sports Review Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum Brad J. Schoenfeld 1,* , Jozo Grgic 2, Derrick W. Van Every 1 and Daniel L. Plotkin 1 1 Department of Health Sciences, CUNY Lehman College, Bronx, NY 10468, USA; [email protected] (D.W.V.E.); [email protected] (D.L.P.) 2 Institute for Health and Sport, Victoria University, Melbourne, VIC 8001, Australia; [email protected] * Correspondence: [email protected] Abstract: Loading recommendations for resistance training are typically prescribed along what has come to be known as the “repetition continuum”, which proposes that the number of repetitions performed at a given magnitude of load will result in specific adaptations. Specifically, the theory postulates that heavy load training optimizes increases maximal strength, moderate load training optimizes increases muscle hypertrophy, and low-load training optimizes increases local muscular endurance. However, despite the widespread acceptance of this theory, current research fails to support some of its underlying presumptions. Based on the emerging evidence, we propose a new paradigm whereby muscular adaptations can be obtained, and in some cases optimized, across a wide spectrum of loading zones. The nuances and implications of this paradigm are discussed herein. Keywords: high-load; low-load; strength; hypertrophy; muscular endurance Citation: Schoenfeld, B.J.; Grgic, J.; Van Every, D.W.; Plotkin, D.L. Loading Recommendations for Muscle Strength, Hypertrophy, and 1. Introduction Local Endurance: A Re-Examination of the Repetition Continuum. Sports Resistance training (RT) is well-established as an effective interventional strategy to 2021, 9, 32. https://doi.org/10.3390/ enhance muscular adaptations. These adaptations include, but are not limited to, increases sports9020032 in muscle strength, size, and local muscular endurance. Evidence indicates that optimizing these adaptations requires manipulation of RT variables [1,2]. The magnitude of load, Academic Editor: Silvio Lorenzetti or amount of weight lifted in a set, is widely considered one of the most important of these variables. Evidence indicates that alterations in training load can influence the acute Received: 26 January 2021 metabolic, hormonal, neural, and cardiovascular responses to training [1]. How these acute Accepted: 15 February 2021 responses translate into long-term adaptations remains somewhat contentious. Published: 22 February 2021 Loading recommendations are typically prescribed along what has come to be known as the “repetition continuum,” also known as the “strength-endurance continuum” [3] (see Publisher’s Note: MDPI stays neutral Figure1). The repetition continuum proposes that the number of repetitions performed at with regard to jurisdictional claims in a given magnitude of load will result in specific adaptations as follows: published maps and institutional affil- 1. A low repetition scheme with heavy loads (from 1 to 5 repetitions per set with 80% to iations. 100% of 1-repetition maximum (1RM)) optimizes strength increases. 2. A moderate repetition scheme with moderate loads (from 8 to 12 repetitions per set with 60% to 80% of 1RM) optimizes hypertrophic gains. 3. A high repetition scheme with light loads (15+ repetitions per set with loads below Copyright: © 2021 by the authors. 60% of 1RM) optimizes local muscular endurance improvements. Licensee MDPI, Basel, Switzerland. This article is an open access article Support for the repetition continuum is derived from the seminal work of DeLorme [4], distributed under the terms and who proposed that high-load resistance exercise enhances muscle strength/power while conditions of the Creative Commons low-resistance exercise improves muscular endurance, and that these loading zones are Attribution (CC BY) license (https:// incapable of eliciting adaptations achieved by the other. Subsequent research by Anderson creativecommons.org/licenses/by/ and Kearney from 1982 [5] and Stone et al., 1994 [6] provided, in part, additional support 4.0/). to Delorme’s hypothesis, forming the basis of what is now commonly accepted as theory. Sports 2021, 9, 32. https://doi.org/10.3390/sports9020032 https://www.mdpi.com/journal/sports Sports 2021, 9, 32 2 of 25 Sports 2021, 9, x However, emerging research challenges various aspects of the theory. The purpose of this 2 of 26 paper is to critically scrutinize the research on the repetition continuum, highlight gaps in the current literature, and draw practical conclusions for exercise prescription. Based on the evidence, we propose a new paradigm whereby muscular adaptations can be obtained, and in some cases optimized, across a wide spectrum of loading zones. The nuances and implications of this paradigm are discussed herein. Figure 1. Schematic of the repetition continuum proposing that muscular adaptations are obtained in a load-specific manner. Repetition maximum (RM). Figure 1. Schematic of the repetition continuum proposing that muscular adaptations are obtained 2. Strength in a load-specific manner. Repetition maximum (RM). Strength can be broadly defined as the ability to produce maximum force against an external resistance [7]. The leftward aspect of the repetition continuum has been referred toSupport as the “strength for the zone”repetition (see Figure continuum1), indicating is derived optimum from gains the in thisseminal parameter work are of DeLorme [4], whoattained proposed by the performance that high of-load 1 to 5 repetitionsresistance per exercise set. It is theorizedenhances that muscle training strength/power in the while“strength low-resistance zone” enhances exercise neuromuscular improves adaptations muscularthat endurance, facilitate force and production that these [3 loading]. In zones are incapablesupport of thisof eliciting theory, Jenkins adaptations et al. [8] demonstrated achieved by greater the increasesother. Subsequent in percent voluntary research by An- muscle activation and electromyographic amplitude when performing leg extension RT to dersonfailure and with Kearney 80% 1RM from compared 1982 [5] to 30%and 1RM Stone over et a al., 6-week 1994 study [6] provided, period. Psychological in part, additional supportfactors to areDelorme’s believed to hypothesis, be involved asforming well, as repeatedthe basis heavy of what load lifting is now may commonly help lifters accepted as theory.acclimate However, to exerting emerging a maximal research effort; however, challenges the psychological various aspects contribution of the to strength- theory. The pur- poserelated of this adaptations paper is to remains critically equivocal scrutinize [9]. the research on the repetition continuum, high- light gapsStrength in the is current most commonly literature, assessed and via draw 1RM testingpractical that conclusions involves the performance for exercise of prescrip- dynamic constant external resistance exercise using either free weights or exercise machines. tion.Meta-analytic Based on the data evidence, of this metric we propose shows a clear a new advantage paradigm to using whereby heavier muscular compared toadaptations can belighter obtained, loads when and thein some number cases of sets optimized, are similar across between a conditions.wide spectrum For example, of loading a zones. The nuanrecentces meta-analysis and implications [10] reported of athis moderate paradigm to large are effect discussed size (ES) differenceherein. (ES = 0.58) favoring high- (>60% 1RM) vs. low- (≤60% 1RM) load training based on pooled data from 14 included studies. Results held true independent of whether testing was conducted in 2. Strength exercises for the upper or lower body. A meta-analysis by Csapo et al. [11] reported similar resultsStrength in older can individuals,be broadly with defined an overall as the pooled ability effect to sizeproduce difference maximum (ES = 0.43) force that against an externalindicated resistance a moderate [7]. The magnitude leftward of effect aspect in favorof the of repetition heavy load continuum training. Importantly, has been referred to asall the included “strength studies zone” showed (see a Figure strength-related 1), indicating advantage optimum to using gains high- comparedin this parameter to are low loads (i.e., effect sizes from all studies resided in the “favors high-load” side of the attainedforest by plot). the performance of 1 to 5 repetitions per set. It is theorized that training in the “strengthThe zone” strength-related enhances benefitsneuromuscular of heavier adaptations loads are generally that facilitate observed independentforce production [3]. In supportof RT volume, of this whethertheory, expressedJenkins et as al. the [8] number demonstrated of sets performed greater or increases the total workin percent vol- untaryperformed, muscle commonlyactivation termed and electromyographic “volume load” (sets ×amplituderepetitions when× load). performing This is an leg exten- important point of note as heavier load training necessarily results in fewer repetitions sion performedRT to failure on a set-equatedwith 80% basis1RM compared compared to light to 30% loads. 1RM Thus, over it can a be 6- inferredweek study that load period. Psy- chologicalis the dominant factors variableare believed for increasing to be involved 1RM, with as other well, variables as repeated seemingly heavy of secondary load lifting may help consequencelifters acclimate [12]. to exerting a maximal effort; however, the