Self Bondage/TipToe Stress Position

I've never done this before, but I'm already sure it's going to be hard. I tie a tight crotchrope to the long spreader bar I'll be using on my ankles, before tying each ankle to one end of the bar.  The combination of crotch rope and ankle restraints will keep my legs wide apart, and my knees bent. But you've ordered me to stay on my toes too, once I stand up and get into the challenging final position.  I have to make it through 5 minutes, and my legs start to cramp up immediately, from the combination of stress on my calf muscles keeping me on tiptoe, and my thighs maintaining the bent-leg position. It's soon agony!