It should be obvious from the name of this site that we’re pretty into science around these parts. When we discuss a particular subject, we try to give a broad, objective overview of all the relevant studies in that area. I’d never claim we’re perfect, but that’s always our aim. However, not everyone is that scrupulous. A common tactic used by many people who aim to appear scientific while still pushing an agenda is called “cherry picking.” Cherry picking refers to discussing only research that supports your point of view, while ignoring or impugning research that disagrees with your biases. In any area of science with a lot of studies being conducted, there are going to be some studies that support one position, and other studies that support the entirely opposite position. The cynic would take that as evidence that science can’t be trusted, but it’s generally much less sinister than that. Simply due to different methodologies, different subject pools, and random chance, you should expect studies to come to differing conclusions. So, how can you avoid cherry picking, but also just avoid saying “some studies say this and some studies say that, so we really have no idea”? Systematic review and meta-analyses.
In a review article, you discuss the findings of many studies instead of primarily just reporting the results of a single study. Not all reviews are created equal, though. In systematic reviews, you follow an extensive set of guidelines to ensure you find and report the results of all of the research in a given area. In non-systematic reviews (sometimes called narrative reviews), you don’t have to report the results of all studies and you have more freedom in how you structure your discussion (i.e. tell a narrative). Some non-systematic reviews are excellent and can be extremely useful because they’re generally a bit more reader-friendly. For example, these are a few very good non-systematic reviews (one, two, three, four). However, non-systematic reviews can also be rife with bias and cherry-picking since they’re not conducted in a systematic way, generally meaning systematic reviews provide a more objective and thorough overview of the literature.
Meta-analyses are simply systematic reviews with the addition of statistical analysis. In a meta-analysis, you pool the results of many studies asking the same (or very similar) research questions to get a quantitative overview of the literature. Maybe 10 studies say A is better than B, 5 say there’s no difference, and 2 say B is better than A. Based on the size of those differences, a meta-analysis may show that, when pooling all results together, A is truly significantly better than B, on average. However, if the 5 studies showing no difference were very large trials, or the two studies in favor of B found very large effects, the meta-analysis may find that there’s no significant difference between A and B, on average, in spite of the majority of studies favoring A.
If you’re familiar with the hierarchy of evidence, systematic reviews and meta-analyses are typically considered the highest quality of evidence. That doesn’t mean they’re perfect – if the literature in a given area is of poor quality, you’re left with a garbage-in-garbage-out scenario – but they’re typically considered to be better and more reliable than individual studies.
Therefore, due to the significance of systematic reviews and meta-analyses, we’ve put together a list and short take-home message of many recent systematic reviews and meta-analyses so you can cut straight to the chase of the results. Many topics related to strength, muscle growth, and nutrition have systematic reviews or meta-analyses covering them. If you’re curious about the research on a given topic, refer back to this list to see if there’s already a systematic review or meta-analysis on the topic. That will give you a better overview than trying to seek out studies one by one (and, if you do want to read the individual studies, it will make your search MUCH easier, since they’ll be referenced in the SR or MA on the topic).
Since there are so many individual systematic reviews or meta-analyses on this list, the overview of each will be really brief. If there are any really major issues, we’ll note them, but for the most part, we’ll just stick to the main findings. Also note that we haven’t included every systematic review or meta-analysis ever done on this list. When there were multiple articles covering the same topic, we went with the one that was more recent or of higher overall quality. If we missed one that you think should be included, let us know in the comments!
To make it easy on you, we split things up by topic. First will be strength, then hypertrophy, then nutrition, then miscellaneous other reviews that are relevant but not neatly categorized.
Just so you’ll know what you’re looking at and reading when viewing the figures below and reading the brief synopses, you’ll need to have an understanding of confidence intervals and forest plots. Confidence intervals (CI) tell you the range of values in which a population average will most likely fall. In meta-analyses, if a confidence interval for comparisons between two different treatments/conditions doesn’t cross zero, then you can state that there’s a statistically significant difference between the two (you have a high level of confidence that the population averages for the two treatments are truly different). Forest plots are figures commonly used in meta-analyses, showing the confidence intervals for multiple studies, along with the pooled average and confidence interval for the entire group of studies.
Here’s an example:
This is a forest plot from a meta-analysis by Schoenfeld et al. looking at the effects of high load vs. low-load training on strength gains. Each black square represents the mean difference in an individual study, while the black bars extending out from that black square represent the confidence interval for that study. The black diamond at the bottom is the confidence interval when pooling the results of all studies. Since the confidence interval doesn’t cross 0, this would be a statistically significant difference, with high-load training leading to significantly larger strength gains than low-load training.
Table of Contents
- Systematic review and meta-analysis of linear and undulating periodized resistance training programs on muscular strength. Harries et al. (2015)
- The Effect of Weekly Set Volume on Strength Gain: A Meta-Analysis. Ralston et al. (2017)
- Comparison of Periodized and Non-Periodized Resistance Training on Maximal Strength: A Meta-Analysis. Williams et al. (2017)
- Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. Wilson et al. (2012)
- The compatibility of concurrent high intensity interval training and resistance training for muscular strength and hypertrophy: a systematic review and meta-analysis. Sabag et al. (2018)
- The Role of Intra-Session Exercise Sequence in the Interference Effect: A Systematic Review with Meta-Analysis. Eddens et al. (2018)
- Strength and Hypertrophy Adaptations Between Low- vs. High-Load Resistance Training: A Systematic Review and Meta-analysis. Schoenfeld et al. (2017)
- Effect of Resistance Training Frequency on Gains in Muscular Strength: A Systematic Review and Meta-Analysis. Grgic et al. (2018)
- Effect of Movement Velocity During Resistance Training on Dynamic Muscular Strength: A Systematic Review and Meta-Analysis. Davies et al. (2017)
- Effect of Training Leading to Repetition Failure on Muscular Strength: A Systematic Review and Meta-Analysis. Davies et al. (2016); also check the erratum
- Effects of Variable Resistance Training on Maximal Strength: A Meta-Analysis. Soria-Gila et al (2015)
- Is inertial flywheel resistance training superior to gravity-dependent resistance training in improving muscle strength? A systematic review with meta-analyses. Vicens-Bordas et al. (2018)
- Effects of weightlifting exercise, traditional resistance and plyometric training on countermovement jump performance: a meta-analysis. Berton et al. (2018)
- The efficacy of resistance training in hypoxia to enhance strength and muscle growth: A systematic review and meta-analysis. Ramos-Campo et al (2018)
- Effects and Dose-Response Relationships of Motor Imagery Practice on Strength Development in Healthy Adult Populations: a Systematic Review and Meta-analysis. Paravlic et al. (2018)
- Dose–Response Relationships of Resistance Training in Healthy Old Adults: A Systematic Review and Meta-Analysis. Borde et al. (2015)
- Effects and dose–response relationships of resistance training on physical performance in youth athletes: a systematic review and meta-analysis. Lesinski et al. (2016)
- A Meta-Analysis of Resistance Training in Female Youth: Its Effect on Muscular Strength, and Shortcomings in the Literature. Moran et al. (2018)
- Supramaximal Eccentrics Versus Traditional Loading in Improving Lower-Body 1RM: A Meta-Analysis. Buskard et al. (2018)
- Muscle growth
- Effects of linear and daily undulating periodized resistance training programs on measures of muscle hypertrophy: a systematic review and meta-analysis. Grgic et al. (2017)
- Should resistance training programs aimed at muscular hypertrophy be periodized? A systematic review of periodized versus non-periodized approaches Grgic et al. (2017)
- Dose-response relationship between weekly resistance training volume and increases in muscle mass: A systematic review and meta-analysis. Schoenfeld et al. (2017)
- Hypertrophic Effects of Concentric vs. Eccentric Muscle Actions: A Systematic Review and Meta-analysis. Schoenfeld et al. (2017)
- Effect of repetition duration during resistance training on muscle hypertrophy: a systematic review and meta-analysis. Schoenfeld et al. (2015)
- Strength and Hypertrophy Adaptations Between Low- vs. High-Load Resistance Training: A Systematic Review and Meta-analysis. Schoenfeld et al (2017)
- Effects of Resistance Training Frequency on Measures of Muscle Hypertrophy: A Systematic Review and Meta-Analysis. Schoenfeld et al. (2016)
- Effect of movement velocity during resistance training on muscle-specific hypertrophy: A systematic review. Hackett et al. (2018)
- The efficacy of resistance training in hypoxia to enhance strength and muscle growth: A systematic review and meta-analysis. Ramos-Campo et al. (2018)
- The effects of short versus long inter-set rest intervals in resistance training on measures of muscle hypertrophy: A systematic review. Grgic et al. (2017)
- A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Morton et al. (2018)
- The effect of protein timing on muscle strength and hypertrophy: a meta-analysis. Schoenfeld et al. (2013)
- Effects of meal frequency on weight loss and body composition: a meta-analysis. Schoenfeld et al. (2015)
- The Effect of Whey Protein Supplementation on the Temporal Recovery of Muscle Function Following Resistance Training: A Systematic Review and Meta-Analysis. Davies et al. (2018)
- Effects of energy-restricted high-protein, low-fat compared with standard-protein, low-fat diets: a meta-analysis of randomized controlled trials. Wycherley et al. (2012)
- The effects of protein supplements on muscle mass, strength, and aerobic and anaerobic power in healthy adults: a systematic review. Pasiakos et al. (2015)
- Effects of protein supplements on muscle damage, soreness and recovery of muscle function and physical performance: a systematic review. Pasiakos et al. (2014)
- Threshold of Energy Deficit and Lower-Body Performance Declines in Military Personnel: A Meta-Regression. Murphy et al. (2018)
- Does high-carbohydrate intake lead to increased risk of obesity? A systematic review and meta-analysis. Sartorius et al. (2018)
- The effect of glutamine supplementation on athletic performance, body composition, and immune function: A systematic review and a meta-analysis of clinical trials. Ahmadi et al. (2018)
- Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Gibson et al. (2015)
- Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. Johnston et al. (2014)
- Weight loss intervention adherence and factors promoting adherence: a meta-analysis. Lemstra et al. (2015)
- Effects of beta-hydroxy-beta-methylbutyrate supplementation on strength and body composition in trained and competitive athletes: A meta-analysis of randomized controlled trials. Sanchez-Martinez et al. (2017)
- Effects of beta-hydroxy-beta-methylbutyrate supplementation during resistance training on strength, body composition, and muscle damage in trained and untrained young men: a meta-analysis. Rowlands et al. (2009)
- Creatine Supplementation and Lower Limb Strength Performance: A Systematic Review and Meta-Analyses. Lanhers et al. (2015)
- Does oral creatine supplementation improve strength? A meta-analysis. Dempsey et al. (2002) and Effect of creatine supplementation on body composition and performance: a meta-analysis. Branch (2003)
- Effects of Whey Protein Alone or as Part of a Multi-ingredient Formulation on Strength, Fat-Free Mass, or Lean Body Mass in Resistance-Trained Individuals: A Meta-analysis. Naclerio et al. (2016)
- Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. Martineau et al. (2017)
- Effects of Vitamin D Supplementation on Serum 25-Hydroxyvitamin D Concentrations and Physical Performance in Athletes: A Systematic Review and Meta-analysis of Randomized Controlled Trials. Farrokhyar et al. (2017).
- Does Fish Oil Have an Anti-Obesity Effect in Overweight/Obese Adults? A Meta-Analysis of Randomized Controlled Trials. Shichun et al. (2015)
- β-alanine supplementation to improve exercise capacity and performance: a systematic review and meta-analysis. Saunders et al. (2018)
- Effects of caffeine intake on muscle strength and power: a systematic review and meta-analysis. Grgic et al. (2018)
- Effects of protein supplements consumed with meals, versus between meals, on resistance training-induced body composition changes in adults: a systematic review. Hudson et al. (2018)
- Branched-chain amino acid supplementation and exercise-induced muscle damage in exercise recovery: A meta-analysis of randomized clinical trials. Rahimi et al. (2017)
- Effect of whey protein supplementation on body composition changes in women: a systematic review and meta-analysis. Bergia et al. (2018)
- Dietary fiber intervention on gut microbiota composition in healthy adults: a systematic review and meta-analysis. So et al. (2018)
- Coffee, Caffeine, and Health Outcomes: An Umbrella Review. Grosso et al. (2017)
- Effect of Dietary Sugar Intake on Biomarkers of Subclinical Inflammation: A Systematic Review and Meta-Analysis of Intervention Studies. Della Corte et al. (2018)
- Glycemic impact of non-nutritive sweeteners: a systematic review and meta-analysis of randomized controlled trials. Nichol et al. (2018)
- Effects of fasted vs fed‐state exercise on performance and post‐exercise metabolism: A systematic review and meta‐analysis. Aird et al. (2018)
- An evidence-based approach for choosing post-exercise recovery techniques to reduce markers of muscle damage, soreness, fatigue and inflammation: a systematic review with meta-analysis. Dupuy et al. (2018)
- Talent Identification in Sport: A Systematic Review. Johnston et al. (2017)
- Blood flow restriction training in clinical musculoskeletal rehabilitation: a systematic review and meta-analysis. Hughes et al. (2017)
- Sleep Interventions Designed to Improve Athletic Performance and Recovery: A Systematic Review of Current Approaches. Bonnar et al. (2018)
- Choking interventions in sports: A systematic review. Gröpel and Mesagno (2018)
- Mindfulness and acceptance approaches to sporting performance enhancement: a systematic review. Noetel et al. (2017)
- Influence of chronic stretching on muscle performance: Systematic review. Medeiros et al (2017)
- The Effects of Injury Prevention Programs on the Biomechanics of Landing Tasks: A Systematic Review With Meta-analysis. Lopes et al. (2017)
- Effects of creatine supplementation on cognitive function of healthy individuals: A systematic review of randomized controlled trials. Avgerinos et al. (2018)
- The effect of resistance exercise on sleep: A systematic review of randomized controlled trials. Kovacevic et al. (2018)
- The epidemiology of injuries across weight-training sports. Keogh and Winwood (2017)
- Is core stability a risk factor for lower extremity injuries in an athletic population? A systematic review. De Blaiser et al. (2018)
- Melatonin for the prevention and treatment of jet lag. Herxheimer and Petrie (2002)
- Association of Efficacy of Resistance Exercise Training With Depressive Symptoms: Meta-analysis and Meta-regression Analysis of Randomized Clinical Trials. Gordon et al. (2018)
- Effects of Strength Training on Running Economy in Highly Trained Runners: A Systematic Review With Meta-Analysis of Controlled Trials. Balsalobre-Fernandez et al. (2016)
- A Systematic Review of the Effects of Resistance Training on Body Image. SantaBarbara et al. (2017)
- The effectiveness of exercise interventions to prevent sports injuries: a systematic review and meta-analysis of randomised controlled trials. Lauersen et al. (2013)
- Muscle Dysmorphia Symptomatology and Associated Psychological Features in Bodybuilders and Non-Bodybuilder Resistance Trainers: A Systematic Review and Meta-Analysis. Mitchell et al. (2016)
- In-house meta-analyses
Linear and undulating periodization approaches led to similar increases in bench press and squat strength. While there was no significant difference, results tended to favor undulating periodization for leg press strength (p=0.07).
The Effect of Weekly Set Volume on Strength Gain: A Meta-Analysis. Ralston et al. (2017)
Higher weekly set volume (5-10+) led to larger strength gains than lower week set volumes (<5). This held true for both compound and single-joint exercises. The differences weren’t quite as large as many people might expect (i.e. substantially higher volume for ~20% faster gains), but they were statistically significant and definitely meaningful for people trying to maximize strength. However, substantial strength gains were also possible with low weekly set volume. These findings mirror those of an earlier meta-analysis by Kreiger: Single versus multiple sets of resistance exercise: a meta-regression. This meta-analysis was discussed in more detail in Volume 1, Issue 6 of MASS.
Periodized training led to significantly larger strength gains than non-periodized training. The difference was considered a small effect. However, there was some evidence of publication bias, with several studies showing outsized results in favor of periodized training beyond what would be expected without bias. When they were removed, the mean effect in favor of periodized training was roughly halved, but it was still significant. This should sound familiar to Stronger By Science readers. This meta-analysis was also discussed in more depth in Volume 1, Issue 4 of MASS.
Concurrent training (doing both strength and endurance training) led to smaller lower body strength and power gains than strength training alone. There were no differences for upper body strength gains. The difference for lower body strength gains depended on the cardio modality used, though. There was a significant difference between concurrent training and strength training alone when running was the cardio modality, but not when cycling was used instead (however, when looking at the raw effect sizes, it does seem that cycling still had some negative impacts, that just weren’t large enough to reach significance). There were no differences for upper body strength gains. Moderating factors included frequency and duration, such that more frequent cardio and longer duration cardio tended to decrease lower body strength and power gains to a greater degree than less frequent or shorter duration cardio.
Much like the Wilson meta-analysis (which primarily used studies employing low-intensity cardio) on the interference effect with concurrent training, this meta-analysis found that combining resistance training and HIIT led to smaller gains in lower body strength than resistance training alone, while upper body strength gains were unaffected. Unlike the Wilson meta-analysis, cycle sprints seemed to negatively affect strength gains more than running sprints (though the difference between modalities wasn’t significant). No interference effect on strength gains was observed in studies allowing at least 24 hours of rest between lifting and HIIT sessions.
If you need to do strength training and aerobic training within the same session, this meta-analysis found that session order (i.e. lifting first or cardio first) didn’t affect gains in aerobic fitness, changes in body fat percentage, or lower body isometric strength, but it did affect lower body dynamic strength. Lifting first in the session, followed by cardio, led to larger strength gains than doing cardio first. The difference wasn’t particularly large (~7% larger strength gains), but it was significant. A 2017 meta-analysis on the same topic by Murlasits et al. came to similar conclusions, but only looked at dynamic strength and aerobic fitness.
This meta-analysis found that, unsurprisingly, heavy training (>60% of 1RM) led to larger gains in dynamic strength than low-load training (≤60% of 1RM). However, there was no significant difference for isometric strength. This meta-analysis was also discussed in more depth in Volume 1, Issue 7 of MASS.
This meta-analysis found that higher training frequencies are associated with larger strength gains. However, in studies where volume was equated despite different frequencies (weekly volume was higher in the higher frequency groups in many studies), higher frequencies weren’t associated with larger strength gains. This meta-analysis was discussed in Volume 2, Issue 4 of MASS, along with an additional analysis of just the studies using trained lifters.
This meta-analysis found that, when controlling for intensity and volume, lifting velocity didn’t significantly affect strength gains. However, it should be noted that several of the studies in this meta-analysis involved training to failure, meaning the velocity differences may have only existed for the first few reps. It also included a few studies using protocols where the training would have been very easy for both groups (i.e. 3×8 at 50% of 1RM) where you wouldn’t expect big strength gains in either group.
This meta-analysis found that training to failure vs. stopping short of failure didn’t significantly affect strength gains. That was true both for studies where volume was controlled, and for studies where volume wasn’t controlled.
This meta-analysis was retracted after a re-analysis found several errors in the original publication. When those errors were corrected, there was effectively no difference between training with straight weight vs. including bands and chains.
This meta-analysis found that flywheel training devices led to similar strength gains compared to gravity-dependent resistance training (i.e. free weights, or most of the machines you’d find a typical gym). This was honestly probably an area of research that wasn’t quite ready for a meta-analysis (only seven studies).
Weightlifting exercises (the clean & jerk, the snatch, and their derivatives) seem to increase countermovement jump performance to a greater degree than traditional resistance exercises. Weightlifting exercises and plyometrics seem to be equally effective at increasing countermovement jump performance.
This meta-analysis found that strength gains after training in hypoxia (i.e. conditions that simulate being at high altitudes using environmental chambers, not using something like “altitude training masks”) were similar in magnitude to strength gains after training with normal oxygen availability. There aren’t many studies on this topic yet, so this meta-analysis may have been a bit premature.
This meta-analysis found that motor imagery training led to significant strength gains compared to no training, but that a combination of motor imagery and physical training didn’t lead to larger strength gains than physical training alone. However, it’s worth noting that a recent study not included in this meta-analysis (since it was published after they’d completed their literature search) did find that a combination of motor imagery and physical training led to larger strength gains than physical training alone. However, that’s still just the fifth study testing a combination of physical training and motor imagery vs. physical training alone, so that’s an area badly in need of more research. As it is, it seems that the main application of motor imagery training would be to aid in maintaining performance when you need to take time off training for some reason (injury, vacation, etc.).
Older people can get stronger and jacked-er too! This meta-analysis was simply intended to determine the training variables associated with the largest increases in strength and muscle size in older adults. I think the most important finding was that the training doses that work best in older adults look really similar to what tends to work best in younger adults as well, except with slightly lower volume and intensity.
Similar to the meta-analysis on older adults, this meta-analysis was simply intended to determine the training variables associated with the largest increases in performance in young athletes. Again, their findings largely mirror what are usually considered good general training practices: long-term training (>23 weeks), with a frequency of 1-3x per exercise per week, high-ish intensities (80-89% of 1RM), high volumes (5 sets per exercise beat out 1-4 sets per exercise), a moderate number of reps per set (6-8), and long rest duration (3-4 minutes between sets) was found to promote the largest strength gains.
Young women can also benefit from resistance training. However, the effects seem to be larger in older (>15 years old) girls than younger girls.
Supramaximal eccentric training involves lowering heavier weights than you can lift concentrically. This meta-analysis found that supramaximal eccentric training didn’t lead to larger 1RM increases than traditional training (i.e. with submaximal eccentrics and concentrics). However, the only study that used a free weight compound exercise (squats) did find a benefit for supramaximal eccentric training. So, while it doesn’t seem that supramaximal eccentrics aid in 1RM strength development in single-joint or machine exercises, it’s probably prudent to wait for most studies using compound free weight exercises before coming to a strong conclusion regarding their efficacy for powerlifting.
This meta-analysis found that both linear and daily undulating periodized training had similar effects on muscle growth. This meta-analysis was discussed in more depth in Volume 1, Issue 7 of MASS.
This systematic review found that, at least in the short term (i.e. a few months), periodized and non-periodized training have similar effects on muscle growth.
This meta-analysis found that higher training volumes were associated with more muscle growth. There was an essentially linear relationship, with <5 sets per week leading to a 5.4% increase in muscle size, 5-9 sets per week leading to a 6.6% increase in muscle size, and 10+ sets per week leading to 9.8% increase in muscle size. However, there was one outlier that strongly influenced the results. When it was removed, the overall trend still held, but the overall effect shrunk. Before removal, each additional set was worth an additional 0.37% increase, on average; after its removal, each additional set was worth an additional 0.25% increase, on average. These results are very similar to those of an earlier meta-analysis by Kreiger: Single vs. multiple sets of resistance exercise for muscle hypertrophy: a meta-analysis.
This meta-analysis found no significant differences between concentric and eccentric training for hypertrophy. However, results tended to favor eccentric training (10% vs. 6.8%; p=0.076). However, since most exercises have both an eccentric and concentric component, this probably isn’t a big deal since you’ll be performing both muscle actions in most of your training. For more on eccentric training, this systematic review by Douglas et al. is also well worth a read.
As with the strength findings from this same meta-analysis (presented earlier in this article), concurrent training led to less lower body hypertrophy than strength training alone. However, this difference was also mediated by aerobic training modality; there was a significant difference when running was the aerobic modality, but not when cycling was the aerobic modality (though nominal effect sizes still tended to favor strength training alone).
Unlike the Wilson meta-analysis (which primarily used studies employing low-intensity cardio) on the interference effect with concurrent training, this meta-analysis found that combining resistance training and HIIT led to just as much hypertrophy as resistance training alone.
Unlike the strength findings from this same meta-analysis, session order didn’t have a significant effect on hypertrophy. If you have to do strength training and cardio in the same session, the order you do them in probably won’t have much of an effect on lower body muscle growth.
This meta-analysis found that repetition duration didn’t significantly affect hypertrophy. As long as you’re training hard, whether you like lifting explosively or purposefully slowing your reps down, muscle growth will probably be similar. It’s worth noting that there weren’t enough studies with really long rep durations (10+ seconds) to meta-analyze, but preliminary results indicate that with super slow reps, hypertrophy may be diminished to some degree.
Unlike the dynamic strength findings from this same meta-analysis, hypertrophy was unaffected by training intensity. Both high load (>60% of 1RM) and low load (≤60% of 1RM) training caused similar muscle growth. It’s worth noting that all of these studies had people train to failure. This conclusion should sound familiar to Stronger By Science readers.
This meta-analysis found that training a muscle group at least twice per week led to more muscle growth than training it just once per week. However, there weren’t enough studies to make meaningful comparisons with even higher frequencies. Higher frequencies were associated with nearly twice as much hypertrophy (3.7% vs. 6.8%). Of note, I think there may have been a problem with the forest plot below because it doesn’t reflect the results presented in the text. The forest plot makes it look like the difference wasn’t significant due to a wide CI, while the text reports of CI from just 0.11-0.28.
This was a systematic review found that moderate-to-slow velocities (2-3 second eccentrics and concentrics) may lead to more quad growth than faster velocities (~1 second eccentrics and concentrics), while faster velocities may lead to more biceps growth. Specifically, three of five studies found greater quad hypertrophy with slower rep velocities (two found no difference), while two studies found greater biceps hypertrophy with faster rep velocities. However, this systematic review covered just six studies, so the findings are very tentative.
Similar to the strength findings from this same meta-analysis (presented earlier in this article), hypertrophy was similar when training under both hypoxic and normoxic conditions. Again, there weren’t many studies included in this meta-analysis, so results are very tentative.
There were six studies included in this analysis comparing short (20-60 seconds) and long (>60 seconds) rest intervals. Hypertrophy tended to be greater with longer rest intervals (9.2% vs. 5.8%), but there was considerable heterogeneity. Ultimately, the authors simply conclude that robust hypertrophy can occur with both short and longer rest intervals, but that more research is needed.
The table for hypertrophy recommendations was previously presented. Again, just notice how the recommendations for older adults largely mirror those for younger adults, except that recommended volume and intensity is a bit lower.
Protein supplementation was found to increase gains in both strength and muscle but didn’t have a significant effect on bone mineral content. The relative benefits of protein supplementation were (unsurprisingly) larger for hypertrophy than for strength, and were larger for hypertrophy in trained individuals than in untrained individuals. The relative benefits also tended to be larger in young people than in older people. Furthermore, it was found that increases in lean body mass tended to plateau at a protein intake of around 1.6g/kg (0.73g/lb). However, the confidence intervals extended up to 2.2g/kg (1g/lb), making that the “better safe than sorry” protein recommendation.
This meta-analysis examined the effects of protein intake in the immediate peri-workout window (within an hour before or after training) versus not consuming protein within that window. Protein timing didn’t significantly affect strength gains. Before adjusting for covariates, the timing did significantly increase hypertrophy. However, many studies didn’t match for total protein intake. After adjusting for higher total protein intakes in the groups consuming protein in the peri-workout window, it didn’t seem that timing significantly affected hypertrophy by itself. In other words, downing a protein shake after your workout may lead to more muscle growth if it increases total protein intake, but it probably won’t make too much of a difference otherwise.
This meta-analysis initially found that higher meal frequencies during weight loss were associated with larger decreases in fat mass and body fat percentage, and smaller decreases in fat-free mass. However, those differences were all driven by a single study; when a sensitivity analysis was performed and that study was removed, there was no significant effect of meal frequency on fat mass, body fat percentage, or fat-free mass.
Protein supplementation was found to increase rate of recovery from training (defined as restoration of muscle function after a training bout). This effect was only significant (p<0.05) for time points <24 hours post-training, and 72 hours post-training; however, effect sizes favored protein supplementation at all time points (g = 0.4-0.7).
High protein diets during weight loss contributed to larger decreases in weight, fat mass, and triglycerides, and smaller decreases in fat-free mass compared to lower protein diets. However, of note, the mitigation in FFM loss only applied to studies lasting more than 12 weeks.
In terms of strength and hypertrophy, Morton (2018) provides a more up-to-date overview of the literature. However, this systematic review also adds another element. Protein supplementation also may increase gains in aerobic and anaerobic power after aerobic or anaerobic training.
In terms of recovery of muscle function, refer to Davies (2018). However, this systematic review also found that protein supplementation tends to decrease soreness and markers of muscle damage after training.
This meta-analysis found that, in military personnel, neither length of time in an energy deficit nor daily energy deficit were independently associated with decreases in lower body strength or power. However, total energy deficit was strongly associated with decreases in lower body strength and power. It’s not clear whether these findings would directly apply to strength or physique athletes trying to cut weight (while military personnel are very active, it’s not like the bulk of their exercise comes from lifting weights), but they’re at least worth taking note of. If they did apply, the implication would be that a quick cut with a large daily energy deficit and a slow cut with a small daily energy deficit would ultimately have similar impacts on performance. This is an area of research that’s been largely ignored in the context of resistance training, unfortunately (this is the only study I’m aware of, and it was pretty poorly controlled).
Short answer: No.
Longer answer: In studies looking at high vs. low absolute carbohydrate intake, and in studies looking at high vs. low carbohydrate intake expressed as a percentage of total calorie intake, carb intake was not associated with increased or decreased odds of obesity.
Glutamine supplementation doesn’t seem to affect any measurable aspect of athletic performance, or any proxy for muscle damage and recovery (i.e creatine kinase). Glutamine supplementation may help a bit with weight loss (which surprised me, honestly). Interestingly, it seemed to nearly lead to a significant increase in fat mass (CI: -0.19-2.22kg), without having any effect on lean mass. This perplexing result can be explained by the fact that one particularly large study measured weight (reporting a decrease) and fat mass (reporting an increase), but didn’t measure lean mass. It had by far the largest weight in the analyses, so it really drove the results. This one study was allowed to have such undue weight because the authors used a fixed-effects model when they should have used a random-effects model. If they used a random effects model, glutamine probably wouldn’t have been found to effect body weight or body composition either.
A surprising outcome in obesity research is that unlike moderate energy-restricted diets, after initial increases, very low energy diets (VLED; < 800 kcal/day) actually reduce hunger (though long-term adherence is problematic). The same claim is made for very low carbohydrate diets (VLCD). This meta analysis assessed appetite response to both VLED and VLCD (< 10 % kcal or < 50 g/day, ad libitum consumption of protein and fat). VLED increased satiety and decreased hunger without changing desire to eat or the anticipated energy that would or could be eaten. VLCD increased satiety and decreased hunger, and also decreased desire to eat.
This meta-analysis included 48 RCTs in overweight individuals and categorized diets based on whether or not they were lower carbohydrate (< 40% kcal), “balanced macronutrients,” or low fat (< 20% kcal). At diet conclusion, lower carbohydrate were 83% likely to produce the most weight loss and produced significantly more weight loss than balanced macronutrient diets, but not more weight loss than low-fat diets. At 1 year follow up, low-fat diets were most likely (50%) among the three diets to result in the most weight loss retention. Adverse events incidence was higher during low-carbohydrate versus low-fat diets: constipation (68% vs 35%, respectively), headache (60% vs 40%), halitosis (38% vs 8%), muscle cramps (35% vs 7%), diarrhea (23% vs 7%), general weakness (25% vs 8%), and rash (13% vs 0%; P < .006). However, weight loss differences among diets were not clinically meaningful (1-2 kg over 6-12 months), and the authors suggested individuals follow whichever diet they can adhere to.
Supervised weight loss attempts tend to have about 65% higher adherence than unsupervised attempts, and interventions with a social support component tend to have about 29% higher adherence than interventions without a social support component. Furthermore, dietary interventions tend to have about 27% higher adherence than exercise interventions.
In trained athletes, HMB supplementation doesn’t seem to significantly affect either strength gains or changes in body composition (fat mass or fat-free mass).
Like the more recent Sanchez-Martinez meta-analysis, HMB still didn’t do anything for trained athletes back in 2009. However, this meta-analysis did find that HMB supplementation significantly increased lower body strength gains in untrained lifters, though it didn’t affect body composition.
Creatine supplementation leads to significantly larger strength gains in both the squat and leg press. The effect was larger for squat (8%) than leg press (3%).
Creatine supplementation also significantly increases lean body mass and bench press strength, and generally improves performance in tasks lasting ≤30 seconds. It may also improve performance in some tasks lasting 30-150 seconds. It may not affect biceps curl strength. It seems to be effective in both men and women (though it may be more effective in men), and in both trained and untrained subjects. It doesn’t seem to reliably affect performance for tests lasting >150 seconds.
While both whey protein and creatine enhance strength gains and hypertrophy independently, they may have even larger effects when taken together.
Vitamin D supplementation seems to decrease risk of respiratory tract infections by about 20%. The reduction in risk may be larger in people with low vitamin D levels.
Even though vitamin D supplementation increases blood concentrations of vitamin D, supplementation doesn’t seem to reliably affect physical performance in athletes (though it may increase handgrip strength).
Fish oil supplementation on top of lifestyle modification doesn’t seem to decrease body weight or BMI more than lifestyle modification alone, but it does lead to significantly larger decreases in waist circumference and waist-to-hip ratio.
β-alanine supplementation significantly increases performance for tests lasting 1-10 minutes but doesn’t significantly affect performance for tests lasting <1 minute or for tests lasting 10+ minutes. This makes sense given β-alanine’s mechanism of action – increasing muscle carnosine content, to help buffer against pH decreases. Short-duration activities (i.e. lifting) are unlikely to be limited by inadequate cellular buffering, and long-duration activities aren’t going to rely as much on anaerobic metabolism in the first place.
Acute caffeine supplementation increases maximal strength and power, though the overall effect is pretty small. The strength increase is more consistent for upper body strength than lower body strength. The average dose of caffeine used was 4.3-6.5mg/kg.
Consuming protein supplements with meals instead of between meals may be slightly more beneficial for improving body composition. However, due to heterogeity between studies, few actual head-to-head comparisons, and lack of biological plausibility, my hunch is that it doesn’t actually matter too much, as long as you’re eating enough protein.
BCAA supplementation led to significantly lower CK activity <24 hours and 24 hours post-exercise. Soreness and LDH weren’t significantly attenuated at any single time point, but when pooling all time points, BCAA supplementation significantly attenuated both soreness and CK activity, and the attenuation in LDH was nearly significant. However, it’s important to note that these studies simply compared BCAAs to a placebo, not to protein (i.e. this tells us that BCAAs are better than nothing, but not better than protein for attenuating post-exercise muscle damage).
Whey protein supplemention seems to help increase lean mass in women. However, that effect is only significant in studies without resistance training (with resistance training, whey protein doesn’t seem to help women gain additional lean mass), and in studies imposing an energy deficit (whey protein helps women hold on to more lean mass when dieting, but it doesn’t seem to affect lean mass without a calorie deficit). Furthermore, whey protein supplementation doesn’t seem to lead to significantly greater fat loss in women. All mean effects leaned in favor of whey protein supplementation, but most comparisons simply didn’t clear the threshold of statistical significance.
Interventions involving increases in dietary fiber consumption significantly increased the abundance of intestinal bacteria strains that are generally believed to be beneficial (Bifidobacterium and Lactobacillus). Such interventions also increased fecal butyrate concentrations (which is thought to protect against colon cancer). The fibers that had the largest effect on intestinal bacteria were fructans and galacto-oligosaccharides.
Coffee, Caffeine, and Health Outcomes: An Umbrella Review. Grosso et al. (2017)
“Of the 59 unique outcomes examined in the selected 112 meta-analyses of observational studies, coffee was associated with a probable decreased risk of breast, colorectal, colon, endometrial, and prostate cancers; cardiovascular disease and mortality; Parkinson’s disease; and type-2 diabetes. Of the 14 unique outcomes examined in the 20 selected meta-analyses of observational studies, caffeine was associated with a probable decreased risk of Parkinson’s disease and type-2 diabetes and an increased risk of pregnancy loss. Of the 12 unique acute outcomes examined in the selected 9 meta-analyses of RCTs, coffee was associated with a rise in serum lipids, but this result was affected by significant heterogeneity, and caffeine was associated with a rise in blood pressure. Given the spectrum of conditions studied and the robustness of many of the results, these findings indicate that coffee can be part of a healthful diet.”
Fructose doesn’t seem to contribute to systemic inflammation to a greater degree that other dietary sugars (specifically when comparing fructose vs. glucose, and high fructose corn syrup vs. table sugar [sucrose]).
Overall, it doesn’t seem that non-nutritive sweeteners actuely have any meaningful impact on blood glucose, on average. The glycemic impact at some time points seems to be smaller in diabetics, smaller in people with high BMIs, and smaller in older people. By 120 minutes post-consumption, non-nutritive sweeteners significantly decrease blood glucose. All non-nutritive sweeteners seemed to have similar effects on blood glucose.
Eating before exercise improves long-duration aerobic performance (but not short-duration performance). However, fasted endurance exercise leads to larger post-exercises increases in plasma free fatty acids, and may lead to larger increases in cellular signaling associated with aerobic training adaptations.
Several different recovery modalities, including massage, active recovery, compression garments, water immersion, contrast water therapy, and cryotherapy were found to significantly decrease DOMS and attenuate increases in inflammatory markers. Massage was found to be most effective for attenuating both DOMS and perceived fatigue. It’s also worth noting that while cold water immersion was found to be effective for promoting recovery, other research shows that it can decrease muscle growth if used chronically. This study was reviewed in more detail in Volume 2, Issue 6 of MASS.
Talent Identification in Sport: A Systematic Review. Johnston et al. (2017)
We don’t actually know very much about talent identification in sport, generally. However, the quality of the research varies sport-to-sport. The research to-date is summarized in Table 1 of the article; there’s too much to provide a tidy synopsis here, but it’s worth reading for yourself if talent identification in a particular sport matters to you.
Low-load training with blood flow restriction seems to aid in strength recovery after injury better than low-load training without blood flow restriction. However, heavier training was more effective for regaining strength than low-load training with blood flow restriction. If loading is tolerated, heavier training is typically the better option; however, low-load training with blood flow restriction seems to be a better option than plain lo- load training in situations where a tissue isn’t yet ready for heavier loading.
Sleep extension (aiming for 9+ hours of sleep per night) was found to most reliably improve performance. Improving sleep hygiene also tended to improve performance, though to a smaller degree and less reliably than sleep extension. This systematic review was discussed in Volume 2, Issue 3 of MASS.
Choking interventions in sports: A systematic review. Gröpel and Mesagno (2018)
“The most reported effective interventions were pre-performance routines, quiet eye training, left-hand contractions, and acclimatisation training. The use of dual task was beneficial for performance under pressure but harmful when used in training. Mixed evidence was found for analogy learning, and null effects were reported for goal setting, neurofeedback training, and reappraisal cues.”
“Compared to no treatment in randomised trials, large effect sizes were found for improving mindfulness, flow, and performance, and lower competitive anxiety. Evidence was graded to be low quality, meaning further research is very likely to have an important impact on confidence in these effects…A number of studies found positive effects for mindfulness and acceptance interventions; however, with limited internal validity across studies, it is difficult to make strong causal claims about the benefits these strategies offer for athletes.”
Influence of chronic stretching on muscle performance: Systematic review. Medeiros et al (2017)
In 28 studies that examined the effects of chronic stretching on muscular performance, 14 reported increases in performance, while the rest reported no difference (none reported decreases in performance). All of the studies reported improvements in performance used dynamic tests, while no measures of isometric strength improved. This systematic review was discussed in Volume 1, Issue 4 of MASS.
Injury prevention programs aimed at altering biomechanical profiles associated with ACL injury risk seem to be effective. Specifically, they seem to help increase knee and hip flexion angles during landing tasks (meaning people are absorbing force more effectively, rather than getting a big “shock” when they land), and decrease knee abduction moments (knee caving). The fact that landing mechanics are trainable is important information for coaches who train athletes (specifically female athletes in sports that require a lot of jumping).
Creatine supplementation may improve short-term memory and reasoning ability, especially in older adults. The effects on memory are larger in vegetarians than omnivores. Creatine supplementation doesn’t seem to affect other cognitive domains.
“Chronic resistance exercise improves all aspects of sleep, with the greatest benefit for sleep quality. These benefits of isolated resistance exercise are attenuated when resistance exercise is combined with aerobic exercise and compared to aerobic exercise alone. However, the acute effects of resistance exercise on sleep remain poorly studied and inconsistent. In addition to the sleep benefits, resistance exercise training improves anxiety and depression. These results suggest that resistance exercise may be an effective intervention to improve sleep quality.”
The epidemiology of injuries across weight-training sports. Keogh and Winwood (2017)
Bodybuilding has the lowest injury risk (0.24-1 injuries per 1,000 hours) and strongman and highland games have the highest injury risk (4.5-7.5 injuries per 1,000 hours), while weightlifting and powerlifting fall in the middle.
Low core stability may increase athletes’ risk of lower extremity injuries.
Melatonin for the prevention and treatment of jet lag. Herxheimer and Petrie (2002)
Melatonin taken before bed helps you adapt to a new time zone when traveling, reducing symptoms of jet lag. Doses between 0.5mg and 5mg seem to be effective (with 5mg helping more than lower doses), and fast-release pills seem to work better than slow-release pills. While melatonin can help when only crossing a couple time zones, it appears that its effects are most notable when crossing 5+ time zones.
Association of Efficacy of Resistance Exercise Training With Depressive Symptoms: Meta-analysis and Meta-regression Analysis of Randomized Clinical Trials. Gordon et al. (2018)
Resistance training seems to aid in decreasing depressive symptoms. This is rather unsurprising, as exercise in general helps decrease depressive symptoms. However, resistance training seems to work just as well as aerobic training, if not slightly better (“When directly comparing the effects of resistance exercise training and aerobic exercise training, a small, nonsignificant mean effect change favoring resistance exercise training was found”). It’s possible that this meta-analysis overestimates the effects of resistance exercise on depression, however. The authors note that higher-quality studies tended to find smaller (though still positive) effects than lower-quality studies.
In really well-trained runners, resistance training (generally a combination of lifting and plyometrics) has a large, beneficial effect on running economy. A 2018 systematic review by Blagrove et al. on trained (but not quite as well-trained) broadly agrees with these findings, adding, “Time trial (TT) performance (1.5–10 km) and anaerobic speed qualities also tended to improve following ST. Other parameters [maximal oxygen uptake (VO2max), velocity at VO2max, blood lactate, body composition] were typically unaffected by ST.” In other words, it appears that strength training primarily improves running performance by increasing speed and running economy, without negatively affecting purely aerobic variables.
A Systematic Review of the Effects of Resistance Training on Body Image. SantaBarbara et al. (2017)
“The majority (8 of 11) of studies concluded that resistance training can significantly improve multiple dimensions of body image, including body satisfaction, appearance evaluation, and social physique anxiety…Overall, resistance training seems to have the potential to improve body image in adults, but future high-quality studies with more rigorous testing methods and study designs are needed.”
Stretching interventions didn’t affect injury risk. Proprioceptive training, strength training, and multi-modal training all significantly decreased injury risk. Strength training had the largest nominal effect.
Bodybuilders display greater symptoms of muscle dysmorphia on all aspects of Muscle Dysmorphia Inventory (MDI) than people who lift weights but aren’t competitive bodybuilders. Furthermore, higher levels of muscle dysmorphia were positively associated with increased anxiety, depression, and neuroticism, and negatively associated with self-concept and self-esteem.
Periodized training leads to larger strength gains than non-periodized training, and undulating periodized training leads to larger strength gains than linear periodized training. When stratifying by training status, periodized training leads to larger strength gains in both trained and untrained lifters. However, undulating periodized training only leads to larger strength gains in trained lifters, but not untrained lifters. When stratifying by lift, periodized training and undulating periodized training lead to significantly larger bench press strength gains than non-periodized and linear periodized training, respectively. Periodized and periodization style don’t seem to significantly affect squat strength gains.
Relative (%) strength gains tend to be larger in women than in men. When stratifying by age relative strength gains are larger for young women than young men, while relative strength gains aren’t significantly different in older men and women. When splitting upper and lower body strength gains, relative gains in upper body strength are larger for young women than young men, while relative gains in lower body strength aren’t significantly different between sexes. Relative hypertrophy is similar in men and women. Obviously, absolute strength gains and hypertrophy are larger in men.
That’s all we’ve got! We’ll update this page as new systematic review and meta-analyses are published. Feel free to bookmark this page and refer back to it when you want to get a quick overview of a given area of research.
If you made it this far, you’re clearly very passionate about staying up-to-date with the latest research in strength, muscle growth, and body composition. You should check out our research review: Monthly Applications in Strength Sport (MASS). Each month, we review the best and most relevant research for strength and physique athletes and coaches, helping you stay on the cutting edge. You can check out a free issue here, if you’re interested.