Muscle protein synthesis (MPS) is the process where your body assembles amino acids into new muscle protein, repairing and building tissue. Two things switch it on: resistance training and eating protein. After a hard session it stays elevated for roughly 24 to 48 hours. Muscle grows when synthesis outpaces breakdown over weeks and months, which makes MPS the actual target of every hypertrophy plan.
Why it matters
Because your body is constantly running the opposite process too. Muscle protein breakdown never stops, so your muscle mass is a running tally: synthesis minus breakdown, every day, forever. Lifting alone spikes MPS. Protein alone spikes it. Stack them and the effect is bigger than either. That 24-to-48-hour window also explains a core programming rule: if MPS returns to baseline within two days, a muscle trained only on Mondays spends most of the week not growing. Training each muscle twice a week keeps the machinery switched on more often.
How to use it in training
Three levers, in order of importance. First, train hard enough to trigger the signal: sets taken close to failure, per muscle, at least twice a week. Second, eat enough total protein, somewhere around 1.6 to 2.2 grams per kilogram of bodyweight daily for people chasing growth. Third, spread it out. Research suggests roughly 0.4 grams per kilogram per meal across 3 to 5 meals beats cramming the day's protein into one giant dinner, since each protein feeding triggers its own MPS response.
And relax about the clock. The post-workout anabolic window is far wider than the old 30-minute panic suggested. Total daily protein and consistent training move the needle. Timing is a detail.
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Go deeper
Want the studies on how to spread protein across the day? Read our full breakdown: Protein distribution research.