By its pure nature, swimming is a major cardio workout. Swimming any of the major strokes such as freestyle or breaststroke requires an extra effort to breathe. This makes swimming one of the best cardio workouts with the least strain on the body. While one must learn to swim correctly, there are various forms of swimming that will create a workout, even if you are not using one of the common strokes.
The true value of engaging in cardio swimming is the buoyancy and resistance provided by the water, which allows for a workout that does not strain the joints of the body. The resistance allows you to burn calories, improve endurance and increase strength in the extremities.
Even a non-swimmer can engage in a cardio workout in the pool. Using a kick-board or the side of the pool, extend your arms out in front of you, and with straight legs, kick legs in an alternate up down rhythm. If you know how to use a frog kick, this is a good alternate exercise.
If you are a swimmer, then it is beneficial to determine either how far you wish to swim or how long of time. Establish goals, time yourself and increase the distance you cover in the pool with each workout. Alternating between using the kick-board and engaging in one of the strokes are a good way to keep it interesting. You also might repeat with a positive affirmation as you reach each side of the pool.
There are also workouts called aqua aerobics that are often offered in pools. Using a flotation device such as a noodle, you can jog in the pool. If the pool is waist deep, walking in the pool will offer resistance and give you a good workout. Doing push-ups on the side of the pool with your hands on the edge of the pool gives you an advantage of buoyancy. You may also place your legs on the deck and body in the water and do sit-ups to build core muscles. Bobbing in the water is also a good workout.
Essentially any cardio workout that you can do on land can be done in the pool, including kickboxing, lunging, dancing and using flotation weights. There are many aqua aerobic training videos on You Tube.
If you want to rapidly increase your fitness while being easy on the joints of the body, engage in some cardio swimming at your nearest heated pool.