The best set of back muscles to use is the large set located between the tip of the scapula and the spine. The easiest way I have found to use these is to draw with the bow pointing down at 45 degrees, with the a high draw elbow, then bring the elbow up and back. If it is done right your scapula should stick out of your back, you can then squeeze the large muscle to bring the draw scapula down and in, while putting full pressure into the bow arm. I will attach a photo I took of my brother after I taught him this technique. </p>
Now I have mastered the back I have a big problem collapsing with my bow shoulder. If I align it before the shot I cant use the back effectively and end up using the# less efficient muscles between the upper scapula, it looks like a crease in my back, which is bad. As soon as I can mix the stable bow shoulder with the back I will rule the world!
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I highly reccomend you buy Total Archery. It is explained in detail there. Together with the draw - holding transfer phase and expansion. It is thought bad to continue pulling as you need to effectively stop in order to transfer the bow weight into the important muscles before the push pull. Maybe Marcus can chip in and add his wisdom.</p>