Leucine triggers mTORC1, initiating 24-48 hours of elevated myofibrillar protein synthesis after resistance training that builds contractile tissue.
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Myofibrillar Protein Synthesis
Mechanical force
mTOR signal
New protein
Leucine
Myofibrillar protein synthesis (MPS) is the specific process of incorporating amino acids into actin, myosin, titin, and other contractile proteins within the sarcomere. It is the direct cellular mechanism of muscle hypertrophy, and it is distinct from sarcoplasmic protein synthesis (synthesis of non-contractile cytoplasmic proteins). Measuring MPS specifically (via deuterium oxide or phenylalanine tracer techniques) rather than mixed MPS allows researchers to track contractile protein accretion independently of fluid and glycogen changes.
Resistance training elevates myofibrillar MPS through two independent and additive pathways: mechanical activation (via FAK, Rho, and mTORC1 activation from myofibril tension and titin mechanosensing) and nutritional activation (via the leucine/amino acid sensing by Sestrin2/GATOR2 upstream of mTORC1). Both must be present for maximal MPS. The post-training window is when the muscle is most sensitive to amino acid availability, because mTORC1 is already mechanically activated; additional leucine from a protein meal amplifies the translational response.
The leucine threshold refers to the minimum leucine content per meal required to maximally stimulate MPS. Below approximately 1.7-2.0 g leucine, MPS is sub-maximally activated. Above this threshold (achieved by roughly 35-40 g of high-quality protein such as whey, chicken, or egg), additional leucine does not further increase MPS acutely, because mTORC1 capacity is saturated. This threshold is the reason protein distribution across the day (3-4 meals each exceeding the leucine threshold) produces better muscle protein accretion than the same daily protein concentrated in one or two large meals.
The duration of elevated MPS post-exercise varies with training status and session characteristics. In untrained individuals, MPS can remain elevated for 36-48 hours after a single session. In well-trained subjects, the window shortens to 24-36 hours as adaptation increases the efficiency of the response. This is the physiological basis for training frequency recommendations: each muscle group needs to be trained approximately every 48-72 hours to keep MPS elevated without waiting for a full trough before the next stimulus, while avoiding training the muscle before the previous MPS elevation has fully resolved into contractile protein accretion.
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Source & accuracy
This myofibrillar protein synthesis is an editorial illustration built to represent the concept accurately. Where it shows figures, they are typical or representative values chosen to make the relationship clear, not a single underlying dataset. The diagram and its explainer are reviewed and maintained centrally, and updated over time as understanding improves.
Leucine, mTORC1, and the synthesis response
Myofibrillar protein synthesis is the building of the contractile proteins, mainly actin and myosin, that make up the working part of muscle. Resistance exercise and protein intake both raise its rate. The amino acid leucine is a particularly strong trigger, helping activate the mTORC1 signaling complex that initiates the synthesis of new muscle protein.
After a resistance training session, myofibrillar protein synthesis is elevated for roughly a day or two in trained individuals, with the response generally larger and more prolonged the more unaccustomed or demanding the stimulus. Exact duration varies with training status, the muscle worked, and nutrition.
How synthesis relates to actual muscle growth
Muscle grows when synthesis exceeds breakdown over time. A single elevated bout does not guarantee growth; it is the repeated net positive balance across many sessions, supported by adequate protein and energy, that builds contractile tissue. Spreading protein across meals and getting enough total daily protein support this balance.
This is general educational information, not medical or nutrition advice. Individual protein needs vary, and anyone with kidney concerns or other conditions should consult a qualified professional before changing intake substantially.
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