Biking -- the human-powered kind -- is an example of aerobic exercise, which is any activity you do that raises your heart rate and increases your body's demand for oxygen. Aerobic activity lasting longer than about 20 minutes will start to burn fat because at that point, or sooner, your body's store of carbohydrate energy has been depleted -- and fat is stored energy waiting to be used. The Centers for Disease Control recommends at least 30 minutes of biking or other aerobic activity every day to support a longer and healthier life.
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Biking involves repetitive and dynamic muscle contractions that require muscular endurance. Muscular endurance activities like biking work muscle fibers that can sustain contractions for longer periods of time. The muscle fibers that can sustain the longer contractions required for biking also contain more fat-burning components than fibers that perform shorter contractions.
You contract muscles in your lower body while biking and to a lesser extent in your midsection. These contractions require energy. Cellular components known as mitochondria produce energy that makes it possible for your muscles to contract while biking. Mitochondria produce energy by burning fat and converting it to cellular energy known as adenosine triphosphate or ATP. Biking can burn fat substantially, because this kind of muscular endurance activity recruits muscle fibers that contain the highest numbers of fat-burning mitochondria.
Aerobic activities like biking keep your heart rate above your resting heart rate. These activities recruit energy from glycogen stored in your muscle tissues and switch to energy from fat after you deplete glycogen stores. Biking and other aerobic activities may burn fat if your biking sessions last long enough for your body to break down fat for energy.
Your heart rate ranges from 60 to 70 percent of your maximum heart rate during a low-intensity aerobic activity like biking. Estimate your maximum heart rate by subtracting your age from 220. According the National Federation of Personal Trainers, your body actually uses more energy from stored fat with lower-intensity aerobic activity like biking than it does from high-intensity shorter workouts. Frequent low-intensity biking can burn enough fat to contribute to a healthy fat loss regimen if your biking sessions last an hour or more.
Higher intensity biking can enhance the amount of fat that your muscles burn. You can achieve a higher intensity by biking uphill against greater resistance; the goal is to cause your heart rate to exceed 70 percent of your maximum rate. Higher intensity biking may cause lactic acid to build up in your muscle fibers, which progressively inhibits the body chemistry that facilitates muscular contraction. Lactic acid build up is what produces that burning sensation in your muscles during a workout. Mitochondrial failure occurs when you deplete the energy in your muscle fibers and cannot contract your muscles. This can enhance the amount of fat you burn while biking, because your muscle fibers will adapt to mitochondrial failure by producing more mitochondria during post-biking recovery.