10x model strength not strong enough for SO or DO despite normal joint moments

[Bas Van Hooren]

Hi,

I want to compute muscle forces from running experiments with both static optimization and dynamic optimization. While the joint moments are roughly in line with the literature (see below for a comparison), I get a message that the model is not strong enough for static optimization even when I increase its strength by 10x. How is this possible/what might I be doing wrong?

Similarly, during dynamic optimization the model is not strong enough when max force is increased 2x as indicated by the muscle activation maxing out for some muscles, although the contribution of the reserve actuators is <5%. Should I in this case further increase the model's strength or can I leave it since the contribution of the reserve actuators is <5%?

[Aaron Fox]

Hi Bas,

To start with, I don't think <5% contribution from reserve actuators is too problematic - but there are probably always means to reduce this.

I've found that quite often in situations where the muscles can't produce enough force, perhaps unintuitively the 'strength' of the muscles isn't actually the problem. The common issues I've come across are muscles producing too much passive tension due to going to extreme lengths (e.g. quadriceps muscles during deep knee flexion). For that particular example, Lai et al. developed a model with adjusted tendon slack lengths to allow deep knee and hip flexion. When there are these extreme passive forces being developed, no matter how high you strengthen the muscles it still won't solve well.

Other issues are operating lengths and contractile velocity properties. For example, I don't think I've ever got a simulation of running/sprinting to work without increasing the max contractile velocity of the plantarflexors up to 15-20. If you're muscles are operating at lengths or velocities that are towards the end of the force-length/force-velocity curves, even dramatic increases in max force still won't allow them to produce sufficient force. In these cases I'd suggest performing some sort of muscle analysis to examine the normalised lengths and velocities muscles are working at.

Hope that helps.

Aaron

[Bas Van Hooren]

Hi Aron,

Thanks a lot for your reply, this is very helpful.

I am using the Catelli model, which is a further improvement on the Lai model so I would expect the muscle properties and moment arms to be ok for a relatively slow running speed of 2.78 m/s. Therefore I have not looked at the muscle operating lengths and velocities with static optimization. However based on your comment I definitely will look into this, thanks!