Plank jacks are a deceptive exercise. Do them for a brief interval of time and they are easy enough. Increase the length of time you do them for and they become immeasurably harder. Do it slowly and it becomes a strengthening exercise. Perform it really fast and soon enough you start to breathe hard too. 

What's involved: Shoulders, triceps, lower back, core, lower abs. 

Powered by: Shoulders, triceps, adductors and core.

Tip: Do not raise your feet too high off the floor. As you bring your legs apart and back again try to just skim the surface of the floor and back again fast. This helps work your adductors (inside of your thigh) as well.