The crunch is a simple trunk flexion exercise.
Lie supine with the knees and hips bent to place the feet flat on the floor. Hold the fingers at the sides of the head or crossed over the chest. Squeeze the abs to curl the trunk upward as high as possible without flexing the hip—this movement is strictly flexing the spine. Return to lying flat while maintaining tension in the abs and repeat.
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.
Crunches should be done for sets of 10-30+ reps.
Crunches can be done with the feet elevated on a bench, or with the feet being held in the air to increase the difficulty.