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Resistance Training for Muscle Size: How Many Days Per Week Is Best?

Clients' age and fitness status play a crucial role in finding the right frequency.

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STUDY REVIEWED: Grgic, J., Schoenfeld, B.J., & Latella, C. 2018. Resistance training frequency and skeletal muscle hypertrophy: A review of the available evidence. Journal of Science and Medicine in Sport, S1440–2440 (18), 30862–64.

To build a great resistance training program, fitness professionals manipulate a number of variables. The process requires striking a deft balance of sets, repetitions, intensity, contraction type, contraction speed, rest between sets and exercise frequency. And then there’s another common challenge: What about people specifically looking for muscle hypertrophy?

The standard recommendation is to train each muscle group 2–3 days a week with at least 48 hours of rest for each muscle group trained (ACSM 2014). But some fitness experts have questioned how strongly the science supports this recommendation, especially for people attempting to increase skeletal muscle size. Grgic, Schoenfeld & Latella (2018) reviewed the literature, and their findings help to clarify how exercise frequency affects success in gaining muscle mass.

The review showed that age and fitness level are key factors in determining a client’s optimum training intervals for building bigger muscles.

Study Process

Grgic and colleagues analyzed results of published peer-reviewed studies investigating links between muscle growth and training frequency, defined as the number of training sessions per muscle group per week. The studies compared resistance training frequencies in healthy adults, based on direct and indirect measures of changes in muscle size.

Training interventions were a minimum of 4 weeks, and subjects were resistance-trained and untrained people grouped by age (young: 18–39; middle-aged: 40–64; older adults: 65-plus). “Resistance trained” meant having at least 6 months of weight training experience. Training volume—total weight × reps × sets—proved to be one of the most crucial data points in muscle growth studies (see “Training Volume Is Crucial to Frequency Research,” below).

Specific Guidance for Untrained Clients

Resistance training research has produced helpful guidance for working with untrained clients. From the perspective of muscle protein synthesis—the metabolic “building block” process for increasing muscle size—some evidence suggests that the American College of Sports Medicine’s recommendation to train 2–3 times per week is appropriate (Saric et al. 2018). But other studies suggest a frequency of 1 or 2 days a week may be all that is needed for boosting muscle size.

A 10-week study by Gentil and colleagues (2015) compared 1 versus 2 days per week of resistance training in untrained men. Results showed that equal volumes of resistance training produced similar gains in muscle mass and strength, regardless of whether people trained once or twice a week. Moreover, Grgic and team’s literature review also concluded that training each muscle group once per week can elicit sufficient muscle growth in untrained clients. The authors added that further research is needed to validate these findings.

What is the take-home message? Grgic et al. noted that people often drop out of exercise because of a lack of time. Therefore, when the goal is muscle size, fitness pros can be flexible with entry-level training enthusiasts, as training a muscle group once a week elicits meaningful improvements in muscle growth. On the other hand, if resistance training is for health, fitness professionals are encouraged to follow guidelines from respected organizations, such as the American Heart Association, the American Diabetes Association, etc.

Trainers might also consider designing different types of resistance training programs—for example, using shorter weight training sessions, focused on one or two muscle groups daily, and spreading multiple sessions (covering all muscle groups) throughout the week.

Training Frequency: What Direct-Measures Research Tells Us

Resistance training research uses two kinds of measures—direct and indirect—to quantify the results of a study. We’ll start by looking at direct measures and then summarize indirect measures in the next section.

Direct measures use ultrasound, magnetic resonance imaging and muscle biopsies to measure muscle growth. Grgic and colleagues analyzed 10 direct-measure studies lasting 6–30 weeks with 289 resistance-trained and untrained participants (55 women; 234 men). Findings suggest that training a muscle group twice a week is optimal for young and middle-aged adults. It does not appear that any additional gains in muscle size result from training a muscle more than twice a week. The team noted that different muscle groups in the body are likely to respond uniquely to the frequency stimulus; thus, specific research comparing various muscle groups of the body is warranted.

Surprisingly, Grgic’s team found that although resistance training is very important for older populations (owing to loss of muscle mass from sedentary, aging lifestyles), there’s no clear optimal training frequency for gaining muscle size in this population. This suggests that fitness pros should vary frequency and track progress to find the best fit for older clients when the goal is hypertrophy. However, Grgic et al. cautioned that resistance training on too many days of the week may inhibit adequate recovery in older adults.

What About Indirect-Measures Research?

Indirect measures of muscle gain in resistance training research include body mass, dual-energy X-ray absorptiometry, skinfolds, bioelectrical impedance and whole-body densitometry. Grgic and team assessed the effect of resistance training frequency in 21 studies lasting 6–30 weeks with 2,472 resistance-trained and untrained participants (1,772 women; 700 men).

This review found no significant difference in the effectiveness of various training frequencies when training volumes were matched. According to Grgic et al., the results imply that a broad spectrum of training frequencies—1–4 per week, for example—will produce comparable changes in lean body mass. Grgic and colleagues further explained that indirect measures may not be sensitive enough to detect subtle changes in muscle growth. Thus, the best program design for increasing muscle size may be the one that heeds a client’s personal workout preferences. As long as there’s no delay in recovery from exercise or too much muscle soreness, fitness pros should individualize training frequency for each muscle group.

Key Benefits of Resistance Training

Resistance training promotes strength gains in both the elderly and young adults. Exercise-induced improvements in skeletal muscle are associated with these outcomes:

  • greater strength, power and functional independence
  • reduced risk of cardiovascular disease and type 2 diabetes
  • lower likelihood of needing to use the healthcare system
  • improved well-being without the use of prescription drugs

Source: Phillips 2007.

Training Volume Is Crucial to Frequency Research

Research finds that resistance training volume is one of the most influential variables for eliciting optimal gains in muscle size.

Researchers calculate training volume by multiplying weight x repetitions x sets. Studies find that larger training volumes yield greater improvements in muscle size.

In studies on resistance training frequency, researchers calculate and equate the intervention’s total training volume and then just change the frequency. This methodology enables an accurate assessment of the influence of training frequency on muscle size gains.

Sources: Grgic, Schoenfeld & Latella 2018; Saric et al. 2018.

What It All Means to Fitness Pros

Here’s a quick wrap-up of recommendations from Grgic and team’s results:

  • Untrained people can satisfactorily increase muscle size by training each muscle group 1 day per week.
  • Trained people seeking muscle size increases should work a muscle group twice a week if they are young or middle-aged.
  • For older exercisers, the research is unclear on the optimal training frequency for muscle size improvements. Personal trainers should individualize resistance training programs for older clients, focusing on long-term adherence that builds muscle, preserves bone mass and ensures effective recovery.
  • Training volume (reps x sets x load) has a profound effect on muscle gains. (We’ll summarize new
    research on training vol­ume in a future column.)

In designing resistance training programs, fitness pros should always focus on the goals of the client—weight management, health, osteoporosis prevention and so on. This new research on resistance training for increasing muscle size provides personal trainers with evidence-based ways to create time-efficient, effective designs.


ACSM (American College of Sports Medicine). 2014. ACSM’s Guidelines for Exercise Testing and Prescription (9th ed.). Philadelphia: Wolters Kluwer.

Gentil, P., et al. 2015. Effects of equal-volume resistance training performed one or two times a week in upper body muscle size and strength of untrained young men. Journal of Sports Medicine and Physical Fitness, 55 (3), 144–49.

Phillips, S.M. 2007. Resistance exercise: Good for more than just grandma and grandpa’s muscles. Applied Physiology, Nutrition, and Metabolism, 32 (6), 1198–205.

Saric, J., et al. 2018. Resistance training frequencies of 3 and 6 times per week produce similar muscular adaptations in resistance-trained men. Journal of Strength and Conditioning Research. doi:10.1519/JSC.0000000000002909.

Zachary Mang, PhD

Zachary Mang, PhD, is a postdoctoral research associate for the wellness program at the Los Alamos National Lab where he specializes in strength and conditioning for structural firefighters. His research interests include resistance training for hypertrophy, oxidative adaptations to resistance training, and the use of resistance training as a frontline defense to prevent chronic disease.

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