Recently the question of closed versus open fingers came up on the Facebook Swim Coaches group (what an incredible collection of coaching minds!), and it amazed me as to the diversity of opinion on what I thought was more or less a closed question. So I went looking to not only find out the present state of the research, but also to see what coaches and athletes were thinking. Here’s just a sampling.

Most said keep the fingers and thumb relaxed and in a natural position. The next theme involved variations on fingers tightly together so no water would ‘slip’ through. And many specified exact finger spacings or a small range of spacings. Here are some other comments from coaches.

  • thumb should be at 90° with four tight fingers
  • thumb anywhere but 90°
  • cup your hand so that the water doesn’t ‘spill’ over the sides
  • finger spacing should be with width of your fingers
  • swimmers should wear finger spacing gloves to train the fingers
  • (and my favourite) open fingers mean the palm of the hand will move through the water more quickly, so attention has to be paid to moving the fingers faster to catch up

The general idea is that a large number of swim sites discuss the issue but simplify the scientific study results horribly, resulting in incorrect generalizations and bad explanations. As a result, far too many professional coaches and swimmers are still confused about optimal finger spacing.

The Physics

So let’s start with a basic understanding of the problem, and then we’ll get to the studies that have been done.

Most of the problems come from a misunderstanding of how fluid dynamics work. At the simplest level, there are 3 factors affecting the drag coefficient of the hand (roughly equating to the effective surface area of the hand).

  • the larger the surface area of the hand, the higher the drag coefficient. The only time this will change is a) when the fingers are tightly closed, essentially squishing the fingers and reducing the surface area, and b) when the fingers and thumb are spread, the webbing between the digits will spread apart increasing the surface area a very tiny and probably insignificant amount.
  • there is a thin boundary layer of water surrounding the fingers. This layer resists movement, and acts as an extension of each finger, increasing the effective surface area of the hand. The thickness of the boundary layer depends upon a lot of factors, including speed of the fingers through the water, shape of the fingers, proximity to other fingers, etc.
  • any water flowing around or between the fingers will create vortices on the other side of the hand, and this will impact the drag coefficient. However, I believe we can ignore this completely, as the effect will be negligible in a real world scenario of a turbulent swimming pool

What this means is that the larger the total surface area, the greater the pulling force. And if we leave a slight separation between fingers equal to at least the thickness of twice the boundary layer, then we can increase the effective surface area of the hand.

The Studies

There have been dozens of hand positioning studies done over the past few decades, some rigorous and precise, and some far, far less so. Of the literature I could find, four studies stand out, all using computational fluid dynamic [CFD] simulations. I should point out that all of these simulations use highly idealized situations (lack of turbulent water or cross currents, simplified modelling of digits, etc).

The Optimum Finger Spacing in Human Swimming, by Alberto Minetti and others (here).

This was the most interesting study, as the CFD simulations were the most complex and sophisticated, and involved the whole hand. Eight different finger spacings and multiple water flow scenarios were modeled.

The primary finding was that the best finger spacing was at 8 mm, with the second best at 3 mm. Interestingly, finger spacings between 3 and 8 mm produced far worse results, but I chalk this up to the highly idealized nature of the digital simulations. Spacing of 8 mm produced an effective surface area 8.8% above wide open and fully closed. This spacing turns out to be the equivalent of a relaxed hand.

The Constructal-Law Physics of Why Swimmers Must Spread Their Fingers and Toes, by Lorente and others, (here)

The CFD simulations used very simplified models (only 2 fingers represented as ideal cylinders). While far less sophisticated than Minetti paper, this study is important as it confirms that the optimal finger spacing is between 20% and 40% of the width of the finger. This turns out to be roughly 4 to 8 mm.

Swimming Propulsion Forces are Enhanced by a Small Finger Spread, by Marinho and others (here).

These CFD simulations also used far less complex models, but included 7 different angles of attack for the hand, but only three finger spacings. Not surprisingly the best angle of attack was exactly 90°, while the best finger spacing was at 3.2 mm.

Hydrodynamic analysis of different thumb positions in swimming, Marinho and others, 2009 (here)

This CFD study is interesting in that it is one of the few to analyze the role of the thumb in the pull. The results showed slightly better results with the thumb adducted (towards the hand) than abducted (away from the hand). Unfortunately, only 3 thumb positions were analyzed, and so an optimal angle wasn’t investigated.

Summary and Remaining Questions

The four studies above represent the most rigorous ones I could get hold of. All studies support optimal finger spacings roughly between 3 and 8 mm, which all correspond to a relaxed hand position as best. The study on thumbs supports a relaxed position closer to the hand. The optimal hand positions are estimated to increase the effective surface area of the hand by roughly 9%. Although it should be noted that all studies use highly idealized conditions.

Surprisingly, there are still some remaining questions that have not been adequately answered in studies. Here are two of them:

What is the optimal angle for the thumb? The most rigorous study merely found that a thumb closer to the hand is better. But I couldn’t find any thumb studies that searched for an optimal angle or range of angles.

What is the optimal hand and finger configuration for hand entry? Video analysis shows elite swimmers with a very relaxed hand position on entry, even though intuitively we might think that a more compact hand upon entry might be better.