Anchoring your feet during situps increases the chances of back pain.
Since grade school, you've been taught that if you want a strong, flat abdomen, all you need are situps. Situps, though, are not an effective way to lose belly fat or tone your midsection. You need a combination of diet, cardio and stabilization and rotation abdominal exercises to reach those goals. It turns out that situps' effects are minimally positive and mostly negative.
Thigh and Hip Activation
One muscle used during the situp is your rectus abdominis, which is the front sheath of your abdominal muscles. The problem is that this muscle doesn't act alone. When you perform the classic situp with your knees bent, your rectus femoris at the front of your thighs and the muscles of the hip flexors, which attach this muscle to your abdomen, also work extra hard. Over time, this can lead to tight, shortened quad and hip flexor muscles, which pull on your back. In turn, this can potentially cause back pain when these tight muscles pull on the muscles of your lower spine.
The classic situp has you place your hands behind your head to support the cervical spine. If you pull the head to help you get your upper shoulders off the floor, you risk injuring the muscles and connective tissue of the neck. If you don't place your hands behind your head, holding your head up without that support can lead to neck strain.
Potential Disc Problems
Situps require you to repeatedly flex your lower spine. For some people, this repeated flexion puts too much compressive force on the spine and can lead to a bulging or herniated disc. If you combine daily situp practice with a job in which you spend much of your day seated, or if you combine it with numerous other exercises like crunches and toe touches that also involve flexion, you are at risk of this injury.
Ab Muscle Imbalance
Situps may help you strengthen your ab muscles somewhat, but they are limited in their scope. The abdominal region is made up of multiple, interweaving muscles that should all be targeted to achieve optimal strength. Situps only target the superficial rectus abdominis and use the obliques for assistance. If you choose to only train with situps, your abdominal muscles will be imbalanced because you aren't thoroughly activating the internal and external obliques or the transverse abdominis. Other exercises -- including full-body, compound exercises such as squats and deadlifts along with rotation and stabilization movements such as medicine ball twists and planks -- are better choices when it comes to balanced abdominal conditioning.