Exercising in the water is a great way to get a good workout in without excessive impact on your limbs and joints, not to mention that it's a lot of fun. Using the water to work your abdominal muscles can give you a greater variety of exercises than doing so on land.
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For a killer ab workout, try doing crunches in the pool. To do water crunches, float in the water on your back perpendicular to the side of the pool. Put your legs up on the deck of the pool up to your knees. Lie flat in the water, then use your abdominal muscles to pull your upper body up out of the water as far as you can. Use your muscles again to lower your body back into the water without splashing. Do 10 to 15 repetitions and rest. The water will help you get a better range of motion than doing regular crunches on the ground.
Using the water as resistance is an effective way to maximize your abdominal exercise in the pool. Leg lifts are done by first finding a spot of the pool where the water is about chest high. Stand with your back to the pool and lift yourself out of the water so you are resting on the deck of the pool by your elbows and forearms. Keeping your legs together and straight, lift them up so they are parallel with the floor of the pool and hold for 10 seconds. Return your feet to the ground and repeat 10 to 15 times.
The otter roll may look unusual, but it's a great workout that can only be done in the water. The otter roll is done by floating on your back in the pool, plenty of distance away from any of the sides of the pool. Cross your arms over your chest and also cross your legs over one another. Use your ab muscles to quickly roll your body over in the water to the right, and then back over again. Do 10 to 15 rolls to one side, then rest and repeat for the other side.
The dolphin kick gives you an effective ab workout while at the same time targeting your glutes and leg muscles. To do the dolphin kick, face the edge of the pool and hold on with both hands. Bring your body up so it is floating face down in the water. Keeping your legs together, kick your legs up and down in a fluid motion, as a dolphin would. Perform the dolphin kick for 30 seconds, then rest.