Don't use inertia when doing sit-ups.
This change in your body shows that when you do sit-ups, most of your strength comes from the upper part of your abdominal muscles.
Don't do this. It's totally wrong.
In fact, standard sit-ups are very difficult and dangerous.
Because you haven't learned how to exert strength on your hips, the psoas muscle is above your hips to protect your spine.
I don't know that doing sit-ups with the explosive power of abdominal muscles will hurt the spine.
The standard sit-ups should be as slow as possible, and the slower the safer, but the more difficult it is.
In fact, the trump card for exercising abdominal muscles is leg lifting.
You can look up the book Prisoner's Fitness, which has a complete leg lift tutorial.
If you don't believe me, you can keep practicing, but you must have read Bian Que meets Cai Huangong.