Long Jump with a Sport Prosthesis | Dr Johannes Funken

Welcome to Lecture 4 of the Sports Biomechanics Lecture Series, supported by the International Society of Biomechanics in Sports, and sponsored by Vicon. I've personally been looking forward to today's lecture for quite a while, so hopefully you'll enjoy it. We have Dr. Johannes Funken from the German Sport University Cologne, and Johannes is the current ISBS student representative, and he's going to be presenting on a really interesting topic about which he recently completed his PhD studies. So I'll allow Johannes to introduce the topic, but I hope you enjoy. Okay. Hello, everybody, and thank you, Stuart, for the kind introduction and for your great work on putting this lecture series together. I would also like to thank ISBS for giving me the opportunity to give this inviting lecture. I will talk about recent insight in the biomechanical characteristics of the long jump with the below-the-knee prosthesis. And as we cannot see each other live or talk to each other after my presentation, I would like to say some brief words about myself to give you an impression of who I am. My name is Johannes Funken, and I received my doctorate degree from the German Sport University Cologne in 2019. Currently, I work as a post-doctoral researcher at the Institute of Biomechanics and Orthopedics of the German Sport University Cologne. And since my first year as a bachelor student, I've been fascinated by the interaction between the human musculoskeletal system and any technological devices. And therefore, I'm very happy to talk about the long jump with the prosthesis. And it is a great pleasure for me to present some of my main findings from my doctoral project for you. When we think about the long jump, many of us might have Mike Powell in mind. In 1991, he set an incredible world record for non-amputee athletes of 895 meters. But there's also this guy, Markus Rehm. During an accident, he lost his lower right leg, resulting in a trans-tibial amputation. With a personal best of 848, he's the current world record holder of long jump of athletes with a below-the-knee amputation, which we now call BKA. Just to get you an impression of how far this is, it's about half a meter longer than your side of a beach volleyball court. Let's have a quick look into the development of the long jump distances needed to become the Olympic champion during the last nine Olympic Games. And as you can see, Mike Powell never actually won a gold medal at the Olympics. Distances to win a gold medal were usually around 850 to 870, dropped a little during the last three Olympic Games. For many years, the distances to jump to become the Paralympic champion or Paralympic champion with a leg amputation were way shorter compared to distances achieved by Olympic champions. But then along came Marcus Rehn, winning the Paralympics with 735 meters in London and with 821 in Rio. This world record of 848 from August 2018 would have been sufficient to win the last three Olympic Games and become at least third in all Olympic Games ever. After this great achievement, questions arose of whether Marcus should be allowed to compete with non-amputee athletes and being ranked in the same ranking system or not. In an attempt to answer this question from a biomechanical perspective, we first had to ask ourselves, what do we actually know about the long jump with and without prosthesis? For non-amputee athletes, there's a great basis of knowledge. To jump far, the athletes need to run up fast and then have an effective take-off step for redirecting horizontal run-up velocity into horizontal and vertical take-off velocity. Both seem to be equally important as a good long jump performance for non-amputee athletes is not possible without one or the other. A good take-off technique involves lowering the center of mass during the last strides and then the athlete really tries to use the take-off leg as a rigid lever to rotate over the foot. In the literature, this is called pivoting. The overall goal should be to generate a vertical take-off velocity without losing too much of the horizontal run-up speed. And what about the long jump with prosthesis? There's some comprehensive work from Nolan and colleagues and one very recent paper from Padulas about run-up and take-off step kinematics. The problem was we did not have any very recent data at all. The research mentioned above is from a time when athletes jumped about one or two meters shorter than they do now. And also in the meantime, athletes switched from taking off from the biological leg to taking off from the prosthesis now. And actually today almost all top-level amputee long jumpers use their prosthesis for the take-off step. And also we do not have any kinetic data, no ground reaction forces, no joint energy, no joint loadings. But it should also be mentioned that already back then Nolan and colleagues already compared the jumping technique of athletes who use a below-the-knee prosthesis for the take-off step with using a springboard. However, as jump distances improved a lot since then, we didn't know if this is still true and what are the underlying biomechanics of the long jump with the prosthesis to achieve distances over seven or even eight meters. Therefore, the purpose of our multinational research project was the biomechanical comparison of the long jump of athletes with and without a below-the-knee amputation. And in my thesis, I worked on three primary research questions. But due to the limited time I have during today's presentation, I will focus on the first two only. And that would be, does the use of a prosthesis by athletes with BKA lead to an advantage or disadvantage during the long jump compared to non-amputee athletes? And the second, are the long jump with and without the prosthesis based on the same biomechanical physical mechanisms? In total, we analyzed ten athletes, three out of the four best long jumpers with a below-the-knee amputation who competed in the 2016 Paralympics, including the Paralympic champion. We also analyzed seven non-amputee long jumpers on different performance levels, including the 2016 Olympic champion. And as you can see, the two groups matched pretty well in terms of performance and anthropometrics. Data capturing was conducted in Cologne and in Tokyo for capturing the kinematics. We used a 3D motion capturing system, a Vicon system. And for the kinematics, we used a force plate implemented below the take-off board. We measured running velocity with a laser gun and ensured valid force plates strikes with high-speed video footage. Marker coordinates together with individual anthropometrics were then used for creating a full-body model of each long jumper. For the athlete with BKA, the lower part of the shank was substituted with the model of the prosthesis and the prosthetic ankle joint was defined at the point of the greatest curvature of the blade. From our model, we extracted and calculated the following measures. First, we calculated the theoretical jump distance. We did that because we wanted to avoid the influence of our landing technique on performance. And theoretical jump distance is defined as the distance from where the foot leaves the ground to the intersection of the COM flight path with ground level. Beside others, we calculated the following kinematic and kinetic parameters. We calculated run-up and take-off velocity, COM kinematics, joint kinematics, COM kinetics, joint kinetics and energy distribution, ground reaction force orientation, ground reaction force, neural arms and vertical stiffness. And just to give you an impression of how our model looked in action, here's a short video of two athletes doing the last few steps before taking off. In red, the athlete with BKA and in black, the non-amputee athletes. Okay, coming to the results. Theoretical jump distance was not different between groups and all athletes with BKA used their affected leg as their take-off leg. On this slide, you can see the individual horizontal velocity indicated on the y-axis for each athlete at touchdown and at toe-off for the take-off step. Left for the non-amputee athletes and on the right for athletes with BKA. Non-amputee athletes run up fast and had a higher horizontal velocity when touching the board for the take-off step. During the take-off step, non-amputee athletes lost about 1.1 meters per second of horizontal velocity. Athletes with BKA on the other side had lower horizontal velocity when touching the board for the take-off, but then only lost about 0.6 meters per second. At the instant of take-off in the actual jump, horizontal velocity was not significantly different between both groups of athletes. For the vertical velocity, there was no significant difference between the two groups, neither at touchdown or at take-off. So, where do those differences in deceleration come from? Since we know that acceleration, or in this case deceleration, is directly linked to the force acting on a body, we looked into the ground reaction forces. Non-amputee athletes had a pronounced impact peak in vertical direction and in horizontal braking direction. Athletes with BKA had a more or less synodial vertical force curve and lower braking forces compared to non-amputee athletes. However, vertical impulses between both groups were similar, but horizontal braking impulse was greater for non-amputee athletes. As I said at the beginning, the overall goal of the take-off step should be to generate a high vertical take-off velocity without losing too much of the horizontal run-up velocity. If we now plot the ratio of the vertical to the net horizontal impulses, it reveals that athletes with BKA had a higher ratio, indicating a more effective take-off step. To understand this effectiveness, we now looked even a bit deeper into the take-off step and analyzed the COM energy during the take-off step. What you see here are individual curves for the COM work during the take-off step, zero-leveled for the instant of touchdown. COM energy is a combination of kinetic and potential energy and during the take-off step, athletes lose kinetic energy because of the braking. As we can see, non-amputee athletes lost some of their COM energy during the first two-thirds of the stance phase and then regained some of this energy during the last third of the take-off step. However, they only regained 56% of the energy they lost. In athletes with BKA, that was different. They also lost some of their COM energy during the first half of the take-off step. But then, during the second half of the stance phase, they were able to increase the total COM energy on an even higher level and it was at touchdown. How are they able to do that? Athletes with BKA store most of the COM energy in the prosthesis. Because the prosthesis is a passive elastic component, some of the energy gets lost here. But then, the athletes with BKA are able to add some energy to the system by use of the active system, in this case, the hip muscles. That's something we could not see in non-amputee athletes. Or, with other words, athletes with BKA are able to store a lot of kinetic run-up energy in the blade in form of strain energy, regain most of this energy and then add some energy with muscular work around the hip. So, as a quick sum-up, athletes with BKA have a disadvantage during the run-up due to a lower run-up velocity but have more efficient take-off step, which is an advantage. Quantifying either the advantage or the disadvantage in meters, which is the unit the overall long-jump performance is measured in, was hard and especially for the run-up, not possible based on current scientific knowledge. So, at the end, when it comes to the question of overall advantage or overall disadvantage, we had to admit that we cannot give a clear answer here because we could not weigh the advantage against the disadvantage in the same units. But still, there were some open questions. Specifically, how can athletes with BKA store that much energy in their prosthesis and at the same time put their hip muscles in action? We now had a look in the ground reaction force alignment relative to the leg joints and the COM. The previously mentioned differences regarding ground reaction force amplitude also became obvious when comparing ground reaction force alignment of the two groups during the take-off step. Athletes with BKA had their ground reaction force vector more or less passing through the knee and hip joint and close to the COM. Non-amputee athletes, on the other hand, had their ground reaction force vector with some distance to the joint centers and to the COM, especially during the impact phase of the take-off step. This was true for the sagittal plane, but also for the frontal plane. Please notice that athletes with BKA are indicated in red jersey and non-amputee athletes with a black jersey. That's also the color coding for our next figures. This qualitative analysis was supported when we calculated mean distances between joint centers and the resultant ground reaction force vector. Compared to non-amputee athletes, athletes with BKA had longer lever arms between the ground reaction force vector and the prosthetic ankle joint, but shorter lever arms between ground reaction force vector and knee joints. In other words, athletes with BKA increased the load on the prosthesis, but decreased the load on the biological tissues at the more proximal, non-affected joints. Furthermore, and as stated by Bivena, a closer alignment of the limb and the ground reaction force vector increases the muscle's effective mechanical advantage at the joint. This kind of selective loading and unloading indicates that the energy changes necessary for the take-off step in athletes with BKA can be provided more effectively with the prosthesis compared to the muscles surrounding their residual limb joints. Also, it should be noted that the energy stored in the prosthesis exceeds the energy storing capacities of the human muscle tendon complex. This kind of movement execution also has some consequences for the vertical COAM movement during the take-off step. When storing energy in the prosthesis, athletes with BKA really sink into their prosthesis and have a pronounced and more extended downward movement compared to non-amputees. Non-amputee athletes hardly have any COAM downward movement, but really pivot over their leg lever. This is also reflected in a higher vertical stiffness during the take-off step in non-amputee athletes compared to athletes with BKA. The higher vertical stiffness is even more remarkable when noticing that knee joint flexion is much smaller in athletes with BKA. So even though athletes with BKA try to keep their residual leg stiff, their COAM vertical downward displacement is much greater than in non-amputee athletes. Athletes with BKA cannot actively stiffen their lower leg, but have to deal with the stiffness of the prosthesis. In other words, by relying on energy storage and return capacities of the prosthesis, athletes with BKA are also confronted with some mechanical constraints of the prosthesis, which they have to deal with. In total, we identified two very different take-off step mechanisms. One is based on redirection of the movement by rotating about a relatively rigid leg lever. The other one is based on energy storage and return, and indeed reminds of the use of a springboard. Additionally, there are two important facts we were able to distill. First, non-amputee long jumpers cannot adopt the technique elicited by athletes with BKA due to their limited capacity of storing and returning energy in the biological structures. And second, long jumpers with BKA cannot adopt the take-off technique used by non-amputee athletes due to mechanical constraints of the prosthesis. And to my mind, the combination of those two facts underlines how fundamentally different both movement mechanisms are and provides important knowledge for future rules and regulations. Now, let me take you on a short excursion and highlight some parallels of the movement mechanisms used by athletes with BKA similar to mechanisms in sports and biology. First, just remember how athletes with BKA store most of the kinetic run-up energy in the prosthesis and then get most of it back and additionally add energy by muscular work. Something similar was observed by Aram Paxes and colleagues during the pole vault. The athlete stores kinetic run-up energy in the pole. The pole releases this energy and then the athlete adds some energy to the system by using his or her active system. In this case, probably arm and core muscles. And for the second example, we remember the ground reaction force curves of athletes with BKA during the take-off step. When looking into the literature, we found remarkably similar ground reaction force curves in popping of yellow rock wallabies. From wallabies, we know that their movement is based on energy storage and return from elastic structures. And of course, we don't want to say that athletes with BKA move like wallabies or like kangaroos. But to our mind, this and the pole vault example underline our finding that the take-off mechanism of athletes with BKA is based on energy storage and return. When we come back to our two main research questions, we had to admit that we could not quantify an overall advantage or overall disadvantage based on the data we had and the current scientific knowledge. For the second question, we were able to identify two fundamentally different movement mechanisms, one based on pivoting and the other based on energy storage and return. And indeed, reminds of the use of a springboard. The knowledge we gained has direct implications for prosthetic design and also training protocols, training equipment, and performance diagnostics might be adapted to the individual movement execution and specific demands of athletes with a below-the-knee prosthesis. And at the end, and as I said before, the knowledge about two fundamentally different movement mechanisms might have implications for future rules and regulations and might help decision makers. From a biomechanical perspective, we were able to gain a variable data set and build a sound fundamental objective and more data-driven discussion on whether or not athletes with BKA should compete in the same ranking with non-amputees. Before I close, please let me add two comments. First, I am not saying one or the other movement mechanism is more or less demanding. They are just based on very different biomechanical physical mechanisms. And the second, this goes out to those who say, Marcos is just using a spring and it's all about the spring. If this was true, Marcos would not jump about 80 centimeters longer than the second best long jumper with a prosthesis. If it was all about the spring, all athletes with BKA would be able to jump that far. To my mind, the long jump with a prosthesis, just like all other sports and disciplines, is all about passion, talent, and really hard work. Thank you for your attention. I would like to thank a few people for their help during my doctoral project and many fruitful discussions during the manuscript writing phase. That would be Dr. Hiroaki Obara, Professor Lina Gurbauski, Dr. Steffen Billwacher, Dr. Kai Heinrich, Mr. Ralph Böhle, and of course, a special thank you to my doctoral supervisor, Professor Wolfgang Potthast. I would also like to thank all student helpers and colleagues from the Institute of Biomechanics and Orthopedics at the German Sport University Cologne. I would like to thank the Japan Institute of Sports Sciences for their help during data collection. I would like to thank the German Sport University Cologne for providing me with a graduate fellowship grant. And a special thanks goes to Japan Broadcasting Corporation, NHK, for financially supporting this study. Thank you very much, Johans. And I really hope that everyone else got as much out of that as I did. It was really fascinating. If anybody's got any follow-up questions to that for Johans, then please either use the comments section on YouTube or if you take to Twitter and use the hashtag at the bottom of the screen, we'll keep an eye on both of those and try and make sure we can get a response for you. And then finally, please don't forget, on Friday we've got another really interesting talk, this time from Wouter Hoegekamer, talking about running footwear and the two-hour marathon. So again, hopefully another really interesting topic. Thank you very much and see you soon.