptations in maximal strength [1]. In a systematic review and meta-analysis of 15 studies (6 examining load autoregulation), no statistically significant difference in 1RM strength adaptations was observed between autoregulated load prescription (via RIR-based RPE or velocity-based training) and fixed percentage-based loading (MD = 2.07 kg, 95% CI -0.32 to 4.46 kg, p = 0.09, SMD = 0.21) [1].

These findings align with chronic randomized trials. In an 8-week daily undulating resistance training trial in 21 resistance-trained men, Helms et al. compared RIR-based RPE load assignment against fixed %1RM assignment [2, 10]. Squat 1RM increased by +17.05 ± 5.44 kg in the RPE group versus +13.91 ± 5.89 kg in the %1RM group, while bench press 1RM increased by +10.70 ± 3.30 kg versus +9.64 ± 5.36 kg [2, 10]. Magnitude-based inference analyses indicated a 79% and 57% probability of a small effect size advantage favoring RPE for the squat (ES = 0.50 ± 0.63) and bench press (ES = 0.28 ± 0.73), respectively, though null-hypothesis significance testing revealed no statistically significant between-group differences (p > 0.05) [2, 10].

Hypertrophic adaptations show equivalent parity between loading strategies [2, 10]. In the same 8-week trial, ultrasound-measured muscle thickness showed no significant differences between %1RM and RPE conditions for the pectoralis major (+1.59 ± 1.33 mm vs. +1.90 ± 1.91 mm), vastus lateralis at 50% femur length (+2.13 ± 1.95 mm vs. +1.85 ± 1.97 mm), or vastus lateralis at 70% femur length (+2.40 ± 2.22 mm vs. +2.31 ± 2.27 mm) [10]. When total volume load and proximity to failure are matched over time, both methods elicit comparable muscle cross-sectional area (CSA) and thickness gains [2, 10].

Physiological Rationale and Limitations of Fixed %1RM

Fixed %1RM prescriptions assume that a given percentage of 1RM elicits an identical proximity to failure across all trainees. However, the number of repetitions completed to failure at a specific percentage of 1RM exhibits substantial inter-individual variability [7, 17]. For example, when performing back squats at 70% 1RM, repetitions to failure vary between 6 and 28 repetitions across trained individuals [7, 17]. Similarly, on the leg press at 70% 1RM, trained weightlifters averaged 20 repetitions compared to 40 repetitions performed by endurance runners [17]. Consequently, assigning a fixed rep scheme (e.g., sets of 8 at 70% 1RM) leaves some lifters far from failure while pushing others to task failure [7, 17].

Furthermore, day-to-day neuromuscular performance fluctuates by up to ±18% around a previously established 1RM (a total variance of up to 36%) due to factors such as sleep quality, life stress, and accumulated fatigue [4, 20]. Fixed percentage loading fails to adjust for these daily shifts, potentially leading to under-stimulation on high-readiness days or excessive fatigue and technique breakdown on low-readiness days [4, 20]. In fatiguing sessions, autoregulated loading protocols allow participants to complete significantly more repetitions than static %1RM prescriptions by adjusting the load to maintain the intended physiological stimulus [21].

Precision and Validity of RIR-Based Scales

The contemporary 10-point resistance training RPE scale directly incorporates RIR: an RPE of 10 indicates maximal effort (0 RIR), RPE 9 corresponds to 1 RIR, and RPE 8 corresponds to approximately 2 RIR [4, 9]. Three primary RIR scales have been evaluated in the literature: Tuchscherer's 7-point scale with top-end RIR anchors, Hackett et al.'s Estimated Repetitions to Failure (ERF) scale, and Zourdos et al.'s 10-point RPE-RIR scale [4].

Subjective RIR ratings demonstrate strong construct validity against objective movement velocity [10, 14, 16]. RIR-based RPE scores correlate inversely with barbell velocity during resistance exercise, with correlation coefficients of r = -0.87 for the back squat and r = -0.79 for the bench press [10]. Mean propulsive bar velocity correlates with perceived RIR (average r² = 0.3 across squats and bench presses) across thousands of repetitions, demonstrating that subjective perception reflects objective movement deceleration [18].

The practical utility of RPE/RIR depends on estimation accuracy [4, 17]:

• Proximity to Failure: Lifters underestimate RIR by an average of approximately ~1 repetition [14, 17]. Estimation accuracy improves significantly when sets terminate within 0 to 3 RIR, whereas precision degrades when sets are stopped >4–5 repetitions from failure [12, 17].
• Repetition Ranges: Estimation precision is higher during lower-repetition sets (≤12 reps) compared to higher-repetition protocols [14, 17].
• Experience and Demographics: While experienced lifters rate RPE at 1RM more accurately than novices (9.80 ± 0.18 vs. 8.96 ± 0.43) and show stronger velocity-RPE correlations (r = -0.88 vs. r = -0.77) [16], broader syntheses indicate that neither training status nor biological sex systematically impairs RIR prediction accuracy during standard sets [14, 15, 18].

Velocity Loss, Proximity to Failure, and Volume Autoregulation

Proximity to failure (RPE/RIR) and intra-set fatigue (percentage velocity loss, %VL) are distinct variables dictated by repetition target and load [19]. For instance, reaching an 8 RPE (a last repetition velocity of ~0.20 m·s⁻¹) results in a 51% velocity loss across 8 repetitions, but only a 23% velocity loss across 3 repetitions at ~87.5% 1RM [19].

In volume autoregulation research, velocity loss thresholds illustrate the differing physiological requirements for strength versus hypertrophy [1, 18]:

• Strength Adaptations: Velocity loss thresholds ≤ 25% produce significantly greater 1RM strength gains compared to thresholds > 25% (MD = 2.32 kg, 95% CI 0.33 to 4.31 kg, p = 0.02, SMD = 0.23) [1]. Minimizing velocity loss limits excessive neuromuscular fatigue and maintains movement velocity, which supports maximal strength and power [1, 5, 18].
• Hypertrophy Adaptations: Velocity loss thresholds > 25% yield significantly greater muscle CSA hypertrophy than thresholds ≤ 20% (MD = 0.64 cm², 95% CI 0.07 to 1.20 cm², p = 0.03, SMD = 0.34) and thresholds ≤ 25% (MD = 0.61 cm², 95% CI 0.05 to 1.16 cm², p = 0.03, SMD = 0.28) [1]. However, thresholds > 25% show no significant advantage over moderate thresholds of 20–25% (MD = 0.36 cm², 95% CI -0.29 to 1.00 cm², p = 0.28, SMD = 0.13) [1]. Training to true muscular failure is not required for optimal hypertrophy, as moderate-to-high proximity to failure (1–3 RIR) provides sufficient motor unit recruitment without disproportionate fatigue [5, 9].

Practical Application Guidelines

1. Load Assignment: When targeting strength or hypertrophy, RIR-based RPE provides a practical, low-cost alternative to %1RM that accommodates fluctuations in acute readiness [4, 22]. Target loads between 1 and 3 RIR (RPE 7–9) to maximize stimulus while avoiding excessive fatigue associated with repeated failure [5, 9].
2. Constraint of Repetition Ranges: To maintain RIR accuracy, apply subjective RPE primarily to compound movements performed in rep ranges ≤12 repetitions, where estimation error remains within ~1 repetition [14, 17].
3. Hybrid Programming: Practitioners can combine both systems by prescribing a target percentage of 1RM as a baseline alongside an RPE target (e.g., 75% 1RM for 5 reps @ RPE 7–8), allowing minor load adjustments on the day based on warm-up performance and subjective readiness [4, 20].