Plank
Main muscle: Abs
Equipment: no equipment
Mechanics type: isolation
Fitness level: beginner
Category: Increase strength

  1. Get into a prone position on the floor, supporting your weight on your toes and your forearms. Your arms are bent and directly below the shoulder.
  2. Keep your body straight at all times, and hold this position as long as possible. To increase difficulty, an arm or leg can be raised.

Source: bodybuilding.com