I bent to trunk model (FacetJointModel in AMMR) towards left (40 degrees) from standing position using AnyKinSimpleDriver and observed the left External Oblique muscle activities. The left External Oblique muscle should provide part of active forces to drive the trunk towards left oblique. So the forces should be larger with the increase of left oblique. However, the activities of left External Oblique muscles are getting smaller. I think it may due to the reduction of the length of left External Oblique muscle bundle. But the decrease of muscle activities seems paradoxical to the increase of left External Oblique muscle force. What is the problem with this understanding?
That is a curious problem indeed. The length of the muscle is unlikely to affect the upper limit on muscle forces unless you are using a 3 element muscle model. If you are using the simple muscle model, this should not happen.
Secondly, if the trunk was bent towards the left and if gravity was acting downward (I assume!), shouldn’t the muscles on the right be more active to keep the trunk from collapsing downwards?
In this case, I could expect the left obliques to only provide the initial leftward acceleration when the motion begins. This could also explain why the muscle activations die out as the motion progresses.