If you want amateurish looking snapshots with greenish skin tones from overhead flourescent lights and deep shadows in the eye sockets, then by all means use their existing lights.
But if you want anything resembling professional looking results, then think about controling both the qualtity and quality of light. I would never even consider doing a shoot such as you describe without the ability to control the light, ie side light to show the musculature, fill to light the face and a back light to outline contours. There is absolutely no diference in lighting requirements between shoot in in a studio environment and on location. On location, you need to bring at least a minimal studio with you. Beyond that, I would personally include baby oil or glycerine to simulate sweat along with full makeup.
If shot during business hours, how anoyed will those waiting for the exrecise equipent be because of delays you cause? If there is any nudity at all involved, how will that be received by the regular jocks present and watching?
I suspect you would gain more of a chance of follow up shoots from having the photos shown in the gym than from those waiting for the equipment you have tied up during the shoot.
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