A skydiver's parachute fails to deploy. After some struggle, his reserve chute deploys in time to save his life. However, his landing is harder than normal and he feels his knee slightly hyperextend. He shakes it off and goes about his normal routine. Later that day he begins experiencing knee pain. Two days later he notices that his knee "pops" when he crouches and soon discovers he has dislocated his femur on the knee. What muscle was likely injured initially?