Why is muscle force from static optimization and Actuation analyses different?

[Hari S]

Hello
1. I used SO to find muscle forces and activation. But when I was trying to find muscle power using the Analyze tool, I found that the muscle forces from that simulation is different from that of SO. Why are they different?
Is it the correct method to find the muscle power during one particular motion?

2. Negative power means that the actuator is absorbing energy from the model. But does muscle absorb energy?

Thank you.

[Dimitar Stanev]

Hi,

1. Maybe the SO and the Analyze tool have different settings (e.g. step_interval in the Property Edit) or you haven't included the same reserve actuators as in SO or you haven't provided the external forces if available.

2. Negative power means that the muscle is lengthening (v_m < 0) while the muscle force is always positive (f_m > 0). When the muscle is shortening its velocity is positive thus the power is positive. When the direction of action is against the movement then the muscle absorbs energy from the system.

Best

[Hari S]

Thank you.
My confusion is how does muscle absorb energy? What happens to the absorbed energy? The problem I am facing is that I cannot imagine how it happens physically. Thank you.

[Thomas Uchida]

Muscle performs positive work during active shortening and negative work during active lengthening. You might think of active lengthening as trying to swim against a strong current: you're still spending energy but you're making negative progress.