It’s a common complaint amongst newcomers to the sport that they get out of breath quickly, despite being able to do other sports with no such problems. It’s often put down to technical issues with their stroke, or just not being a good swimmer, but in most cases it comes down to breathing. People having this issue are commonly holding their breath and then trying both to exhale and inhale whilst their mouth is out of the water. This means that the time to breathe in is reduced and an oxygen debt builds up leading to the out of breath state. This isn’t necessarily to do with fitness – running whilst holding your breath and taking only small short breaths every few seconds would no doubt lead to a similar outcome. The important thing is to make sure that all of the time the mouth is out of the water is spent breathing in, and not wasting any of that time breathing out.
To achieve this most competitive swimmers exhale underwater, leaving themselves only to breathe in once their mouth is above water level. There are two main ways of achieving this, either continuously exhaling in a trickle between breaths (humming can be a useful way of practising this), or by holding one’s breath and then exhaling vigorously just before taking the inward breath. Which method is better will depend on the frequency of breathing (short durations between breaths can often mean there is little difference between the two methods) and personal preference.
The video here shows an example of explosive exhalation between breaths. Note the fast stream of bubbles exiting the swimmer’s nose. In this case the swimmer is breathing every stoke to her right so there is no difference between trickle and explosive exhalation and the bubbles start to appear almost as soon as her face is back in the water. The critical point is that all the air is gone before she takes her next breath.