DON'T DO SIT UPS, they're ineffective, read my post on why!

The military, the gym classes in schools and still athletes and others are still doing sit ups anywhere, WHY? It's because nobody is educated enough or has the common sense knowledge about the muscles involved or not being involved. People think it's doing something for them, they think it's an ab work out when it's not!! 

I provide a picture here that shows the iliopsoas (iliacus and psoas major), these are the muscles more involved in sit ups. The iliopsoas muscles help you sit up in a chair for posture and also initiate hip flexion. The abs aren't isolated, they maybe pulled on a little bit but ideally the iliopsoas is sitting you up! Look at the picture and you'll see the attachments for iliopsaos. You'll also see the different in the rectus abdominis.  

The iliacus orginates on the iliac fossa as in the picture and the psoas major orginates off the transverse processes and bodies of the lumbar vertebrae, and both insert on the lesser trochanter, where that iliopsoas tendon comes down to in the picture. The last arrow on the left. 

 

In this picture of the rectus abdominis you can see why it wouldn't be involved in sit ups, they don't help you sit up. The rectus abdominis helps compress your abdominal contents, helps for deification, trunk flexion and inhalation. The rectus abdominis orginates on the pubis and inserts onto the xiphoid process and 5-7 ribs via the cartilages. You'll feel the abs pull but it's the iliopsoas helping you sit up, feel for these muscles.