Primary muscles involved when knee is straight?!

Question:

Answers:
Knee extension (what would be holding the leg straight out against gravity) would be the quadratus muscle group, consisting of rectus femoris, vastus medialis, vastus lateralus, and quadratus femoris.

To get very specific, rectus femoris and quadratus femoris soley do the action, while the to vasti have secondary actions with leg approximation and deviation along with extension.

Other Answers:
Quad muscles, on the front of your leg above your knee. You can feel them when you flex your leg straight.
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