Effects of bi-articular muscles

[Kamal Gautam]

Hello,

I have a question concerning the Static Optimization (SO) workflow in OpenSim and its treatment of bi-articular muscles.

I understand that the static optimization process calculates the joint moments at each time point based on the input motion data, then uses an optimization algorithm to distribute these moments to individual muscles, aiming to minimize the sum of squared muscle activations.

However, when it comes to bi-articular muscles, such as the biceps brachii or the long head of the triceps, these muscles have the ability to transfer forces and moments between the joints they cross.

Could you please clarify the following points:


1) During the calculation of joint moments, does OpenSim consider the capacity of bi-articular muscles to transfer forces and moments between joints?

2) In the process of static optimization, how does OpenSim determine the distribution of forces of bi-articular muscles among the joints they span?

I look forward to your insights on this matter.

Thank you in advance.

Kamal Gautam

[Carmichael Ong]

Bi-articular muscles will affect multiple joints because the model will calculate moment arms for each joint (or coordinate) that it spans. Then, it will distribute force based on the equations in the How Static Optimization Works documentation page.