This is the latest tidbit from a recent Harvard Medical School health newsletter:
Sit-ups were once thought to be the trick to tighter abs and a slimmer waistline, according to the newsletter. But planks, when you hold your body in a push-up like position, have become the gold standard.
Sit-ups can be hard on your back because you push your curved spine against the floor and because you work your hip flexors, muscles that run from the thighs to the lumbar vertebrae in the lower back, according to the newsletter. When too strong or overly tight, hip flexors tug on these vertebrae, which can be a source of lower back discomfort.
Besides, planks exercise a better balance of muscles on the front, sides, and back of the body than sit-ups, which target just a few muscles.
— Anne Aurand, The Bulletin