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Resistance Training for Muscle Size: How Many Days Per Week is Best?
Zachary Mang, M.S. and Len Kravitz, PhD

Resistance training (RT) is a popular training modality used by a wide variety of exercisers. It has been shown to promote strength gains in both the elderly and young adults (Philips 2007). Philips highlights that the exercise-induced improvements in skeletal muscle are associated with numerous benefits including strength, power, and functional independence as well as reducing the risk for cardiovascular disease and type 2 diabetes. Philips continues that resistance training programs have also been shown to reduce an individual's use of the health care system, while also providing overall well-being improvements not observed with a pharmacological intervention.

The training variables manipulated by fitness pros designing RT programs include the number of sets, number of repetitions, intensity, contraction type, contraction speed, rest intervals between sets and frequency of exercise. Specific to frequency of exercise, when programming adults for health-related resistance training programs, the American College of Sports Medicine (ACSM 2014) recommends that each muscle group should be trained 2-3 days/week, with at least 48 hours separating the exercise training sessions for the same muscle group. In regards to increasing skeletal muscle size (i.e., hypertrophy), particularly for trained individuals, experts in the field have questioned whether the evidence fully supports these ACSM guidelines (Grgic et al., 2018). In addition, Grgic and others have deliberated on the precise role that frequency of exercise has on increasing gains in muscle mass. Given these unanswered questions, and since frequency of exercise is a major recommendation fitness pros use regularly, Grgic et al. collected and synthesized the most recent research review on RT frequency for muscular hypertrophy.

Study Reviewed:
Grgic, J, Schoenfeld, B.J., and Latella, C. (2018). Resistance training frequency and skeletal muscle hypertrophy: A review of the available evidence. Journal of Science and Medicine in Sport (Available online 13 September 2018).

The Study Process
Grgic et al. (2018) searched the main databases utilized in the field of RT for peer-review published studies investigating the effect of frequency on hypertrophy. For clarity, RT frequency was defined by the researchers as the number of training sessions performed per muscle group within a week. Studies included were investigations that compared RT frequencies in healthy adults using direct measures and indirect measures for changes in muscle size. Training interventions were a minimum of four weeks in resistance trained and untrained individuals (grouped by young = 18-39yrs; middle-aged = 40-64yrs, and older adults= &Mac179;65yrs). Resistance trained individuals were defined as having at least six months of RT experience.

Understanding the Importance of Equating Training Volumes for RT Frequency Research
When it comes to RT variables most influential for eliciting optimal gains in muscle size, training volume has been indicated to be decisively important (Grgic et al., 2018). Training volume is calculated in research in the following way: load (i.e., weight) X repetitions X sets (Saric et al., 2018). Grgic and colleagues highlight that recent studies show that greater volumes of training result in greater improvements in muscle size. In studies on frequency of training sessions per week it is therefore imperative that RT study designs equate the total volume of the interventions (Grgic et al., 2018). By equating the total volume of training with the interventions, the only variable manipulated is RT frequency. This methodology allows for the accurate way to assess the influence of frequency of training for muscle size gains.

What Frequency of RT Exercise is Best for Untrained Adults?
Researchers are quite interested in RT with men and women who are identified as trained (i.e., having at least six months of RT experience) and those who are untrained. This is great because all fitness pros work with entry level untrained RT enthusiasts regularly as well as with those trained participants. Saric et al. (2018) propose that from a muscle protein synthesis perspective, which is the metabolic 'building block' process for an increase in muscle size, some evidence suggests that the ACSM recommendation for a training frequency of 2-3 times per week is may be a satisfactory training goal for untrained individuals. Interestingly, in a 10-week study by Gentil and colleagues, comparing one versus two day per week of RT training in untrained men, the researchers showed similar gains in muscle mass and strength with equal volume of RT. In their review of literature, Grgic et al. (2018) concur that training a muscle group once per week can elicit sufficient muscle growth. What is the take home message? In their summary, Grgic first highlight that a primary reason people drop out of exercise is due to a lack of time. Therefore, fitness pros can be very flexible with entry level RT enthusiasts, as training a muscle group once per week elicits very meaningful improvements in muscle growth. This suggests that individualizing the RT program for the beginner client's schedule should be a priority. Perhaps this also suggests initiating different types of RT training program designs, such as using shorter sessions of training one or two muscle groups on a daily basis, with multiple resistance training sessions (covering all muscle groups) spread out throughout the week.

What Frequency of RT Exercise is best from Direct Measures Research?
In RT studies that measured hypertrophy with direct measures (e.g., ultrasound, magnetic resonance imaging, and muscle biopsy) Grgic found 10 studies that met their inclusion requirements. This data represented studies ranging from 6 to 30 weeks with a total of 289 participants (55 females and 234 males). After reviewing all of the research, Grgic and others suggest that with trained individuals it appears that training a muscle group twice a week is optimal for young and middle-aged adult populations. The researchers state that it does not appear there will be additional muscle size gains by training a muscle more that twice a week. One limitation Grgic et al. note is that it is very likely that different muscle groups in the body respond uniquely to the frequency stimulus, and thus specific research comparing various muscle groups of the body is warranted. Surprisingly, from their research review the Grgic research team summarize that although resistance training is very important for older populations (due to the loss of muscle mass with a sedentary lifestyle as someone ages), it remains UNCLEAR what is the optimal frequency of training per muscle group each week to attain the best muscle size gains for this population. This suggests that fitness pros should vary frequency and track progress of gains in their older clients, to best see what frequency stimulus leads to the best gains. However, Grgic et al. caution that RT on too many day of the week may delay adequate recovery from exercise with older adults.

What Frequency of RT Exercise is best from Indirect Measures Research?
Grgic et al. 2018 assessed the effect of RT frequency in 21 studies that used indirect measures (e.g., body mass, DXA, skinfolds, bioelectrical impedance, and whole body densitometry). This totaled 2472 participants (1772 females and 700 males). The range of the studies was 6 to 30 weeks. Most of the studies assessed muscle gains with training frequencies from 1 to 4 days per week among untrained individuals. The results of this review with these indirect measures of hypertrophy is there is no significant difference with the training frequencies when training volumes are matched (Grgic). Grgic summarize that the results imply comparable changes in lean body mass can be attained by a broad spectrum of frequencies per week (i.e., 1 to 4) and that program design may best be determined on a personal preference of client training. As long as there does not appear to be a delay in recovery from exercise, or to much muscle soreness, fitness pros are encouraged to individualize the frequency of training for each muscle group for their clients.

When volume is equated, the current research indicates for untrained individuals a training frequency of one day per week is quite effective for increases in muscle size. Grgic and others (2018) conclude that with trained individuals, the evidence indicates that training a muscle group twice a week is optimal for young and middle-aged adult populations. For increases in muscle size with an older population the research is currently unclear. Fitness pros are encouraged to individualize the RT program with each older client, perhaps designing RT programs more to ensure long-term adherence to exercise, that lead to gains in muscle and preservation of bone mass, and that allow for effective recovery from sessions. Importantly, the research appears to show that training volume (reps X sets X load) has the profound effect on muscle gains and that frequency of exercise has more of a secondary effect. In a future research column we will summarize this new research on training volume. Alas, one final take home message to fitness pros, in regards to weekly frequency of RT muscle groups, is to explore time-efficient exercise program designs on an individual basis for each client. Determine what frequency is most enjoyed, which seems to elicit the best targeted gains, which results in the least amount of soreness, and which has the best healthful recovery from the RT.

Zachary Mang, M.S. is a youth baseball coach in Albuquerque, NM and a doctoral student in Exercise Science at the University of New Mexico. His research interests include metabolic adaptations to HIIT, resistance training interventions in obese youth populations, and molecular adaptations to exercise as they pertain to health and fitness.

Len Kravitz, PhD, CSCS, is the program coordinator of exercise science at the University of New Mexico, where he received the Outstanding Teacher of the Year and Presidential Award of Distinction. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Len was awarded the 2016 CanFitPro Specialty Presenter Award.

Additisonal References:
ACSM. (2014) ACSM's Guidelines for Exercise Testing and Prescription 9th Ed. Philadelphia: Wolters Kluwer.
Gentil, P., Fischer, B., Martorelli, A.S. 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), 144-149.
Philips, S.M. (2007). Resistance exercise: good for more than just Grandma and Grandpa's muscles. Applied Physiology, Nutrition, and Metabolism, 32; 1198-1205.
Saric, J., Lisica, D., Orlic, I., 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 (in Press).