GTA San Andreas: Losing muscle

Stabshauptmann

My Character has too much mussle, need to lose some. How?

Ive been told to just run him ragged on the treadmill, he'll start loosing muscle once all the fat is gone, but that takes far too long.

Skellington

As you were told already, run him ragged, and swimming on the surface (i.e not diving under) will start to work after a couple minutes.
Edit: also, stop feeding him, even when it says he's getting hungry.

Borat_Sagdiyev

There's an easier way. Just don't do anything that involves him being fed for a few days, you'll lose muscle fast that way. List of things that feed him:

Saving
Going on dates
Buying any type of food (duh)
Dying
Getting Busted

There may be more that don't come to mind right now. If you're looking for something to do while you're waiting for his muscle to come down, then do the taxi missions or the ambulance missions or the firefighter missions or the pimp missions or the vigilante missions.

The best would probably be the taxi missions or the ambulance missions because in general you aren't in much of a hurry in either of these missions so you won't be in danger of doing anything silly which might get a cop after you.