Tendon

Tendons are dense cords of collagen linking muscle to bone. They are strong in tension and slightly elastic, storing and returning energy during movements like running and jumping — a genuinely useful spring, not just a cable.

It matters because tendons adapt far more slowly than muscle. Their blood supply is sparse and collagen turnover is slow, so strength can outpace tendon capacity in a few keen months — which is a common route into tendinopathy in people who progress load rapidly.

In practice, tendon problems respond best to progressive loading rather than rest: heavy slow resistance and isometric holds have the strongest evidence in Achilles and patellar tendinopathy. Complete rest reduces pain temporarily and leaves the tendon less capable than before.