developing the abdominal muscles for size rather than capability I find to be such a difference. even one set of situps to failure and the next day my vagus nerve is canked, which always sucks. for me, the core muscle seem to be best trained by twisting movements, fluid movements like kettlebell swings, and isometric holds. and they're very forgiving even if I do a lot. whereas the contraction-to-failure situps for muscle growth approach requires way more restraint. one set, near to failure.