Tendons on linear motors seems like a good approach. http://www.roboy.org/

Also, there are shape memory alloys and they are essentially a strand of synthetic muscle for a few dollars. You could concievable bundle them together to make a larger muscle, but you will probably have to do some active cooling if you want any sort of decent response curve and even then motors can be a lot faster.

What I think we could really do with is a linear actuator with a simple built in clutch that doesn't take power to hold a position.