Muscle protein synthesis
Muscle protein synthesis (MPS) is the cellular process of assembling new proteins in muscle tissue from amino acids. It continually replaces and remodels proteins and can contribute to the addition of contractile and other muscle proteins over time.
MPS is not one single product. Myofibrillar protein synthesis concerns proteins in the force-producing machinery, while other fractions include mitochondrial and sarcoplasmic proteins. Researchers measure synthesis rates with stable-isotope tracers and tissue samples over a defined period. A percentage change in one fraction therefore cannot be read as the percentage of new muscle a person has gained.
Resistance exercise and dietary protein can raise myofibrillar protein synthesis. Protein breakdown occurs at the same time, so the balance between synthesis and breakdown determines whether muscle protein is being gained or lost during a period. Repeated positive balances can support hypertrophy, but whole-muscle growth also reflects training history, recovery, energy availability, and how consistently the relevant fibers are loaded.
An acute MPS rise is a biological response, not a direct score for workout quality or a dependable forecast of an individual's long-term growth. A human training study of acute MPS found that the post-exercise response after an initial bout did not correlate with later hypertrophy. In previously untrained muscle, early synthesis may be directed substantially toward repair and remodeling. Another longitudinal human study found that integrated myofibrillar synthesis related more closely to hypertrophy after muscle damage had attenuated.
MPS also should not be reduced to an on/off "anabolic switch." Its rate changes over time and depends on the proteins measured, training stimulus, prior training, food intake, age, and other factors. Consuming adequate total protein and distributing protein-containing meals can supply the required amino acids, while progressive resistance training supplies the recurring demand. No sensation, pump, wearable, or workout log directly measures muscle protein synthesis.