Sorry, i wasn't clear. There is a force of 35-40# pushing on your bow hand towards your body. That force is transmitted through the skeleton, and is eventually taken by the back muscles. I didn't mean to suggest that the arm muscles should be doing the work of resisting that force, quite the opposite.
For anybody who is interested in the nitty gritty detail, there are some excellent articles on tenzone, including this one on the use of the bow arm
http://www.tenzone.u-net.com/Technique/pdfs/BowArm.PDF