Your abdominal muscles (in the front of your trunk, not in your back) do the real work when it comes to sit-ups. Many structures are of course still involved with this exercise, though, as with any movement. When someone tells me that sit-ups hurt his back my thought is that repeated joint activity (flexion) causes his back pain. With sit-ups, the spinal joints flex, or bend forward. If repeated muscle activity was the problem causing the pain, we’d expect the pain to be in the abdomen, where the muscles actually are.

Location of pain and provoking factors are just two pieces of the puzzle when it comes to diagnosing, but they do tell me two important things. One, in this scenario the person likely has the diagnosis of joint derangement (versus a muscle or other problem). And, two, it’s unlikely the person will need spinal flexion to get better, since it is clearly provocative. (Thus it’s more likely he’ll need another spinal direction to get his joints working properly again - and therefore be able to soon tolerate all the flexion in sit-ups.) -- Laura

So sit-ups flex your spine? So what? We flex our spine thousands of times a day and sit flexed an awful lot as well. As long as you (1) know how to discern if an activity (such as sit-ups) is harming you and (2) know how to correct it if it is, you are good to go. The best way to know if an activity is harming you outside of symptoms is ascertaining if it decreases your motion. (If we’re talking about the low back, that means flexion, extension, left sideglide, and right sideglide.)

I incorporate sit-ups into my workouts as well as other spinal flexion exercises. I like sit-ups with my legs straight and ones with my legs bent. But, what do I also do? I spend 10 seconds every day checking if I have my normal low back motion and also perform prophylactic movements. (I’m lying on my stomach propped on my elbows right now.) If I were just starting sit-ups, I would check right after to make sure they didn’t cause me to lose motion (they never did).

Are there certain things certain people can’t do? Of course. Sit-ups could hurt your back just like brushing your teeth could or a long car ride could; they’re not extra scary. By understanding the concepts of how to keep joints healthy, we don’t have to avoid nor fear specific exercises. I’m here to teach. -- Laura

In orthopedics, the core comprises a specific group of muscles in the trunk/pelvis. (Others use core generally to mean trunk.) Core muscle strength is beneficial. Just as arm, chest, and foot strength are beneficial! Core muscles are not exemplary. They’re no more our “foundation” than our foot muscles or those running the length of our spine.

Many erroneously treat orthopedic low back pathology by strengthening the core. Assuming core muscle strength can be accurately assessed, if one or more of them is weak, the question is why. Muscles become weak (and painful) from pulls/tears. However, these are very rare when it comes to the large muscles of the core. (Tears follow a consistent, predictable pattern, too, which should make them obvious to an attentive clinician.) Pain can create weakness, but absent a clear tear, the pain usually originates from something other than the muscle.
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The number one reason any muscle is weak (the large majority of cases) is because its electricity from nerves has been inhibited – either at the spine or extremity joints. It’s a joint problem. Therefore, in most cases strengthening a weak muscle (or entire group!) is simply attacking a symptom, which won’t fully resolve the problem. -- Laura