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.