 Hello 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. Again, as always all views are those of the individual speakers, but today we've got a speaker and some views that I'm really looking forward to hearing. I know some other people are as well. I think when I first set up this series, one of the main goals was to try and help as many biomechanists as possible, ranging from students through to researchers. And hopefully with the topic today, it's something that will benefit quite a broad range of people. So, without further ado, I'm joined by Professor Bill Balzopoulos, who is head of the Research Institute for Sport and Exercise Sciences at Liverpool John Bowles University. His main research interests and work are focused on the biomechanics of the musculoskeletal system. Today, he's going to give a talk on a really common area within biomechanics, where there are a lot of kind of potential misconceptions. So it could be really good to try and clarify those and maybe add in a little common sense to the situation. So he's going to give a talk on inverse dynamics, joint reaction forces and loading in the musculoskeletal system with various mechanical misconceptions. So thank you very much. And over to you, Bill. Thank you very much. And hello everyone. I hope you're all well. Thank you for tuning in either live or watching this recording later. Thank you very much for the introduction and your kind words, Stuart. And I can assure all people watching that I am the same person as the Sport Biomechanics lecture series flyer that Stuart circulated. This lecture was from a long time ago. But thank you for all your hard work and therefore Stuart it's really appreciated it's an excellent initiative, especially in these strange and difficult times. And thank you also to ISBS and Viking for supporting and contributing to this work. And briefly explained in this lecture I just want to address some important issues in my view linked to inverse dynamics musculoskeletal loading and related terminology. Now I appreciate that this is perhaps a dry technical subjects and perhaps not not as interesting or exciting as the other more applied sessions. Yes, I believe that it is very important as it underpins most of the other work that we do and we must all understand and communicate with the same mechanical language. So, the session hopefully will satisfy everyone. It's a little bit difficult to plan when you don't really know the experience of the audience. So I try to balance the talk. And I'm going to start with some basic principles were needed so that we all understand the same things. I also included a basic modeling example, just to make some points. So, I hope you excuse me for sort of following a rather simplistic approach to make some some points, but I'm just going to start with some general aspects about the assessment of loading. Just concentrate on, as I said, some basic steps I'm also going to go back to some seminal papers in this respect because I think this is very important. And then I'm going to concentrate on just the main steps of inverse dynamics. And in my view, why some mechanical misconceptions have crept in over the years, and are not very helpful in my experience. And I'm just going to end up with some conclusions and recommendations for the way forward in my view. I think by mechanics. Just a big picture, the work that all of us are doing are basically either to improve performance or training or to prevent injuries. So I'm sure we all want to help athletes achieve great results. So in the process, trying to avoid injuries. Now, whether we want to improve performance or prevent injury, we are interested in forces in biomechanics that's why we obsessed with understanding the effects of the forces, and spend a lot of time trying to use forces either to improve performance or understand the effects on biological tissues. But the problem of course is that forces are not visible. We can only see their effects, because we can't directly measure them. So, obviously, there's a broken tibia there with a clean fracture. So that's clearly the effect of some overload, a force that was applied. So the only thing that we can do is to measure the effects of the forces on the musculoskeletal system. And these effects are usually deformation in the tissue or some tissue strain. Now, it is actually possible to measure in vivo the effects of forces, that is to say, what deformation or strain they cause in tissues in vivo. However, this is a special sort of tensiometry that's been developed. What obviously instrumented prosthesis or some strain or optical transducers that can be implanted in the tissues. And you have some really nice pictures there of the work that was by me by Taya Fini and Paavo Komi assessing the strain of the patella, but also the Achilles tendon using optical fibers. So the optical fiber goes through the tendon, and obviously as the tendon develops force and the strain is changed, the quantity of light that goes through the fiber is calibrated to determine the force or to reflect the force that goes through the tendon. There's a lot of other methods that have been developed over the years for the assessment of loading in the musculoskeletal system by somehow assessing the in vivo tissue strain. So I'm just going to go through some of these techniques. I don't want to spend a lot of time. It's just an indication of how using some of these invasive techniques, we can actually have some estimation or direct measurement of the strain in tissues. If we look at tendons, again, there've been techniques that use ultrasound, for example. This is the paper, seminal paper in my view that Costis Maganaris did with Professor Paul. In this particular one, they measured the mechanical properties of the tibialis anterior tendon, and just by tracking my tendon's junction of the tendon, they were able to, in essence, measure the strain in the tendon, and with some measurement based on the measurement of moment time, they were able to relate that displacement of the tendon or the tendon strain, if you like, to the stress of the tendon. More recently, obviously, Daryl Fellen and his group presented some amazing new techniques that rely on shear wave tensiometry. So just by tapping the tendon and measuring the speed with which the waves sort of travel through the tendon, either initially through ultrasound or with some accelerometers. They're able, as you can see, to relate the speed with which the waves are travelling through the tendon with the torque, because obviously the speed will be proportional to the strain in the tendon. Some really, really exciting ideas that basically allows you to, with this external device, measure the wave speed, and as you can see here, the Achilles wave speed squared is following very nicely the ankle torque developed during GATE. So again, very exciting and promising recent techniques. A little bit further, we can see that you can actually look at ACL strain. We're doing a lot of work and a lot of people are interested in preventing ACL injuries. And it actually, it is possible, obviously, under very restricted conditions to implant a transducer and the work of Bruce Payne in the 90s. They've started with a whole effect transducer, but this is an improved method. And you can see here that obviously in these conditions, you can see the ACL strain in different flexion extension examples in the knee. These days, a similar idea for the ACL strain, but based on biplane x-rays. So this work is using high resolution CT3D image. And obviously, if you identify the proximal and distal ends of the Achilles, sorry, the anterior cruciate ligament, these points can be tracked in real time during the movement from the x-rays. And obviously, you can measure the 3D distance between the attachment sites of the ligament. And therefore calculate the lengthening of the ligament in this case during walking and running. Again, under x-rays, there are restrictions, but very promising techniques to be able to look at the different strains in this case, not only of the whole ligament, but the different bundles of the ligament as well. A different group. This is Louis De Freight's group, obviously, based on MRIs this time. So the 3D model is based on MRI, but again, if you know the location of the attachment in the femur and the tibia, then those can be tracked in real time with the x-ray system during the movement. And again, you can see approximately 7% maximum strain in the ACL. Again, very promising techniques. And the last piece, I don't know whether you were in the CAMHS NE workshop that Bill Taylor organized in February, but again, some pioneering and excellent work using the data at the Tarete Hospital and the Eulens Wolf Institute together with Gail Bergman, the pioneering work of Gail and Gail Duda now, and obviously Bill did also some pioneering work tracking ACL with bi-planar x-rays, but this is an exciting development, but again allows the assessment of loads directly using these instrumented prostheses. To measure in vivo effects of forces in tissues, but they require highly specialized techniques that have various problems. There's obviously ethical issues either with invasive techniques or x-rays and ionizing radiation, although the dosage is quite low these days. There's obviously infection complications if you have invasive techniques. There's clearly disruption of normal movement, there's only restricted movements that you can perform under these conditions and restricted activities. Obviously if someone is having an instrumented knee prosthesis, they're unlikely to be able to perform perhaps highly dynamic movement that might be injured in sports biomechanics, and obviously there are various calibration problems. So just a quick introduction and of course the alternative and given that we're talking about inverse dynamics, the alternative to all those direct in vivo methods is of course to use biomechanical models. So whether we use a rigid body model or a more complex model, modeling is an attempt to represent reality in a simplified way. So if I was interested in what joint moments are exerted in a person that is performing a vertical jump, then obviously I can use a simplified model or a more complex muscle skeletal model. So I just want to spend a little bit of time on the basics of this process because I think that's where most of the problems start. And the question that I will follow, obviously there are a lot of different ways to classify biomechanical models. But the type of classification that I'm going to use here is basically divide models to either conceptual, statistical or regression models or full mathematical sometimes also called computer models. And if we look at sort of a graphical representation of those models, I'm not going to talk about at all about sort of hierarchical or conceptual models of technique or regression models that we sort of measure a lot of parameters and we're trying to sort of fit relationship to those. But I'm just to concentrate on the types of models where we represent the human body with a rigid segment and using the equations of motion, we're trying to understand something about the forces that either cause the movement or loaded the various tissues. I'm not going to go into detail into highly complex three dimensional musculoskeletal models. I'm just going to show you some brief output of the work that we do. But for this lecture I want to concentrate on simple models because I want to, as I said, explain some principles with very basic examples that you can probably do by hand. In Liverpool John Moores University we have a big osteoarthritis project, the o-active projects and in fact we apply quite complex techniques to understand multi-scale effects of forces. So we might start with joint angles and kinematics in the body, we then try and determine internal joint loads and muscle forces but we then also apply these to finite element models of the menisci and the ligaments and cartilage in the knee to understand cartilage stresses in people that don't have osteoarthritis and those that develop osteoarthritis. Obviously the complexity is increased dramatically if you try and understand the stresses in various tissues but obviously the clinical relevance is also increasing because this type of information is very important for a clinician. And as I said a lot of this, although this is a clinical application and not a sport application, similar techniques can be applied in sport. So we use a pipeline using the open sim system using motion analysis of normal gait and people with osteoarthritis and using this pipeline we can calculate knee joint loading but also if we have MRI or CT scans from patients then we can also combine the kinematics with the finite element model that was developed from the segmentation of the MRIs and that provides the stresses in cartilage and tissues in the knee. But for this talk what I wanted to separate is the concept of running inverse dynamics techniques and that provides us with the joint moments and only the partial joint reaction forces and obviously in a complex system like this when you have many muscles the knee joint moment calculated will be originating by the activation of a number of muscles and the forces produced by a lot of different muscles. So through static optimization we calculate the individual muscle forces and then with some force system analysis we calculate the total joint reaction forces. But what I wanted to stress here is that to arrive at the point where somebody can calculate the actual joint reaction forces having calculated muscle forces you need to go through inverse dynamics first. And that allows us to as I said calculate cartilage just a quick example here that shows that a healthy person the stresses are distributed both on the lateral and the medial side. But if you see with as you progress through early way or develop to osteoarthritis, then the stresses, especially in the medial side tend to be concentrating on a much smaller area. Again, an example of multi-scale modeling and how you progress from inverse dynamics to cartilage stresses in specific tissues. But generally speaking, the procedures, the general procedures in modeling follow all these steps. And I'm using the example here from Beno Nigs and Walter Herzog's book, which I think is very clear in these issues. And you can see that irrespective of the type of model that you need to use, whether whether it's a regression model or a rigid body model or even a more complex three-dimensional muscle sleepy model, you have to go through these general steps. And one of the questions that you might want to ask, for example, is what is the loading the need during weight training? And according to these general modeling procedures, you need to start thinking about what part of the body do I need to model? What I need to know about the way the forces are applied by muscles and tendons in this movement and what kind of model will be appropriate, but as simple as possible. In other words, can a two-dimensional rigid model of lower leg will be sufficient or do I need to go to a full three-dimensional musculoskeletal model? Now, again, I'm going back to one of the first papers that applied and explain the application of inverse dynamics in biomechanics using free-bodied diagrams. And if you look at these papers, the process, the general modeling processes applied to the analysis of forces using a free-bodied diagram, obviously you have to look at the system of interest for the research question. We always have to start with a relevant research question and then see what type of model would be appropriate, but once you decide that you still need to do some assumptions and simplifications. You said a model is an attempt to represent reality, so you have to accept that you're going to perform some or accept some assumptions and make some simplifications to be able to do that. And then you're ready to build the free-bodied diagram, which is basically a depiction of the rigid body, the reference frame and all the external forces and moments acting on the body. And what kind of forces are we talking about? Generally speaking, and again you can go back to the original references, there are just two types of forces that you need to think about in applying in a free-bodied diagram. There are more forces that are applied without any contact and in biomechanics, we're basically talking about the weight, which we normally apply at the center of gravity and contact forces. In other words, forces that are applied on your segment on your free-bodied diagram of the segment through some sort of contact. And these contact forces are described as two types. If you go back to the paper by Andrews, for example, he just clearly states all we need is contact forces at the proximal and distal joints and these are because of the contact with other segments in the kinetic chain or contact forces on the free-bodied diagram between the joints. So in the beginning, in the very beginning, there was no other sort of terminology used, it was either remote or contact forces. And then obviously if you have those forces applied, you develop the equations of motion depending on the complexity of the model, and then you just solve the equations of motion to calculate the various mechanical parameters that you need. And this is an example from the Andrews paper for three dimensions and two dimensions as well. I just want to again make the point that what we're talking about here is either contact forces at the proximal and distal joints and contact forces in between. Anywhere in between in addition to the remotely applied weight force vector. So when we're talking about inverse, what does the term inverse mean? What exactly is inverted in inverse dynamics? I started up on a six when you pulled through the clouds and then I moved in above them. Well, if you were directly above him, how could you see him? Because I was inverted. I don't know whether you're a top gun fan, like myself, but the new top gun movies coming out while he was coming out this July, hopefully that won't be delayed. So basically what we're talking about is not being upside down. What we're talking about is using the equations of motion for a simple two dimensional model here, but in the inverse sense, that is to say, we start with a movement. And if we calculate the kinematics of the movement, then we can use the equations of motion to determine the forces or the total force that cause that movement and then obviously that can give us the loading. Again, remember that the only forces that we're talking about are either contact forces at the joints and contact forces anywhere else on the free body diagram. There are also questions such as what is the loading in the ligaments, for example, during this exercise or what is the loading in the different joints. If I calculate the total joint moment, if I, if I calculate forces in a certain direction. So if I look at a very specific example that you can probably do by hand, I just want to do a very simple knee extension exercise. Just imagine that a person is just holding the lower leg in a horizontal position. So they only have to overcome the weight of the lower leg segment and the keeping the leg in this position. For this example, I'm just going to assume that the horizontal axis is aligned with a compressive or the long axis of the segment and the vertical axis, the y axis is is aligned with the shear axis of the segment. So there's the rigid body depiction. I put my reference frame system, I assume that these are the positive directions in the compressive and shear or the x and y, if you like. And then I apply the external forces and moments, as we said, there's the remote force of the weight and then the joint contact forces. We don't have a distal joint here. We assume that this is the last, the terminal segment in the kinetic chain. So I just apply two forces compressive and the shear component in the proximal joint. And these are the joint reactions, because this segment obviously has contact with the upper leg at the knee joint here. When I'm coming to draw the contact forces on the free body diagram, we basically have two options and we can follow two different approaches. You can either draw those forces as they act. And this is what Nick calls actual forces approach in the Megan heads book, which is very clear on this subject, or you can use what's called the equivalent moment and force. So instead of applying the forces as they act, you just apply a resultant moment and force at a suitable position. And normally this is is the origin or the joint point through which the axis of rotation is supposed to act. Again, the only terms that we use is contact forces and for this category of forces in the free body diagram, I need to make this choice. If I want to apply the forces as they act, then obviously I need to have that information, and we can either get this information from the literature. So for example, if I want to know how is the patella tendon acting on in this example, then I can look in the literature we have done a lot of work on this subject. So you can see here that we can measure the knee joint kinematics using x rays. And in this picture you can see that I can almost faintly make the outline of the patella tendon. So I can see how the tendon, the patella tendon is attached. So how the quadriceps force will be applied in relation, for example, to the TBL plateau. So here's the x-ray. I hope that will show play on the video. So, okay, the video is clearly doesn't play, but I'll basically explain here that with the x-ray we can actually determine the orientation of the patella tendon relative to the TBL plateau, which happens to be our shear axis in the joint. Because obviously that will be important information if we wanted to model this. So this is from the work of Demetriopoulos PhD. So if I want to go in passive conditions or in contraction conditions, obviously if I'm just holding my weight there will be somewhere in between here. And I can see that near full extension angle would be just over 120 degrees. So I can get this type of information to model the way the muscle force is acting here. To have this information I can also assume that there will be a joint moment exerted by the muscle force, although I don't have information to model the force as it acts. I can just replace it with a moment and some force that will simply change the overall compressive and shear components of the contact forces at the joint. So if I apply the equations of motion, then I can solve them to calculate those contact forces and either the moment or the force that is applied here. So here's the equations of motion for this simple example. Obviously we can only have one unknown muscle moment and one unknown force for each equation, otherwise we won't be able to solve these equations. I'll talk about this a little bit later the system will be indeterminate. But if there's only one unknown force acting, all the others are known. So I can sequentially first calculate the overall joint moment and then again sequentially go to the compressive direction and apply the equations of motion to calculate the overall reaction force in the compressive direction and do the same for the shear direction. What I want to make here is that whether I'm using a very simple two dimensional free body diagram that I can do the calculations by hand, or whether I'm using a very complex three dimensional musculoskeletal model. So these are the two processes of determining the free body diagram and all the forces acting and all the muscles. This is musculoskeletal modeling aspects that we have to go through irrespective of the complexity of the model. If I see those two examples, we can see here what is called the resultant moments approach or the actual forces approach, and you can see that the equations of motion are almost identical. And the only thing that will be changing obviously is that the number of forces that you will have in a resultant moments approach will be different to the number of forces that you will have where you apply the actual forces. So the only difference is to be different. So normally the maximum number of forces in the compressive direction will be less than the maximum number of forces in the compressive direction in this approach. So here's the detailed expanded form of these general equations. And as you can see, you can probably solve these by hand, but it's an important point here to make, and is that both approaches are mechanically equivalent. So I can examine this simple near extension example using either this set of equation or this set of equation. And the mechanical behavior will be equivalent. That is to say, the mechanical system motion will be exactly the same whether I apply this set of equations or this set of equations. And the other point to make is that the inverse dynamics output is always the forces that are applied or the unknown moment and the two reaction forces. So the equivalent of the of the reaction force in the X and Y direction, although they're normally initially drone generically to point towards the positive direction. Obviously the calculations will will give us the actual reaction forces in the way they act that is to say, if the RY calculated is negative it means that it's actually acting in the negative direction. So these calculations and again I sort of kept that simple so that we can do these calculations by hand. And as you can see, if I follow the actual forces approach, I will come up with the answer that the muscle force applied. So obviously the moment arm. Then the muscle force is 243 Newtons. And you can see that the compressive force is 225 Newtons in this example, as shown here, where's the shear force the component of the reaction force, or the joint contact force is minus which means that in this example it will be acting along the negative direction, whereas with the resultant moments approach. Again, I calculate the overall joint moment in this case, and the RX and RY in this case they have different values. So no compressive force and the shear force or the component along the Y axis or the shear direction is positive in this case, meaning that it's acting in the positive direction. So you can see that there's already some differences depending on the complexity of the model and the approach that we take in modeling the way the forces, the one category of contact forces the contact forces in between the joints are applied. The important thing here is that this information say if I look at the actual forces example. So I've drawn here roughly to scale the compressive and the shear force. And what we can see is that this information, if I think of the model of the free body diagram, it tells me that there is a force applied in this direction, and obviously an external the other segment that is attached to this segment, the ligament that can create a force in this direction, mainly, obviously there's other tissues, but it's mainly the anterior cruciate ligament. So with this information, I can start getting some assessment if you like, or some idea of what are the ligaments that are likely to be loaded in the joint. If I was following this simple example. Obviously, this will give me a completely different answer because this will probably indicate that is actually the posterior cruciate ligament that is loaded. So you can see that I can get a different answer, depending on how I model the forces and which inverse dynamics approach I'm following here. This is an excellent paper recently to present a sort of a similar more generic example that makes these points. I think the inverse dynamics. Nomenclature that is recommended. In my view, sort of reproduces some concepts that have been introduced over the years, and Andrew has done a very good job in sort of summarizing the different technology in a magnitude that is applied to classifying the different aspects, if you like, of joint forces. But as you can see here, the situation, if you look at the literature is quite complex, was if you go back to the way the free body diagram was originally described, you only had two types of forces, remote and contact forces, and just two categories of contact forces was here we start seeing that over the years, we have things such as resultant joint force, bone on bone forces, or net joint forces. And in my view, that attempt to sort of make things perhaps more complex goes in the other direction, whereas I think we should be keeping things simple, and trying to simplify the way we apply inverse dynamics. There's another very good recent paper by team and collaborators. In fact, we wanted to discuss these issues with Alberto in the ISPS conference. But again, you can see here that there are concepts about intersegmental forces, net forces, resultant forces, and so forth. So, my view is that we started in the 1960s with some sort of seminal papers by Andrews or Paul, for example, in his paper in 1966 about hip joint loading, and the only term was joint contact forces. And here we are in 2020, and we have this classification of sort of put this information from Andrews paper, but I've added their own suggestions and also the ISP recommendations. And you can see, for example, that some of these sort of appear on both sides of the table if you like. So somebody might be calling this joint reaction force, and somebody might be calling this joint reaction force. So although there was a great development of technology in biomechanics over these years, I think the terminology actually is a retrograde step. And perhaps, if you like me, you probably think that it's probably a situation with the tariff bubble than making things more clear. So I think my view is that we need to simplify this. This is not helping this classification is not helping the work that we do. And I just wanted to sort of follow some arguments. So let's say that I was applying the actual force, and then I also was able to estimate how much the antagonist force was. So using a model applying the antagonist force in this example. It is possible to do this we've done it before. Another of other groups have done it before. If you use an EMG force model during the flexion, then you should be able to apply the flexor antagonist force during the extension. So the question here is, is this calculation, the joint reaction force will be obviously different, because it'll probably reduce the shear load and increase the compressive load. But is this another category or another flavor of the joint reaction force? In my view, that's not the case. The joint reaction forces are only one set. And in the same way as we have ground reaction forces in the terminal segment, we should only be talking about these contact forces at the joint and calling them joint reaction forces. And the magnitude and direction would depend on the complexity of the model. It's not a sort of a dual system, depending on the complexity and the approach that we use. The overall joint reaction force will have a different magnitude and direction and obviously the components in the two different axes will be different, but it's one and the same joint reaction force in the context of the free body diagram. So my view is that all these terms net joint forces or resultant joint forces or intersegmental joint forces are inappropriate for differentiating joint reaction forces if we use an actual forces approach or resultant moments approach, because a net or resultant simply means that is the vector sum of something. And if you look at this diagram, what we're actually doing is that if I calculate the net of all the forces applied on this free body diagram, the net or resultant is actually applied on the next segment. So all the forces applied on the lower leg have a net force that is actually applied on the upper segment. But what happens to this upper leg segment is of no interest to me when I'm analyzing the lower leg. And what I'm interested in is the reaction of the net force that is applied on this segment by the the other segment. So what I'm calculating is actually the reaction to the net force of all the forces applied in the lower leg. So irrespective of the approach, the joint reaction forces will be net and resultant so I can use those two to differentiate. And obviously intersegmental means that is acting between or across segments. And as I explained, the reaction force is obviously applied because of the application of the net force to the other segment. So the reason that we can't, in my view, differentiate with these terms is that I said the components of the reaction force are net or resultant and intersegmental irrespective of the approach used. That is to say Rx and Ry either in this approach or that approach can be described as net resultant and intersegmental. So I shouldn't be using these terms to differentiate those two. There's also another example of supposedly the existence of the bone and bone force that is different to the joint reaction force. And in various books not only in winter. I mean this is a great book, but this sort of concept has been reproduced in another of biomechanics books to supposedly describe the existence of the bone and bone force that is different to the joint reaction force. However, in my view, this is not a rational argument. And in fact is a flawed foundational premise to support the existence of a different category of force that is called bone and bone force. What is actually being described as a joint reaction force that's different to this bone and bone and bone force is in fact a joint reaction force that was incorrectly calculated because if you say that there's a difference between a relaxed segment or when somebody is contracting the muscles, then what you're saying is that I know that there's some forces acting, but I'm not going to bother including them in the free body diagram. Well, obviously, if you know that there's forces acting and you don't include them, then the joint reaction force that you will calculate will be wrong. But that's not a reason to say that there's a bone and bone force that is different to the joint reaction force. What is actually described as a bone and bone force is the correct joint reaction force that you will calculate if you included those forces. Now obviously, that's technically might be challenging but as I said, even if you calculated the force of one of them or estimated, then the inverse dynamics will predict the other one and will calculate the correct joint reaction force. In conclusion, in inverse dynamics, free body diagrams include the actual forces either as applied or as equivalent resultant force and moments, and these lead to those two different approaches. But irrespective of the approach used, the inverse dynamics output always include the joint reaction forces representing the interactions with the adjacent segments. Although the resultant moments approach is very convenient because you can use it to model a multi-segment system so do multi-body inverse dynamics because you don't have to worry about modelling the forces in different joints. They avoid indeterminate systems because you always have only one unknown in each segment. But the joint reaction force calculated is only the partial joint contact force because it does not contain the contribution from muscles and other internal joints. As I said, it's very convenient in inverse dynamics these days. After all, if you're interested in the loading in the hip joint, there's no point calculating in detail how the muscles are acting around the ankle. You can easily use resultant moments approach in the foot and move up the kinetic chain and then just do a more detailed analysis when you arrive at the hip joint. If you're interested in the total joint reaction force, in other words, the total joint load, you have to make the effort to apply the muscle forces in particular as they act. As I explained, all the other terms that are suggested are inappropriate in my view because whether the joint reaction force is calculated from the actual forces or the resultant moments can be described by all those terms. So I just want to dispel some myths and misconceptions in my view. There are no different flavors of joint forces. There's only a single joint contact force calculated from the inverse dynamics, but obviously its magnitude and direction will depend on the complexity of the musculoskeletal model and the inverse dynamics approach that was implemented. And musculoskeletal modeling is always required in inverse dynamics, irrespective of the approach that you use and the complexity of the model that you implement. And the last point that I want to make is that there's normally an accusation that all these misconceptions and inappropriate terminology are linked to sport or sport science or sports biomechanics only, but I just want to make it clear that in my view, these terminology issues and misconception crept through other areas and not exclusive or certainly not originated in sports biomechanics. So I'll leave you with some guidelines and recommendations in my view again for using accurate terminology for inverse dynamics. You should always report the inverse dynamics approach used and whether you use an actual forces approach or a resultant moments approach. The joint reaction forces. If you use the resultant moments approach are only partial joint reaction or contact forces, you should not be using those to determine joint loading. And as I said, all these terms, in my view are inappropriate, because they can be inappropriate to distinguish joint reaction forces, because they relate to the calculated joint reaction forces irrespective of the inverse dynamics approach used to calculate them. And if you want to calculate accurately the joint loading, then you need to somehow determine the muscle forces and the way they act. So joint reaction forces from resultant moments should not be used for joint loading estimation. Again, if I go back to the way, an example from the active project. Again, although we calculate the partial joint reaction forces here through static optimization in this case because it's a very complex model we're able to calculate the muscle forces, and then through a four system analysis basically the actual way of applying the forces were able to calculate the total joint reaction force and then apply this force to the finite element models. So, if it was up to me, my suggestion for inverse dynamics is instead of having a complex table is simply to state the inverse dynamics approach that you use, and just refer to these joint reaction forces if you use the resultant as partial joint reaction forces was if you made the effort, either through simplification and just including one muscle force or through optimization and calculating the muscle, all the muscle forces acting is to refer this to total joint reaction force. Thank you very much for tuning in. Thanks, Bill. That was brilliant. And yeah, firstly, I love the top gun reference. But yeah, I think I'm massively in favor of anything that simplifies either terminology or application and I love that you brought it all together at the end of a series of really clear and concise recommendations is basically just saying, we have to state whether we're using an actual forces approach or a resultant moments approach, and we have to state whether we're using total or partial joint reaction forces. And I think when you can sum it up in a really simple and clear message, but actually have all of the kind of theory and justification behind it to support those recommendations. And yeah, I thought that was brilliant. Thank you. Thank you very much. Yeah, I think the only kind of real question for me as a follow up to that was, could you give any examples of potential problems that can occur as a result of people using the wrong terminology. There are there are several examples and again, if you look at Andrews paper, we've got to tell paper that they cite various examples where people basically in essence they use the resultant moments approach. And this is this is the typical approach that you would use in a multi segment inverse dynamics, especially with a musculoskeletal modeling software. And initially you have to calculate the joint moments, and the joint reaction forces that you calculate from, from this part of the inverse dynamics process, as I explained, are only the partial joint contact forces or the partial joint reaction forces. And you can't, you can't use that system that that calculation to start making inferences about what would be the load in the different tissues, such as the cartilage for example or the cruciate ligaments and so forth. I think I think there's numerous examples of papers, as listed in Andrews said, of applications where people are using a partially calculated joint reaction force to make inferences about the joint loading, and that is the point that we shouldn't be doing this. And obviously, if the interest is only on the joint moment, then you can stop at that stage. And because the joint moment will be accurately asked and say, you know, the joint moments was such and such and that's how the joint moment is attributed in the, in the joints of the low limbs, for example. But if you wanted to go any deeper and calculate loading in the ligaments or the cartilage, then you need to make sure that the joint reaction force, which again is the only output in terms of forces in the joint from the inverse dynamics approach that that is the force that be related to the actual loading in the various tissues around the joint. So that that that is the point. Thanks. I think, yeah, again, another set of kind of really elegantly expressed but clear and concise recommendations. I think again it comes back to that general point we always go back to of your methods stemming from your actual research question. And what are you actually trying to work out. Is it the joint moment or is it loading in cartilage or ligaments and then your methods have to apply to that. Yeah, and one of the things that for example, that is clear is that if we look at the terminal segment, we all we all understand that, you know, you have the action to the ground and then you have the reaction from the ground to the segment. And we happy to sort of just call this the ground, the ground reaction force. And, you know, there was no, you know, reason to, you know, change this and, you know, call it net net ground force or result and ground force. You know, we're all we all understand that it's the reaction to the force that we apply to the ground. And it should be the same on the other side, you know, with the proximal joint. We're looking at that segment at that point in time. And it's the reaction from the other segment that is applied on the segment we're analyzing and therefore it's a reaction force at the joint and therefore it's a joint reaction force. I think we should just stick with a joint reaction force and whether it's the partial or the total depending on the complexity of the musculoskeletal model and avoid any other misleading terms in my view that sort of are not helpful in trying to sort of discuss issues about loading in the musculoskeletal system. Yeah, perfect and clearer, more concise and more simplistic is always beneficial for everyone. Yeah, I think to have got a comment on YouTube I think sums up sums that up perfectly. So, for pronounces right Athanasios Visas says, thanks Bill that was great, in particular the clear recommendations we need as a scientific community to clarify these issues and move forward using the same conventions. I think that sums up perfectly both what you've said and what I feel Sahel is. And I think sometimes it's also important to be going to some of these, you know, older but seminal papers because I think the crystal clear crystal clear. I'm sorry my, and the terminology is unambiguous and very very clear. Yeah, agreed and if anyone is interested in any of the papers mentioned especially the more seminal articles. There are links below the video on YouTube to most of the papers mentioned in the talk, and then the others there's a full reference there as well. So hopefully encourage some more kind of wider follow up reading as well. And then, but yes on that I think all the remains really is just for me a huge thank you to Bill because I've said that was really really excellent and useful for a lot of people, and just to people watching and don't forget that all of the previous lectures are available to view on the same channel and keep an eye out for lectures in the coming weeks on tennis biomechanics and bit more a practical based guide or session from Vicon on various things relating to motion capture. Yeah, thanks again Bill. Thank you very much and thank you all for watching.