All of the major swimming strokes strengthen and rely on your core or abdominal muscles. The most common swim stroke -- freestyle -- is the easiest to master and most efficient. It uses body rotation, which relies on your abdominal muscles to keep you aligned in the water while breathing. Breaststroke and butterfly, if you swim them efficiently, also work your core muscles and provide a more intense workout; however, breaststroke and butterfly are more challenging strokes best left to advanced swimmers. Talk with your primary care physician before embarking on a new swimming routine.
Incorporate different swimming drills to improve technique and work on strengthening core muscles. Hold a kickboard in front of your body either at arm's length or hugged to your chest. Do not lie on top of the kickboard to keep you afloat. Tighten your core muscles while you flutter kick or dolphin kick across the length of the pool. Intermediate swimmers should use the streamlined position with their arms over their head while lying on their back instead of a kickboard. The streamlined position relies more heavily on your abdominal and leg muscles for a more intense exercise. Beginners should complete 150 meters of kicking, while intermediate swimmers should complete 400 meters.
Use a pull buoy, which is a flotation device placed between your legs to keep you from kicking, to help you strengthen your core and arm muscles. Swim freestyle across the pool without the use of your legs. Tighten your core muscles and focus on the rotation of your body and breathing. One arm should be almost fully extended out in front of you, and the other arm should be at your side when you take a breath. Breaststroke and butterfly swimmers can opt to use these strokes instead of freestyle for a more intense abdominal workout. Beginners should complete six 25-meter swims with 15-second rest intervals, and intermediate swimmers should complete four, 100-meter swims.
More advanced swimmers can perform drills with the breaststroke and butterfly to engage core muscles and improve endurance and speed. Breaststroke swimmers should perform one arm pull for every three leg kicks. Butterfly swimmers should use one arm pull for every three dolphin kicks. As with freestyle, tighten your core muscles to help you bring your arms out of the water. Advanced swimmers should complete ten 25-meter swims with 15-second rest intervals between each.
Swimming regular freestyle allows you to consistently engage your core muscles while also building up your cardiovascular fitness. Swim at a consistent pace for a designated period of time, starting with 30 minutes for beginners to one hour for intermediate and advanced swimmers. Take short rest periods if needed.