Graphing Physical Exertion

Last week I did 30 minutes on the rotating stair machine at level 10 while holding an 8 pound medicine ball. At certain intervals (when the bars were more than 5) I would hold the ball above my head, the rest of the time I held it near my stomach. At the end of that workout I was sweating really profusely and felt like I had had a great workout, but that I had some juice left. This week I thought I would crank it up by going to level 14 and using a 10 pound medicine ball for 20 minutes. This seemed reasonable to me at the time. The problem was that I had made a fatal assumption that the exertion required to complete the exercise was a linear factor of the speed, weight, and time variables I had adjusted. In other words y=x. The actual equation is graphed here for clarity: