I learned from
@Playalot to either drink a protein shake (made at a bar
in Energized mood) or the protein plate (cooked while in Energized mood) before exercising on the workout machine.
For me, I formerly couldn't achieve maximum body potential without also using the weight lifting machine. In a recent game, however, my sim achieved it after several consecutive days of yoga before maxing Fitness. In this case, though, he'd been to the gym for cardio and weightlifting on previous days as well.
Here's a link to the thread on achieving maximum body potential:
http://www.carls-sims-4-guide.com/forum/index.php?topic=23148.0Edited: Added a strikethrough in accordance with
@Playalot's correction that a protein shake can be made without being in Energized mood