Let's say you have a significant other, work out buddy, best friend, with whom your relationship entails some physical activity routine together. Let's say that traditionally that physical activity has been chronic cardio. It gives you time to chat and interact while getting in your exercise. Very multi-task oriented, right?
Let's also say you wanted to change the routine. That your efforts to change it up have been met with resistance and now you've decided to toss out a challenge specifically related to your exercise routine.
You let your workout buddy do his/her usual thing and you adopt a routine oriented more to resistance exercises and intervals.
What is the duration of the challenge? 4 wks? 6wks?
What are your parameters? Before and after photos? Weight reduction? Body fat %?
Keep in mind both parties will continue their other regular dietary habits and hobbies to reduce confounding variables and we are only going to compare the effect of this exercise routine change. Granted there may be some unconscious effort to eat "better" or put more oomph into your regular routine as a result of the challenge but those things can't really be controlled for in this design.
This could be fun.
Weight reduction? Don't think you want to go there. If chronic cardio is your norm and you start sprinting and resistance training you're most likely going to gain weight, not lose. It will be good weight but toss the scale out the window.
Bodyfat %? Unless you have hydrostatic weighing available to you it will be unreliable at best. Even calipers used in 10 different places produce very inaccurate numbers in most cases.
Photos? This is probably the best option you've mentioned. Just be sure you do the before and after photos together so they're done in the same lighting, etc in both instances. Only shortcoming of photos would be subjective nature of how somebody looks in a photo. Your 'opponent' might think really skinny looks more healthy...if cardio is their game I'd bet on it.
Could you put together a challenge incorporating several activities (pushups, situps, 100 meter sprint, 1 mile run, standing long jump, etc.) like your own decathalon? Perform them all on Day 1, then perform them all again on Day 60. You earn one point for every percentage point you improve in each exercise. No sandbagging on Day 1...this only works if you're competing against somebody you really trust.
Good luck, looking forward to hearing how it goes!
C-reactive Protein? or some such inflammation marker? That'd be a good comparison. And it is something that does matter.
Shoot for 3 months ideally, to make some changes.
Why don't you do a cross -over experiment. Agree that you will both follow partner A's preferred exercise routine for 30 days, then you both follow Partner B's exercise routine for the next 30 days. Do it as a dare or whatever - at the end of 30 days if one routine is better it should be obvious and the other aprtner will agree. Unless they are really stubborn but it doesn't sound like it.