The 'hamstrings' or 'hamstring muscles' are the muscles at the back of the thigh - semimembranosus, semitendinosus and biceps femoris.
View from the back of the two heads of the biceps femoris component of the hamstrings inserting on the lateral side of the knee.
Why are they called the 'hamstrings'?
They are so called because in pigs they are the muscles used to make ham, and the hams were strung up using hooks through the long string-like tendons.
Late swing or early stance? A narrative review of hamstring injury mechanisms during high-speed running. Kenneally-Dabrowski CJB, Brown NAT, Lai AKM, Perriman D, Spratford W and Serpell BG. Scand J Med Sci Sports. 2019 Apr 29. doi: 10.1111/sms.13437. [Epub ahead of print].