How Much to Bend the Knees Doing Sit Ups?

Upper-Body Position

Before you start to perform a set of sit-ups, get your body into the correct position to tackle the exercise. Lie flat on the floor on your back and bend your arms so that your hands are together behind your head or neck and your elbows are pointing away from your shoulders. As an alternative to holding your hands behind your head or neck, cross your arms over your chest.

Lower-Body Position

When your upper body is in position, focus on the correct placement of your lower body. Bend your knees and place your feet flat on the floor. The American Council on Exercise recommends having your heels 10 to 12 inches from your backside and Top End Sports suggests positioning your feet about 12 inches apart. When you're in correct position, your legs will be bent at roughly 90 degrees. Having them bent at slightly more or less than 90 degrees won't negatively impact the exercise.

Sit-up Execution

When ready to perform your first sit-up, exhale and contract your core muscles to begin lifting your shoulders and upper body off the floor. While keeping your tailbone firmly on the floor, pull your chest to your knees. Inhale and slowly lower your shoulders toward the floor while keeping your core muscles contracted and your shoulders and neck relaxed. Once you reach the floor, you've completed one rep of this exercise.

Sit-up Variations

A standard sit-up is an effective way to build your abs, but if you want extra challenge in your workout, you can try any number of sit-up variations. Modifications to consider include weighted sit-ups, in which you hold a weight plate behind your head or against your chest, or incline sit-ups, when you lie on a bench with your head and shoulders lower than your knees.