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Most bodybuilders stick to traditional bodyweight exercises for abs, such as crunches, bicycle crunches and hanging leg raises at about 20 reps per set. Yet the abs are muscles just like the biceps in that you need to vary your weight and rep ranges to strengthen and better develop the muscles and avoid stagnation.
Researchers from Youngstown State University (Ohio) had male and female subjects perform a bodyweight ab workout three or six times per week, and others did no ab training at all. They did three sets of 20 reps for all exercises with 30 seconds of rest between sets. They measured their abdominal muscle strength at the start of the study and after 11 weeks of following the program.
The researchers were surprised to find that regardless of whether the subjects did the ab workout three times per week or six times per week, their abdominal strength did not increase significantly from their starting strength.
Sticking with the same resistance and repetition range even for abs appears to cause stagnation and prevents significant gains in muscle strength.
Regardless of how often you train abs, the more important factor appears to be cycling the resistance (weight) and rep range. Be sure to include weighted exercises, such as weighted crunches, cable crunches and machine crunches, and alter between heavy weight and low reps (6–8), moderate weight and moderate reps (8–15), and light or no extra weight and high reps (15-plus reps).