Plank jacks are a dynamic exercise that mostly works the core. Expect to raise your heart rate and work up a sweat. In addition to strengthening the core, plank jacks engage the hips and back, areas that are important to strengthen if you spend a lot of time sitting.
Keep your abs tight and hips even, and imagine a line moving through your skull, neck, and spine. The motion for this exercise is all in the lower body; don't let your back sag as you jump out with your feet — it is better to jump the feet out less far, or step them out one at a time instead. Keep your eyes on the floor through the entire motion. Looking too far forward or up will affect the position of your entire upper body and can hurt your spine.

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