Full mechanism, in text

How tension switches on growth

1 · Mechanical tension = signal

Mechanical tension is the main stimulus for muscle growth; a signaling chain in the cell turns it into a chemical signal, a model drawn mostly from cell and animal experiments.Schoenfeld 2010 splits why training makes muscle grow into three stimuli: mechanical tension, metabolic stress and muscle damage. Mechanical tension is the main one.

How tension becomes a signal: the chain below is one of the more-studied explanations. Most of it comes from cell and animal experiments, and some middle steps have not been fully confirmed in human muscle, so read it as a model that makes sense.
· When a fiber bears load, the structures linking the sarcomeres to the cell membrane are pulled
· Integrins in the membrane grip the connective-tissue mesh outside at one end and the cell skeleton at the other, bringing the pull inside
· Under load, a small platform gathers on the inside of the membrane (a focal adhesion), and FAK (focal adhesion kinase) is thought to help translate the pull into a chemical signal
· Phosphatidic acid (PA) is also thought to be part of passing the signal on

Key idea: turning force into a chemical signal is the core of muscle growth. The pull itself does not make muscle grow directly; it works through a signaling chain inside the cell.

This also explains:
· Lighter loads taken close to failure still build muscle: the last few reps approach maximal tension
· Heavy loads for low reps build muscle too: every rep carries high tension
· Schoenfeld 2017 : when sets are taken close to failure, light and heavy loads build muscle about equally

2 · FAK + PA → mTORC1

Mechanical tension, amino acids (especially leucine) and insulin converge on , the master switch for protein synthesis, so training and protein work together. is the cell's master switch for protein synthesis.

3 upstream signals converge on it:
· Mechanical tension (the training route; the model from the previous step)
· Amino acids, especially leucine (the protein route; a common rule of thumb is about 2.5-3 g of leucine per meal, see protein-and-lifting)
· Insulin / → PI3K → AKT → mTORC1 (the well-fed route)

Key: training and protein work together.
· Training makes the switch easier to flip, but synthesis needs raw material (amino acids)
· After a protein meal, synthesis stays raised for a few hours; that does not mean you must eat within 30 minutes of training
· Aragon 2013: keeping the meals before and after training within about 3-4 hours of each other is enough; Morton 2018 also found timing, per-dose amount and source play a minor role at most. Eating enough across the day matters more

The other direction:
· (switched on when energy is short) inhibits mTORC1. This is often used to explain the interference effect when heavy endurance and strength training are combined; it is one explanation, and how much interference actually happens depends on the person and the program

3 · mTORC1 → S6K1 + 4E-BP1 → synthesis

Once on, phosphorylates S6K1, which adds ribosomes, and 4E-BP1, which releases eIF4E to start translation, so protein synthesis in the fiber speeds up.Once is switched on, it drives two pathways at once that speed up protein synthesis:

Pathway 1 · S6K1 (ribosomes):
· mTORC1 → phosphorylates S6K1 → more ribosomes are made and translation becomes more efficient
· Ribosomes are the protein-making machines: more machines raise the ceiling on synthesis

Pathway 2 · 4E-BP1 (translation start):
· mTORC1 → phosphorylates 4E-BP1 → releases eIF4E → the translation-start complex can assemble
· mRNA that was held back can now be translated into protein

The two converge → protein synthesis inside the fiber speeds up.

How it is measured: tracing amino acids labelled with stable isotopes after training shows muscle protein synthesis (MPS) rising after a single session and staying up for a while (hours to a day or two, depending on training status).

Anabolic resistance in older adults: older muscle responds less to the same meal and the same session, so PROT-AGE recommends older people eat somewhat more protein than younger adults (see elderly-resistance-training).

4 · Accumulation over months

Each session lets synthesis briefly outpace breakdown for a tiny net gain, and training each muscle 2-3 times a week for months adds these into a visible change.After one session, synthesis briefly outpaces breakdown, leaving a tiny positive net gain.

Accumulation:
· Each session leaves only a very small net gain
· Training each muscle 2-3 times a week for months is what adds those small gains into a change you can measure and see
· Beginners grow fastest; after many years of training, each year adds less and less as you approach your own ceiling

Constraints:
· Volume: the Schoenfeld 2017 shows a graded dose-response: fewer than 5 sets per muscle per week, 5-9 sets, and 10 or more sets give progressively more growth. The coaching range of 10-20 sets a week is experience, not limits set by the paper
· Protein: in the Morton 2018 meta-analysis, the benefit for muscle levelled off around 1.6 g/kg a day (95% 1.03-2.20); for people trying to maximise gains, the authors suggest about 2.2 g/kg is the safer bet
· Sleep and recovery: see recovery-science
· Energy: in a large calorie deficit, building muscle at the same time is hard

Traps to avoid:
· Piling on sets: beyond what you can recover from, fatigue outweighs the benefit
· Constantly switching exercises: each switch means relearning the movement, and the truly effective volume ends up too low

Full mechanism, in text

1 · Mechanical tension = signal

2 · FAK + PA → mTORC1

3 · mTORC1 → S6K1 + 4E-BP1 → synthesis

4 · Accumulation over months

Mechanical tension → mTOR → hypertrophy

Full mechanism, in text

1 · Mechanical tension = signal

2 · FAK + PA → mTORC1

3 · mTORC1 → S6K1 + 4E-BP1 → synthesis

4 · Accumulation over months

Mechanical tension → mTOR → hypertrophy