In 2015, Brad Schoenfeld (and colleagues) investigated the effects of training muscle groups once per week and training muscle groups multiple times per week on muscle and strength gains in well-trained men 1
This is what they did.
Things to Bear in Mind
• The length of the Study: Although 8 weeks was sufficient to achieve significant increases in strength and muscle gains in both groups, differences could have diminished with a longer term study.
• The Novelty Factor: 16 of the 19 subjects reported training with a split routine on a regular basis before the study. There is evidence to suggest muscular adaptations are enhanced when program variables are altered from the normal stimuli and it’s possible the participants in the TB group benefited from the new stimulus of high-frequency training.
• Higher frequency (training each muscle group 2-3x per week) seems to trump lower frequency (training each muscle group 1x per week) if muscle growth is the goal.
• This doesn’t mean ‘Bro-split’ style training is bunk. Both can and should be used in a periodized program. Example: a period of high-frequency training followed by a period of low-frequency training.
• If you can only train 2-3 x per week, then a full body routine will be more optimal than a body part split.
• Trained individuals benefit more from higher frequency if the goal is to maximise muscle gain.