David Pease, PhD, is a swim coach, scientist, and national lead of Sports Biomechanics at the Australian Institute of Sports (AIS). He is also the author of the Swimming Biomechanics chapter in the book Triathlon Science.

• The key elements of swimming biomechanics: centre of mass, propulsion, and momentum.
• Drag: how it works in swimming and how to minimise it.
• The lowdown on kicking, stroke rate and rotation.
• How far should you pull through with your stroke before exiting the water?
• Technical troubleshooting of body position, dropped elbow and asymmetric stroke. 

00:40 - 

• David is the national lead of sports biomechanics at the Australian Institute of Sport.
• Swimming biomechanics is his speciality. 
• David is a former swimmer himself, and works with many swimmers both professional and amateur. 
• He has done a lot of research on swimming biomechanics.
  • He knows the practical aspects, and the scientific research aspects!
• He is the author of the swimming biomechnics chapter in the book Triathlon Science. 
• David got his bachelors and masters in exercise science at the University of Southern California and the University of Colorado respectively. 
• David then moved to New Zealand and did his PhD there.
• He now lives in Australia.

5:00 -

• The easiest way to think about centre of mass is a single point of your body which allows you to represent the entire mass of your body in one place.
• For most people, when you're standing with your arms by your side it's right around your belly button.
  • It does move around - if you raise your arms above your head the centre of mass will move up. 
• In swimming, the centre of mass works in combination with the centre of buoyancy. 
  • How those two points align dictates how you balance yourself in the water. 
• The centre of buoyancy is the centre of your underwater mass.
• If your centre of mass is closer to your feet than the centre of buoyancy, your feet will sink. 
  • If your centre of mass is closer to your head then the centre of buoyancy, your head is going to come down and your legs will rise higher. 
• In a good swimming position:
  • Centre of mass and centre of buoyancy should be on top of one another.
    • It will ideally be a few cm's above your belly button.
  • This reduces torque or turning action caused by the centre of mass around the buoyancy. 

7:42 - 

• Propulsion is the force that moves you through the water. 
• Propulsion, in combination with drag, is a key aspect of swimming.
  • You need to look at generating propulsion to maximise performance. 
• A lot of technique issues are really focused on maximising the amount of propulsion you generate. 
• You generate propulsion with your arm stroke and your kick in freestyle swimming. 
  • The percentage contribution of these is subject to perpetual debate because it's difficult to directly quantify. 
  • In a triathlete population, at least 80-90% of your propulsion is coming from your upper body.
    • That's your hands, but also your forearm and a little bit of your upper arm if using the right technique well. 
  • With the kick, depending on how flexible your ankles are, you don't tend to generate as much propulsion. 
    • Ankle flexibility is not normally a huge characteristic in a triathlon population. 
  • Focus on maximising the amount of propulsion from the upper body in triathletes.
• Propulsion can be quantified using watts, but we tend to measure it in pure force. 
  • The forces are quite low, an elite swimmer would only be generate about 10-12kg of force max.
  • We would generally use power if we were doing a complete biomechanical analysis and wanted to look at joint power.
    • Power going through each of the individual joints in combination with the velocities.
• Higher level swimmers have generally got a solid technique already so the propulsive forces are already there.
  • A lot of the little tweaks needed are more around drag reduction.
• Less skilled athletes are often not as efficient movers in the water, e.g. through bad body position, poor arm pull etc. 
  • More time is then needed to get the proper technique and get the propulsion.
  • Once that has been attained, then you play around with characteristics of body position to try and get the drag force down. 
    • E.g. How they sit in the water, what material their suits are made of.
• The bigger the difference between the propulsive force and the drag force, the faster you will go. 
• Unless you have a seriously bad body position, which would need addressing, it's better to address propulsion first. 

13:57 - 

• Due to the nature of the stroke, if you track your velocity of your centre of mass through the water you will see a lot of fluctuation. 
• Momentum is the mass of the swimmer times the velocity they're travelling at.
  • Mass is constant, so changes in momentum are due to changes in velocity.
  • This becomes inefficient if the velocity change keeps going up and down dramatically. 
• You will often see swimmers with common technique flaws only getting a propulsive force in the middle of the pull.
  • In that situation they have a lot of time where their velocity is slowing down, and they'll need to accelerate to get their average speed back up again. 
• You want to try and keep the momentum as even and smooth as possible, to make the effort last as long as it can.

16:06 - 

• Drag is any force that tries to stop you from moving forwards.
• There's a bunch of different types of drag if you go into the physics of swimming. 
• From a practical point of view we're looking at three types:
  • Friction drag: water rubbing up against the body or suit etc. 
  • Form drag: head on frontal resistance that you get from the water 
    • If you look at someone head on when they're swimming towards you and draw an outline of all the parts of the body you see, that's their form drag. 
    • You want the frontal area as small as you can.
    • The biggest way to reduce this is how high you can keep your legs and hips. 
    • Rotation won't change form drag a great deal. 
    • Personally I like 30-40 degrees of rotation to each side because you put the arm into a better position when pulling and it allows a better exit of the hand through the back of the stroke. 
  • Wave drag: when you generate a wave in the water. You have to use energy from your movements to lift the water, which is energy not being used to generate propulsion. 
    • Wave drag can be as much as form drag, and in some cases up to 60% of the total drag can come from waves.
    • You want to be nice and clean in the water to reduce this drag. 

20:34 - 

• There are a few things that are different between swimmers and triathletes. 
  • In general, swimmers tend to be more flexible, especially in the shoulders.
  • Swim coaches will advise early vertical forearm and high elbows for example, which are important but difficult to do if you're not flexible.
• This needs to be modulated for triathletes.
  • The easiest way to picture this is imagine your arm has entered the water and you're stretching out in front and you're just about to catch the water. 
  • If you draw a line from the wrist to the shoulder, I'd want to see your elbow just above that line. 
  • In a triathlete you often see the elbow just below the line. 
  • This means the hand pushes down on the water, which isn't helping propulsion and could raise the front of your body which creates more wave drag and form drag. 
  • You won't then get into a propulsive part of the stroke until the middle. 
  • If you can keep that elbow above the imaginary line, the hand tends to want to pull backwards on the water more.
• You also want an S pattern pull.
  • You pull out a bit, then come back in towards the mid-line of your body, then back out again.
  • What you often see in athletes without a body roll is exaggerating the S pull.
  • This causes a lot of lateral movement and you snake around in the water. 
• If you have a good amount of body roll, it will naturally create the S shaped pattern in a much more controlled way. 
  • From the athletes point of view the pull feels straight, but from the outside you can see the S shape pattern. 
  • This has advantages in terms of getting clean water under the hand, it makes the mechanics of the pull more effective. 
• We also want to look for symmetry of the stroke - the sweep of the stroke of both sides should be basically the same. 
  • This is a common problem in triathletes and competitive swimmers. 
  • The out-sweep at the front of the catch is going to be just outside the shoulder, the maximum part of the in-sweep should hit down the centre midline of the body, and then the out-sweep back past the hip. 
  • The issue especially with triathletes swimming in open water is drifting off to one side because of an imbalance.
  • You then have to spend more time on navigation, or rely more on other athletes to keep you in line. 
  • This also causes wasted energy because you are having to make corrections. 
• Symmetry doesn't have to be the perfect S shape if it's not possible, but the balance between the two sides is the most critical thing.
• Once you start the pull backwards, you should be accelerating the hand all the way back to the finish of the stroke. 
• I normally say the stroke finishes when your thumb hits just below the hip, it just brushes the top of the thigh. 
• Sometimes if you decide you're going to finish the stroke early, there's a change of direction that needs to happen and because of this you slow the arm down. 
  • So a lot of people who are trying to pull out too early decrease the amount of force they exert because they're slowing the hand down to get the change in direction. 
• You're always going to have this, but if you can carry on the acceleration to just before your arm hits the full extension, that tends to be ideal.  
• Acceleration through the stroke is usually something you need to think about until you are used to it.
  • If you're used to a constant speed of the hand through the pull motion it will feel quite different.
  • One of the main reasons it's important is once you've started the catch, in order to maintain the force under the hand you've got to speed it up. 
  • You've already started to get moving, if you keep at the same speed you won't get as much force under it.

30:56 - 

• Most triathletes are fundamentally bad at kicking which is a problem.
• A lot of this is to do with the level of ankle flexibility triathletes have. 
  • People from running and cycling backgrounds tend not to be able to get into the really big toe extension positions.
  • They've never developed the ankle flexibility that allows you to push back on the water with the top of the foot.
• Triathletes tend to try and achieve this using an excessive knee bend, which is inefficient as well.
  • You increase drag and there's only a small period of time where you get propulsion. 
• Another advantage of keeping the legs long is getting lift force as well as propulsion which keeps the legs up. 
• However, triathletes without ankle flexibility who keep their legs long will sometimes kick and move backwards, which isn't helpful. 
• Ankle flexibility is actually one of the harder joints to get flexible, so you're almost stuck with what you've got. 
  • As a result we try and get athletes who don't have that flexibility to minimise the kick and focus on getting the legs up.
• You get additional buoyancy in a wetsuit which helps in races. 
• If you don't have ankle flexibility, kicking hard will just lead to blowing up. 
  • You won't get anything beneficial out of it so try and not get sucked into the trap.
• The kick is a counterbalance to the arms to help keep you strong and streamlined through the body. 

34:37 - 

• There tends to be a difference between stroke rate in the pool compared to open water. 
  • Stroke rate may increase in open water, often because you're dealing with swells and tides etc.
  • With a higher rate you are more flexible in making necessary adjustments. 
  • In the pool, it's a more controlled environment so stroke rate can decrease and focus on the length of the stroke. 
• Reaching an ideal stroke rate is an optimisation process.
• Ideally you want to get as much stroke length out of your stroke as you can.
  • Similar to pushing a massive gear on your bike, you can generate a lot of force and get speed going but the rate is low. 
  • Or you can do this with a smaller gear and have a higher pedalling frequency and get the same velocity using less energy. 
  • It's the same in swimming. 
• You need to find the balance point for your physiology and technique.
  • The rate the you're using should give you an optimised length. 
• You can do this by doing stroke counts at different paces in the pool.
  • Especially if you combine this with a heart rate measurement to assess the physiological cost. 
  • E.g. If you're doing a 40-50m swim, you take 30 strokes and have a 120bpm heart rate. If you then do a 40m swim but take 20 strokes but your heart rate is 160bpm, you can see this is a big physiological cost. 
  • You'll tend to see a point that you hit and get a big spike in heart rate, so you know your physiological limit.

38:51 - 

• Head position can be a problem, maintaining a good body position is important.
  • Ideally, if you're in a pool environment, you want the water level to hit just above your forehead. 
  • When you start swimming in open water and you need to do navigation strokes, you tend to lift the head a little higher.
  • You want to swim as little as possible with your head higher than the water hitting just above your goggles. 
  • If you lift the head, you tend to drop the lower body.
  • A cruising kick should provide enough lift to keep the legs up, and the head down. 
• Elbow above the line on the catch is the second biggest mistake we see. 
  • A lot of this can be helped using visualisation. 
  • The old adage: once you've entered the water imagine you're reaching over the top of a barrel under your arm. 
  • Or you're trying to reach down a drainpipe. 
  • These help you get into that position. 
  • There are training aids - e.g. the forearm fulcrum (figure of 8). 
  • When triathletes are trying to work on their high elbow catch they might think they're doing it, when actually they're just flexing at the wrist. 
  • This means they're just relying on the hand to make propulsion which is cheating yourself of the forearm strength. 
  • The fulcrum, or something similar, keeps the wrist straight and forces you to keep your elbow high up at the catch.
  • Holding tennis balls in your hands, or swimming with fists can also help this technique. 
  • Swimming doggy paddle might also be useful to practice. 
• Drill work is really important.
  • However it can be a little bit of a trap because you can get into pretty bad habits. 
  • The first thing I tell any athlete is to take the wrist strap off paddles, because this can introduce some really lazy habits as the paddle won't move from your hand if you do something wrong. 
  • If you take the wrist strap off it forces you to use a better propulsive technique. 
• Try and make sure the motions you are doing are going to resemble the bit of the stroke you're trying to work on. 
  • E.g. if you're doing doggy paddle, remember the focus is to get the elbow high and use the forearm as much as you can. 
  • Try to keep everything else as normal as possible 
• The other big mistake is side to side imbalance.
  • If you're going to be navigating a lot you'll add a lot of extra distance onto your swim if you're not balanced. 
  • A good test for this: I get athletes to get into the pool, push off and take a few strokes, then close their eyes and keep swimming.
  • Once they close their eyes they lose visual references, you'll get the natural behaviour of the stroke. 
  • It's very common for athletes to go two strokes and then hang an arm over a lane line. 
  • If you tend to veer to the right, once you go back and look at what they're doing you'll usually see the left arm pulls really wide. 
  • It helps you focus down and see where in the stroke the side to side motion is happening.
  • Also watch for breathing because it could be coming from there - with the pulling arm sliding way out as they breathe. 

46:41 - 

• There is a trend in devices such as the TrainSense Smart Paddle that measure stroke trajectory and power from a device on your finger. 
• I haven't used one so can't speak too much to how effective they are. 
• That area of technology using inertial sensors to get the orientation of the hand has gone crazy recently.
• It raises an issue of getting orientation of the hand from those sensors can be touchy. 
• They look interesting and it's a big improvement on similar systems just looking at pressure under the hand.
  • Pressure curves aren't helpful if you have a bad technique. 
  • Orientation is a bonus. 
• Back in the late 80's, early 90's when I was working with US swimming we did this stuff all the time.
  • It wasn't with a sensor - we'd use 3D video and do calculations on the positions of the hand. 
  • That kind of information can be hugely useful to get an idea of how efficient athletes are.
• I would want a play with one and compare it against other measures to see how reliable and accurate it really is. 

49:52 - 

• What is your favourite book, blog or resource related to triathlon or endurance sports.
  • Science of Swimming by James Counsilman.
• What is your favourite piece of gear or equipment?
  • As a swimmer, my kick board. 
• What do you wish you had known or done differently at some point in your career?
  • In my swimming career I wish I knew as much about technique as I do now because I would have saved myself a lot of pain in my shoulders. 
  • Professionally I should have finished my PhD a lot faster than I did. 

Links and resources mentioned

• Triathlon Science book
• All previous triathlon swimming episodes on That Triathlon Show on one page

Connect with David Pease

• On his Research Gate page.

Connect with host Mikael Eriksson

Hi! I'm your host Mikael,

I am a full-time triathlon coach and an ambitious age-group triathlete. My goal is podium at the Finnish national championships within the next few years.

I first started the website Scientific Triathlon in autumn 2015 as a passion project to share my learnings with a larger triathlon audience. Later on, in early 2017 I started the podcast That Triathlon Show. 

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