A sit-up is an exercise where you lie on the ground and lift your shoulders and upper back off the ground to flex your spine. The movement creates a compression in your outer abdominal muscles, stimulating some muscle growth. Sit-ups help tone and strengthen your abdominals; however, they do not burn belly fat.
Fat Metabolism
Fat metabolism occurs in your cells' mitochondria, which are organelles inside the cells that break down nutrients into energy. During exercise, fat is broken down into fatty acid chains and glycerol, its basic components, exercise physiologist William McArdle explains. These substances are released into the bloodstream and muscle cells, where fatty acids are transported into the mitochondria. They are broken down by complex series of chemical reactions to produce energy. Fat metabolism continues as long as oxygen and glucose are present in your cells -- also known as aerobic metabolism. Otherwise, lactate will build up in your muscles from a lack of oxygen, causing fatigue and muscle cramps.
Misconceptions
Many people believe that doing sit-ups and other abdominal exercises help reduce body fat in their belly. However, these exercises last between 30 seconds and 90 seconds, which is not enough time for your body to enter aerobic metabolism. According to dietitian Ellen Coleman, author of "Ultimate Sports Nutrition," you need between 10 and 15 minutes of continuous exercise at a moderate intensity -- about 55 to 65 percent of your maximum heart rate -- to get into pure aerobic metabolism. When you do sit-ups and other strength exercises, you are using the anaerobic system, which uses about 90 percent carbohydrates and 10 percent from fat and protein. Coleman states that the only way to reduce body fat -- including your belly -- is by burning more calories than you eat.
After-Burn
Although you do not burn fat while doing anaerobic exercises, these exercises tend to be higher-intensity in nature than lower-intensity exercises, such as jogging, walking and cycling. After a bout of high-intensity exercise, your body transitions from anaerobic metabolism to aerobic metabolism. This is called excess post-oxygen consumption, or EPOC, a state where you breathe hard to recover between sets of strength or high-intensity exercises, according to exercise physiologist Chantal Vella of the University of New Mexico. During EPOC, your body uses fat as the main fuel source to repair muscle tissues, replenish nutrients to your cells and bring your metabolism back to its resting state. In other words, you continue to burn fat throughout your body after a high-intensity workout.
Considerations
Instead of doing sit-ups, Rodney Corn, co-founder of PTA Global, recommends that you do full-body exercises to burn the most calories in less time. Although you can incorporate ab exercises into your workout, add full-body exercises, such as push-ups on a stability ball, pull-ups, kettlebell swings, sprinting, rope jumping and medicine ball throws.
Warning
Too many sit-ups can cause your lower spine to lose its natural curve and cause your upper spine to curve out excessively. Anthony Carey, owner of Function First in San Diego, explains that most people already experience a tight chest, shoulders and abdominal muscles from working at a desk all day. Sit-ups further tighten the front of your body more and weaken and stress your back and neck muscles.
References
- "Exercise Physiology: Energy, Nutrition, and Human Performance;" William McArdle; 2001
- "IDEA Fitness Journal;" Exercise After-Burn; Chantal Vella; November 2004
- "Ultimate Sports Nutrition;" Ellen Coleman; 2004
- "IDEA Fitness Journal;" Creative Total-Body Exercises; Rodney Corn; February 2010
- "Pain-Free Program;" Anthony Carey; 2005
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