Bodybuilders have always done reps in a slow and controlled manner, because it better works the target muscle rather than relying on momentum to “cheat” on the exercise. Therefore, crunches should be performed at a speed no faster than 2 seconds on the positive (concentric) reps and 2 seconds on the negative (eccentric) reps.


Fast reps (at a speed of 1 second or less on the positive) can help to recruit more muscle fibers, which can lead to better muscle development, including the abs.


  • Spanish researchers tested the muscle activity of subjects’ rectus abdominis, external obliques, internal obliques and spinal erectors while they performed crunches at a rep speed of 4 seconds, 2 seconds, 1.5 seconds, 1 second or as fast as possible.
  • They reported that as the rep speed increased, the muscle activity of all four muscles increased. The greatest boost in muscle activity occurred in the external obliques, which were hardly involved in the crunch at the slower speeds, but rose by over six times at the fastest speed.


The fast reps will help to recruit more muscle fibers in the midsection and will particularly help to turn an ab exercise, such as the crunch, into a fairly effective oblique exercise.


Be sure to mix up your rep speed during ab exercises — such as the crunch — going slow and controlled for some workouts, and as fast as possible for other workouts. This can help you build better-developed abs that are also strong and powerful.