Key difference: Crunches and sit-ups are both common abdominal exercises. A sit-up is a strength training exercise which strengths the hip flexors and abdominal muscles, in addition to chest, neck, lower-back and lower-leg muscles. A crunch, on the other hand, is considered a half-sit up. It only exercises the abdominal muscles.

Crunches and sit-ups are both common abdominal exercises. The problem while differentiating them is that they tend to overlap. However, they are two different types of exercises that target different muscle groups. They should both be included in a core body workout as they each have different functions.

A sit-up is a strength training exercise which strengths the hip flexors and abdominal muscles, in addition to chest, neck, lower-back and lower-leg muscles. A crunch, on the other hand, is considered a half-sit up. It only exercises the abdominal muscles. Let’s see how crunches and sit-ups differ physically.

BCLiving defines the proper method of doing a crunch and a sit-up:

How to do a Proper Crunch

• Lie on the floor, legs bent at the knees, feet shoulder-width apart and flat on the floor. Rest your arms on the floor at your sides, cross them on your chest or hold your hands lightly at your ears.
• Raise only your head and shoulders from the floor to feel the abdominal muscles contract. Avoid pulling or flexing your head forward; keep your eyes looking above and ahead of you.
• Return to your starting position. Exhale as you rise and inhale as you lower.

How to do a Proper Sit-up

• You can hook your feet under a secure object for more stability, but this will use muscles in the hips and legs, putting less stress on the abs.
• Place your fingertips at your ears, or rest your hands lightly behind your head, crossed on your chest or lying at your sides. The neck should be slightly flexed (head tucked forward) for less involvement of the back.
• Concentrate on using abdominal strength to curl your upper body off the floor until you are upright in a semi-seated position. Exhale as you rise. Inhale as you lower back to the floor.

Unlike a sit-up, in a crunch, the lower back stays on the floor. Also, strength exercises such as sit-ups and push-ups do not cause the spot reduction of fat. Fat should be reduced all over the body, including the abdomen, to develop it.

Furthermore, as a sit-up works more muscles that the crunch, it can cause stress in the lower back. Excessive sit-ups may cause a condition known as Lordosis or saddle-back. Hence, some argue that sit-ups should be replaced with the crunch in exercise programs.

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