The main function of sit-ups is to strengthen abdominal muscles. Doing sit-ups can not only improve the elasticity of abdominal muscles, but also protect the back and improve the posture. On the other hand, doing sit-ups is a waste and even harmful.
Do sit-ups:
Lie on your back and bend your knees about 90 degrees. Keep your feet flat. Don't fix your feet (for example, your partner presses your ankle with his hand). Otherwise, the leg and hip flexors will join the work to reduce the workload of abdominal muscles.
Moreover, straight leg sit-ups increase the burden on the back and easily cause back injuries.
Determine the position of your hand according to the strength of your abdominal muscles. The closer your hands are to your body during sit-ups, the more difficult it is for beginners to lean their hands on their sides to adapt to or improve their physical fitness, so they will cross their hands and stick them on your chest.
Try crossing your hands on your face, too. Each hand should be placed on the other shoulder of your body. Hands and fingers are often crossed on the face to avoid neck muscle tension and reduce the workload of abdominal muscles.