Had a thought - just the one :) How about those spring loaded grip developer things (can't think the exact term) you use to strengthen your hands. If he had one secured to each hand so he couldn't drop them and the only way to stop the electro to his cock was to keep them closed he would have someting else to think about when on the treadmill. Unclenching his hands would break a circuit which would switch on the estim. Same principal as those self bondage items/key release that use electro magnets so if the power goes out they release as a safety measure. 