Alberto Minetti is fascinated by machines that improve human physical performance. He describes the bicycle as a `marvellous invention' which increased man's top terrestrial speed by approximately 300% at the energetic cost of a light jog. But the story isn't so impressive in water. The only tools that humans have developed to boost swimming performance are fins. Bikes increase muscle efficiency by taking the legwork out of supporting our own body weight. But a swimmer's weight is already supported by the water, so how do the fins benefit their performance? Minetti and his team began scrutinising every detail of swimmer's leg strokes as they swam with and without fins (p. 2665) and discovered that the appendages increase the size of the propulsive jet of water that pushes a swimmer forwards with less wasted energy.
Compared with air, water is a dense and viscous medium. With every kick, a swimmer thrusts a propulsive jet of water backwards that overcomes the effect of drag. But some of the swimmer's energy is wasted simply displacing water sideways. Minetti needed to find out how efficient swimmers are before he could see how fins improve our performance.
Minetti teamed up with Dave Pendergast in Buffalo New York. Pendergast has a 60m long swimming channel where he monitors swimmers' metabolic and mechanical performances as they swim along the channel. Paola Zamparo, Minetti's student, travelled to Buffalo where she videoed elite swimmers' kicking action while measuring their oxygen consumption as they swam at different speeds with and without the fins.
Swimmers have to overcome drag, so the strength of their backwards propulsive jet must balance the amount of drag from the water. Because the strength of the propulsive jet is equal to the drag acting against the swimmer, she measured the amount of drag acting on each swimmer to find out how strong the propulsive jet was. She also calculated the amount of energy wasted by kicking water sideways based on the movements of the swimmers limbs. From these values, she could determine the efficiency of the swimmers as they performed with and without fins. By comparing the swimming efficiencies with and without fins, they realised that the swimmers with fins were able to push more water into their propulsive jet, increasing the efficiency from 61% to 70%.
Minetti and his team also realised that the fins slowed the swimmer's kick rate at a given speed. They realised that the finned swimmers were moving more efficiently because their kicks had been slowed to a speed where their muscles generated work more efficiently. So, each leg was moving more water at a lower physical cost to the swimmer, giving them spare energy to boost their speed.
Minetti accepts that other types of fin might offer bigger improvements in human aquatic performance, but he doesn't hold out much hope for future developments in small-fin technology. He believes that it's going to take a much more radical development before we see human swimmers keeping pace with the dolphins.
- © The Company of Biologists Limited 2002