The Plank by itself is one of the great exercises you can do. It builds endurance, strengthens the abdominals and lower back, and works the stabilizers.
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If you haven't performed the Plank itself yet you can begin the 3 Levels of Progression for it:
Level 1 : hands & feet, tight abs, tight glutes, straight line from head to heels
Level 2 : Elbows & knees, tight abs & glutes, straight line from head to knees.
Level 3 : Elbows & Feet, tight abs & glutes, straight line from head to heels.
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