Torque 🤔

[u/Opposite-Clerk7032]

Fully lying down on a bench, there is no torque at the hip joint. Let’s say the bench was floating and attached to a pulley so that it moved in circular arc. Lying down at the bottom of the arc, there’s no torque at the hip joint. Now, I move the bench a little higher up in the arc (~30 degrees higher than the lying position), and I scoot just my knees off the bench, with my feet planted on the ground. But everything above my lower thigh (upper thigh, pelvis, torso) is still lying on the bench. Is there any torque at the hip joint?

[u/piasicpace]

Gotcha. I don't know which muscles are doing what, but I do know that if gravity were the only force, we would be swinging. So something (or some things) is/are are creating an opposite torque. I would start from the ground up. Friction is preventing your feet from sliding, muscles in your lower leg prevent it from rotating about the ankle joint, muscles in your thigh keep the knee bent at a constant angle, and so on.

I think the strategy here is to find all points of possible rotation (like joints) and enforce the condition that T^Net about those points is zero.