I had many recent injuries from a head on car accident (7 herniated discs, fractured sternum and broken bones), torn rotator cuff in my bow arm, tennis elbow in both elbows and arthritis in both shoulders.
I had to step down in weight quite a bit until I over came these injuries.
I wanted to work out to increase accuracy at first by making my current bow easy to draw and hold when hunting. I didn't start out to shoot heavy bows again.
Therefore I used this exercise with the Formaster to increase bow strength so I could be more bow strong at my current hunting weight.
I used a bow 5# heavier than my hunting bow.
http://www.texasarchery.org/Documents/FMaster/formaster_exercises.htm At this time I didn't have the desire to increase bow draw weight since my hunting bow was 60# and I was working out with 65#.
By doing this exercise I eliminated my drawing arm tennis elbow. Until I did this exercise I never realized that I wasn't drawing the bow back with my shoulder and back muscles. My tendonitis in my drawing arm was from using my fingers, wrist, forearm and bicep to draw the bow. Once I realized this, I concentrated on keeping a relaxed drawing hand, wrist, forearm and bicep and drawing with my back and shoulder. This not only eliminated my tennis elbow, it made me a much better shot. Using my back and shoulder muscles properly to draw the bow in of itself made me more bow strong and able to shoot heavier weight bows. In addition it trained me to ensure I used back tension at my anchor to prevent arrow creep and short drawing the bow.
Then I went to Bob Wesley's. Bob corrected my form with my bow arm. He taught me to keep a low and forward bow arm shoulder, a bent elbow and modified where I hold the bow in my bow hand. This eliminated tennis elbow in my bow arm and the pain in my bow arm shoulder from prior injuries. It also dramatically improved my shooting, to where I shoot better now than I did before all my injuries.
Once my form was straightened out and all the pain was gone I continued to shoot my 60#.
I had a collection of Hill longbows. I posted in the HH thread. One was a special one and was 71#. Everyone asked me how it shot. I went out and shot and it felt like it was in the 56# range to me. I thought the weight may be wrong since it was reworked by John Schulz to Howard Hills specs for Bob Wesley. I had an 80# Hill that the first time I shot gave me a cyst in my bow arm wrist. I thought I would compare the 71# to 80#. When I drew and shot the 80# it was comfortable and I was shocked I could shoot it.
I am 58 and had many injuries over the years that disrupted my shooting. I would have never dreamed that at my age and after all my injuries I could shoot a heavy bow again. In fact I can shoot heavier bows now than when I was a young bull.
By exercising using the exercise above and correcting my form, I found that I had gotten very bow strong now. I started shooting 80# every other day 73# on off days. My hunting bow is 73# now. When I go back to my 60#, I feel that the release almost seems like a delayed reaction now.
I feel to improve your bow strength, while maintaining and improving accuracy without injury, is to first straighten out any form issues you have.
In my case I believe that if I first attempted to start drawing heavier bows, without fixing form problems I had I would have amplified the injuries I already had due to poor form.
By using the above exercise you train yourself to draw with your back muscles which automatically allows you to draw heavier bows and keep a relaxed drawing arm reducing the chance of injury.
My advice would be to take care of any current injuries first, then straighten out any form problems you may have, followed by a low and slow exercise program.
I like shooting heavy bows because I hunt, enjoy the flat trajectory and cleaner release. In my case you wouldn't believe me when I say this, but when I shoot a lighter bow my bow arm shakes like crazy, yet when I shoot heavy everything is locked in solid. I guess from all the injuries the heavy weight is the glue to keep me from coming apart at the seams.