Sit Up vs Crunch, know the difference. A crunch is an isolation exercise that targets your abdominals. It has a short range of motion with only your shoulders coming off the floor while your lower back stays down. A sit up is a compound exercise where your entire upper body comes off the floor. It targets multiple core muscles including hip flexors as well as abdominals. #situps #crunches #abs #sixpack #core #fittok