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I am using versions of the Arnold lower body model (with mods) and also the new model from Apoorva. In order to make the model strong enough to execute loaded single-leg squats in static optimization runs, it becomes necessary to either increase the Max Isometric strength of the muscles in the model, or to simply allow activation to span from 0 to 5.0 instead of 0 to 1.0. I am wondering about the pros and cons of each approach. The model seems better behaved with the wider activation range, but I don't want to invalidate the results.

In my experience the Arnold model gives high passive muscle forces especially at extreme joint angles (quads in deep knee flexion for example). If you increase the the max iso strength 5x, I think you will find some extreme passive forces, so be sure to double check this. One option to fix this is to modify the force length curve for passive forces.

Thanks, Colin, the reason you cite is why I am leaning toward expanding the activation range for the muscles to allow them to generate higher active forces. But I am still wondering if that approach has pitfalls, too.

i hesitate to modify the model itself by altering the passive force-length curve, because how do you know what to change it to, and what are the unintended consequences of doing that?

Static optimization ignores the passive fiber force. Also, I would think such passive force would help the model complete the squats by decreasing the required active force (since I would think a squat requires a large knee extension moment).

Excessively high passive knee extension forces are a known problem with these types of models. Look at Fig 6 in the V&V paper (http://nmbl.stanford.edu/publications/pdf/Hicks2015.pdf). If you adjust the model, you could look at the experimentally measured passive knee moment for guidance. Still, I don't think adjusting the passive knee force will help you with static optimization.

If I recall, the knee extensors are reaching maximum activation. It would be good to run a muscle analysis to see what the moment arms, fiber-length and fiber-velocity multipliers are in that scenario. Perhaps the fibers are so long that the active fiber-length multiplier is very low. If this is true, you can see what happens if you increase the tendon slack length, which would allow the fibers to operate closer to their optimal length. Tendon slack lengths have a fair amount of uncertainty.