Mostafa Sorry, I wasn't very clear.
What I intended to say was this:
- have the neutral pose of your model with the elbow slightly bent
- have one LOA cure for the biceps
- have another LOA curve for the triceps
Now when you straighten the arm, the curve used for the triceps will shorten, and the triceps will fire.
When you bend the arm, the curve used for the biceps will shorten, and the biceps will fire.
I hope this helps!
-jj