Stand with your feet together and your arms by your sides. Jump your feet out wide while swinging both arms up overhead, then jump your feet back together as the arms come back down - one smooth, continuous rhythm. Land on the balls of your feet with soft knees every time, and let the arms swing through their full arc: sides to overhead and back, not half-hearted flaps at shoulder height.

Works: quads, glutes, calves, shoulders

Breathing: Steady and rhythmic - one breath cycle every couple of jacks, held from the first rep to the last.

Common Mistakes: Landing hard on flat feet with straight knees, cutting the arm swing short, and letting the rhythm fall apart as fatigue builds instead of slowing down and keeping it clean.

Make Easier: Take the impact out - step one foot out at a time while the arms swing - or drop the arms and do Half Jacks.

Make Harder: Speed up while keeping the full range, or vary the pattern with Split Jacks or Seal Jacks.