der Knöchel
ankle
The joint that connects the foot to the lower leg.
Beim Sport ist sie umgeknickt und hat sich den Knöchel verstaucht.
While playing sports, she twisted her ankle and sprained it.

ankle / knuckle
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ankle
The joint that connects the foot to the lower leg.
Beim Sport ist sie umgeknickt und hat sich den Knöchel verstaucht.
While playing sports, she twisted her ankle and sprained it.

You can tell from context whether 'Knöchel' means the joint on the foot (ankle) or on the finger (knuckle). Words like 'Fuß' (foot), 'umknicken' (to twist one's ankle), or 'verstauchen' (to sprain) point to the foot, while 'Hand' (hand), 'Finger' (finger), or 'Faust' (fist) point to the finger.
Sie hat sich den Knöchel am Fuß gebrochen.
She broke her ankle.
In connection with the ankle, typical verbs are 'sich den Knöchel verstauchen' (to sprain one's ankle), 'sich den Knöchel brechen' (to break one's ankle), or simply 'umknicken' (to twist one's ankle).
Ich bin gestern beim Laufen umgeknickt und mein Knöchel ist jetzt geschwollen.
I twisted my ankle while running yesterday, and now my ankle is swollen.
When the finger joints are meant, 'Knöchel' is often connected with actions like 'eine Faust machen' (to make a fist) or 'mit den Knöcheln auf den Tisch schlagen' (to hit the table with one's knuckles).
Beim Boxen muss man lernen, mit den Knöcheln zu treffen.
In boxing, you have to learn to hit with your knuckles.
heel
'Ferse' and 'Knöchel' both refer to parts of the foot, so they are easy to confuse. The 'Ferse' is the lower back part of the foot, the part you put weight on at the back when standing. The 'Knöchel' is higher up, where the foot meets the leg. It is part of the joint and often sticks out a little on the side. If a shoe rubs at the back, it is usually the heel that hurts. If you twist your foot, it is usually the ankle that is affected.

Beim Fußball ist Tom umgeknickt und hat sich den Knöchel verletzt.
While playing soccer, Tom twisted his foot and injured his ankle.
Nach der langen Wanderung hatte Max eine Blase an der Ferse.
After the long hike, Max had a blister on his heel.