When performing a basic crunch, lift your upper body off the mat until your should blades come off the floor. This is about 30 – 45 degrees of movement.
Your abs are the main muscles working up until this point; past 30 degrees of movement, your hip flexors take over and do most of the work. (Your hip flexors are a group of muscles that lift your knee or thigh up)
You may notice your feet starting to come off the ground once you curl up past a certain point. This is a sign that your hip flexors are taking over.
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