The genes your tendons are made of

Two runners follow the same training plan. One ramps up mileage and feels great; the other picks up an Achilles niggle at week six, again. Training errors explain most of that difference. But connective tissue itself is built from proteins your genes specify, and a small genetic layer sits underneath who strains what.

Collagen, with a version number

Tendons and ligaments are mostly collagen, and COL5A1 encodes a chain of type V collagen, a minor component with an outsized role: it helps regulate how collagen fibers assemble, like a site foreman for the main structure. The best-studied variant here is rs12722. A meta-analysis of musculoskeletal soft-tissue injury studies found carriers of the T version leaning toward more soft-tissue injuries, with two-C carriers leaning toward more resilient connective tissue. The lean shows up mainly in people carrying two T copies, and it is a lean: a shifted probability across populations, not a diagnosis of fragile tendons.

The matrix protein with a small-print finding

TNC encodes tenascin-C, a protein of the tendon matrix that is especially active where tendons handle mechanical load, and it spikes during tissue repair. Small studies link a common TNC variant to tendinopathy risk, with one version leaning resilient and the other leaning susceptible. This one rests on much thinner evidence than COL5A1, small studies that have not been broadly replicated, and both our report and this article treat it as an early signal, not a settled fact.

What actually protects tendons

Here is the part that matters more than any genotype, and the research community is unambiguous about it: tendons adapt to load slower than muscles do. Cardiovascular fitness improves in weeks, muscle in months, connective tissue on its own longer timeline. Most soft-tissue injuries happen in the gap between what your engine can do and what your chassis has adapted to. The boring protective habits, gradual volume increases, warm-ups, recovery days, not stacking intensity jumps, dwarf any genetic lean in both directions.

So what is a strain lean actually for? Calibration. If your report shows the resilient genotypes, it is not a license to skip the ramp-up. If it shows the strain lean, it is not a sentence, it is a reason to be the runner who adds ten percent a week instead of thirty, and who takes the calf tightness seriously the first time instead of the third.

Your Helisoma report reads COL5A1, TNC, and the rest of your training profile, muscle fiber type, aerobic response, recovery markers, shows your genotype at each, and links the studies, including the ones below. Free preview, file never leaves your browser.

Sources

1. Meta-analysis of COL5A1 polymorphisms and musculoskeletal soft-tissue injuries. PubMed 35241120
2. TNC (tenascin-C) variant studies in tendinopathy. PubMed 40869983