To quote:
1) The origins and insertions of the hamstrings and rectus femoris allow them to extend the hip and knee simultaneously, even though their actions oppose each other.
2) Two joint muscles allow force from single joint muscles to be transmitted to joints they wouldn’t otherwise be able to effect. For example, the rectus femoris allows the glutes to help extend the knee.
3) You can put these principles to work for you by learning the best way to grind through your sticking point on a squat
4) Because two joint muscles transmit force throughout all of your hip and thigh musculature, squats aren’t truly knee or hip dominant, regardless of how they look or what the external torques at the joints are.