 Hi, I'm Stuart McCurley-Naylor from the University of Suffolk and welcome back to the Sports Biomechanics Lecture Series. As always, supported by the International Society of Biomechanics in Sports and kindly sponsored by Vicon. Today we're joined by another very special guest. This is someone who in the last 24 hours has been described by separate people on Twitter as one of the original grandfathers of Sports Biomechanics and as one of the all-time greats. So it's a real treat to be joined today by this person who is of course Professor Bruce Elliott from the University of Western Australia. Bruce is synonymous with the research area of tennis biomechanics. So much so that actually one of my colleagues in the office when I was doing my own PhD was researching tennis biomechanics and it became a standing joke that almost her entire reference list was categorized under E for Elliott and every time she found a new paper it was again Elliott. So it's a real honor to be joined by Bruce today to talk about a topic that he knows so well in the biomechanics of tennis. Yes, so thank you all for joining and I really hope you enjoy. Thank you Bruce and over to you. Good morning everybody. My name is Professor Bruce Elliott. I'm here this morning to deliver a presentation on the biomechanics of tennis. In this presentation I will be ably assisted by Professor Jacqueline Alderson. Good morning everyone. I'm here as overpriced technical support. Tennis is a global sport widely considered the most popular individual sport in the world. So it's important that we understand how we develop players because obviously people are interested not only in performing at all levels but in watching this game. So what do you need to be successful in tennis? You need talent. You obviously need the appropriate equipment to perform at all levels and at all stages of development and you need effective coaching and this coaching has to be linked to the appreciation of sports science. This talk today however is on biomechanics and biomechanics links closely to tennis because most strokes have a fundamental mechanical base. An injury primarily has a functional mechanical cause that is either misuse or overuse. So where do we go from there when we look at tennis? What parts can tennis play in being linked to biomechanics? We can look at stroke progression through the development pathway. We can look at injury reduction from both equipment and mechanical loading perspectives and today with Hawkeye tactics is forever becoming more important and the data we get back from Hawkeye allows us to modify player tactics. This presentation however will address biomechanics and how it plays a part in selected aspects of the game. We will look at equipment used by beginner players. It's so important if the game is to flourish that beginners both enjoy and develop. We will then look at shoulder internal rotation and how it's a generator of velocity in the serve followed by the role of trunk rotation in forehand racket velocity. How does it play a role? What type of role does it play? And then finally we'll look at the role of variability training in ground stroke development. So let's look at what research tells us about young children. More logically today we find that equipment should be linked to the stature of young kids. If you go back to when I learned to play the game everyone used adult balls and almost always used adult rackets. Today that is different and you can see in this picture of this young girl. She's using a modified racket and the ball is modified compared with the adult version. So if we look at rackets there are two aspects that can be looked at with reference to children. One is the swing axis and it's clearly evident that the racket is smaller. That is shorter in length for the young player. You can see this young girl has even moved up the grip to shorten the racket even further to make it easier for the moment of inertia. That is the swing moment of inertia to play a role and allow her to swing the racket effectively. However, if you look at the polar moment of inertia, which is the twist axis, you can see that the difference between adult and children's rackets isn't as great as the difference you would see with reference to the length. So kids need a reasonably large polar moment of inertia to increase stability. To enable them then to swing the racket and allow off-center impacts not to play too much of a role in stroke production. Today balls are different and we have three colour balls that kids will use differently over different stages of their development. There's the red, orange and then green ball. You can see the difference between these is that they have different weights and that's very important for young girls. We may not have a great deal of strength and therefore the weight enables them from a momentum viewpoint to play more effectively but equally the coefficient of restitution of the three balls is different. The concept here is that we want the ball to bounce so that it lands at about and is able to be hit at about hip height. So different coefficient of restitution, different bounce heights permit kids to use the same grip and hit the ball in the same impact location over the development pathway. Bruce, are they regulation sized tennis balls? They're not regulation tennis balls. They've got fur exactly like other balls. They, as I said, the differences are in mass and in the actual coefficient of restitution. Other than that, they look like a tennis ball so a kid would pick it up and it wouldn't be like they were playing with a different ball. The red ball has an actual factor, a bigger version for really young kids and it's more foam where the three balls we're talking about today of texture similar to that of a tennis ball. I saw this brochure on the wall in a clubhouse in Europe and it's interesting if you look at it here, someone would arrive for a lesson, they would know what group they were classified as. They might be a Mickey 19. So you can see it's linked to Disney characters and a Mickey 19 is a shorter length racket than any of the others. So it obviously relates to a younger player. That person would then presumably use a red ball and they would play on the red court. So if you look at the picture, you can see that not only do we change the racket, we modify the ball. We also modify the actual size of the court. Research has clearly shown that if you use equipment that is more related to the size of the person, then you're going to have better stroke production, better improvement in stroke production and more enjoyment. So this is really something that has changed dramatically over the last decade or so. Let's look now at the service and talk about internal rotation and the serve. You can see from this image that the person's leg drive is finished. I want you to look at the hips. Notice how the hips are angled so that the back hip moves vertically upwards and doesn't move horizontally around. This enables then with the appropriate backswing for the racket to move into external rotation. When it nears completion of external rotation at the shoulder, we have an eccentric contraction of the internal rotators. Key aspect of how the tennis serve works. You can then see the person moves from maximum external rotation at the shoulder through to internal rotation. And here there's a concentric contraction of the internal rotators. Later in the talk, I'll talk about joint power and how you can actually get free energy from this type of service action. Let me ask you a question. What percentage do you think internal rotation plays in racket speed or racket velocity at impact in the mature service action? No percent and that's possible. 20%, 40% as much as 60%. Look at the images below and you can see that internal rotation occurs in both the 15 year old serve and that of Roger Federer. The answer is about 40%. Now, this is going to vary clearly from person to person, but research has shown that it will provide about 40% of racket velocity at impact if you use internal rotation. So you can imagine if you take internal rotation out of the service action, where do you find this extra 40%? Some people can try and develop it in other ways, but typically this will lead to misuse and overuse injuries. You can see that the 15 year old boy also has the correct service action for his age and he probably at 15 would be starting to get about 40% of his racket velocity at impact from internal rotation. If you look at the actions of the two, this is a 15 year old versus a mature professional. Again, look at that back hip. Look how the back hip moves vertically upwards. Both hips are driven up from lead drive, but the back hip moves up higher. In other words, with higher velocity, that's vertical velocity and the frontier. So you stay almost side on for much of the action until you get near impact and there's only then that you start to move horizontally with your trunk into the ball. This is the only way that you can effectively use internal rotation to create velocity. So let's look at male versus female. It's always an interesting question. It's one that Jackie and I talk about quite consistently. If you look at the fastest serve that's been recorded on the ATP or WTA circuits and you can see the two numbers there in kilometers brown. The difference between the two is about 18%. So this is the fastest that's ever been recorded in an official match. If you then look at the typical grand slam tournament, what I've done is taken the Australian Open and I've looked at data from the Australian Open and males typically are in the 190, 195 km per hour range. Females about 165 to 170. There's clearly females that serve higher in velocity than some males but generally the continuum works to this level and again the difference is about 14%. Not that different to that which would record from the higher speed record. Do you think this is a strength, pure strength issue or is it more related to perhaps the greater stature of male players in terms of their labor? I think there's three things. One is certainly strength. Two is certainly stature. And the third actually is technique. And third is a question of technique, Jackie. The next slide helps give more clarity to how this works. You can see here we have data collected for males and females and these are professional tennis players. The females were all Australians and we recorded data ourselves at the IAS. The males are a collection of data across a number of research studies. So it's not really comparing an apple with an apple but it's very close. So if you look here at internal rotation velocity, in other words, this is peak internal rotation velocity for male and female professional players. You can see the males are typically higher than the females. The 17% difference does make a difference and it reflects the same level of difference that we saw before. In other words, if we looked at the highest velocity ever recorded, if we looked at the average velocity at a Grand Slam tournament and now we're looking at the difference in velocity. This is shoulder internal rotation velocity of the two groups. Again, about 15% different. So this is what we really should use if you like as a rule of thumb. There's about 15% difference between the two. So the take-home message on this is that if females want to serve harder with higher velocity, then they really need to work on their internal rotation. That's the technique of internal rotation. I fully accept that the strength levels, the stature will also make a difference but it's a way of making the difference smaller. And that's certainly the case that there are a number of females that serve with higher velocity is then some of the males on the circuit. We're just generally talking about a way of improving one's service velocity. A lovely study done recently in Spain compared Spanish youth and the idea here was to look at internal rotation across the development pathway. So we're looking at internal rotation strength and service velocity for Spanish 13-year-olds and for Spanish 15-year-olds, males and females in both categories. If we compare these, it's interesting that prior to puberty in the under-13 group there was no significant difference in their internal rotation strength nor was there any difference in their serving velocity. So prior to puberty in the under-13 group, they were as one, males and females were as one group. And then as we move to 15 and compared that group with the 15-year-old group I accept they're not the same players, different cohort of 15-year-olds. We now find two things. We find that service velocity increases for both sexes. So it was the girls and the boys are higher in their service velocity than the under-13 cohort. And now we have a difference between the males and the females. The under-15 boys serve at a higher velocity and have greater internal rotation strength. So what's our take-home message from this? Two things. Prior to puberty, we should work on technique. What we say to coaches is that get technique such that the feet are in the right location, the trunk is rotating in the right directions around the right axis, that the movement of the racket and arm are such that you will allow increases in velocity to naturally occur once puberty occurs. In other words, once you move through puberty, these things will change naturally. Secondly, if we want girls, these are the 15-year-old girls to serve with a higher velocity then we should address internal rotation as a selected aspect of tennis development. In other words, don't forget that this is the power generator in the serve. You as a coach must deliver and must work on these types of action. The final slide looking at internal rotation considers energy. And I talked before about eccentric and concentric contraction about the shoulder for the internal rotators. This is a joint power study. If you look to the two images, I fully accept these are both retired Grand Slam champions. But if you can look at what you can see in the pictures you see, that they are in a position of external rotation at the shoulder ready to move into internal rotation. Research has shown that if you look at a group of players, and these were high-performance players, not professionals, that the joint power at maximum external rotation was negative. You can see it's negative 220. So what has happened here is you have the arm moving backwards into external rotation at the same time near the completion of external rotation that the internal rotators switch on. In other words, the internal rotation torque is pushing forward while the arm is moving backwards. In other words, you get a negative joint power, an eccentric contraction, a storage of energy. I've described this as free energy. If you move from there to from external rotation then into internal rotation towards impact, you can see the number there. This is a concentric contraction. So you have the torque, the internal rotator torque that is, and the joint angular velocity being in the same direction. Then you can see it's a very large power value. So for those that in actual fact wish to serve with a high velocity, it is imperative that we get this free energy. We use the joint power both at the completion of external rotation and then move this into the concentric traction of internal rotation. Let's change now. We've looked enough at internal rotation. Let's look at racket velocity in the tennis forehand and compare it to trunk rotation. Now, when I talk about trunk rotation here, I'm talking about trunk alignment. So if you draw a line between the two shoulder joints in these pictures, you can see that the shoulders clearly move from the backswing position to impact. What role does this play? Does it play an important role in the development of racket velocity in the forehand? A lovely study here using American players, they ask people to actually hit at different velocities. So they hit at slow velocity, medium velocity and high velocity. And in general terms, the ball velocity changed from about 20 meters per second to 40 meters per second. In saying this, they then looked at how the trunk rotation varied between these three impact velocities. Again, they found that the trunk shoulder alignment rotation increased as they were required to hit the ball harder. As a general rule, it's just a rule of thumb. As you've got a doubling of ball velocity, there was a doubling of shoulder alignment rotational velocity. So in other words, the player was using the trunk to assist the development of racket velocity more effectively. And certainly they used it more than they did at the lower speeds. Another study, this time we're looking at professional players. We were looking at coaches. So it was a variety of players from high level or high performance players through to intermediate level using different stances. Some used an open stance and then they used a square stance. And you can see from the graph that the linear velocity of the racket increased in an almost linear fashion with trunk angular velocity. So it supports what we talked about in the previous slide. Again, this is for US players. So as you increase the velocity of your shoulder rotation, you increase the velocity of the racket at impact. An Austrian study compared high performance players who were professionals. This was six ATP ranked players, 23 years of age, with seven high performance juniors. And these were about 16 years of age. Interestingly, if you look at racket velocity, look at the table. The difference in racket velocity was only 6%. In other words, they were almost hitting with the same velocity. However, the upper trunk angular velocity, in other words, the alignment of the shoulders, the rotation of the alignment of the shoulders increased dramatically. So what can we learn from this? One would assume that the professional players using the larger trunk to assist the development of racket velocity far more than the young juniors. Even though we have to accept that these juniors were high performance juniors ranked on the ATP circuit. So again, it's evident that the trunk plays a more important role if you wish to both hit the ball with a higher velocity and consistently hit the ball with a higher velocity. In other words, you don't want overuse to become an issue. Bruce, there's very little difference here, really, between the racket velocity to cohorts. How would you explain with how the juniors are developing that racket velocity if it's not at the trunk? I think what they do, I think lots, if you watch lots of high performance junior players, that they use their upper limb. They use the segments of the upper limb and they use internal rotation and they use other aspects of movement around the shoulder joint to actually increase velocity. They may use more wrist, but it's logical to assume that if you look at someone who's trying to develop all the velocity out of their arm, comparing that with a player that's developing a similar velocity, both with the arm and the trunk, it's not that difficult to imagine why the person who uses the trunk can in actual fact maintain that velocity over the entire match. In other words, over three sets, even over five sets, where the young players, obviously their velocity would drop off as they were required to play longer matches and they become more fatigued. Let's finally look now at stroke variability. Here I'm comparing Nadal versus Nadal. So when I'm talking about variability in this particular case, I'm not talking about Nadal compared to Federer or Nadal compared with any other player. So we're looking at Nadal versus Nadal and we're looking at Nadal versus Nadal in a similar tactical environment. In other words, the ball needs to be hit from a similar location, a similar height to a similar court location. So it's not like one's asking a person to hit the ball down the line and comparing that with the ball that's hit across court. It's the same shot. It's Nadal versus Nadal in a similar tactical environment. So here we go with a lovely study that was done by Nudson and you can see, you can read the different parts. You can see risk velocity and acceleration, they're variable. So if you look at the path to the ball, the velocity of the risk and its acceleration profile are variable. The racket orientation and impact is consistent. So these players are able to get the racket to a similar position, even though we said the risk velocity and acceleration was variable as if you look at the others, the elbow velocity and acceleration are also variable but the shoulder angular movements are consistent. So what do we learn from this and what can we take home from this? We can take home that the racket position and impact is relatively constant for the same tactical stroke. However, how the racket gets there is variable. No, the scientists, heavy duty scientists in the group won't like me saying and certainly those right in the motor control weren't like me saying, I'd like to talk to coaches and say, this is like the joints talk to each other. So in other words, you get the racket to the same location but the joints and the velocity profiles and acceleration profiles of the joints differ to allow the racket to get to this position for different tactical environments. For what reason? The reason and let me answer that in the next question because it really comes down to how I've changed my view and I think most people that are as old as I have have changed their view on stroke production and this would relate to many, many sports and take to almost all sports. Let's go back to the 70s and 80s when I was trained and I was a young man. On a tennis court, I was put in a line and a coach fed me a ball into the same location and I was told to hit it to the same location. So everything was really a repetition. We tried to make everything repeatable. A stroke was meant to be exactly the same as the stroke you played before. You went across court then you compared an across court with an across court. I could never understand how a player that had the ball fed into the impact zone would then walk onto the court and hit the ball wide, short. They go forward. They go back. They hit the ball with different spins. They hit the ball at different heights and all of a sudden more recently and I was today but today really now reflects more from the 2000s on. We realize and David's has done lots of work on this area that variability is an essential feature of human motor behavior affording the necessary flexibility and adaptability needed in skill performance. So in other words that's the variability that Munson saw in the forehand. There's variability in how you get to that point. It's not necessarily how you teach it. You have to create an environment where variability becomes part of the learning environment. Let me look at this. How would you do this on a court? It sort of gives a better example. I think let's talk about the feeder and the tactical situation. Two things that talk the tactical situation. First, it's very important. The motor behavior literature is suggested that you need target hitting. You need your brain to be prepared so it knows that you've been successful or not successful. So I've always encouraged people to use different size squares, rectangles, whatever level you are at. You can change the size of the square. And you need different locations rather than a witch's hat. You can always cheat your brain by telling the brain that yes, that was close. And therefore that was a good shot where if you're heading for an actual factor location on an area, you know full well whether the ball has gone into that area or not. In other words, you're getting feedback. Your brain is getting feedback every time it hits the ball to a given position on the court. The feeder and this doesn't necessarily have to be a feeder. It can be a live drill. I fully accept that. But if there is a feeder as you start the development process balls should be fed slow, medium, fast. Low, medium and high. And you should be hit from a stationary location. You should be forced to move laterally and you should be forced to move forward and back. It's not that difficult today with professional players being so happy to stay on the baseline. They are so consistent from the baseline. So many of them aren't happy about hitting the ball when they are moving forward towards the net. In other words, they're hitting the short ball and must practice this. And they must practice it with a tactical situation. And one way you can do that is to set up a court with obviously targets on the court. I've just got five targets on here of a given area. As I said before, these target areas can be larger or they can be smaller depending on the level of development. They can be in different locations depending on what the tactical structure of the way you want your player to perform would be. From that, you have a player and a feeder and then you feed balls to different locations. So you may have to hit three balls to location one and three to balls to location three. And these would be a game. Let's look back to the feeder. Some might be high, some might be low. I've certainly worked with professional players who have excellent forehands when in a stationary position but when forced back or forward all of a sudden errors are forced into their game. They hit the ball out of court. They hit the ball into the net. One way to get around us is this concept of variability. I fully understand. No, I don't fully understand but I have a better appreciation now for what's going on in the brain when one teaches someone to hit the ball that is hit differently to them rather than having the ball fed always to the same location. So biomechanics clearly plays an important role in tennis growth production. I've only given you four examples really who've looked at different examples I accept and I hope that they have been of interest to you. I haven't tried to bog down on technique. I've looked at different things that might appeal. If you are part of the ISPS structure or if you believe that research findings should not be kept on a shelf and should be put into operation then it is important that you actually broadcast your findings. You interpret your findings. You put them into a terminology that coaches understand you and actually just don't assume that coaches understand biomechanics the way you do. What we have tried to do and the research team that I've worked with with Jacqueline and others at the University of West Australia we've published a number of books. We work closely with the ITF. In fact, there's another book coming out in June this year that looks again at how stroke production can be developed across the pathway. So often we have just collected data on high performance and professional players and from that we've assumed we can then go back and use this data with juniors with 15 year olds. That's been the guide by which most of us has worked. Luckily more recently people certainly acknowledge the fact that research must be performed on appropriate ages and we have teams now looking at how stroke development occurs through the ages. Thank you. Thank you, Bruce. That was fantastic and it's always a real honour to hear from I'll say a true legend such as yourself but it's always a delight to hear from someone so knowledgeable about their topic but also someone who's so passionate about not only the topic they're speaking about but sports biomechanics in general. So yeah, a huge thank you from me for that and for your time in preparing and giving this talk. So everybody watching at home I hope you're all keeping safe and well and I hope that you enjoyed that as much as I did. Yeah, and just watch out for next week's talk which will be by Vicon themselves giving a bit of a practical overview on 3D motion capture. So I'm aware a lot of people especially students are potentially missing out on practical hands-on experience during these difficult times and so Vicon are hopefully going to provide a bit of an overview into what is motion capture some of the hardware and software and the modeling used in motion capture and then in a few weeks time there'll be a follow-up as well with a bit more of a demonstration around that but hopefully we can all benefit from. So thank you very much and hopefully see you next week.