Why it's on the list: Muscle activation studies consider the plank a mid-level exercise. But in this case, that doesn't tell the whole story. It's first and foremost a great transverse abdominis move, but that muscle is deep, so can't be measured by EMG.

Planks primarily made the list because of how easily you can use different variations to change the level of difficulty. If an elbow plank is too difficult, perform it with the arms straight or simply drop to your knees. Too easy? Lift an arm or a leg—or an arm and a leg. Put your feet into suspension straps or on a stability ball. Each one of these progressions leads to a greater training stimulus to the abs.

Plank Variations for Strength and Growth:

• Elbow plank  
• Side plank  
• Plank leg raise  
• Feet-elevated plank on ball  
• Elbows-elevated plank on ball  
• Side plank hip dip

In your workout: You can do these last on ab day, but you can also do them at home just about any time you want. Since it's an isometric move, shoot for time rather than reps. Doing 3-5 sets of 30-90 seconds in whatever variation is toughest for you should get you shaking!