There are a number of variables that can be manipulated to stimulate training adaptations in athletes. These include volume, intensity, rest periods, tempo, movement, frequency and so forth. When training specifically for muscular hypertrophy, the overwhelming majority of lifters opt for a low training frequency (training muscles only once or twice per week) with high volumes. Body parts are often split so that upper body and lower body workouts are alternated. Alternatively, some prefer to split the body into push (i.e., pecs, delts and triceps), pull (i.e., lats, traps and biceps) and legs with a training frequency that allows muscle groups to be trained only once per week. Training with a higher frequency is generally avoided to allow for sufficient recovery prior to the next training session. Unfortunately, there is limited research in this area to really determine how frequently one should train a muscle to optimize size and strength improvements.
A new study published ahead of print in the Journal of Strength and Conditioning Research sought to compare traditional hypertrophy training (body part split, muscles trained once per week) to a higher frequency training protocol (full-body workouts, muscles trained 3 times per week). Twenty well trained college-aged males were split into one of the two training programs. The training program lasted 8 weeks and involved 3 training sessions per week on non-consecutive days. The groups were matched for baseline strength and body mass. Before and after the training program, the researchers evaluated muscle thickness of the forearm and of the vastus lateralis in addition to 1 rep max in the bench press and parallel back squat. Volume was equated and nutrition was monitored to control for confounding variables.
After the 8 week training period the results tended to favor the higher frequency training group. 1RM bench press improved by 10.6% in the high frequency group compared to a 6.3% increase in the body part split group. Improvements in 1RM squat strength did not differ between groups (~11% increase in both). Though not all reached significance, muscle thickness measures were greater in the high frequency training relative to the body part split group. Collectively, these results suggest that training muscles three times per week is likely superior to training them once per week for strength and hypertrophy gains. The authors suggest periodizing training by varying training frequency to maintain the effectiveness of the stimulus.
Schoenfeld, B. J., Ratamess, N. A., Peterson, M. D., Contreras, B., & Tiryaki-Sonmez, G. (2015). Influence of Resistance Training Frequency on Muscular Adaptations in Well-Trained Men. The Journal of Strength & Conditioning Research. Ahead of print.