Plank jacks combine core engagement with cardio ... Lift your legs and upper body, reaching your hands toward your toes. Lower back down with control, keeping core tight.

Sit-ups, on the other hand, engage corresponding muscles (hips, lower back) a bit more than crunches, but not as much as a plank would. Plus, if you have a previously existing lower back or neck ...

Hold a plank on your forearms with your body in a straight line. Engage your core, glutes, and quads. Run your knees towards ...

Sit-ups, on the other hand, engage corresponding muscles (hips, lower back) a bit more than crunches, but not as much as a plank would. Plus, if you have a previously existing lower back or neck ...