Does working out twice a day help build muscle?

Not in the one study that tested it directly in trained lifters. When trained men did the same weekly sets as one session a day or two, muscle thickness grew the same over 8 weeks; only the squat max rose more with two sessions4.

So twice a day is a way to arrange work, not a way to add muscle on its own. Below is what was tested, what moved, and why the squat result is easy to over-read.

What was studied

The key study is a randomised trial from Brazil, published in 20224. Randomised means a draw, not the lifters, decided who went into which group. Resistance-trained men were split into two groups:

• Once a day: every muscle group trained in one daily session (11 men).
• Twice a day: every muscle group trained in two daily sessions (12 men).

Both groups trained for 8 weeks. Before and after, the researchers measured three things4:

• One-rep max (1RM): the heaviest load a person can lift once with good form, on the bench press and the parallel back squat. (More on it in what an estimated 1RM is.)
• Muscular endurance: how many reps a person can do at a lighter load, here 60% of their 1RM.
• Muscle thickness: the depth of a muscle measured on a scan, a standard stand-in for muscle size. It was taken at five sites: biceps, triceps, outer thigh, front thigh and chest.

The published summary does not say whether weekly work was matched. A later meta-analysis team extracted that detail from the full paper and posted it as open data5. Their table lists the same 32 weekly sets for both groups, 4 sessions a week for the once-a-day group and 8 for the twice-a-day group, every set taken to failure (the point where another rep with good form is not possible), and 1.5 minutes of rest between sets.

In plain terms: same work, same weeks, same kind of lifter. The only thing that changed was whether a day's training came in one block or two.

What the study found

One result differed. The squat 1RM rose 16.1% with two sessions a day and 7.8% with one, a statistically significant gap4. That is about twice the gain.

Everything else came out the same. Both groups improved their bench press 1RM (4.6% once a day, 6.8% twice a day), squat endurance (19.0% vs 24.3%), bench endurance (15.4% vs 24.0%) and muscle thickness at all five sites, "with no differences between experimental groups"4.

The authors' own conclusion was careful: twice-daily training "augments lower-body muscular strength", but daily frequency "does not seem to have any additive effect" on upper-body strength, endurance or muscle size in trained men4.

In plain terms: two sessions a day grew the same amount of muscle as one. The squat was the only measure that moved more.

Why the squat moved and the muscle did not

The study does not explain the squat result. A common reading is that a heavy lift is partly a skill, and more practice sessions give more chances to rehearse it. That is a hypothesis, not a finding here. The bench press, also a practised skill, did not show the same gap, so the study cannot settle it.

Two details make the squat number less tidy than it looks:

• The groups started at different levels. In the meta-analysts' extracted table, the twice-a-day group's average squat 1RM at the start was lower than the once-a-day group's5. A group that starts lower can have more room to gain in percentage terms.
• Small groups. With 11 and 12 men randomised (9 per group in the extracted outcome rows), one or two lifters can swing a group average.

The endurance numbers lean toward the twice-a-day group too. The study reported no significant difference, so reading them as "more endurance" would go beyond what was found.

In plain terms: the squat result is real in this study, but it is one outcome in one small trial, and its cause was not tested.

What older studies of split sessions found

Two earlier studies looked at splitting the same training volume into two daily sessions. Both were short and small.

In 1994, ten female athletes trained for 3 weeks with one daily session, then for a second 3 weeks with the same total volume split into two daily sessions1. In the one-session period there were no systematic changes in isometric leg strength (pushing against an immovable resistance) or in the cross-sectional area of the front thigh. In the two-session period, isometric strength rose from 2,493 to 2,620 newtons, about 5%, and the thigh's cross-sectional area increased1. The design has a catch: the split period always came second, so it cannot separate the split from simply training longer.

In 2007, ten nationally competitive male weightlifters were randomly assigned to once-daily or twice-daily sessions with similar volume for 3 weeks2. There were no significant differences between groups on any measure, including muscle cross-sectional area and weightlifting performance. Isometric knee-extension strength rose 5.1% with two sessions and 3.2% with one, a gap that was not statistically significant2.

In plain terms: the older studies hint at small strength effects. The 1994 authors read their result as a sign that splitting may favour growth too, but with a fixed order and no randomisation, and a null 2007 trial, neither shows a reliable muscle-size advantage.

How this fits the bigger picture on frequency

Twice a day is one extreme of a broader question: how often should a muscle be trained? The research on weekly frequency gives useful context.

A 2019 meta-analysis pooled 25 studies3. When weekly volume was equated, higher and lower frequencies produced no significant difference in muscle growth, in trained lifters or in upper- or lower-body muscles. When volume was not equated, higher frequency came out ahead, which the authors attribute mostly to the extra work. They concluded that, for a given volume, people can choose a weekly frequency "based on personal preference"3.

A 2026 meta-regression of 67 studies and 2,058 participants reached a similar split5. More weekly sets meant more growth and more strength, with diminishing returns. More frequent training showed a consistent link with strength gains, also with diminishing returns, while its effect on muscle size was "compatible with negligible effects"5.

The twice-a-day trial fits that pattern: frequency showed up in a strength measure, and total work, which was held equal, drove size. Weekly sets are covered in how many sets per muscle group per week.

What this means in practice

This is description, not a recommendation to train twice a day. What the studies show:

• Muscle size followed total work. In the direct trial and the frequency meta-analyses, matching weekly sets matched growth43.
• Strength on a practised lift may respond to more frequent exposure. That fits both the squat result and the 2026 meta-regression, with the caveats above5.
• Two sessions a day is a scheduling choice. If a single long session does not fit a day, the trials that split it kept the same total sets. The question of how many days to train is in how many days a week should you lift, and the related question of whether three short workouts can replace one gym session has its own article on this site.

Nothing in these studies measured fatigue, sleep, hormones or anything beyond 8 weeks in the direct trial. Two sessions a day was not tested in women in a randomised design, nor in beginners or older adults.

In plain terms: if the weekly work is the same, splitting it into two sessions a day did not add muscle in the studies that tested it.

What the studies found

Limits worth knowing

• One direct trial in trained lifters. Twenty-three men randomised, fewer analysed, over 8 weeks4.
• Men only in the randomised trials. The only study in women had no control group and a fixed order1.
• Abstracts only. The Corrêa paper is not open access. We read its abstract; the matched weekly sets and session counts come from the meta-analysts' extraction of the full text, not from a page we read5. We also read only the abstracts of the other four papers.
• Short studies. The two older trials lasted 3 weeks each, too short to say much about muscle growth12.
• Not measured. Fatigue, sleep, soreness, injuries, time costs and anything past 8 weeks.

Latest research

The newest study we opened is the 2026 dose-response meta-regression in *Sports Medicine*5. It is not a twice-a-day trial; it pools 67 studies of weekly volume and frequency. Its finding that frequency tracks strength but not clearly size matches the direct trial's pattern. We found no randomised twice-a-day trial newer than the 2022 study. When one appears, this page will be updated and its date will change.

The short version

• In trained men on the same weekly sets, two sessions a day grew the same muscle as one over 8 weeks.
• The squat max rose more with two sessions (16.1% vs 7.8%), with a lower starting point and a small sample as caveats.
• Bench strength and endurance showed no difference.
• Across frequency studies, total weekly work, not how it is split, tracks muscle growth.

Educational overview only — not medical advice.