The tendon clock: the cable is older than the muscle

A fibre can be a different organ in eight weeks. Myofibrillar protein turns over on a scale of days, satellite cells donate nuclei, and a squat that was heavy in January is ordinary in March. The tendon on the end of that fibre did not get the same memo. Collagen in the core of an adult tendon is one of the slowest proteins in the body. Estimates for the half-life of that core run to years. The outside of the tendon, the sheath and the bit near the bone, turns over faster, months rather than decades, and that is where a lot of the adaptation you can actually measure lives. Stiffness can rise in a training block. The bulk of the rope you are pulling on was built a long time ago. This is why a rapid jump in muscle strength is a tendon problem waiting for a date. The motor got ahead of the cable.

In short. Muscle rebuilds in days and weeks. Tendon collagen is much older. Strength can arrive before the cable is ready for it.

A tendon is mostly type I collagen, wound as fibrils, fibres and fascicles, with a small population of tenocytes sitting in rows between them. The tenocyte is the only cell that maintains the rope. It feels strain through its cytoskeleton and writes collagen, and it writes the enzymes that cross-link that collagen. Lysyl oxidase needs copper to make those cross-links. Prolyl hydroxylase needs vitamin C and iron to make the hydroxyproline that lets the triple helix hold. A tendon without load goes slack and loses organised collagen. A tendon with sudden enormous load, the kind a new squat cycle or a new dose of enthusiasm produces, sees strain at a rate the tenocyte cannot match in the same week. The mid-portion of the Achilles and the patellar tendon are poorly supplied with blood, which is another reason a tear there heals on a calendar that feels insulting. Muscle bleeds and rebuilds. Tendon sulks and cross-links.

In short. Tenocytes maintain the rope, and they need load, vitamin C and copper to build and cross-link collagen. The middle of a big tendon has little blood, so it heals slowly.

Collagen synthesis in a tendon rises after a loading session and stays up for a couple of days. Miller, Magnusson and Kjaer measured that window. The useful picture is a short, heavy strain, then a day or two in which the tenocyte is actually writing, then another strain. Hours of bouncing on a sore tendon is a different stimulus: lots of compression and friction, not much organised new collagen. Heavy slow resistance, the kind Kongsgaard used on patellar tendons, works because the strain is large enough for the cell to notice and slow enough that the fibrils are loaded rather than snapped. Isometrics, the kind Rio used for patellar pain, work in the short term because they turn the pain down and let you load the tendon without the explosive bit that irritates it. Rest, as a months-long strategy, starves the tenocyte of the only signal it trusts. A tendon that hurts often wants less stupidity and more heavy, slow, repeatable strain, not a chair.

In short. After a heavy session the tendon makes collagen for a day or two. Slow heavy load is the signal. Endless bouncing irritates it. Total rest, for months, starves it.

Shaw and Baar's feeding work is the nutritional half, and it is narrower than a supplement advert. Collagen and gelatin are poor in leucine, so they do almost nothing for the myofibril. They are rich in glycine and proline, which is the amino acid weather a tenocyte needs. Vitamin C in the same window is the cofactor for the hydroxylase. A dose of gelatin plus vitamin C, taken before a short loading session, raised markers of collagen synthesis in their human study. That is a blood marker and an engineered tendon construct, not a guarantee that a patellar tendon will be new in a fortnight. It does say the tenocyte will use those amino acids if they are in the blood while the strain is happening. The muscle wants leucine and tension. The tendon wants glycine, proline, vitamin C, and a slower strain. One shake aimed only at the muscle is a complete meal for one of those tissues.

In short. Gelatin is useless as a muscle protein and useful as tendon raw material, if vitamin C is there and you load the tendon while those amino acids are in the blood.

The injuries that follow from ignoring the clock are dull and predictable. Proximal hamstring tendons in people who discovered deficit deadlifts. Patellar tendons in people who added a hundred kilograms to a squat in a season, or who returned from a lay-off at the old weight because the muscle memory essay was right about the nuclei and wrong if you applied it to the tendon. Biceps tendons in people who chased a peak curl. Achilles tendons in people who mixed a big calf and a lot of jumping. Androgens and growth hormone change this picture, and not always kindly. Muscle and fluid arrive faster, collagen synthesis can rise, and the stiffness of the tendon does not automatically keep the ratio. A stronger pull on an older rope is the whole mechanism of a rupture that 'came out of nowhere' on a warm-up set. The warm-up set was the first heavy strain of a tissue that had been losing the race for months.

In short. Tendons fail when strength, or a drug-driven strength, gets ahead of a rope that turns over in months. The warm-up set is often just the first heavy pull the old collagen has seen.

Add load to the ranges the tendon feels, and add it on a tendon calendar. That means the deep squat, the full hinge, the calf at a stretch, progressed over months, not jumped because the muscle could take it on Tuesday. Keep some heavy slow work in the week even when you are chasing a pump, because the tenocyte does not read the pump. If a tendon is already sore in a precise spot, especially in the morning or at the start of a session before it warms, that is tendinopathy: a disorganised collagen and a grumbly tenocyte, more than a simple inflammation you can ice away. The session that helps is usually heavy enough to be a strain signal and boring enough to recover from. The session that keeps it is the one with a novel violent movement on a tissue that already hurts. Muscle soreness in the belly is the other essay. A hot line from bone to muscle, worse on the first steps, is this one.

In short. Progress the ranges a tendon actually feels, over months. A precise morning pain at the tendon is a different problem from muscle soreness, and it wants heavy slow load more than it wants a new violent exercise.