Unstable musculoskeletal model, what to do?

[Antonio Parziale]

Dear all,

We want to use a simplified version of the "Upper Limb Dynamic Model", where only 6 Millard's muscles are considered (Pectoral Major, Deltoid, Brachialis, Bicpes short, Triceps Long, Triceps Lateral). We are interested in elbow flexion/extension and shoulder horizontal adduction/abduction movements.

By applying any kind of muscle activation, we notice that musculoskeletal model performs unusual and "crazy" movements. One would expect that with the minimum muscle activation, no movements should be performed. Instead, arm rotates around the axes, moves back and forth and never stops.

1) We imagine that muscle parameters need to be changed, at the moment the ones provided with the original model are used. Is it right?

2) We are evaluating to follow one of the procedures described in
- Kurt Manal, Thomas S. Buchanan, Subject-Specific Estimates of Tendon Slack Length: A Numerical Method, 2003
- Garner, B.A., & Pandy, M.G. (2003). Estimation of musculotendon properties in the human upper limb. Annals of Biomedical Engineering, 31, 207-230.
Is it correct? Both the procedures request data measured on subjects, can you suggest papers or dataset?

3) Can you provide any other useful information for helping us to be on track?

Best regards,
Antonio