Alarm bells rang in my head while visiting my parents today when Mum told me she does sit ups as part of her exercise routine. I have never ever suggested she does sit ups. I asked her to show me and she laid down on the floor, and before I knew it she had clasped her hands behind her head, brought her elbows in, pulled her head in and swiftly raised her upper body up to her knees. I gasped in horror, jumped up, exclaiming “No Mum, stop!”. After I had explained that sit ups can potentially be harmful for the back and neck, and that other exercises would be more beneficial and safer, she accepted my advice to avoid the sit up.
The sit up, bent or straight leg, does strengthen the rectus abdominis, but it is not a muscle we should concentrate on strengthening, as it does not help to control rotation at the lumbar spine so does not help protect it, and over working the rectus abdominis contributes to thoracic kyphosis.
The hip flexor muscles which attach the top of the femurs to the front of the lumbar spine contract strongly during a sit up, which can cause anterior shear forces on the hyperextended lumbar spine, especially when abdominal strength is low or the lumbar spine is stiff. Bending the legs doesn’t make it much safer as it still demands considerable hip flexor involvement. Pilates and yoga use a version of sit up called roll-up which can be safe if taught in a one to one situation and closely monitored, but overall there are superior methods of creating abdominal muscle balance and stability for the spine.
Emphasis should be placed on strengthening the external obliques and transversus abdominis. Examples of good abdominals exercises are the side bridge, plank, wood chops, knee rolls, supine bridges, ball roll outs, and all standing exercises where stability is required.
I’m not saying ‘never do a sit up again’ but I would ask why would you do it when other options are safer, and, is it right for you? Maybe not.