I have performed inverse dynamics operation for normal level ground walking. I would like to clarify a few doubts regarding the outputs.
Are the muscle forces normalized with respect to the body weight in the output? Also, may I know what’s the unit for it if its not normalized? (N)
Is the muscle activity normalized with respect to the maximum voluntary contraction? If so how is the maximum voluntary contraction determined? Are the values taken from the literature or is it the maximum peak from the trial? Also, what is the unit of muscle activity obtained from AnyBody if its not normalized? (milliVolt)
Briefly, the muscle activity from the model is a dimensionless quantity, which reflects the normalized level of muscle force. The normalization is to the maximum force production capacity of the muscle, which in general is [f(l)*f(v)*F0] where f(l) and f(v) represent the force-length and force-velocity relationships of the CE in the 3-element muscle model, respectively, and F0 is muscle maximum isometric force parameter.
If you choose simple muscle, basically it is equivalent to set f(l)=f(v)=1 and therefore a=Fm/F0 where Fm is muscle force.
For more details, I highly encourage you to read about muscle models in the literature e.g. the following:
Zajac, F.E.: Muscle and Tendon: Properties, Models, Scling, and Application to Biomechanics and Motor Control. Critical Reviews in Biomedical Engineering, 17, 359-410, 1989.