Soccer Kicking Biomechanics | Dr Neal Smith

Hi there all, welcome to my little talk I'm going to give today. Thank you very much for Stuart and his team for organising these during these difficult times. It really is a privilege to be involved in this and I'm glad to share with you some of the ideas that we've gained over the years in our laboratory, myself and colleagues from around the world that are hopefully trying to unpack this very very complex skill of soccer kicking. So we're going to give you a little bit of a journey today and try and unpack what we know and hopefully give you some ideas of what you might want to do should you start to look into this area yourself in terms of research or or indeed being a practitioner. So where are we going to go? I think it's fair to say that the game of soccer is extremely popular, a global game and to many people this actually even seems a little bit more than this, it's almost a religion in the way that they follow this. So it can affect many people's lives in a lot of ways. Now as you can see from the images, these type of kicks, the accurate powerful kicks for goal, have created some tremendous sporting memories for many of us. You can pick out your own, just a couple. For me here, Beckham against Greece, there was a Ronaldo knuckleball kick or straight non-spinning kick against Portsmouth in the Premier League many years ago. Again, two great memories in terms of technical proficiency. But they're very very useful in terms of maximum powerful kicks, in terms of scoring a shot, in terms of taking a penalty or in terms of really being able to kick the ball hard, switching play, trying to take the time away from the opposition. Hopefully this means that we can give ourselves a greater chance of scoring a goal. If we give ourselves a greater chance of scoring a goal, hopefully we're going to have more chances of actually winning the game. So really important for us to be able to understand the fundamentals of this skill and exactly what is going on. Now there's several ways in which researchers have approached this, a lot of different facets to it. So what I'm going to do today really is try to cover a little bit about what we know from the research, both inside and outside of our own laboratories here in Chichester. We'll look at things such as approach, we'll look at some bits on foot placement, we shall concentrate on the main body of research in terms of the kicking leg, but we shall also look at other aspects of the kick as well. So hopefully we'll understand by the end of the presentation that what we've got is a whole body action rather than just this focus on the one leg that strikes the ball. We'll deal with some aspects of the mathematics and the biomechanics behind it as well, so some considerations you will need to have if you're going to research in this area, namely that be whenever we collect some data we end up with certain errors in that, so we'll talk about smoothing and hopefully then we can start to unpack how we would use this information, these biomechanical techniques or these mechanisms that are at play here, to try and design an intervention to help people improve their performance. So we'll try and talk through this in terms of an intervention study and the type of things we might want to think about when we're devising those. And finally we shall talk about usability, so what happens when we try to work with players, what is the best way to do that, what is the best way to affect performance in terms of your day-to-day practice. So just an example here of a pretty typical motion analysis setup, this is a some data collection we were involved with many years ago, we're lucky enough to be involved but it gives a very very good example of what might happen when we try to collect 3D motion capture data. You can see the small reflective dots that are placed, anatomical landmarks all over the player, these are then collected by these cameras in the background, they'll collect data usually up to 500 times per second, sometimes in our lab now some of the data we'll contact on later is up to a thousand times a second, but as I mentioned these data will be collected direct to the computer but they will have certain amounts of errors in there, there's going to be certain ways in which the skin wobbles, in which the muscle wobbles over the skeleton and especially when we're dealing with impact situations like kicking, this becomes even more important, so we'll try and cover a little bit of that too. So I said we're going to start really with some of the notions of what might happen in terms of the data that we already know, we start to think of our approach towards the ball, this is the first thing that we do when we're kicking a ball and there's certain aspects of that approach we need to consider, so what is going to be the speed in the approach, what is going to be the angle of the approach, very many different options for this, mostly dependent on player preference but also dependent on the situation that arises within the game, there might well be opponents in the way, there may be a defensive wall, we may have certain angles and wins to deal with, so there's many different facets to this, what we do know is that typically players tend to have short curved runs and these somewhere list between two and five steps, just before we get to that impact with the ball we can see the footprints there that come towards that standing support leg that's placed beside the ball ready to impact. Studies have found that a range between a zero angle of approach right through to a 90 degree angle of approach at the optimum tends to be between 30 and 45 degrees, now of course we can alter that approach angle and this basically will end up with a very very similar outcome in terms of the ball velocity, however it's going to change the way in which the individual body segments contribute to that overall ball velocity, so if we come from wider angles approaches, so I'm saying here towards the 90 degree angle approach, so maybe we're talking 75 degrees, the wider angles give us a much greater movement outside of that sagittal plane, so we're looking at things like adduction, abduction of the hip, we're looking at internal rotation of the femur and we're looking at transverse rotation around the vertical axis then of the pelvis itself, so in other words we've got much more frontal and transverse plane motion when we're coming from a steeper angle, if we're coming from a straighter angle what we tend to find is that it's more sagittal plane movements that are prevalent, so we've got more flexion extension that's occurring in the hip and also in the area towards ball contact. In terms of approach speed, we do have a playoff here, so the quicker we can approach the ball, the more momentum in theory, and if we take momentum as being mass multiplied by velocity, our body mass is going to be standard, so the quicker we can approach the ball, hopefully the more momentum we can impart to that ball and therefore the ball travels faster, seems pretty logical. However, there is going to be a playoff here, if we go too quickly we're going to lose that inter-segmental control that we get in the timing aspect between the flexion, extension, abduction, adduction, rotations of those body parts towards that ball impact, so the optimum approach is going to be individualized and probably going to be a little bit less than their maximum approach speed. Right, so the main body of focus tends to have looked at the kicking leg during high velocity maximal instep kicks. This is an example from an old Brazilian hero, Roberto Carlos, he's got some fantastic free kick goals from his left foot. This all happens really, really quickly, so what we hope we can do here in the grand scheme of things is to try and break down this kicking skill into discrete phases. It's so fast we can't necessarily absorb all this information with the naked eye, so we need to use our high-speed video, we need to use optoelectronic systems to try and break it down into its constituent parts. Mainly what we tend to find in this kicking action, and we're talking here predominantly about the kicking leg, is going to be the main bit that occurs to contribute to foot speed is going to be hip flexion, so that forward action of the thigh is going to be an extremely important movement for us, and those thigh flexors, the hip flexor muscles are going to be the key for us in terms of generating ball speed and foot speed. When we're on that initial stages, the take back and the initiation, the great examples of some mechanical principles here, we look at principles of inertia, and then as we come down towards the downswing, a lot of energetics come into play, and we'll look a little bit more about how that can happen. A good example here, if I break this down into the individual phases of the kick, what we'll see, we tend to start from the top left hand side of the screen here, we'll start as the striking foot just leaves the ground, this first phase will come down to the point where support foot touchdown occurs, as we can see at this phase here, the kicking leg has been taken back, the knee is flexed, the hip is slightly extended, but what then happens from that point onwards is that the knee becomes even more flexed in that next phase, so this phase going from the middle picture at the top towards the picture at the right hand side at the top, the knee becomes a little bit more flexed as the hip begins to move forward, or that thigh is brought forwards. What is happening there is we're trying to reduce the amount of inertia within that leg, and for those of you again that remember that phrase inertia, inertia is going to be our resistance to angular change, so the smaller the amount of inertia we can create there, the quicker we're going to be able to accelerate that limb forwards, so that is why that knee is then flexed quite tightly at that position. As we move down to the bottom rung of pictures on the bottom left, we then see the downswing taking control, we reach at that position of 90 degrees at the knee, which we'll talk about a little bit more detail in the coming slides and then down to ball impact and follow through. So the muscular activity that we talk about here in terms of the thigh is going to be extremely important at this the top part of this downswing, so we talk about strong quadricep muscles being required to kick a long way, yes they are required but they're not going to be the main muscles, as we've spoken about the hip flexors are going to be more key with that. The quadriceps are definitely involved in the initiation of that movement and we see the images here on the left hand side, just around that first image on the top left is where those quadriceps are going to start to fire, the leg is then swinging downwards and in images two, three and four, the lower leg is traveling so fast that it then needs to be controlled around and about that point of impact. If we're talking about what speeds here, really the leg can reach somewhere over 1500 degrees per second, I guess if we put somebody in a measuring device we'd get them to extend their knee as quick as they possibly can, there's no way we can get anywhere near 1500 degrees per second in terms of isolated knee extension, it just cannot happen this quickly from muscle contraction alone. So the knee extensors actually do little from that point onwards, so anywhere from that 90 degrees downwards they do little to enhance the speed of the foot, they just really place for that initial firing, they place that lower leg in a position so that energy transfer can then occur. So in the initiation of the downswing what occurs, as I say we've got quadricep activity, this is this red arrow it's pointing up the thigh, that causes that initial rotation of the leg, from there we've got that's the active part and we've also got some passive energy transfer that occurs there, we can see the guy Indiana Jones here down on the bottom right using his whip, it's a very similar action that occurs here with our soccer players, as we bring the whip forward we stop the hand and the whip carries on faster, same here we bring the thigh forward and the thigh then stops and we whip the lower leg through down towards ball contact. The way that is done amongst many other different contributing mechanisms within there, some of that is actually through the contacting ligaments that hold the thigh and the shank together, we can see the star here is representing the centre of mass of that lower leg segment and what happens from this 90 degree point here which we said was key before, we can see here Ronaldo performing his kick at a similar point, we then got these contact forces within the knee, as the pelvis moves upwards we're going to get this force lifting the thigh upwards, as the thigh slows down we're going to get this force pushing backwards again on this lower leg segment, both of these forces together will act eccentrically, so they'll act to the side of this centre of mass of the shank and will serve to accelerate it down through towards ball contact. If we try and put some numbers on that, looking at the hip first of all, we realise that what we're talking about here is hip velocities and joint moments, for those of you again just a refresher that the joint moment is actually the turning force, so when we're talking about forces usually those work in a linear or a straight line way, we're talking about forces that cause turning which all do at joints, we're talking about those being called moments. So the moment is that turning force around the joint that's mainly coming from the muscles, we see here that the hip flexor moment which is the green line that we come across here, basically the hip flexors are going to be dominant until just prior to the point of impact, but these hip flexors are nearly always positive right the way through the swing phase, so the hip flexors are going to be the main power generators if you like for our kick. That energy is then transferred down to what then happens at the knee and the shin comes forward quite quickly as we mentioned, the knee extends the moment, so this one where we initiate the forward swing of that thigh can be seen in the second image here and will correlate really with this knee moment that we get here, this little this positive aspect of this which basically is your extension moment. Then just as we come down towards impact what we see is a very very large extension velocity around about 1500 degrees per second as I've already mentioned and then the impact point occurring here where the dotted line is. Now interesting point to note here with that impact point because what we are getting around that point of impact is an awful lot of wobble in terms of our little spherical reflective markers, those markers will vibrate as the foot hits the ball, the muscle will wobble, the skin will wobble and we find ourselves in a difficult situation where we're trying to unpick the real signal out of these movements of these markers and any sort of impact in any sport causes this problem but we must make sure that mathematically we deal with that signal quite correctly. Simon Augustus is a researcher here at our university here in Chichester, just published some of his data regarding this of how to treat this particular type of dirt set. We realise that these ball contacts are basically occurring around about 10 milliseconds so if we're sampling at quick enough speeds we will have a number of frames to deal with over this ball contact period but this impact will generate a high frequency movement and our typical conventional filters which will have a cut-off frequency and they'll try and get rid of any frequency component that is above that cut-off frequency will basically get rid of a lot of that high frequency component that is inherent within that signal from the impact. Our new fractional Fourier filter that we look at here actually takes into account the length of that impact time but also the amount of high frequency content that was within that within that particular signal itself. Without going into too many details of that it then enables us to keep the main frequency content of the signal within our measurements without throwing it out the window and absorbing it through the use of a traditional filter. This means hopefully that as practitioners we can interpret that data a little bit more correctly around those points of impact and this can be of much use as well in other different sporting scenarios. We also have a problem here in football or in soccer where we do have that ball impact force so again those peaks that we're seeing generate what we might look at as false muscle moments around those points of impact. What we can see here we took to the laboratory and tried to recreate this looking at the turning forces around the ankle, the knee and the hip. When we kicked a ball of normal mass we then try to take away that ball impact force by using a foam ball so the foam ball was in the red lines whereas the standard regular ball was in the blue lines. We can see around that impact force around the impact time between the two dotted lines there that we're drastically reducing the amount of muscle moment that is measured due to that false impact force that is being inherent within our calculations. Our calculations that we use are called inverse dynamics so this is the process that we use to estimate the internal forces of the joints and relies on the kind of the mass and the inertia of the leg but for that 10 milliseconds not only we've got the mass of the leg but we've also got the mass of the ball attached to that as well. So we can see inherently we have a problem when we're trying to interpret this data around about impact. An extra 450 grams is hitting the foot and this is going to cause that reaction force and this is going to affect those calculations as we see from the graphs. So as a very swift guide for that if we're trying to use any sort of energetic calculations or any calculations of joint force what we really need to do is try to reflect and extrapolate that data through the impact along with using these new more complex filters to try and help us maintain some of the high frequency content of that signal. Okay so that's enough on the kicking leg I want to move along a little bit more now towards what might happen to the rest of the body mainly towards the torso and the pelvis and I think that it's pointless it's a very good point to note what might happen here with relation to the upper body and a possible generation of a tension arc within the upper body. We'll notice the arm movement within kicking usually goes in this type of pattern where the non-kicking arm will both abduct and will extend throughout that preparatory phase towards the kick and it creates a tension arc which is designated around where this bone arrow is concerned here just before impacting that downswing there's a tension across the torso and in addition to that we get the pelvis that is tilting forward in that final downswing and also has a transverse rotation so that's the rotation around the vertical axis. Now the chances are these two things are not happening by chance at the same time they're happening in sequence that torso and then the pelvis rotation are more likely to give us some stored energy and we'll release and pass on that energy to optimise the function of that kicking leg during the downswing. Brings us on to the one part of the system we haven't really looked at I guess which is the support leg. So the support leg is the leg that we stand on during the kick as we can imagine from the approach we have pretty large braking forces occurring here so we stop the body from moving forwards we slow the motion of the body we have to stabilise the movement and hopefully then during that period of contact of the support leg we can transfer all of that energy we've just been speaking about the tension arc the movement of the pelvis and the movement of the kicking link through into the ball. If we look at some of the work that's done by some of our colleagues over in Japan this is Koichiro Inoue's work back from 2014 who tried to model what might happen during the support leg during this contact phase. It's very very important that we see that there is a joint reaction force here during that position of body contact and if you notice that the kicking leg is reaching around about that 90 degree point again so if we can get this reaction force at the joint bearing in mind the ground will be giving a ground reaction force at the foot from our impact this will pass up the limb and hopefully then this will cause this support side of the pelvis to raise up in the air with the pelvis being a solid entity it's not only going to cause the support leg pelvis to raise it's going to cause the pelvis of the kicking leg to raise too and if we can do this round about we can extend this support leg around about the time when the kicking leg is reaching the 90 degree mark it can further aid those motion dependent energy transfers. This kind of mechanism where we're hopefully transferring energy from the support leg through the pelvis to the kicking leg is not necessarily clear from the coaching literature so really we thought in our laboratory thought it'd be a very very useful thing to try and improve people's kicking performance and perhaps through that mechanism of extending our supporting leg. Now if anybody's ever tried to alter somebody's technique through biomechanics it's a very very difficult thing to do. I guess our natural instinct is to talk very much in terms of flexions, extensions, raising, lowering, stretching these type of things whereas if we start to talk very particular about body parts and some of the research that's been conducted in this area and I'll give an example here of Carson and Collins at times at UCLan looking at a 5a model for refining and regaining skills there is a motor control and motor learning strategy to try to alter players technique without breaking that technique down and the major point of that is not to talk specifically about body parts during that process. So we tried to look at what that process might entail and the 5a's give five distinct phases for us to go through. The analysis is going to be where we use our three-dimensional motion capture to try and understand what exactly is going on in the skill and how it compares to elite performers. The awareness is going to be to get to show the players where they are currently and hopefully where they want to be. Their adjustment phase is going to be when we get them to actually alter the skill and we'll show you how we did this in a second and the final two phases of re-automation where we try to introduce them back to a game-like scenario hopefully even with some more challenging pressure drills and opposition in there and finally we return to measure with our 3d kit again maybe three or four months down the line to make sure that that changing technique has been made permanent. So we looked in our laboratory again this is some of Simon's early work and the support leg that can contribute to maximal in-step soccer kick performance. We use this intervention and looking at top players the example of Steven Gerrard here was a fine exponent of this and that point there in the downswing he had a very forceful extension of his supporting knee and supporting hip which would raise that pelvis and hopefully produce what anecdotally has been told to be the most powerful kick in the Premier League at the time. So we're going to use those type of anecdotes and try to increase the player's prowess in terms of that maximum performance or that maximum power soccer kick. So what did we do? It's easy to say that we're going to follow the five A's but how did we translate that exactly to the way in which these people were performing their skill? So we looked at that we're going to give them a brief overview we're going to show them clips of elite performers. These were the current professionals that the players would be looking at on the tv day in day out and we were trying to focus on that final approach stride so our real idea here was to get the players to come in low and end up high. So we rather than talk about that we talk about a long final kicking stride and very similar to long jump performance this would tend to lower the centre of mass and then we're talking about a low to high transition of your of your body frame you're setting your belly button as you kick in stride and the follow through. The coaching point that we're giving here if we can get both feet to leave the ground after contact that will definitely mean that they're raising their body centre of mass and moving upwards. So using those skeleton reconstructions I showed you earlier we gave those that animation of a previous performer using this desired technique and further highlight these points in during a slow motion example so they can see exactly what it is that we wanted and what we were aspiring to. The cues that we presented so again we try not to talk about individual body parts but we're saying we want you to approach the ball with increasing step length move your body weight from low to high striking the ball as forcefully as possible and follow through fully leaving the ground and landing again on the kicking leg. So that was our coaching cue to hopefully get the change in technique that we were we were hoping for. During this adjustment phase how we get them to do that the participant tries to practice and discover the refined techniques for themselves so we give them verbal feedback by the researcher in relation to those cues we've just we've just spoken about. Also we used instant video feedback so as soon as a kick has been performed we let them see that performance and allow them to further refine their kick and get them also to self-rate those kicks so one being the poorest ten being the most perfect the three questions that we gave them all the time throughout their adjustment phase well how well do you think you produce the best possible ball contact how well do you think you perform the coordinated kicking motion and how well do you think you perform the kick in relation to those cues that we've given you beforehand and the idea being as the participant improves if you're consistent consistently scoring over eight in all of those for five consecutive consecutive kicks then we are happy that that transition has then occurred so hopefully you can see the method that we try to employ to help that that translation of new skill coming in as you can see here we just ran a few statistics on this and we can see we had an increase in ball velocity after the intervention and we also as our key variable to see how that increase in ball velocity occurred we see that the vertical displacement of the kipping the kicking hit but also increased significantly so what what might we do then if we're going to work with players on a day-to-day basis um i think it's very very important to stick and adhere to these technique change guidelines um as i say not going gung-ho like you might expect us as biomechanists to talk very technically about individual body parts we need those global cues and we need to give the participants chance to self-rate to understand and feel that kind of aesthetic understanding of what it is they're doing in this new technique we also must make sure that we're not taking this away from the coach we're going to do these kind of technique changes in conjunction with the coach we're here as a sport science service to try and help we're not here to take over somebody's job in terms of a coaching scenario so try to do this in conjunction with a coach if you're going to work with players in this sport or in others try to adhere to the scientific evidence as well so how do we maintain that technique alteration we only really monitored within our intervention study those first three phases what we needed to really do for that is to increase a introduce a pressure drill scenario and also to go back to those players three four months later just to make sure that our 3d kit that they are genuinely doing the action in a new and improved way and that has stuck with them throughout their training procedures um it's very very expensive and complex to have 3d analysis so not everybody has uh access to that most people will however have access to literature so hopefully they can get the basic understanding of the mechanics of the action that they need and perhaps use a video analysis for instant feedback to try and help the players could it be that simpler measures maybe give us an indication of the quality of the ball contact for example so if we have the speed of the of the ball after it leaves the foot we know the mass of the ball we can work out then the quality or the ratio of the of the foot to ball ratio and from our high speed video and that'll give us an idea uh roughly of the quality of the impact can we also combine that perhaps with high speed video focusing in on the impact point so we can see the quality of that impact point and where the foot's contacting with the ball i'll give you some examples here again this was uh this was another um slow motion impact of ronaldo's free kick we see this in situ on the top right here when he scored the knuckle ball against against portsmouth and we see here on the bottom left when we stop the frame just on his impact in the laboratory and what they're trying to do really then is is to focus if we're hitting some of these type of kicks powerful kicks with no spin we're trying to align the center of mass of the foot with the center of mass of the ball and a peacock ball some colleagues over in australia have done some very complex analysis and tried to model the actual shape of the foot and give a very very precise impact location of that foot in relation to the to the ball center that they did in aussie rules kicking we can do a similar thing in soccer but quite often we don't have the time to do that in situ with a player so perhaps this high speed imaging can give us a to look kick after kick with the player give them feedback comparing with what it is that they're looking for whether they're looking at doing a knuckleball whether they're looking at doing a curling free kick and whether they're doing an inswing or an outswing it these impact points can change and we can alter that uh alter our feedback in relation to that with the coach themselves so in summary i think i've gone on long enough for you guys um hopefully this has been a whistle stop journey there's loads of information out there um but just in summary um there is a good solid literature base for describing good kicking skill so there's plenty of data out there for you to look at when you're combat when you're comparing your your own particular players to those of the elite level um most of that focus as we've seen is on the kicking leg but hopefully our little journey here has helped us realize that it should be a whole body skill especially when we're in and we're using pelvis we're using torso we're using our support leg as well and if we are getting involved with athletes and we're hopefully trying to improve their technique make sure we use the correct methods trying to alter those athletes technique and remember there's not always those um very very high uh technical components of 3d analysis that are going to going to be um the best obviously there will be a gold standard from that but we can have quick and effective feedback to the athlete that is underpinned by biomechanical principles just from the use of high speed video with those guys as well so thank you very much for listening also thank you to simon for some of his access to some of his data during this particular talk i'm just going to leave you with some text there as well and that will give you a little bit further insight into what might happen in football biomechanics not just in soccer but in the other football codes once again i'd also like to thank stuart very much for inviting me to come and do the talk and i'd like to thank all you guys for um for listening and thank you