I had to go to therapy once for an upper shoulder problem... I only went for a week, (daily), and I used a rowing machine. And NOTE: this was when I was in VERY good shape... running 6 miles a day for 6 days and on the 7th day running near 15... PLUS riding a bike 25 miles, PLUS an hour on a stair stepper, PLUS an hour with free weights. all of the daily and 7 days a week. I was in VERY good shape.

And the rowing machine wore me out. Seriously... that was the hardest workout I can recall. 86% you say? 99% wouldn't have surprised me.