Tendon
Tough connective tissue that transmits force from muscle to bone.
A tendon connects muscle to bone. It is dense connective tissue, mostly collagen fibres aligned along the line of pull, which makes it extremely strong in tension and relatively poor at handling shear or compression.
It is not a passive rope. Tendons store and return elastic energy, which is why running is far more economical than it would be if muscle did all the work, and why the Achilles behaves like a spring during each stride.
They adapt to load, and they adapt more slowly than muscle does. That mismatch is behind a common pattern: someone whose strength climbs quickly in the first months and whose tendons pick up a problem in month four. Progressive loading, rather than heroic jumps, is the answer.
Tendinopathy, the usual injury, is not primarily inflammation despite the older name tendinitis. It is a failed adaptation of the tissue, and the treatment with the best evidence is progressive loading rather than rest. Rest reduces pain and reduces capacity further, so the pain returns when activity resumes.
Recovery is measured in months. Trial protocols typically run twelve weeks with improvement often continuing beyond, and heavy slow resistance performs at least as well as eccentric-only protocols, with reviews finding little basis for isolating the lowering phase.
Corticosteroid injections deserve a specific caution: they relieve pain in the short term and are associated with worse longer term outcomes in several tendinopathies, so they are a clinical decision rather than a first move.
In practice, load tendons progressively, expect the timeline of a season rather than a fortnight, and treat pain that improves as you warm up and returns the next morning as a tendon problem worth managing rather than ignoring.