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I am having a problem regarding the forward dynamics simulation of a hand closing around a ball. The hand is supposed to be initially at rest with finger joint angles of 45, 45, and 10 degrees (proximal to distal), and is then supposed to close around the ball. One muscle actuates all 4 fingers. No matter what type of muscle excitation profile I use, the fingers always extend until they are fully extended and then they contract as expected. I have no idea how to solve this issue so any help would be greatly appreciated.

Also, I am working on a more complex version of the same model but when I try to run the simulation, I get to about 30 percent and OpenSim crashes. Any advice on how to speed up my simulation would also be incredibly helpful.

Some ideas:
1) Could gravity be extending the fingers? Or any other force?
2) Do your joints have default initial generalized speeds larger than zero, which initially extend the fingers?
3) Have you tried using simple pathActuators instead of muscles?

The simulation speed problem could be due to very high initial accelerations. You could try to lower the integrator accuracy.
If you cannot figure it out perhaps post your model/code.