Draw the diagram correctly for us.
If you consider the foot to be its own linkage attached to the shank by the ankle, then the normal force and its resulting moment would be applied on the foot and the shank would only experience internal moments which was my interpretation of the diagram.That's why this diagram is confusing. It considers the foot and shank to be one single rigid link -- thereby allowing the normal force to produce an external moment on the knee.
Of course the forces across the patellar are dependent on the load, but those are internal forces.
Well that's just blatantly wrong. The moments most definitely do not sum to zero. Besides I don't even know what you mean by sum? Across a single joint? Because you cannot sum moments across different axes.
Even across a single joint it's obvious there is a net moment --> there's a non-zero angular acceleration and M = I * alpha.
Everything that I'm reading in these replies is worse than the original diagram -- which is only correct under the assumption the shank and the foot form a rigid linkage and these assumptions are not clear.
This is kinematics not statics.
Draw the diagram correctly for us.
I would draw it like this: diagram.jpg
Your diagram isn't wrong, just confusing.
Exactly, an internal moment across the knee joint caused by the external force of the loaded barbell...Of course the forces across the patellar are dependent on the load, but those are internal forces.
For those of us with a shitty understanding of physics, is the OP saying that because regardless of where on the back the bar sits, the only force known to the femur is whatever force exerted at he hip joint? Therefore (according to the OP), there is no moment arm on the femur other than the forces of the hip joint (or knee joint, looking at it fronm the other side)? Im trying to follow this discussion....
Oh really?
What happens out of the hole? The hips and knees begin to extend, shortening the distance of each from the vertical line centered over the mid-foot. A reduction in distance results in a reduction of moment.
Of what use is studying these transient cases in which the moment is less than the maximum that occurs at the very bottom of the squat?
No, it's not...we are concerned with the Forces / Torque / Moments present during an instant in time, which is what the diagram is illustrating.
Kinematics: the branch of mechanics concerned with the motion of objects without reference to the forces which cause the motion.
Statics: the branch of mechanics that is concerned with the analysis of loads (force and torque, or "moment") acting on physical systems that do not experience an acceleration (a=0), but rather, are in static equilibrium with their environment.
Dynamics: the branch of mechanics concerned with the study of forces and their effects on motion. (a does not = 0)
You might find this hard to believe, but a number of us SSCs have backgrounds in Engineering or Physics. This material has been scrubbed, discussed, argued over, etc.
So, for the edification of the layman (such as myself)... the descriptor provided in the diagram is statics. What would it be if you calculated the moment, leverage, torque and compression on each joint for a period or instance of the movement?
Who cares, David? The diagram is designed to show you that moment force is calculated from the gravity vector, that there are 2 points of rotation on either end of the thigh and the shank, and that there are therefore 2 moment arms on each of these segments, the lengths of which can be manipulated by your position/technique to variously affect the muscle groups that operate the moment arms. Does it do that? I didn't write an engineering book because I'm not that smart. I was trying to illustrate a concept. Did I succeed?