Why cutting the wetsuit will give you a better running economy

The topic on cutting the wetsuit or not, is under constant debate. Especially now with all the new swimrun wetsuits hitting the market in 2017. Should you cut it and if so, what are the reasons for doing it? Read more about it further down.

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/The WoS Team

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This text will mainly focus on the running economy in swimrun, but everyone who runs should read it! Here you’ll learn a lot of things about your running efficiency, regardless if you like to run in rubber or not.

Approaching my debut in swimrun, having trained a lot of people running in various running contests, including several ÖtillÖ challengers and winners, I thought it’d be a good idea to try this myself. Especially since I give courses on how to best use the biomechanics to become the most energy efficient runner as possible in forests and on rocks. I put out a question on Facebook on whether I should cut or not, and most racers seemed to agree to cut the wetsuit above your knees, but failed to give any further explanation on the underlying reasons to do this, apart from the fact that it will become easier to run? As a nerd I decided to investigate this myself.

Pros and Cons

Looking at the pros and cons to cutting, there are some factors talking against cutting the suit. If it’s cold in the water the leg will become stiffer. This makes it more difficult to exploit the elasticity found in muscles and tendons in the feet and lower leg that provides ‘free energy’, which is a big part of how energy efficient they are. In addition, you lose buoyancy provided by the uncut legs that help the feet (and shoes) from sinking too much when swimming. On the pro side, cutting the legs will help prevent possible overheating and of course the above described ‘undefined’ feeling of being able to run more freely.

As soon as I put on the newly cut suit, I realized what I should have understood from the beginning: It should be quite obvious to cut the suit, especially the legs if you focus on the running efficiency.

Cut legs

Some people believe that you save energy by having a low gait (close to the ground), that it should cost less energy than having to lift the heel up higher. But that’s not how it works. It’s really a question of fundamental physics. A long pendulum (leg and foot) takes longer to swing across and costs more energy to move than a short pendulum. It’s a bit like how an old fashioned metronome works. Adjusting the weight up and down you will find the shorter pendulum moving faster than the long one. Or if you imagine yourself waving a four-meter-long broomstick through the air. Its pendulum frequency will be slower compared to waving four one-meter pieces stacked together.

Are you with me?

If we apply this idea on one leg, the long leg pendulum (when not lifting your feet so high off the ground) will cost more power to the front pulling muscles compared to a short pendulum, that you’ll get from having your heel dangling up and behind you. The long gait almost always leads to landing in front of your centre of gravity, which is bad in so many ways that it’s not fit to take this discussion here.

– But don’t you have to spend power to lift up the heel then?

Well no. In the gait, the foot and lower leg should ‘dangle’ on its way up behind, and not be lifted up by muscle power. It works like this: imagine you have a pair of Bruce Lee nunchucks, i.e. two sticks that are ca 30-40 centimetres long and connected by a thin chain. Your femur is one stick and your lower leg (shin) the second where the knee is the chain in between. Now holding the femoral pin firmly and swinging it back, the effect gained will come from the chain swinging back the lower leg-pin up behind, just like the curved arrow (1.) in the picture below (Picture 1).

pic 1
Picture 1 – Running technique

This movement of course requires relaxation, i.e. not to be super tight in the front thigh muscles, to save power in the hip flexor pulling muscles. Remember however that the movement is related to the speed. At higher speed, the greater the movement.

When the lower leg dangles up there it will shoot the knee down / forward (2. Picture 1) from the weight of the rotational force, and with the ‘freely’ gained power, pushing the knee forward. The lifting of the heel (the dangling) triggers what is called the ‘Stretch-Shortening Cycle’ (SSC) in the hip and thanks to the elasticity of the muscles and tendons provides ‘free energy’ to lift up the knee

Do I cut the sleeves?

The arms and legs are related. How the arms move when running will directly affect the legs. The nerve connections from the arms to the legs can be easily noticed if you run on-the-spot (not moving forward), and increase the frequency of your arm rotations back and forth, the frequency of the legs will automatically follow (it doesn’t work the other way around though). It’s the same principle as with the legs, the longer arm pendulum you have, the more energy you will use. By cutting the sleeves above the elbow joints, it will be easier to keep your hands high enough and avoid running with relatively straight arms, which are heavier and often makes you land too far in front of the body (centre of gravity). Further, the blood vessels on the forearm are close to the surface and if it’s a moderately hot summer, uncovered arms can help reduce the risk of overheating. But, you will lose some buoyancy and pull force in the swimming.

The weight you will carry

Not having to lift that extra weight is important and the reason for normal racing shoes always being designed to be as light as possible. Cutting away the suit on the lower legs can be beneficial thanks to the reduced weight you have to carry in your gait. To compensate, many swimrunners add floating aid in their compression socks, e.g., cut foam pieces or wear wetsuit claves, all to get better buoyancy when swimming. This however might not provide the effect wanted since adding foam or similar will require you to use high compression sock to hold it in place, whereas the wetsuit calves gives the option to wear short socks, which weigh less.

Weighing the different options, the wetsuit calf and short sock weighed approximately 100 grams, while the long compression sock plus the added foam weighed in on 42 grams, allowing a profit of 58 grams per leg! So if you want to save weight yet maintain a decent level of buoyancy, long socks and foam pieces are preferable over wetsuit calves which you’ll save for when racing in cold conditions.

Picture 2 – A short sock and wetsuit calf = 100 grams.
Picture 3 – A long compression sock and a piece of foam = 42 grams.

The hip flexors (Iliopsoas)

The latest (and most expensive) swimrun wetsuits are designed so that it will be easier for the rubber to bend at the hip when running. It’s very tiring having to lift your knees up when the fabric of the suit is resisting this movement. It’s therefore good for swimrunners to strengthen theses muscles through exercises like this.


Exempting the knees from the restraint of the wetsuit is excellent for your biomechanics. It will save a lot of power and energy through improved mobility in the knee joint, reduced restrain on the hip flexors (the psoas and illiacus) and muscles in the front thighs. Free knees will also facilitate an optimal frequency for the best use of the intrinsic (muscle) elasticity that provides the ‘free energy’.

  • The knee and hip joints will most likely be less burdened by the increased mobility.
  • The person who, technically speaking, runs in the most energy efficient way, taking advantage of the increased mobility, has the most to gain.
  • Free knees also reduces the risk of overheating.
  • Save the cut wetsuit parts and use them when it is very cold in the water which can decrease your elasticity.
  • If it’s not it cold, it is better to use long compression socks with foam pieces (or similar lightweight material) compared to the heavier neoprene calves alternative.
  • And remember to strengthen the hip flexors!

So there you have it!

This article is a summary translated from its Swedish original (found here), written by the legendary Swedish specialist running coach, Fredrik Zillén.

Fredrik, the Founder of Spring Snabbare (Run Faster), has coached several ÖtillÖ medalists with their running technique.