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Lie on your back with your knees bent and feet flat. Press your hips up.. The goal is to get your body straight from knees to shoulders. Hold this position for 3-5 seconds, then slowly lower back down. When this is easy you can progress to alternately lifting one leg at a time as if "marching in place". The focus then is to not allow your hips to drop as you alternately lift one leg at a time about an inch off the floor.
When counting reps, lift - march right, left, right , left - lower.... that's one rep.
Repeat as directed by your coach.