In the FAQs, you raise an interesting question about how AnyBody computes joint moments, which is : Does the AnyBody Modeling System compute joint moments?. Based on your answer to this question (http://www.anybodytech.com/index.php?id=651), I wonder if estimating the required activation of bi-articular muscles by first estimating the joint torques necessarily leads to an innacurate estimation, or just introduces an uncertainty ?

That is a good question. If you are looking at an open chain of segments, then you can safely compute the joint moments first and resolve them into muscles forces afterwards. The reason is that, in an open chain, the joint moments are a unique solution, i.e. there is only one set of joint moments that will carry the exteral load.

It is more complicated when there are closed chains of segments in the model as with the radius and ulna of the forearm or the shoulder girdle, or even when the body is just supported against the environment at multiple points, then the joint moment solution is not unique anymore and will depend on the mutual strengths of the joints. These strengths depend again in a complex way on posture, and because of multi-articulate muscles, the posture of one joint affects the strength of another.

This is the reason why AnyBody does not use joint moments as an intermediate result for computing muscle forces. We form equilibrium including the muscle forces right away and solve those equations leading directly to muscle forces.

Really interesting ! I shall look at this concept a little more deeply because it is something that I have to understand and explain clearly in my PhD thesis.

The best way to really understand it to do a hand calculation. Try first with, for instance, two segments in an open chain. Fix the chain to ground at one end and load the other with with a known force. Then set up equilibrium equations and compute joint forces and moments. It is quite simple.

Secondly, make a closed chain and try again. Now you will be missing an equation and cannot determine all of the joint reaction forces and moments.