One of the key foundations of a great freestyle stroke is good exhalation. You might have been previously taught to hold your breath to increase your buoyancy. While this can increase your upper body buoyancy, it causes the legs to sink resulting in drag. Holding your breath also makes swimming feel harder as your body stores carbon dioxide, making you feel ‘out of breath.’
The ability to exhale comfortably in the water enables you to feel relaxed and develop a rhythm so that you can swim continuously without needing to stop at the end of every lap.
Try this next time you swim. As you swim freestyle repeat the mantra with each stroke: Bubble-bubble-breathe; One, Two, Breathe; or even Exhale, Exhale, Breathe. Say “Bubble” into the water to help remind you to breathe out constantly for two arm pulls. On the third stroke say, “breathe” in your mind's eye as you rotate and take a breath. These mantras help you coordinate exhalation and bilateral breath timing in a three-stroke cycle. Focus on exhaling by contracting your stomach muscles as you push air out. Inhale by relaxing your diaphragm.