Rich,
I've worked on "The Firm" workout video shoots for about fifteen years and also did post-mixing and video editing on the last series. We originally used a Fisher boom to drop a painted directional lav mic into the shot. The mic and it's cable were painted light grey in an attempt to camouflage it. I forget what model of mic (ESM-55 I think), but it worked ok for the first few episodes. The problem was when the instructor walked and talked, she very often did so quickly and we lost the level as the boom operator tried to follow.
The more recent shoots utilized a wireless system of various configurations, but they all worked better in most situations. I like to use a Lectrosonics 400 series Hybrid with a Countryman B6 capsule. The small size of both the 400 transmitter and the B6 works great and the thin cable can be run through tight spandex without being too obvious. The transmitter is put into a small fanny pack at the waist and kept in the small of the back. I use a modified iPod armband for this purpose.
There are a few situations where mic and/or transmitter placement will need to be tweaked, such as when the talent is laying chest first on the floor. The mic gets hit and the voice is already throaty.
I find an excellent placement for a small mic such as the B6 to be in the hair just above the forehead. If you tell the makeup/hair stylist ahead of time, they will usually clip it in for you. Just check to make sure that there is plenty of slack in the cable for head turns and don't let them spray your mic with hairspray!
Other options would be to just let the instructor wear a headset boom mic. I remove the windscreen and pull the boom back towards the ear to minimize proximity effect boominess and to give the cameras clean face shot.
If you don't have access to the equipment I've mentioned, you can use other models, just try and use an omni lav with a thin cable and small head. The transmitter should be as small as possible as well. Mic placement, a strong voice and a quiet studio are more important than brands. Remember that the sound take will be masked by an unusually loud music track and intelligibility is key.
I hope this helps your situation.
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