Stand with your feet together and your arms by your sides. Jump your feet out wide while raising both arms out to the sides to shoulder height - forming a T at the top - then jump your feet back together as the arms lower. It's a jumping jack with a shorter, stricter arm path: the arms stop dead level with your shoulders, which keeps constant tension on the shoulders instead of letting the swing carry them.

Works: shoulders, quads, glutes, calves

Breathing: Steady and rhythmic, in time with the jumps.

Common Mistakes: Letting the arms drift above or sag below shoulder height, landing heavy on flat feet, and shrugging the shoulders up as the set wears on.

Make Easier: Step out side to side instead of jumping while the arms keep the T rhythm, or take a break from the arms with Half Jacks.

Make Harder: Pick up the pace and hold the T crisp at exactly shoulder height, or move on to Jumping Jacks or Seal Jacks.