Crunches are a popular abdominal exercise that primarily target the rectus abdominis muscle, which is the main muscle responsible for the "six-pack" appearance of the abs

. They are performed by lying on your back with knees bent and feet flat on the floor, then lifting your head and shoulders off the ground by contracting your abdominal muscles, while keeping your lower back on the floor

. This movement isolates the abs without involving the hip flexors, making crunches effective for toning and strengthening the abdominal muscles specifically

. Crunches are commonly used to build and define abdominal muscles and can be done anywhere without equipment, making them accessible and beginner-friendly

. Proper form includes folding arms across the chest or placing fingertips lightly behind the head without pulling on the neck, lifting the shoulders smoothly, and avoiding neck strain or back injury

. They are less comprehensive than other core exercises like planks because they focus only on the abs and do not engage other core muscles such as the obliques or lower back

. While crunches help strengthen the core and improve posture, they do not burn fat by themselves; achieving a flat stomach requires combining crunches with aerobic exercise and a healthy diet

. There is some risk of back and neck strain if done improperly, so attention to technique is important

. In summary, crunches are a bodyweight exercise designed to isolate and strengthen the abdominal muscles, useful for toning the belly and building core strength when performed with proper form