Joint Reaction Analysis

[Najoua Assila]

Hi,

I know that this question has been raised on the forum many times, but I still can't find an answer.
It's related to the evaluation of the joint reaction load. Why are the results dependant on muscle forces acting on both sides of the joint. The joint is in equilibrium, so of course, the loads should be equivalent, but from a mathematical point of view, solving the system, as explained on Opensim, from the furthest body to the closest, the results shouldn't depend on the muscles that are "still to come".

I raise this question, because I am trying to implement my own joint reaction analysis, and as a validation step, I am trying to get similar results to the ones obtained via Opensim, using the same forces file (SO), however, while the evolution of the forces is similar, the amplitude isn't.

On the core, I couldn't find the implementation of the analysis. I suppose it is done in Simbody, I would appreciate if you could also point out where exactly.

Thanks,

Najoua

[Michael Sherman]

Hi, Najoua. Here is the Simbody method OpenSim uses to calculate joint reaction forces:
calcMobilizerReactionForces(). I've linked to the doxygen for that method which has a fairly detailed explanation of what it means. Note that this is defined in terms of straightforward Newtonian mechanics -- there is no funny business going on with muscles.

Regards,
Sherm