“In fact, water in general can suggest danger and annihilation, because we are not well adapted to aquatic life. (Michael Phelps’s top sprinting speed is about a third of a dolphin’s.) Breathing is hampered as we swim. Studies suggest that the water compresses the chest, making it more difficult to inhale. While floating horizontally takes pressure off breathing muscles, blood pools in the lungs, leaving less room for oxygen. In freestyle, we also burn more energy for less oxygen, because of the short, over-the-shoulder breaths – what poet Maxine Kumin, in 400-Meter Freestyle, calls “little sips carefully expended”. As a result, our lungs tire more quickly during swimming than during exercises like running or rowing – in a matter of minutes we suffer what researchers call “inspiratory fatigue”. This, in turn, leaves us with less air in our lungs, and oxygen in our cells. This then has flow-on effects for the whole body: our muscles become weaker or slower, and have more trouble co-ordinating.”