Find a beautiful setting to walk in.
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While walking is not the most intense physical activity you can do, when it comes to flattening your stomach it can be very useful. Of course, to really speed up the process of flattening your abdominal area, eat healthy foods in sensible portions, and drink plenty of water. Combining this with regular, brisk walks will help to flatten your stomach over time.
Keep The Fires Burning
Walking, although generally not a high intensity exercise, can burn off significant amounts of energy. A Harvard study found that brisk walking could burn up to 600 calories per hour -- on par with whitewater rafting and aerobics classes. Burning off enough energy over the course of the day is important to help you reduce belly fat and flatten out your abdominal area. However, keep in mind that burning off energy will only have the desired outcome if you are not overeating -- using a macronutrient calculator can give you a good idea of how much you should be eating day by day for your body and your goals.
Add Some Mass
Walking helps to increase your lean muscle mass -- the total amount of muscle on your body. The increase from walking won't be as significant as from weight lifting or rowing, but the increase will be enough to also boost the amount of energy you use every day -- muscle requires more energy to sustain than stored fat. Walking also improves the muscle performance of your core. In order to keep you upright and balanced, your rectus abdominus -- the "six-pack" muscle -- and your obliques need to be engaged, which in turn will help to tighten and flatten the area.
Just as the rectus abdominus and oblique muscles help to keep your body upright and stable, they also dictate your posture. A flabby-looking stomach can often be the result of poor posture -- which is often caused by weak abdominal muscles. Walking, particularly uphill or over challenging terrain, forces these muscles to engage and strengthen, which in turn improves your posture. Having a straighter, more upright posture will improve the appearance of your stomach, making it seem flatter and tighter.
Stress No More
Walking is very effective for reducing stress. This is key in helping to flatten your stomach; stress triggers the production of a hormone called cortisol. Too much cortisol will cause a pocket of fat to gather in your lower abdomen, just below your navel, and can also cause the puffy "muffin-top" over your obliques. Walking in peaceful, natural environments -- free of vehicles, lots of noise and distractions -- will help to reduce your stress levels quickly, and help reduce some of the cortisol concentration in your body.