According to Sayer, the crunch is the superior exercise. “In terms of actually targeting your abs, crunches would be better than sit-ups, because sit-ups require the use of more of your hip flexors than abs muscles,” she says.
The way people often perform sit-ups can accentuate this problem too. “My concern with sit-ups is that a lot of people will hook their feet under something to keep their feet down,” says Sayer. “Then they’re really just using momentum and kind of pulling up with their hip flexors. That can cause injury and it ends up not really targeting the abs well.”
That said, Sayer notes that if you have spinal stenosis, osteoporosis of the spine or you’re pregnant, you should avoid crunches. Otherwise, crunches beat sit-ups for training your abs any day of the week.
The crunch has one other major advantage over the sit-up: It trains more of your core than the sit-up, and you’re likely to be weaker in that area than your rectus abdominus. “Performing [the crunch] properly will work other core muscles like the transverse abdominis and pelvic floor,” says strength and conditioning coach Fiona Scott, who contributed to our guide to how to do a crunch.