Why can muscles in Evolution both expand and contract?
As you know, muscles in real life can only contract and loosen up, but not actively expand. In order to be able to move a bone in two opposite directions, there needs to be a muscle pair with each muscle pulling in the opposite direction.
Since you actually want such bidirectional movement most of the time, I decided to combine both muscles into one. Not only do you now have to place fewer muscles in order to allow for basic movement but this also drastically increases the chances for a successful simulation.
This is because the more muscles you place on a creature, the more likely it is for these muscles to cancel each other out which would cause the creature to barely move at all. If you had to place muscle pairs around each joint, then the first thing each creature would have to learn would be to identify such pairs and make sure the muscles don‘t both contract at the same time.
Merging a muscle pair into a single muscle eliminates these issues.