My new thread is about the new muscles recruitment criterias available in AMS 4.0 and above. My question is quite simple : Is there a recruitment criteria well suited to a musculoskeletal deficiency ? Let me explain :
Take the shoulder for example. When dealing with a massive cuff tear, the central nervous system (CNS) deals with less muscles available to abduct the arm and to stabilize it. Thus, I guess the CNS is not acting normally and maybe modifies its recruitment strategy in order to raise the arm as high as possible or to minimize the contact forces at the acromion.
Personally, I think one of the criterias is better than other, but even after reading the tutorials, I cannot say which one, as you only deal with asymptomatic joints in the examples.
As these kind of musculoskeletal deficiencies are more and more studied, I think this question might interest more AnyBody users, including myself and other colleagues.
I don’t think anyone knows the correct muscle recruitment criterion for pathological conditions, and probably not for non-pathological conditions either. But we can at least say something about the consequences of different criteria based on the mathematics.
If we consider polynomial criteria in general and bear in mind that the min/max criterion corresponds to a polynomial criterion of infinite order, then we can say the following:
Criteria of low orders tend to favor muscles with large moment arms, which means that they can produce a given joint moment with relatively small joint reaction forces. If there is pain in the joint, then it is likely that the CNS will learn to recruit the muscles to minimize the joint force and hence favor a low-order criterion. The downside to this is that when the muscles with smaller moment arms are not recruited to their full potential, then the system can carry less external moment before the first muscle reaches its upper limit.
Criteria of high order maximize the synergism between the muscles. This means that they maximize the strength of the organism by recruiting even small muscles and muscles with unfavorable moment arms. This also leads to maximization of the joint reaction forces. This criterion is reasonable if the muscle configuration is deficient and has trouble carrying the external load, but it is not a good idea in the presence of joint pain.
If I understand you right, every pathology is different and requires that the recruitment criteria take into account what we know about that pathology (low contact forces, degree of synergism, etc.). That’s really interesting.
Thus, if my model is asymptomatic, I use a min/max or high order polynomial criteria and if I need to minimize the contact forces, I use a low order criteria. Is is that simple ?
In the near future, would it be possible to consider a recruitment criterion based on a condition directly defined by the user (e.g. minimize a particular force, prioritize a particular muscle, etc.) instead of a condition that cannot be chosen ?
You can actually do a lot of that already if you study the formulation of the optimization problem. The strength you assign to muscles works exactly as weight factors on that muscle in the optimization, and if you want to mimic sore joints, you an replace their reaction forces with AnyGeneralMuscle’s with a low strength.
That’s a very good idea. I’ll try decrease both the recruitment criterion order (use the MR_Quadratic) and the reaction force muscles strength at the glenoid and at the acromion.
Can I directly change the strength values in the GHReactions.any file ? For the moment, I have a value of 5000 N in these reactions muscles, but this values isn’t even reached during the simulation. I guess that if I want to see a difference, I will have to decrease to a lower value compared to the maximum contact force that I get, which is more or less 600 N.
The existing reaction muscles have a high strength precisely because we do not want them to influence the recruitment, except by virtue of their inability to provide tension. So you can effectively simulate limited load-carrying capability of the joint by reducing this strength.