Lie on your back with the knees and hips bent to place the feet flat on the floor. Hold the fingers at the sides of the head. Squeeze the abs to curl the trunk upward as high as possible without hinging at the hip—lift the upper and mid back off the floor while the lower back pushes down into it. Return to the bottom while maintaining tension in the abs and repeat.
 
Purpose
The crunch is the simplest and easiest trunk flexion ab exercise and is good for developing stamina in the abs, as it will generally be used with high reps and should have continuous tension in the abs through a set.
 
Programming
Crunches should be done for high reps—generally this will be 30 or more.
 
Variations
Crunches can be done with the feet elevated on a bench, or with the feet being held in the air to increase the difficulty. Slow tempos and pauses can also be added.