Muscles that act on the knee joint form most of the mass of the .

thigh

Explanation:

The muscles that act on the knee joints majorly include the hamstring muscles (semimembranosus, biceps femoris, and semitendinosis) and the quadriceps muscles (vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris).

The quadriceps muscles and the hamstring muscles are the major muscles that make up most of the mass of the thigh. The quadriceps are located at the front of the thigh, while the hamstring muscles are located at the back of the thigh.

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so whats the answer