How a fibre grows: tension, time, and the protein ledger

A muscle gets bigger when the fibres inside it keep more protein than they throw away, and they do it for weeks. The two rates have names. Muscle protein synthesis is the building. Muscle protein breakdown is the demolition. You never see either one in the mirror. You see the integral: cross-sectional area, the thickness of the fibre, months later. A session can double the building rate for a day. A meal can raise it for a few hours. A drug can raise the gain on both. None of those is the growth. Growth is the building staying ahead of the demolition until the fibre has to lay down more sarcomeres, which are the repeating contractile units, side by side.

In short. Muscle grows when you build protein faster than you break it for long enough that the fibre gets thicker. A pump or one meal is not that. Months are.

The measurement is a tracer. Give a person a labelled amino acid, take a biopsy, and the fraction of new protein that contains the label is the fractional synthetic rate. After a hard session that rate rises, often for something like a day in people who already train, and longer in people who have never lifted, because their fibres are also busy repairing. Damas and colleagues made the awkward point cleanly: early on, a lot of that 'synthesis' is patching damage, and it does not track the growth you measure later. Once the person is trained and the damage has settled, the rise in myofibrillar synthesis — the contractile proteins, actin and myosin — lines up better with the fibre getting bigger. So a phospho-blot the afternoon of your first squat is not a prophecy. The tape measure in week twelve is closer.

In short. Labs measure building with a labelled amino acid and a biopsy. Right at the start, a lot of that building is repair. Later, it is the contractile protein you actually wanted.

Three stories get told about why a set causes this, and they are not equal. Mechanical tension is the one with the clean evidence. The fibre is loaded, costameres and integrins feel the pull, phosphatidic acid and a few other local signals lean on mTORC1, and the ribosome is allowed to keep going. Metabolic stress is the burn and the pump: lactate, hydrogen ions, a swollen cell. It probably helps by fatiguing the fibre so more motor units have to join, and a swollen cell is a mild anabolic hint. It is not a substitute for tension. Muscle damage is the third story and the most oversold. Soreness means connective tissue and membrane complained. Eccentric work, the lowering, causes more of it. You can grow from concentric-only lifting, which damages less, and trained people grow while soreness fades. Chasing pain is chasing the repair bill, not the sarcomere.

In short. The pull on the fibre is what grows it. The burn is a side effect that may help a little. Soreness is mostly damage, and damage is not the goal.

There are two ways to add sarcomeres, and the mirror confuses them. Add them in parallel and the fibre gets thicker. That is hypertrophy, and it is most of what a bodybuilding session is for. Add them in series, end to end, and the fibre gets longer, the fascicle length changes, and the joint angle where you are strong can shift. Heavy slow lowers and work at long muscle lengths — the stretched part of a curl, a deep squat, a chest movement that actually reaches the bottom — bias the fibre toward more growth in the human trials of the last few years, Pedrosa, Wolf, Kassiano and others. The finding is young and it is not a religion. The practical reading is already old: do not cut the range short on a muscle that is loaded when it is long. A half-rep that never stretches anything is a smaller tension signal, whatever the weight on the bar says.

In short. Thicker fibres come from sarcomeres side by side. Longer fibres come from sarcomeres end to end. Using the stretched part of a lift tends to grow more muscle than stopping halfway.

A fibre is one cell with many nuclei, each nucleus serving a volume of cytoplasm. For a long time the teaching was that this myonuclear domain had a fixed size, so any serious growth required satellite cells to fuse and donate nuclei. The picture has softened. A fibre can enlarge some way on the nuclei it already has. Large growth, the kind that changes a limb, still brings satellite cells in, especially in the type II fibres the first essay described. The old single number for the domain was a sketch. Treat it as a sketch. What is not a sketch is the timetable: a signalling pulse is minutes, a synthesis spike is hours to a day, new nuclei and a visible change in thickness are weeks.

In short. Each nucleus looks after a share of the fibre. You can grow a bit without new nuclei. Big growth brings in stem cells from the surface. That takes weeks, not one workout.

Ribosomes are the other capacity problem, and people skip them because they are less famous than mTOR. mTORC1 decides whether the ribosomes you have are allowed to work. Ribosome biogenesis decides how many ribosomes you have. Repeated training raises ribosomal RNA. A fibre that has been asked, for months, to build myosin owns more of the machines that build it. That is why a trained muscle answers a meal and a set differently from an untrained one, and why one heroic week does not look like six quiet months. The kinase is the switch. The ribosome count is the factory size. Both have to move if you want a different arm in the autumn.

In short. One switch says 'build now'. Another process builds more protein factories. Months of training do the second thing. One huge week only flips the switch.

The meal's job inside this window is narrow, and the protein essay is the long version. Essential amino acids, leucine in particular, let mTORC1 sit at the lysosome. About twenty to forty grams of high-quality protein, depending on the person and their age, saturates that switch for a few hours. After that the muscle is full, Atherton's phrase, and another identical shake mostly becomes urea. The session has already opened a period, roughly a day, in which the fibre will use those amino acids for contractile protein if they show up. Miss the day entirely and the tension signal still happened. You just failed to pay it. Carbohydrate's job is less romantic and just as real: it refills glycogen so the next session can still recruit the fast fibres, and the insulin from a mixed meal holds breakdown down. Fat in that meal is calories and hormones. It is not the leucine.

In short. After you lift, a normal protein meal is enough. A second shake an hour later is mostly wasted. Carbohydrate is there so the next session still works.

People do not answer this ledger equally. Hubal's famous training study found men who gained a great deal of muscle and men who gained almost none, on the same programme. Some of that is satellite cells, androgen receptor content, and how many type II fibres you were born with — a census that is partly genetic and only partly trainable. A lot of it, in the follow-up work, is that the 'non-responder' was under-dosed on sets. Add volume, or take the sets closer to the point where another rep is not there, and many of them start growing. Sleep debt, a deficit you did not mean, and a tendon that made you afraid of the last reps will all look like a genetic ceiling. Sometimes they are. Often they are a week.

In short. Some people grow easily and some barely grow, on the same plan. Genetics are real. So is simply not doing enough hard work, or not eating and sleeping.

An androgen raises synthesis and leans on breakdown, and it makes satellite cells more willing, which is a way of saying it pushes both the daily ledger and the nuclear capacity. Growth hormone and IGF-1 push the Akt side of the same growth switch, and they push collagen harder than they push myosin. Insulin from a meal, or from a pancreas doing its job, holds breakdown down and stores glycogen. None of them creates a sarcomere out of a fibre that never felt tension, and none of them supplies the amino acids. Read them as amplifiers of this page. The next essays name the amplifiers one receptor at a time. This page was the thing being amplified: a fibre that kept more protein than it lost, at long lengths, for long enough to need more nuclei.

In short. Steroids and growth-hormone signals turn the volume up on building. They still need the tension and the food on this page. They do not replace either.