Situps vs. Crunches
Lying on your back with your knees bent and your feet flat on the floor is the beginning position for situps and crunches. However, crunches have a much smaller range of motion. For situps, lift your torso all the way off the floor. Your low back should not be in contact with the floor at the top of the movement. To do crunches, perform the first third of a situp. Lift your head and shoulder blades off the floor, but keep your low back in contact with the floor at all times.
Spinal flexion is when you pull your ribs toward your pelvic bone -- your waist bends and your low back rounds. The abdominal muscles are the primary movers for the first 30 to 45 degrees of spinal flexion. After that point, the hip flexors take over the movement. The hip flexors, or iliopsoas muscles, attach to the spine and extend down to the pelvic bone. Once you lift your low back off the floor and sit up all the way, you are not targeting your abdominal muscles, but rather your hip flexors, so you won't feel the exercise in your abs as much. Situps also involve high levels of activity in the rectus femoris, or quadriceps, taking even more of the load away from the abdominals.
Because of the involvement of the hip flexors, the situp exercise places undue stress on the lower spine. When overworked, the hip flexors shorten and pull on the lower spine, causing back pain and excessive pressure on the lumbar discs. This means the situp exercise may be contraindicated if you have low-back problems or pain.
To feel your abdominal muscles work, perform crunches in a slow, controlled manner. Do not pull on your neck. Press your low back into the floor and bend at the waist, pulling your ribs toward your pelvic bone to fully activate the abs. Include a range of crunch exercises in your program that involve spinal flexion, torso rotation and lateral flexion -- bending to the side -- to thoroughly work all your abdominal muscles.