Elite swimmers should use their backstroke and freestyle kick to make themselves go faster, not waste it trying to keep themselves upright if they want to get an edge in the pool, say Australian sport scientists.
The findings are based on a new computer model of the rotational effects of buoyancy on drag, published in the Journal of Applied Biomechanics.
"Kicking is very inefficient at generating propulsion so every little bit that you can get out of the kick is going to be a good thing," said senior author, David Pease, from the Australian Institute of Sport.
Swimmers try to minimize their drag by keeping a streamlined horizontal alignment. But hand and arm movement tends to cause the lower body of a swimmer to rotate around so their legs sink.
It has previously been hypothesized that an upwards buoyancy force, operating at a distance from the swimmer's center of mass helps counteract this. But to date there has been limited evidence for the drag-counteracting effects of this "buoyancy torqu."
Pease and colleagues developed a computer model of the rotational effects of buoyancy during freestyle, backstroke, butterfly and breaststroke.
They took laser body scans and video footage of national-level athletes to capture their motion and produce 3D biomechanical computer models of their swimming. The researchers then calculated the position of each swimmer's centers of mass and buoyancy. They found evidence to support the idea that a buoyancy torque helps keep swimmers horizontal.
During swimming the center of buoyancy is closer to the feet than the center of mass, and buoyancy torque causes the legs to rise and the head to lower, says co-author Dr Raymond Cohen, a research scientist at CSIRO.
"This is surprising because if you are just lying horizontally in water your legs tend to sink," he said.
"But when you swim your arms and upper body tend to go out of the water and the position of the center of buoyancy ends up being closer towards your feet than your center of mass.
"The buoyancy torque assists the legs in counteracting the torques generated by the arms and hands which are typically in the opposite direction."
The researchers say the findings are most relevant to freestyle and backstroke which naturally have better buoyancy torque since more of the upper body is out of the water in these strokes.
According to Pease the findings have important implications for the way swimmers are coached on kicking technique.
"A lot of people think they have to kick up and down hard just to keep their legs up (but) their legs are going to stay up anyway because of the way all the torques are working."
Pease says the findings suggest swimmers should waste less of their kicking energy keeping horizontal, and more of it on moving forward faster through the water.
"We can work on adjusting the technique of the kick in particular so that it can be more propulsive," he said.
"The best way to do this is to maintain good ankle flexibility so the swimmers are pressing water backwards."
Less than 10 percent of a kick is propulsive, says Pease, so even if this gives just a small increase in speed it is going to be an advantage.