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

​Depending on where a muscle originates and where it inserts, it can move the bones of your body in a specific way. It’s rare that a muscle even works in isolation. A certain muscle or muscle group may need to be a focus of rehab in the case of a problem, but there’s no hierarchy of prestige in the human body. (I’m not talking about the heart, diaphragm, etc.) Your psoas is not the key, nor is your core (a group of several muscles). No one muscle will ensure health or prevent injury.

​The relative importance of a muscle is circumstantial, based on what a particular individual’s needs are. For example, a shot putter, to improve performance, will focus on different muscles than a cyclist. Someone whose goal is to be able to get up easily from a low couch will have different needs as well. -- 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