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A great way to tone your abs while walking is to do upper body exercises while you walk. Though these exercises may seem to be focusing on your arms, they engage different muscle groups in your core as you walk. Moving your upper body and lower body at the same time engages core muscles as you work to keep your balance.  These exercises are meant to be completed as you walk. You shouldn't stop to do these exercises. In an hour long walk, incorporate three or four sets of these exercises. Do each exercise for a one-minute interval, spaced about seven to eight minutes apart. If you need to start by doing each exercise for 20 or 30 seconds to begin, do that. You can add additional time once one minute feels too easy. You may also try doing groups of three or four exercises together without rest two to four times through your walk. A good way to strengthen your ab muscles while walking, or doing any other daily activity, is to pull your navel toward your spine. This exercise works the transverse abs, which run from your sides to the front of your abs.  As you walk, pull your bellybutton in, like you’re drawing it towards your spine. Hold your abs like that. Make sure you keep breathing as you contract your abs. Don’t suck in your stomach and stop breathing. Start by holding your abs for a few seconds, or until you get to certain objects before you. The stronger your abs get, the longer you’ll be able to hold it. One simple exercise to add to your walking routine is an arm curl. Start by hanging your arms at your side. Bring your hands up towards your shoulder, like you are doing a bicep curl. Make sure to keep your elbows close to your body.  An alternate way to do this exercise is to hold your arms at shoulder height. Curl your elbows up, bringing your hands towards your shoulder. This is also a bicep curl, but with your arms parallel to the ground, making your torso work harder.  If you want an extra challenge, add hand weights to the curls. You can start with three or five pound weights. Doing punches as you walk is a great way to engage multiple groups of core muscles as you walk. They also help raise your heart rate, which gives you a better workout and helps boost your metabolism.  You can try doing front jabs or uppercuts. To do a simple jab, hold your hands in fists at shoulder height. Punch forward with one arm, then the other. You can twist slightly as you punch. Make sure to engage your abs while you do these exercises by contracting your abdominal muscles. You can incorporate these into one minute intervals with other exercises. You can also do sets of 20 jabs and uppercuts for every few minutes you walk. Another good core-toning exercise you can do while you walk is to reach your arms overhead. To do this, lift one arm over your head as you step with your opposite foot. As you take the next step, drop that arm and reach the other hand overhead. Continue to alternate with each step.  To add a bit more core work, you can lean slightly towards the opposite side as you lift your arm. You can also do this exercise with hand weights.
Incorporate short bursts of additional exercises to your walking. Walk while pulling in your core. Do arm curls. Punch as you walk. Alternate reaching your arms overhead.