33.3 percent of external obliques were recruited when exercisers increased their rep speed to one rep per rep. Translation: For hard-gaining abdominal muscles like the external obliques, faster is better. The study from Universidad Miguel Hernandez de Elche measured the effects of rep speed on muscle recruitment and coactivation of the rectus abdominis, internal obliques, external obliques, and erector spinae during curl-ups, at speeds of 4, 2, 1 1/2 and 1 second per repitition.
SEE ALSO: 7 Best Moves For Crazy Obliques