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Just doing the exercise isn't enough. Engage your core muscles during each exercise to receive the most benefits.  To find your core muscles, hold yourself in push-up position for about a minute or two and pay attention to what feels tired. It's usually not your arms. When you drop into a push-up position, or do any core-tightening exercise, tighten your ab muscles during each rep. Those are the muscles we're talking about. To do these exercise properly, breathe in when you're contracting the muscles and breathe out when you're releasing them. o planks. Planks are simple and engage all your core abdominal muscles, making them an excellent exercise for tightening your core. To do one, get in push-up position. Raise your feet to about shoulder height by balancing them on an exercise ball, or a stool. Keep your arms flexed just slightly, not locked, and hold that position with your core muscles engaged for one minute. When you're first starting out, aim for 2-3 sets of these, holding each for about a minute if you can. If that's too hard, hold it for at least 30 seconds, or go as long as you can. Lie on one side and brace yourself on your elbow. Put your legs on top of each other. Place your other arm on top of your hip or reach it up to the ceiling. Tighten your abs, lifting your hips off the floor. Keep your back straight, forming a triangle with the floor. Hold for 30 to 60 seconds, then repeat on the other side. Try to do 3-5 sets of these, on both sides. o burpees. Start in push-up position, your core muscles engaged and your back straight. In one swift motion, jump forward into a squatting position with your feet and stand up. Then drop back into a squat and kick your legs back out into push-up position. You should do this as quickly as you can, comfortably. When you're first starting out, try to do three sets of 15 burpees. If you want more of a challenge, do jumping burpees, or do them while you're holding hand weights. Start in push-up position, your core muscles engaged and you back very straight. With one leg, take a big step, bringing your foot up toward your waist, then alternating, kicking your other foot forward as you bring the first foot back. Do these as quickly as feels comfortable. Try to hold this position and do these mountain-climbers for about 30 seconds. Try to do three sets if you can. . A variety of leg-lifting exercises can be done to strengthen all your core muscles. To start, lay flat on your back with your hands under your butt. Keep your feet together and raise them about six inches off the ground. Raise them to about a 45 degree angle, then lower them to six inches above the ground again. Try to do as many reps as you can in 30 seconds, and repeat this three times. You can also do bicycles by putting your hands on the sides of your head, as if you were going to do a crunch, and sitting up with your back straight, a few inches off the ground. Raise one leg at a time, bringing your knee up and twisting that side of your body toward it. Keep your back straight. Get on the floor in a push-up position and place your hands a little wider than your shoulders. Keep your feet in place, then walk slowly with your hands. Go as far out as you can. Try for 10 times, if you can. Sit down with your legs extended out in front of you and your feet turned out in a V position. Point your toes. Contract your core muscles and roll your spine into a C-curve. Lift your arms up and move them as if you were climbing a rope twisting slightly with each reach. Do 20 reaches with each arm. Lay on your back with your knees bent and your feet flat on the floor. Put your hands on the sides of your head, or across your chest. Keeping your back and neck very straight, sit up by engaging your abdominal muscles. Raise yourself to about 45 degrees, then lower yourself, but not all the way back to the ground. Repeat.  When you're first starting out, aim for a couple sets of 30 crunches. Do them slowly, keeping your muscles engaged the whole time. Crunches should be difficult, not something to blow through as quickly as possible. Many people make the mistake of thinking that doing a couple hundred crunches every night will yield rock-hard abs in a couple of weeks. If that's all you're doing, it's unlikely you'll notice results. Crunches strengthen muscle, but don't burn much fat.
Keep your core muscles engaged during all workouts. . Do side planks. . Do mountain-climbers. Do leg lifts Do push-up walkouts. Do rope climbs. Do fewer crunches, but do them properly.