RESULTS

DISCUSSION

Resistance training is a central component of exercise programs. It should be a core component of physical fitness programming, as it has broad-reaching benefits for muscular health and physical function. This overview of reviews summarized 137 systematic reviews to determine the impact of RTx variables on muscle function and hypertrophy in healthy adults. Compared with no exercise, RT improves muscle strength, hypertrophy, power, endurance, contraction velocity, and performance on several physical function tests (balance, gait speed, chair stand, and timed up-and-go; Table 3). Additionally, other forms of RT improve multiple outcomes compared with CTRL, including home-based RT, elastic band RT, power RT (which involves performing the concentric phase quickly), velocity-based RT, and circuit RT (Table 3). Based on these results, it is apparent that many forms of RT work to promote the primary hallmark outcomes of RT: increased strength, hypertrophy, and power. Our primary recommendation is that healthy adults perform RT with high effort (effort can be measured through various scales, but can be achieved with various loads and sets per the FITT-VP principle) at least twice weekly, with all major muscle groups being engaged.

Between distinct RT programs, only some RTx variables can be altered to optimize increases in muscle function and hypertrophy (Tables 4 and 6). Significant and optimal improvements are two different goals; however, participants and facilitators of RT must distinguish between them. Compared with CTRL, significant improvements in muscle function, hypertrophy, and physical performance can be accomplished with the adoption of many RT programs. This overview of reviews enhances the quality of recommendations for healthy adults to significantly increase muscle function, hypertrophy, and physical performance by providing current, comprehensive, and evidence-based insights into the impact of RTx variables.

Progressive overload is a concept considered fundamental to RT programming principles. Progressive overload refers to the need to increase the stimulus (stressor) placed upon the muscle throughout a training program. Progressive overload is often proposed as essential for continued adaptive progress with any form of exercise training as physiological systems adapt to reduce the stress of the exercise and build an increasing adaptive response (i.e., strength). Such adaptation would require increasing the stimulus, such as load, volume, training frequency, exercise selection, or duration (even if only slightly), as the muscle continues to adapt to produce further adaptations (164). Modifying an RTx (e.g., increasing relative load [percentage of one-repetition maximum, %1RM]) can support progressive overload. Still, the same relative load can also be sustained when regular strength testing is performed or perceived exertion scales are used to increase absolute load commensurate with strength gains (113,165). Notably, as the results in Table 4 highlight, progression is not necessary to achieve beneficial outcomes, and overload, or more accurately, increasing the stimulus in some manner, is likely a requirement only for those seeking continued longer term progress. We note that in some populations, due to inexperience (and potential safety concerns), RTx necessitates that loads are necessarily low and progression is a requirement to achieve a meaningful benefit. Continued progression could be a personal decision and part of the individualization of RTx (see below). A similar commentary could be made around the “need” for variation of RTx variables (see below).

The principle of specificity states that training adaptations are specific to the training stimulus applied. Some adaptations can be affected by modifying RTx variables, although there is a considerable carryover of training effects on general muscular performance in various domains in nonadvanced trainees (17,24). Individualization involves modifying RT programs to meet the unique goals, needs, and characteristics of each individual, such as their experience and performance level. Individualized programs can increase exercise adoption and adherence (10), but individualization has been scarcely discussed in previous Position Stands (11,12). With the accumulation of evidence demonstrating that many forms of RT are effective for healthy adults to improve muscle function and health, RT programs would ideally be individualized to maximize adherence, enjoyment, safety, and effectiveness specific to training goals.

Variation is the systematic modification of RTx variables over time to facilitate continued adaptation. One of the most common forms of variation is periodization—intentionally modifying prescription variables (e.g., load, volume, frequency) throughout an RT program. The impact of periodization on strength could not be determined, although one review found that periodized programs were slightly favored over nonperiodized programs to maximize strength gains under volume-equated conditions (117). With appropriate progressive overload, periodization is not significantly superior to nonperiodized programs; thus, periodization is less important than previously hypothesized (11,12) for healthy adults to improve muscle function and hypertrophy. We note, however, that definitions of periodization vary (166,167), but broadly involve the systematic manipulation of RTx variables with the goal of optimizing performance adaptations, managing fatigue, preventing overtraining/injury, and peaking for specific goals. Thus, it may be that the ordering of variables within a given RTx to achieve certain goals is understudied compared with the outcomes contained in the reviews we examined. We also note that periodization and programming are often conflated (168), which may have been the case in some reviews.

Resistance training improves numerous measures of muscle function and can result in hypertrophy compared with no exercise (Fig. 2). Provided the strikingly low RT participation levels (169–173), individuals and practitioners alike should acknowledge the tremendous benefits of completing RT of various forms compared with no exercise. Additionally, nontraditional forms of RT also yield marked benefits—for example, elastic band RT has been shown to increase strength, hypertrophy, and certain components of physical function. Similarly, home-based RT improved strength, muscular endurance, and balance. Nontraditional forms of RT may provide alternative, perhaps more accessible or approachable, strategies for completing RT with appreciable physical benefits.

We build on several aspects of the 2009 ACSM Position Stand (11), which aimed to provide evidence-based RTx guidelines for healthy adults wishing to progress beyond the first 3–4 months of training. Previous guidelines have recommended healthy adults use free weights and machines to complete two to three RT sessions per week, with eight to ten exercises involving major muscle groups per session, one to four sets per exercise, eight to twenty repetitions per set, 2–3 min rest between sets, loads 40%–70% 1RM, and follow the principles of progressive overload, specificity, and variation (10,11). Current estimates (from self-report) are that only ~30% of American adults complete some muscle-strengthening activities at least 2 d/wk, and nearly 60% complete no muscle-strengthening exercise (169–173). Estimates for older persons vary; however, data from the UK, United States, and Australia put participation rates from as low as 1% to as high as 40% (174). We suspect that rates of participation among older persons for the types of RT programs we outline here are likely 10%–15% (175). Notably, others have estimated that developing general muscular fitness (strength, hypertrophy, and power) according to previous ACSM guidelines may, in some cases, require training for 20 h or more per week (176). Thus, the recommendations of the previous Position Stand (10,11) may be less relevant for most adults, particularly older adults, many of whom do not engage in RT. The specific program details outlined in previous Position Stands may be an appropriate starting point for some individuals; however, several other RT programs, as evidenced here, can also be effective.

Deviating from previous guidelines, we propose that individualizing programs to increase RT participation is, from our perspective, more important than conforming to specific RTx criteria outlined in previous Position Stands. Research on RTx reveals that “minimal doses” of RT are able to bring about substantial strength, hypertrophy, and physical functional gains (177). We propose that our stance around encouraging participation in RT is underscored by the expanding knowledge of health benefits, some of which are unique to RT (4–6). The recommendations presented here apply to healthy adults, ranging from complete novices (with no experience with RT) to experienced trainees, and support the adoption of a much broader range of RT programs to improve overall muscular strength, function, and hypertrophy. Advances enable a more comprehensive understanding of how RTx variable manipulation affects outcomes. For example, the previous Position Stand recommended novice lifters complete two to three full-body sessions per week for muscle hypertrophy, and this recommendation was graded as the highest possible quality of evidence (11). However, this recommendation was informed by only three studies (178–180) (n = 59 total participants), and two of the studies did not investigate frequency (178,179). In contrast, we present guidelines suitable for many healthy adults, supported by ample primary evidence, that were formulated with leading evidence synthesis methodologies.

Resistance training is safe for healthy adults of all ages (5), but many adults, particularly older adults, avoid RT due to misperceptions about safety and injury risks (181). In an analysis of >38,000 participants (>6700 RT participants), of which >11,000 were older adults, exercise did not increase the risk of serious adverse events. The risk of nonserious adverse events (e.g., pain, fatigue, bursitis, and edema) was not different than aerobic exercise in terms of injury rate or risk (182). Nonfatal cardiovascular complications also occur much less frequently during RT than aerobic training (5,183). In 23 studies (n = 1174) of adults with coronary heart disease, all 63 nonfatal cardiovascular complications occurred during aerobic training, and the 20 musculoskeletal complications that occurred during RT were caused by preexisting conditions (knee arthritis) and were resolved by changing RT intensity or body position (183). Clearly, RT can be safe and effective for healthy adults of all ages.

This overview of reviews summarized RTx variable-level evidence, an approach that prevents statistical insights into the comparative effectiveness (e.g., dose-response) of individual RT variables. Nuanced details of RT protocols and dose-response data require discussion for practitioners designing RT programs to enhance adaptations, since some features of RTx are not reflected in variable-level summaries. For example, RT frequency was found to impact strength; however, there was insufficient evidence to conclude a dose-response relationship, and the impact is diminished when volume is equated (73,76,90).

Load (often used synonymously with intensity) has repeatedly been shown to impact strength more than other RT adaptations (17,83,184). Load has typically been quantified as a %1RM or repetition maximum. Several methods have been used to convert loads between %1RM and repetition maximum (17,184,185), including a recent analysis that considered exercise-specific conversions and between-subject variability (186). The number of repetitions performed within a given set is inherently and inversely related to load, provided a given load and near-fatiguing effort are applied; however, not all adaptations are enhanced by fatiguing efforts (e.g., power and strength). Practitioners should consider the principle of specificity when designing RT programs to achieve specific outcomes.

Increasing the number of sets (volume) per exercise positively impacted strength (88–93) and hypertrophy (24,128,131–133), which aligns with a recent analysis by Swinton et al. (184). Clearly, one set is superior to zero sets (CTRL), and two sets are superior to one set (17,24,88–93,128,131–133), but the exact number of sets required to optimize adaptations cannot be ascertained. A meta-regression (187) of the effect of volume (weekly) sets showed progression, but as expected, a dose-response that plateaued and showed diminishing returns beyond ~2–3 sets/exercise for strength and ~18–20 weekly sets for hypertrophy with varying loads; thus, healthy adults are advised to complete at least two sets per exercise. Completing more than two sets per exercise may provide additional benefits, but we hypothesize that these benefits diminish with each subsequent set. Weekly volume load—the total amount of load lifted in each week of training, defined by load/rep • reps/set • sets/exercise • exercises/session • sessions/week—which would presumably represent the complete representation of “dose” of RT—was rarely considered in included reviews.

Completing sets to fatigue (momentary muscular failure) does not enhance gains in strength, hypertrophy, and power, and so is not necessary for benefits to occur. It may also be that lifting to fatigue is inadvisable for certain populations (e.g., older individuals) due to risks to vascular health and an increased risk of injury resulting from poor form (24,97–99). We propose that an adequate stimulus (the effort to meet the minimum stressor) is required to induce adaptations. Sufficient effort (assessed using various scales) can be accomplished by completing sets with various RTx and completion of “near-failure” or a target of 2–3 repetitions in reserve (RIR) (24,97–99,128,137,188–190).

Blood flow restriction protocols often compare different loads (e.g., higher load RT versus BFR with lower load), so the individual effect of BFR cannot be distinguished from the load in many cases. Similarly, flywheel RT typically involves eccentric overload, so the individual effect of flywheel RT and contraction type cannot be distinguished. Evidence-based conclusions on these matters cannot be determined with the available data, but consideration is warranted when designing RT programs and conducting future research. Notably, these considerations can be largely overlooked when designing RT programs for most adults who are inexperienced with RT. Untrained individuals will benefit from various RT programs, provided that progression, the variables outlined here as affecting outcomes, and adherence are core principles.

Limitations should be considered when interpreting the results presented in this analysis. Overviews of reviews provide strong (larger or consistent effects) pooled, group-level evidence, so that appropriate deviations from the reported recommendations will be required on various occasions, in accordance with the principle of individualization. We acknowledge that any RT program would need to be tailored to an individual’s needs and goals. Not discussed here, but of paramount importance, is the understanding and acknowledgment of the critical role adherence plays in any RT program. Such considerations are beyond the scope of this review, but work has been done to examine this important area (191,192).

The totality of evidence in overviews of reviews is limited to interventions and outcomes summarized by systematic reviews. For example, the Short Physical Performance Battery (SPPB) is a common assessment used in randomized trials; however, only two eligible reviews have synthesized the results of the SPPB. When evaluating the results of this review, it would lead one to conclude that RT does not improve SPPB performance; however, this conclusion likely reflects the sparseness of evidence, rather than a true null effect. We note that there was ample evidence for individual components of the SPPB—gait speed, balance, and chair stand tests—and RT improved all these components. Overviews of reviews are also limited by overlapping evidence. While we estimated this overlap using the CCA method for strength, we did not do so for other variables. We acknowledge that caution is needed when examining other outcomes for which we provide no CCA analysis. Individual studies included in multiple eligible reviews could inflate the evidence supporting or refuting conclusions; for example, three reviews were included on the impact of RT time of day on hypertrophy (24,102,131), but two reviews (24,131) synthesized one original review (102). Overlapping data are an issue in overviews of reviews, and methods are being developed to improve exercise-related overviews of reviews (193,194). Overviews of reviews do not permit inferences about the comparative effectiveness of different interventions (14). This review summarizes the individual impact of RTx variables; however, alternative data synthesis methods are required to formulate evidence-based conclusions on the relative effects of multiple RTx. For example, network meta-analysis can be leveraged to statistically compare and rank the efficacy of unique RT programs (17). Reviews included in this overview of reviews were limited to healthy adults. Provided sufficient evidence, future guidelines can be developed for additional subpopulations (e.g., older adults and clinical populations). There was insufficient evidence to form recommendations on emerging RTx strategies; for example, various minimal-dose RT programs warrant further investigation and consideration when individualizing RT programs (195). The method of testing muscular endurance (e.g., absolute vs relative, pre- vs postintervention strength) was not accounted for in the data synthesis and should be considered when designing future studies (196).

Methods to quantify load based on proximity to muscular failure (e.g., RIR, perceived exertion) are appealing because they could translate to various RT forms (e.g., elastic band or body weight RT). While training to failure is not obligatory for optimizing results, there is insufficient evidence to quantify exact RIR and perceived exertion targets (189,190,197,198). The available evidence did not compare distinct body regions or muscle groups. In the view of the authors, applying the recommendations herein to the body regions “upper” and “lower” for “push” and “pull” exercises (i.e., four body regions) is sufficient to target major muscle groups. The movement directions “horizontal” and “vertical” may also be considered for upper body exercises (i.e., six body regions). The regions and movements we recommend should align with the FITT-VP principle in terms of RTx, which could be a per-training-session requirement or a per-week requirement, depending on individual goals, training status, age, and other relevant variables.

We also need to acknowledge that randomized trials in sports and exercise medicine-related fields are often poor quality with small sample sizes, poor randomization methodology, lack of preregistration, poor reporting of adverse events, and other shortcomings (199), and much remains to be done in terms of the conduct of evidence syntheses (200,201). Guidelines have been developed to improve the reporting of exercise trials, and we would encourage their adoption (202). Consequently, cohesive QoE evaluations, such as pooling Grading of Recommendations Assessment, Development and Evaluation assessments (14), were not possible. Field-specific guidelines for evidence syntheses have emerged and will continue to be developed (203). Nonetheless, to improve the quality of evidence that informs guidelines and best practices as a field, a higher, more rigorous standard of trial conduct and evidence syntheses are required in the field of RT.

This overview of reviews represents, to date, the most comprehensive summary of the impact of RTx variables on muscle function and hypertrophy in healthy adults. RT greatly enhances overall muscular health compared with no exercise, and only a few RTx variables can be manipulated to enhance adaptations in experienced trainees. Therefore, individuals should prioritize completing any form of RT to improve muscle function and hypertrophy. Exercise and healthcare practitioners can provide invaluable guidance by leveraging these conclusions to promote RT participation and adherence, helping adults of all ages improve their health and fitness.

This article is being published as an official pronouncement of the American College of Sports Medicine. This pronouncement was reviewed for the American College of Sports Medicine by members-at-large and the Pronouncements Committee. Care has been taken to confirm the accuracy of the information presented and to describe generally accepted practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any consequences from the application of the information in this publication and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. The application of this information in a particular situation remains the professional responsibility of the practitioner; the clinical treatments described and recommended may not be considered absolute and universal recommendations. B. S. C. was supported by an Alexander Graham Bell Canada Graduate Scholarship-Doctoral. S. M. P. thanks the Canada Research Chairs program for its support and acknowledges grant support from NSERC and CIHR. No specific source of funding was used for this work. B. J. S. formerly served on the scientific advisory board for Tonal Corporation, a manufacturer of fitness equipment. No other authors reported conflicts relevant to this work. M. A. F. S., E. S. R., B. J. S., A. E. S. R., G. A. T., N. T. T., T. A. W., T. J. W., and S. M. P. designed and conceived the review; B. S. C. undertook the search of articles; B. S. C., A. C. D., C. V. L., and J. S. extracted data; B. S. C. synthesized data; B. S. C., M. A. F. S., E. S. R., B. J. S., A. E. S. R., G. T., N. T. T., T. A. W., T. J. W., and S. M. P. reviewed evidence recommendations; B. S. C. and S. M. P. wrote manuscript; B. S. C., A. C. D., M. A. F. S., C. V. L., E. S. R., B. J. S., A. E. S. R., J. S., G. A. T., N. T. T., T. A. W., T. J. W., and S. M. P. reviewed manuscript. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine.