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What Is Muscle Protein Synthesis? Simply Explained
Muscle Protein Synthesis (MPS) is the metabolic process involving the incorporation of amino acids into new skeletal muscle proteins, thereby rebuilding and strengthening muscle tissue in response to stimuli such as resistance exercise and dietary protein consumption.
4 questions. Published Mar 11, 2026. Updated Oct 19, 2026. By AI Fit Hub.
Definition
Muscle Protein Synthesis
Muscle Protein Synthesis (MPS) is the metabolic process involving the incorporation of amino acids into new skeletal muscle proteins, thereby rebuilding and strengthening muscle tissue in response to stimuli such as resistance exercise and dietary protein consumption.
Why it matters
Muscle grows when protein synthesis outpaces protein breakdown over weeks of training. Each training session and protein-containing meal raises synthesis for a while, so how often and how much protein you eat, and how often you train, shape how much of the day muscle spends in a net building state.
How it works
MPS runs through a cellular signaling pathway. Resistance exercise causes minor muscle damage and activates specific intracellular pathways, most notably mTOR (mammalian Target of Rapamycin). With amino acids available from dietary protein, mTOR activation drives the translational machinery in muscle cells. Messenger RNA (mRNA) carries the protein-coding instructions, and ribosomes translate them into new protein structures using the amino acid pool. MPS rate is shaped by training intensity, protein quantity and quality, and nutrient timing. The net effect on muscle mass is the balance between MPS and Muscle Protein Breakdown (MPB) — when MPS exceeds MPB, muscle grows.
Example
How long muscle protein synthesis stays raised after one heavy session
Basal rate in young and older men (Moore 2015)
about 0.028% per hour
4 hours after heavy resistance training
about 50% above baseline
24 hours after
about 109% above baseline (more than double)
36 hours after
within 14% of baseline
After a heavy session, protein synthesis rises quickly, is more than double at 24 hours and is almost back to baseline by 36 hours (MacDougall 1995). Protein eaten during that window feeds the response, which is one reason to spread protein across the day and to train each muscle again within a few days.
Key takeaways
MPS is the fundamental process for muscle repair, recovery, and growth, driven by both exercise and protein intake.
Protein raises it; 20 g maximised the response over 4 hours in young men (Moore 2009), while 100 g gave a larger and longer response than 25 g over 12 hours (Trommelen 2023).
The net balance between MPS and Muscle Protein Breakdown (MPB) ultimately dictates whether muscle mass is gained, maintained, or lost.
Related terms
Questions people ask next
How long does the elevated MPS response last after a workout?
About a day and a half. In MacDougall's study, synthesis was about 50% above baseline 4 hours after heavy training, more than double at 24 hours, and almost back to baseline by 36 hours. Eating protein during that period, and regularly across the day, keeps feeding the response.
What is the optimal protein intake to maximize MPS?
Gains in muscle from protein supplementation level off at a total intake of about 1.6 g per kilogram of body weight a day, with an upper confidence limit of 2.2 g/kg (Morton 2018). Per meal, 20 g of protein maximised synthesis over 4 hours after exercise in young men (Moore 2009), and older men needed more relative to body weight (about 0.40 g/kg against 0.24 g/kg; Moore 2015).
Is there a limit to how much MPS can be stimulated at once (the "muscle full effect")?
Less than once thought. Measured over 4 hours, synthesis plateaued at 20 g of protein in young men (Moore 2009). Measured over 12 hours with a more complete method, 100 g of protein produced a larger and longer-lasting response than 25 g, with no upper limit found (Trommelen 2023). A big protein meal is not wasted; spreading protein across the day is still a sensible default.
How does age affect Muscle Protein Synthesis?
Older muscle responds less to small protein doses. In Moore's analysis, synthesis plateaued at about 0.40 g of protein per kilogram of body weight in older men (around 71) against 0.24 g/kg in younger men, so older adults need a larger serving per meal, roughly 30 g for a 75 kg person, to get the same response. Resistance training remains the main stimulus at any age.
Sources
- Protein and amino acids for athletes , Journal of Sports Sciences (2004) — Tipton & Wolfe
- Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men , American Journal of Clinical Nutrition (2009) — Moore et al.
- Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men , The Journals of Gerontology: Series A (2015) — Moore et al.
- The time course for elevated muscle protein synthesis following heavy resistance exercise , Canadian Journal of Applied Physiology (1995) — MacDougall et al.
- The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans , Cell Reports Medicine (2023) — Trommelen et al.
- A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults , British Journal of Sports Medicine (2018) — Morton et al.