Science and Swimming: The mysteries of the freestyle leg kick

How important is the leg kick in freestyle swimming? Professor Gatta analyses it scientifically and explains which aspects of the leg kick need to be focused on in training.

According to various anthropologists as man emerged from the water onto dry land, he was forced to change the way in which he moved and – despite being born immersed in his mother’s womb – his return to the water as a swimmer seems to be much trickier than you might think.

There are various rival theories about the events leading us to believe that man is an aquatic creature who moved to dry land and, in contrast, other theories claiming he is completely unsuitable to being in the water. Supporters of the first theory focus on a number of significant events occurring when a person enters the water, for example, when the rise in pressure due to immersion tends to squash us, blood flows to our most internal organs to prevent this from happening (the blood-shift effect). Then there is the slowing down of our heartbeat and selective distribution of oxygen to those parts of the brain where it is most needed when we simply place our face in a bowl of water (the diving-reflex), and how can we fail to be amazed at the way new-born babies spontaneously hold their breath when immersed in water just a few moments after they are born? In other words, there are plenty of events that will inevitably amaze anybody curious enough to read “Your inner fish” written by Neil Shubin, professor of anatomy at Chicago University, in which he informs us that lots of illnesses are due to the fact that…. unfortunately, as we evolved, we stopped being fish.

On the other hand, supporters of the opposing theory point to an equal number of events focusing on the terrible relationship between our body and water. For example, if we start by making a comparison between a man and the mammal best as swimming, then there is absolutely no competition: our movement is slowed down by a huge amount of resistance and we consume more energy than a tank in inner-city traffic. Swimming is the slowest form of human locomotion that exists. In a nutshell, our body is not hydrodynamic and our limbs are too long and thin to be efficient!

And so, moving on from walking to freestyle swimming, it is obvious that our limbs must work in completely opposite ways. When we walk, our legs allow us to move forward and our arms swing in the opposite direction to our legs or, in the words, in time with our opposing leg in order to balance our stride and prevent us from losing balance. In the water, it is our arms that generate most movement providing approximately 85% of forward drive and the legs merely served to “balance” our position, preventing the body from rotating around its axis (Persyn, 1995) and stopping our legs from sinking heavily “behind” us (Zamparo, 1996).

But leaving aside all the intricacies of the evolutionary development of these events, what swimming coaches are most interested in is that 15% of forward drive coming from the lower limbs and, due to our healthy “professional obsessiveness”, we inevitably start thinking straightaway about how to make the most of it and even improve on it to win races.

Legs are much stronger than arms, but the limited mobility of leg joints prevent any useful motion from being generated.

Swimming freestyle using your legs only generates lots of lactic acid (Meyer 1999) and uses up three quarters more oxygen than swimmingly with just your arms (Adrian 1966). So, using your legs is not very advantageous and very energy-consuming. It also seems to be hard to train your legs: Konstantaki (2007) has worked out that, among swimmers training the same way, those devoting 20% of training specifically to just their legs had no “statistically significant” performance gains over 400 m freestyle.

We have also studied this topic and published a scientific paper about it in the magazine Sport Biomechanics (“Power production of the lower limbs in flutter-kick swimming” -11-2012).

In this article, we focused on the percentage of forward propulsion the legs provide in a flat-out sprint. The test underscoring this research involved training 18 national-standard swimmers who first glided impassively through the water and then flutter-kicked at maximum intensity over increasingly long distances, while measuring the resistance to forward motion. The machine used for this purpose (Ben-Hur ApLap Roma) is fitted with an electric motor pulling a nylon cable that the swimmer holds on to while being pulled along the entire length of a 25 m pool.  

On average the legs provide around 50 Newtons (approximately 5 kg) of forward propulsion at speeds of around 1.26 m/s. As the speed increases, the pulling device begins to work and it is easy to calculate the contribution coming from the legs by studying the delta resulting from comparing the resistance measured during tests without a leg kick and tests, at the same speed, with maximum leg kick. The interesting fact resulting from these tests is that the forward drive provided by the legs decreases as speed increases. This effect is due to the dynamics of movement. The legs’ forward drive can be shown in a graph as the force acting along the perpendicular from the back to the foot during the down-beat action.

Breaking down this result into a parallelogram of forces, two components catch the eye: a “lift” vector preventing the legs from sinking and a propulsive vector: “drag” with backwards thrust producing forward motion. As speed increases the down-beat action is no longer vertical but tends to take on a similar direction to the lift vector, due to the swimmer’s forward shift. This inevitably results in a decrease in thrust given by the function: ηL = -48,16x+112,8, where the power given in Watts is a function of the speed (x). This function allows us to estimate that the legs no longer help the swimmer move forward at theoretical speeds of approximately 2.3 m/s or greater.

This suggests that the most important thing for obtaining greater thrust from the lower limbs is not greater muscular power but rather a high level of joint mobility for the plantar flexion of the ankle.

This joint has limited flexibility, so it would seem that this human fin has gradually straightened over time to allow people to stand up and it is strange that we now want to re-bend it again!!!