Sit-ups once ruled as the way to tighter abs and a slimmer waistline, while “planks” were merely flooring. Now plank exercises, in which you assume a position and hold it, are the gold standard for working your core, while classic sit-ups and crunches have fallen out of favor. Why the shift? One reason is that sit-ups are hard on your back – they push your curved spine against the floor and work your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When the hip flexors are too strong or too tight, they tug on the lower spine, which can create lower back discomfort.