Hello Brad
Hope this helps you a little bit.... well u need to strengthen your upper body muscles.Your upper-body strength will naturally develop as you move and maneuver with your wheelchair when self-pr0pelling. Target specific muscle groups to make sure your upper body receives a balanced workout. Lifting free weights or working with cable pulley machines is a way to build specific arm, back, core and shoulder muscles. The sitting pushup increases your entire upper-body musculature. At the same time, the sitting pushup momentarily takes pressure off your buttocks, something you need regularly. Hold the arms of your chair, and push yourself up, holding the position for about 10 to 20 seconds. Lower and repeat 10 times. Include sitting pushups in your regular workout routines, and perform them two or three times through out the day, everyday.
Also have u tried Straight leg Raising exercise(SLR) ? lay flat on ur table/floor/bed(not with a soft spring mattress) with both legs straight and your feet close together. Then bend one knee until the sole of your foot is flat on the floor and lift your other leg off the ground until your heel is about eight inches off the support. Lower your leg slowly until you return to the starting position.... would advise 10 repetitions only for the mean time ..... then repeat the same exercise for ur other leg