Sit ups

Sit ups develop strength in the Rectus Abdominals and the accessory hip flexor muscles. Sit ups shouldn't be performed with the feet hooked under an object or with an assistant holding onto them. This enables the hip flexor muscles to produce the majority of the movement. Instead cross your feet or just allow your knees to hang slightly this will increase the activation of the rectus abdominis.

Functional Benefit

The Rectus Abdominals counter the forces produced by the gluteus maximus and erector spinae during running or exercises like supermans. Abdominal strength is a key component of preventing tight hip flexors, and is important in maintaining core position during bridges positions