hamstring

hamstring   /ˈhæm.strɪŋ/

https://dictionary.cambridge.org/dictionary/english/hamstring

one of the three large muscles at the back of the thigh:
the hamstring muscles

The hamstrings are three muscles at the back of the thigh that affect hip and knee movement. They begin under the gluteus maximus behind the hipbone and attach to the tibia at the knee. They are:

Biceps femoris: This long muscle flexes the knee. It begins in the thigh area and extends to the head of the fibula near the knee.

Semimembranosus: This long muscle extends from the pelvis to the tibia. It extends the thigh, flexes the knee, and helps rotate the tibia.

Semitendinosus: This muscle also extends the thigh and flexes the knee.