Defense Doing ab exercises at a slow and controlled pace is important to enhance ab muscle size and definition. Bodybuilders have long used slow and controlled movements to increase the time under tension and ensure that the target muscle is doing the work without momentum taking over.
Prosecution Fast reps recruit more muscle fibers, so using fast reps for crunches can also provide a similar benefit.

■ Researchers at the Miguel Hernandez University (Elche, Spain) tested the muscle activity of the abs (rectus abdominis), the external and internal obliques and the lower back muscles (spinal erectors) when subjects used different rep speeds — 4 seconds, 2 seconds, 1.5 seconds and 1 second, and as fast as possible (less than 1 second per rep).

■ As speed increased, the muscle activity of all four muscles increased, especially in the external obliques.
■ At slow and controlled speeds, the external obliques were hardly involved, but their muscle activity increased by more
than six times at the fastest speed.

It appears that doing crunches as fast as possible allows you to use more of the ab muscle fibers, especially the external obliques, which turns the crunch into not only a great ab exercise but also a great oblique exercise.

SENTENCING Speed up your reps on crunches from time to time. Perform the first 10-15 reps of a set at a very fast pace. As your ab muscles become fatigued, focus on finishing the set with slow and controlled reps to increase the time under tension. Using fast reps also has been shown to burn more calories, further aiding your quest for a ripped midsection.

— Jim Stoppani, PhD