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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.
VERDICT: FAST REP 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.