Achilles Tendinitis
Achilles tendinitis is inflammation or degenerative damage to the Achilles tendon (which connects the calf muscle to the heel bone) — a typical overuse injury among runners, especially with a sharp increase in volume or intensity.
In detail
The Achilles tendon is the most powerful tendon in the human body, but it is relatively poorly supplied with blood, which is why it heals slowly. The pain is usually located 2-6 cm above the heel, worsens when running uphill or during accelerations, and morning stiffness is characteristic.
How to use it
Eccentric calf exercises (slowly lowering on one leg from an elevated surface) are the gold standard for Achilles tendon rehabilitation with the strongest evidence base; it's not advisable to abruptly return to your previous running volume after experiencing pain.