Training Muscles Twice a Week vs Once: Evidence

This is one of the most quoted findings in the hypertrophy literature, and the quote is a decade out of date. The short version: the advantage of twice a week over once turned out to belong to the extra sets that usually came with it, not to the extra session.

What the 2016 analysis found

Schoenfeld, Ogborn and Krieger identified 10 studies that directly compared weekly training frequencies with morphological measurements — biopsy, imaging, circumference or densitometry — over at least four weeks. Treating frequency as a binary predictor, higher frequency produced a larger effect on hypertrophy than lower frequency, 0.49 against 0.30, with the comparison reaching statistical significance. The authors concluded that major muscle groups should be trained at least twice a week to maximize growth, and explicitly left open whether three times beat twice.

That paper is where the “each muscle 2x per week” rule in every program template comes from.

What the 2019 update found

Three years later, the same lead author with Jozo Grgic and James Krieger repeated the exercise with 25 studies. The result went the other way. On a volume-equated basis there was no significant difference between higher and lower frequency — not for direct measures of growth, not in resistance-trained participants, not when upper body and lower body were separated out. Their conclusion is worth reading literally:

In conclusion, there is strong evidence that resistance training frequency does not significantly or meaningfully impact muscle hypertrophy when volume is equated. Thus, for a given training volume, individuals can choose a weekly frequency per muscle groups based on personal preference.

The same paper found something that explains the confusion. In studies where volume was not equated, the meta-regression did favor higher frequencies — because training a muscle more often is the easiest way to end up doing more sets. The authors described the magnitude of that difference between 1 and 3-plus days per week as modest.

Why both results can be true

Both papers are measuring the same thing from different sides of one variable. Frequency and volume travel together in the real world: nobody adds a second chest day and removes an equal number of sets from the first. Equate the sets in a study design and the frequency effect disappears; let them float, as they do in a gym, and higher frequency looks better because it is carrying more work.

The 2026 dose-response meta-regressions, across 67 studies and 2,058 participants, drew the line cleanly. Volume had a 100% posterior probability of a positive slope for both size and strength, with diminishing returns. Frequency’s effect on hypertrophy was compatible with negligible, while its effect on strength was consistent and identifiable.

What to do with this

• Set the weekly volume first. It is the variable with the evidence behind it.
• Then pick the frequency that lets you complete it. If 18 sets of quads in one session means the last six are junk, that is the argument for splitting them — a scheduling argument, not a physiological one.
• If you are chasing a lift rather than a look, frequency earns its keep on its own. More exposures to the squat means more practice at squatting.

Related pages

Run your own numbers in the training frequency calculator, turn the answer into a program with the workout split calculator, and see muscle recovery time between workouts for how far apart those sessions need to be. If the goal is strength, the strength standards chart is a useful reality check on where you sit.

This content is informational and does not replace advice from a physician or a certified strength coach.