There are several natural signs for exercise. One looks like you are pulling up some dumbbells at the gym, doing bicep curls: starting with your arms down and both hands in fists with thumb and fingers facing you, bend your forearms at the elbow, then pull up toward your body and repeat a few times. Another sign for exercise looks like tricep pull-ups, where you start by holding both fists on either side of your head, fingers facing out, then push them upward aligned with your head and parallel to each other, repeating this up-and-down motion a couple of times.
A third alternative ASL sign for exercise is to rotate both letter 'X' (or hooked) hands on either side of your head, as if you are mimicking jumping rope.
Exercise is an advanced sign that you can use to supplement an activity or location. You can use exercise to refer to energetic activities you do with your baby, whether it be running around the backyard or swinging on the monkey bars. "Let's go to the park and get some exercise."
Teach fitness early on! You can introduce your baby to this sign when doing baby yoga together, or when you go for a walk.