This is such a "worn" idea. In the early nineteen eighties, I was responsible for ways to convert stored energy to generate power in Measurements While Drilling (MWD)Systems. One way was to harness the stored energy in downhole shock absorbers coupled to various generators, magnetic voicecoil motors, piezoelectric crystals, hydraulics, etc. Working models were constructed, but I did not patent them. After leaving the company (NL Industries, Houston. TX) "they" did patent the ideas and took the credit!!!
Now, if only someone would talk to Cybex or some other gym equipment maker and come up with a way to convert that wasted fat enery into energy that can be stored. Some kind of friction-based kinetic transducer, or a pressure transducer instead of dumb weights... Seems a shame to see that huffing and puffing go to waste.