 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. I'm Stuart McCurley Naylor from the University of Suffolk, but much more importantly, I'm joined today by Christoph Kipp, who is an associate professor at Marquette University in the USA. And I know Christoph's had some really cool studies recently that I've enjoyed reading on the topic of weightlifting biomechanics. And for anyone who's had a look through his Twitter feed, there's a lot of kind of really cool graphics and data visualization around weightlifting that sort of a really thought provoking for me. So I was really keen to hear some more thoughts and really happy that Christoph agreed to come and share some thoughts with us today. But yeah, I'm aware weightlifting biomechanics is a very broad title or broad topic. So I'll hand over to Christoph and then he can tell us a little bit more. Thank you. Alrighty, thank you Stu. Thanks for the introduction and thanks everybody for joining us or for those people might be watching a little bit later on. As Stu said, I'll be talking about weightlifting biomechanics today and I gave the title of selected thoughts because I've taught an entire class on this topic before. And so I've just curated a couple of things that I thought might be interesting for people to listen to. And another disclaimer that I typically give if you pick up a little bit of an accent, I was born and raised in Germany, so German is my first language, and I actually learned how to speak English in Canada after my family immigrated to British Columbia in the mid 90s. So if I sound a little funny, it's because it's a mixture of those two. And sometimes when I get excited, I tend to ramble on a little bit. And I have a couple of beers, which probably isn't the case right now because it's 10am. Although, as Stu kind of talked to me about earlier, it's pretty much five o'clock everywhere these days. So today we'll talk a little bit about the sport of weightlifting and I have a couple of slides here and videos just to kind of make a distinction that we're talking about weightlifting as in the snatch and a clean and jerk. Primarily in relation to sport spa mechanics and not necessarily related to strength and conditioning or resistance training exercises, such as everybody else's favorite bench press and bicep curls. So here are a couple of videos just to kind of introduce these these exercises here first we have the we have the clean. Slow motion video of a lifter lifting the barbell from the floor, big triple extension explosive extension onto the shoulders, and then standing up. The second part of this lift is called the jerk that I won't be focusing much on today. And then we also have the snatch where the lifter moves the barbell from the platform to the overhead position and one motion. So, and my name so manual is very happy here. Probably one of the greatest sports achievements of all time in my opinion, the only time that anybody snatched two and a half times their body weight. So I will be pretty happy as well. So from a performance perspective or from a perspective of studying weightlifting by mechanics. It's pretty nice because we have clear performance criteria. We can directly, we have a direct measurement of how much the person is lifting during the exercise. So we can quantify that with with the bar mass. And then also the technique is pretty interesting that it's not just, you know, load up the bar with heavy weights stand up with a dip underneath of it and they're actually some very interesting technical aspects that kind of manifest in this technique in this very stereotypical technique that we observe. So with respect to the weightlifting research that that we do and that most people have focused on from the bar mechanical side there are some focus on the bar mechanics. So we're tracking kind of the movement of the barbell through space. And then also the forces, the forces that are interacting between the lifter and the ground and the lifter lifter and barbell system in the ground. And then also joint mechanics over either looking at the kinematics of different body angles body segments and the kinetics. So just to show you some of the research projects that we focused on or focus on either competition analysis or doing some work with some maximal loads in the effort to kind of look at different warm up techniques for weightlifting. Here we have a picture of kind of our original bar mechanics lab setup, where I still can't believe that my original undergraduate supervisor let us construct this weightlifting platform in the lab around these force plates with very expensive cameras. Right all around weightlifting not very happy as Mike Conroy or weightlifting coaches looking on and saying do it again harder faster so. So with respect to some of the variables that we actually look at and weightlifting bomb mechanics. The first one is bar path. So here I have kind of stick figure drawn in to kind of show you the direction that the lifter is facing in order to provide a little bit more information or context for how the bar moves. So here we have a bar path with an initial rearward displacement during kind of the initial pole phase, followed by a secondary forward movement that is around this what we call the vertical reference line. And this initial backward forward movement that crosses the reference line is typically what we refer to as an a type trajectory. If the lifting exhibits a barbell trajectory produces a barbell trajectory that doesn't follow or that doesn't cross the vertical reference line. We typically call this a V type trajectory. And also C type trajectories were sort of an initial rearward displacement available initial forward displacement was kind of back and forth lift and this will be classified as C type trajectory. And typically what coaches seem to seem to think and what research seems to indicate that type A and B are considered okay type trajectories if we observe these type C trajectories are somewhat considered as efficient as you can imagine too much back and forth movement may not may not be good. I'll come back to that in a little bit. So you can also then move on to some from the barbell trajectory can calculate the acceleration and the velocity profiles of the of the barbell. This is illustrated here at the blue line right from lift off increases pretty steadily and then drops to zero as the barbell reaches its maximum peak. And we also, if we had the same time kind of like the acceleration thought the acceleration we have pretty a pretty steady acceleration profile that only really drops negative once the barbell has obtained its maximum velocity and as the lifters preparing to catch the barbell. I just included a another figure here where the trajectory is slightly different I just wanted to show this to illustrate that there are some individual differences. What's interesting to notice here right is that instead of the steady increase in velocity to a to the overall peak the global peak. The initial increase followed by a slight decrease in velocity. And so this is associated with a negative acceleration phase. And so there might be some applications or implications for whether this denotes a breaking phase and how efficient this is. We've been playing a little bit of how to communicate some of this data a little bit better, especially taking barbell velocity profile or acceleration profiles and providing that back in a format or relating that back to a format of bar path. And I tweeted some of these pictures a while ago and I realized that it probably would be good to also show some of these velocity acceleration profiles so people have a little bit more of an idea of what these bar, what these line graphs look like. With respect to the ground reaction forces, we've measured ground reaction forces during weightlifting exercises. And of course if we're just measuring the ground reaction forces we're looking at the force that acts on the barbell lifter system as a as a unit. So the ground reaction force that's that's applied to the lifter here is obviously related to two parts of acceleration of the barbell, as well as acceleration of the center of the lifter center of mass. And if you can look at the time series data of what this looks like we observe that there's a typically this biphasic profile where we have an initial, this initial peak where the lifter is producing a ground reaction force that exceeds the system weight. A little bit of a decrease and followed by my second and larger peak. And initially this has been kind of discussed by Inoka is that as far back as the 70s that we do observe this this biphasic profile. And in some cases we can even see a decrease below the system, the system weight, whether the ground reaction force falls below that system weight. And so the terminology that we use to kind of denote these phases of weightlifting movements. Here we have from the first phase is called the weighing, the weighing phase, unweighing phase and then the weighing to phase. As Inoka showed in that same papers that we actually have, or that the ground reaction force follows the barbell profile pretty in a pretty similar manner. And so, again, it seems that the ground reaction force does gives us a pretty good picture of what is going on overall with the system as well as the barbell. Let's dive in a little bit deeper, look a little bit more detail of what's going on at the level of the joints. Here we have hip angle plotted against time of what I call percentage of lift. And really, it's not the entire lift that we're plotting that we're looking at here we're looking really more at we refer to as the pole phase. The focus primarily on this phase because after this, we don't really capture ground reaction forces anymore. And so we lose the ability to do to say to apply our favorite hammer and biomechanics which is inverse dynamics. But if we look at the kinematics of the, the joints during this we still see some very interesting interesting patterns. Not much for the hip angle but we look at the angle we started in a flex position here right we pretty much extend until about 60% of the pole phase, maybe a slight depth for some of the lifters. And then we go on to final extension phase. The barring kind of shows the ensemble average of this is either five or six lifters and then the thin gray lines kind of shows individual variations of the, of the lifters. So the hip, we kind of see this primarily extension movement. The knee angle things become a little bit more, a little bit more interesting sort of speak, where if we look at the knee angle, we can start off in an initially flexed position during the starting, during the starting position. We then move towards extension. The initial extension phase is followed by a really market rebending reflexing of the knee is then followed again by an extension. So we have this extension flexion extension pattern that we observe at the knee and we'll come back and talk about this in much more detail later. The ankle angle we see in this extension flexion extension pattern. Not quite as as market is at the knee joint but we definitely see this triphasic triphasic pattern as well. And so another way that we often look at movement phases during the weightlifting pole. We're doing this, doing this main pulling phase is that we denote the first extension phase as quote unquote the first pole. The rebending of the, of the knee, the knee joint reflexing. We call the transition phase or sometimes also the quote unquote second knee bend, which is then followed by the second pole that final extension of the knee joint. Another way to to look at this from maybe a little bit more complex coordination coordination perspective if we look at ankle angle. Angle angle diagrams, we have trunk angle on the horizontal, the angle on the vertical. The lift starts down here. We look at the coordination between these two joints. We see that the initial phases characterized by the initial extension phase at the knee. Where we don't observe much change in the trunk angle. So there isn't a large excursion on the horizontal x axis. Once we once the barbell reaches just about the, the point of the, the knee, right about the patella, we see that rebending of the knee and followed by the final extension phase. We then shift gears and look at the joint mechanics, kind of focus on the kinetic side a little bit. Here we have the hip joint torque or the hip net joint moment across the pole phase. Either if we say the entire pole phase or the first pole transition and second pole phase. We characterize this when we say what do what do we see what do we observe we observe pretty much a steady steady hip extension moment throughout most of the motion. Then progressively goes to zero towards the end when the lifter loses contact with the barbell and drops underneath the bar to catch the bar either the shoulders for the clean or overhead in the snatch. We have the net joint moment at the hip. And we notice that the patterning of the joint moments is a little bit more, a little bit more interesting than what we observe at the hip, where we have, we start off with an initial extension extension moment. Now, see that towards the middle of the exercise, towards the middle of the movement, we actually observe a knee flexion moment is then following in by any extension moment so it seems to match to some extent what we see at the knee joint. In terms of the kinematics where we observe that extension flexion extension pattern. There is that if we're looking at net joint moments right we can also talk about the implications of involvement of the extensor as well as the flexor muscles. And then if we look at the ankle joint moment profile again it seems to have this back and forth extension, maybe coming to almost a flexion moment and then extension pattern again so similar to the knee joint but just not as market of a variation from one to the other. So that's for the planter flexor muscles. So where things get interesting in terms of looking at movement phases movement control or really putting right putting things together from a biomechanical perspective is as if we consider the kinematics and kinetics together. The graph does knee joint angular velocity. So we have, again, wherever this dark line is positive the knee joint is extending right the magnitude just gives us the velocity with which it's extending. Here, if we have a negative knee joint velocity of joint is flexing magnitude because it's how fast joint is flexing. And then we have that extension during the during the second Paul and forgot to have this color coded so we have our little extension face here knee joint flexion followed by the final extension phase. I'm going to try to match this up now with what is going on at the joint moment level. So I've graphed the knee joint moment down here and extension. It's positive in the sense that flip this from convention in order to show this a little bit better. But if we can take any positive net joint moment to be an extensor moment that would tell us that the knee joint that the knee extensor muscles are producing an extension moment that's in that extensor moment. And also the knee flexors are creating a net flexion moment. And then we have that extensor moment towards the end. And so, again, we have three phases but it's kind of interesting that if we multiply these things together knee joint angle velocity knee joint moment, we look at knee joint power we come up with a curve that doesn't look like any of these. And the phases here if I try to line up the blue and yellow boxes, right things actually look kind of complicated. And I think this is where I start losing a lot of my undergrad by mechanics students I've tried to talk through this very slowly and deliberately. And I've realized this step by step. Right. So, again, positive joint power would say that the, we're producing or generating power, we're doing positive work at that joint, negative power, we're absorbing power. And I've labeled on top here what is going on with the kinematics. So I have again an extension motion that matches up with the phases that I have for my angle of velocity, knee flexion motion, and then again the extension motion. And down here, I've kind of added the labels of what is going on with the kinetics within that joint moments at the time so we walk through this right we have these these light blue phases where light blue, light yellow light blue which seem to match extension flexion phases over here but then I have these weird between shaded shaded areas. So, if I start with the initial extension motion, right we see I have an extension of extension motion of an extension net joint moment. So my net, my knee joint extensors are generating power, until I get to this part where you can see that I'm still extending. But now I have this flexion moment. So it's actually the knee flexors that are slowing down the extension, and are therefore creating this absorption of power at the joint. So this is characteristic of an eccentric muscle action or an eccentric action at the joint. The net joint moment continues, continues to exist for a little while longer. Actually, then turn it stays, it stays present during the movement until the joint or as long as the joint brings the joint into flexion. Right, so this again what I have up here so the joint start to flex under the flexion moment. So, here I come back to now the joint is flexing under flexion moment so it's my knee joint flexors are actually generating. Positive power right so they're creating that flexion that that rebending of the knee joints that we see going during that transition phase from first poll to the transition phase and then if we go into the second poll. The new extensor moment starts, or we start producing a new extensor moment before the knee actually is extending so while the needs so flexing I have any extensor moment. So I have a little bit of energy that is absorbed at the knee joint again indicative of an eccentric eccentric action at the joint that then precedes the final extension phase. And this is often what we talked about when we, when we mentioned the double knee bend, one of the stretch shortening cycle action at the knee during weightlifting exercises, right is where as the knees flexing during that double double knee we have knee joint flexion, but as I'm as I'm flexing actually creating this knee joint extension moment. So, we can also look at hip joint powers and ankle joint powers we can calculate those for for the other joints. And so something that's kind of interesting from a control perspective right. If you stayed with me as I explain as I tried to explain what is going on at the knee joint seems fairly complicated but things get easier at the hip joint and at the ankle joint because I don't have these oscillations between positive power and negative power positive power negative power. So where I have five cycles back and forth for the knee joint I only have four for the hip and three for the ankle. Let's us talk about the movement phases during them during the poll in a slightly different way than what's traditionally been been used right so originally when people use primarily force plates and kinematic analyses. So, which is in one of these classic articles by hack in 1984. We're looking at ground reaction forces. Throughout a clean, we're looking at knee joint angle. And we're trying to look at, we're trying to make inferences about muscle action such as concentric, eccentric concentric movements based on based on the kinematics and what we observe overall the ground reaction forces. And look at, we take some of this data from what we get from the joint kinetics, we can get a little bit more detailed information where the solid black line here are my ground reaction forces, the dotted line of the net joint powers. This is for the for the knee joint hip joint and ankle joint over here. I just want to point out here is that, as I have that unweighing and the ground reaction force that is where I temporally observe my peak knee flexion power that seems to coincide with that unweighing phase. Okay, so I was kind of a brief overview of some of the mechanics of weightlifting. I want to try to talk a little bit about some more research. And some, again, more thoughts that I have on where we could maybe have research go in the future. So if we look at bar mechanics, right, oftentimes we're talking about efficiency, how efficient is it left there. And the classic definition for weightlifting efficiency came from Garhammer back in the 80s, we're just a ratio of vertical work to total work that's done on the barbells we're just looking at. Overall, what is what is being done with the barbell and if we look at some data that he presented, one of the seminal papers in 93. We see that these efficiency ratios for different lifts definitely run the on the gamut right from 90 90% all the way to 100% for different lifters different exercises different loads. However, something that always bug me a little bit with this classification or this definition is that the way to get perfect efficiency is that we would just eliminate all horizontal barbell motion. And I just don't know how likely that is if you talk to coaches, for example, or if we look at what is what we see in the literature, like here's data from the 1985 World Championships was presented by Bowman and general journal of applied by mechanics. So if we look at weightlifting barbell trajectories across a lot of different weight classes. There is horizontal motion. There's an inevitable joint horizontal sorry there is an inevitable horizontal barbell motion that barbell motion seems to be the horizontal horizontal barbell motion seems to be less, depending on what group of lifters you're in so a group lifters are better lifters versus be group lifters with kind of the lower class lifters in the study, but we still have some horizontal barbell movement. So, one way that I often think about efficiency and weightlifting or bar mechanics is that it's kind of a way that was presented by veteran doctor in an article 1999. So I really come to this idea that you know the task goal and weightlifting is to move the heaviest bars possible from the floor into this catch position into that support on the shoulders on the overhead position. And in order to do that I need to pull a bar certain height right I need to pull a certain height so that I have enough time to drop under it and support the barbell. And one way that we often talk about this or that I think about this is that I try to lift the barbell to a certain height with a certain velocity. Maybe we can treat the barbell as a quote unquote projectile at that point, and I've used questions like this for undergraduate by mechanics exams because they're fun projectile motion type of questions and they have some application. And really an interesting question is whether or not the velocity at this point at this height actually lets you move the barbell or would produce this travel range that we that we observe or that we need in order to have the time to drop underneath. And when you actually do the math, you find out very quickly that no, the velocity that you have it's not enough. Right so typically the velocity that lifters produce at this point will only produce a quote unquote projectile drift or motion up to a certain height, and they actually have to do some what what these authors called rest work on the bar, some active travel so they have to continue to continue to apply force during this point so it's not quite as simple as projectile motion. So it is that this relationship for this idea pretty much seems to hold across different weight categories so it seems like it would be something that could be universally applied. And so I think back in terms of efficiency, the way that veteran Deutsche presented this is that right, really a way to think about efficiency is that we want to try to come up with. We want to try to maximize the rest work right so we want to try to. We want to try to have a high travel of the bar by itself. We want to have in optimal V max, so vertical maximal velocity. Instead of having an excessive vertical velocity right so, and, in other words to think about this is very inefficient way is to have very high velocities that would lift the bar very high and then with them crashed down on the bar. So one way to think about this is that really it should be a ratio right that I want to try to minimize the velocity that's needed in order to lift the bar bell into that, into that height overhead. So some things that they that the authors proposed is that there should be, there should be a minimum velocity that's needed to move the bar into the overhead position in the snatch or to the shoulders in the clean. One part of training should really be around trying to establish a critical minimum velocity that you're trying to train towards anything above that velocity. Above that critical velocity will be inefficient wasted energy wasted effort however you want to quantify that. It's the one way to think about this and very simple by mechanics terms in terms of kinetic energy. So if I have my equation here for kinetic energy, but a better lifter, whatever kinetic energy they could muster up they would use to lift greater barbell mass. There's mediocre or worse lifters are probably favoring the velocity of their over accelerating the bar producing higher velocities, or they're not optimizing that vertical optimizing vertical velocity. And this kind of also brings me to something that I think about sometimes as a problem with high power outputs right power is pretty ubiquitous and sports by mechanics and people argue about the definition. We should use it if we should use it at all certainly for vertical jumping I think it's falling out of favor. And so, in a way I think that power has probably been an over focused part of what we do and weightlifting biomechanics especially barbell power in relation to weightlifting performance for weight lifters. So this is very easy to artificially increase power outputs by just increasing the velocity rather than lifting heavier weights. So, I did some back of the napkin calculations, while on quarantine. So, since I'm not able to get into my lab. So, my favorite lifters from back back in the day. Sorry. So it was cool about him, but there was actually a series of studies that was published a series of observations by Husky 1997, where he actually put data on a lot of these parameters that we would need for these sort of efficiency calculations right how high this barbell travel what's the height of velocity. What's the height of the barbell at its maximum velocity, what's the drop distance, etc. And so if you look at this performance from the 1994 World Championships was 172 five. So if we use that data from his guess analysis of plug it into the equations that better enjoy to have. You would actually be able to predict that demos would have been able to lift a max of 187.75 kilograms which indicates a reserve of almost almost 10 kilograms almost 20 pounds of weight that he could have been could have been lifting in what he, what he did. And so if we then calculate an efficiency ratio brings us to almost 95%. And so it's interesting that if you look at this overall personal best snatch that came in 1999 actually information performance of 180.5 kilograms. We assume that he used very similar kinematics might plug it into those equations and we will get an efficiency of 99.3%. So, you know, we go from pretty good to to great. And this is something that I think of might be might be a little bit more useful and rephrasing efficiency and performance prediction monitoring assessment of Olympic weight lifters going forward. We were kind of velocity and acceleration profiles and looking more at the time series data. We looked at some of this quite a few years ago. Again, we were interested in looking at acceleration profiles and relationship to weightlifting performance and performance being a measure of somebody's the barbell mass that they lift with respect to their actual body mass. So, here we have the acceleration in the black line, the barbell, the acceleration on the barbell during the pole. And in order to kind of look at the profiles or the patternings we use their principal components analysis where we try to look at patterns of variation in the barbell and try to correlate that to actual performance. One of the patterns that we observed seem to capture this difference between a very steady acceleration profile or very steady acceleration versus an acceleration that was characterized by a decrease in acceleration actually negative acceleration during the transition phase and a increase during the during the second pole phase. So when we ran the correlation analysis we found that better lifters. So those were able to lift heavier weights followed what we observed with these positive symbols rather than these negative symbols in terms of their acceleration profile. So we do some very simple mechanical analysis right if we have weight lifters that only able to produce a finite amount of force, a better lifter, right will lift a heavier mass with a smaller acceleration, whereas a worse lifter or worse technical lifter, right, perhaps lift a smaller mass and use more that force potential to put into acceleration. And side note if we try to tie this into kinematic patterns in this 1999 article. These two authors actually looked at certain movement characteristics that were associated with different acceleration profiles. And one thing that they found is that the acceleration profile that we see in the middle here where we have a pretty much steady acceleration throughout the lift is almost characterized with a kinematic movement pattern during the first pole where the torso angle stays fairly constant. So this is often something that you hear weightlifting coaches talk about hips and shoulders rise at the same rate during the first pole. And this is an indication of something that coaches talk about right from a body posture movement perspective that actually holds over and is related to the acceleration profile. Here we looked at ground reaction force patterns in relationship to again weightlifting performance. We did a similar pattern analysis where we used principal components analysis to extract different patterns and ground reaction forces and correlated those two weightlifting performance. And something that we noted is that better lifters. So those that lifted higher weights with respect to their their body mass actually exhibited smaller decreases in the ground reaction forces during the transition phase than worse lifters or worse lifters again here in the negative. Negative lines versus the positive lines. And better lifters also had a what seemed that kind of a temporal shift in the ground reaction force pattern where we have a quicker transition phase. So this phase from going to the end of the pole first pole to the beginning of the second pole was done with a shorter percentage was part of this as a proportion of the total pole and with less loss and ground reaction forces during that phase. We've also looked at I'll try to apply this same type of pattern analysis to to joint motion so joint kinematic data. Here we're looking at hip angle where positive is our hip flexion angle, or we start in a flex position, and then move towards an extension throughout the throughout the pole. And again, something that we notice is that our better lifters follow the pattern that's denoted by the positive symbols here whereas worse lifters follow the pattern that shown by the negative symbols here. So our better lifters essentially had what we said a more steady hip angle or torso angle during the first pole and less of a rebending reflexing of the torso during the during the transition phase. Right so that again that first first pole was pretty much done with a very steady increase in torso angle. Change in hip flexion angle primarily driven by changes in the thigh segment, not the torso segment. And then, before we go into final extension during the towards the end of the pole. With respect to joint kinetics, the one thing that we've noted is that, and if we apply the same analysis, our same pattern analysis to the net joint moment. So here we're looking at my knee extension flexion extension mode extension patterns in the in the in the moment. We've noticed that our better lifters seem to follow this, this positive pattern or this pattern of positive symbols, where they are producing larger knee extension moments during the during the second pole. Something that I think is worth talking about briefly is that if we just talk about magnitudes of joint kinetics, because I get a question about this sometimes that if we look at the kinetics, the knee joint, either knee joint torques knee joint moments during weightlifting, especially at the knee joint they don't tend to be that large. So here, this is data from Garhammer 1976 so this is a person that I believe this is 142 kilograms snatch. One of the middle weight lifters almost a 300 pounds snatch where the person is only producing and knee joint moment of what hundred new meters, which is not again, but extremely large. It's kind of interesting to consider, but what else might be might be driving driving performance on the kinetic side. So some interesting data that was presented. I'm bringing them back in the 80s by by Bowman as an analysis of the 1985 World Championships that compare data from two lifters during competition so this is crossed versus since on us. Both in the super heavyweight category during this competition. Being the world champion when in the gold medal with a snatch of 202 kilograms and since on us being in the big group with a snatch of 160 kilograms. We see that crossed up is able to produce or was able to produce greater ground reaction forces to lift greater greater weight. It was interesting that if we look at the ground, if we look at the knee extension moment during these lifts we actually see that craft stuff produced a much much smaller. More than the extension moment during the final phase then craft stuff. Something that bombing all moment at all commented on is that it's perhaps related to the fact that craft stuff was able to manipulate the external moment arm in a much more efficient way about the knee joint, then since on us and so we see some external influence or some other influences of controlling the moment arm controlling the effective mechanical advantage during the during the exercise from that same paper also saw that look at joint moments across different weight categories and across different lifts. The best correlation that we have right between increases in joint moments and body mass or body weight plus barbell weight was the was actually the hip joint moment and not the knee joint moments. So this is kind of this is led us to kind of consider how do we how do we get at some of this with kind of future studies. And this is something that we're considering right now, one of my graduate students is working on this at the moment where we're looking at net joint moments, as well as muscle forces we're trying to use musculoskeleton modeling to get muscle forces during different exercises. So I will just say this is during the squat so it's not a weightlifting exercise but we're trying to apply this in the future hopefully to weightlifting exercises where my hip extension moment, my knee extension moment. And the gray area is the joint moment, and then all these individual lines are the joint moments or the muscle moments created by the individual muscles the forces they're producing multiplied by the moment arm of those muscles. And how they're contributing to these joint moments. And so we're doing that for the for the hip and for the knee. So we have this for 0% load. And then we have this across different, again, different loads and so something that's kind of interesting to consider right is even though we have an increase in joint moment, so that gray area increases right so we have hip extension moment increases as we load more weight during a squat. So we have, can these increases and how much does, for example, the group max contribute to the hip extension moment. Right, that seems to increase. We also have hamstring muscle contribution to the hip extension moment, it's the dash dash line here. All right so that changes interesting thing about the hamstrings right hamstrings extend the hip, but they create a knee flexion moment at the knee flexion moment at the knee joint. So that's what we observe down here so at the knee joint in the gray area we have the joint moment that is calculated as a sum of all the individual muscle moments that are produced by the muscles individually. So here for example we have the bassist muscles joint moment that's created by the muscles and we see that that's larger than what we have as the net joint moment, because we're having to offset the hamstring effects of the hamstring muscles. So again just something to consider that even though the net joint moments might not be that large the muscle forces might actually be in themselves fairly large. And something else that we've thought about a lot or that we're thinking about a lot is this idea of absolute versus relative net joint moments and this idea of strength versus capacity is that oftentimes we just focus on absolute joint moments. But we don't really know what percentage of your overall capacity of this represents. So we're not able to perhaps indicate what are the bottlenecks for performance. Or we often term this what is the operating capacity of each muscle group during the exercise. So one way that we're kind of looking at this is looking at relative muscle effort where we're taking the ratio between the net joint moments that we measure dynamically versus the net joint moments from a maximum contraction. One of the natural things to do for weightlifting as we use the isometric met dipole, which is kind of shown here yet we have a rack where we can position the bar weight lifters will do polls at different positions, and we can kind of get an idea of how much net joint moments are people able to produce them during these different positions. And then relating that to the dynamic moments that we measure throughout the motion. So quite problem with that is a one site caveat is that we know that the net joint moment depends not only on the joint angle but also joint angular velocity. So we're trying to use some musculoskeletal modeling, we're using surface regressions, trying to predict what is the maximum joint moment that a person is capable of producing, given the combination of their joint angle joint angle angular velocity, and then we're trying to map that out against the dynamic net joint moments that we're actually measuring. So here in the red line. For example, I've calculated the max net joint moment at the knee joint, given the surface regression equations. And then we plot that against the dynamic net joint moment we look at the knee joint right there is a there's a difference here so that's good so. The knee joint moment is not reaching its max capacity at the ankle joint, however, we do see that the green line seems to exceed the predicted capacity at the net joint, or the predicted capacity that the moment that the ankles. The knee joint reflexors should be able to produce. So, one other issue, right that comes up a lot when we talk about net joint moments, and some of these modeling techniques is influence of two joint muscles issues of co activation etc. And I'll just say, we're trying to get around this by using some more modeling. So this is computed muscle control, where we're again predicting individual muscle forces, calculating our net joint moments, or the actual net joint moment that we observe during an exercise during a task. But then we also take that same motion, we use some more musculoskeletal modeling where instead of where we're artificially setting the activation of the muscle muscle to 100% essentially. So saying giving them all given the motion that we observe, we dial up activation to 100%. We're accounting for muscles ability to produce force based on its active length component passive length and its force velocity properties. And so we get, again, this net joint moment max, that should represent the maximum capacity at the joint. I'm using that then to calculate this relative muscular effort as a ratio between the two. Okay, coming to an end, I just wanted to end on on the abstract form, probably one of the earliest articles on weightlifting that I've been able to find and probably one of the best titles ever the defeat of gravity and weightlifting. So, here it goes, that weightlifting as the art of defeating gravity may seem a statement of the obvious, does very rewarding Harvard as an all battles to consider the nature of the adversary. It's relentless nature means that man can only win for limited time. As we shall see the earlier he engages the enemy, the more spectacular as a short term advantage. The long drawn out battle is to be avoided at all costs. Man, on the other hand is apt to injure himself when he tries to mobilize his forces too rapidly. Like the battle commander and the weightlifters presented with a dilemma and the information available about his own strength and resources is inadequate to eliminate the possibility of disaster. We must be thankful that at least the fall gravity is an entirely predictable one. These are fairly or these words are fairly accurate summary of this paper and hopefully of this presentation so So I'm going to end with thanks to my bond mechanics law group are standing conditioning staff collaborator pretty many over the years, but especially to Michelle civic who gave me first start my, my bond mechanics career. And by com for sponsoring this and stew for making me give this talk on short term notice, but I enjoyed putting this material together. So thank you. Brilliant. Thanks Christoph. Yeah, that was amazing. I think I didn't want to put too much pressure on but I had high hopes for that and it was even better than I was expecting and I love the ending as well that and grieve abstract. So I've got a new idol in terms of writing style. And we don't write like that anymore at least not after axle. Yeah, unfortunately not. Yeah I think just while we wait for the live chat on YouTube to catch up if anyone's got any questions just leave them there. And if you'd look on the screen at what's coming up over the next couple of months really. If you want to keep updated. Apparently I'm supposed to tell you to subscribe and click on the little bell thing and you'll get a notification. I've got a lot of questions already. So apologies in advance for terrible pronunciation as always, but from a superior shower, or that's how I'm going to say it and I'm sticking with it. And says the first one, basically what percentage of one rep max. Should we use for testing the snatch. If we want reliable results in a study. It depends. I mean, not to be facetious but I guess it depends on what you're trying to write what the purpose of the study is if you're trying to look at maximal competition performance you probably need to use loads right that are close to 100% of maximum. Weightlifting research is difficult because you can't ask people to do too many lifts at 90% right we can just say here's a mean of 10 repetitions that somebody did at 90 to 95%. So in a way I think it's the nature of the beast that you're stuck with a few repetitions. And if Priya has more questions feel free to email me I would be happy to chat about this more. But again it's with with competition again in competition you only have three attempts right and those attempts are typically at either different loads, or you have a failed attempt in there and so it just makes it really difficult to to talk about this issue of reliability which maybe we conveniently ignore. Yeah I think the question looking again and doing my job properly and was specifically around studying lift trajectory, which I guess is still a, it depends and even trajectory mine depend on the actual load as well. Yeah, yeah and I think it's, it's interesting because I didn't have that slide in there, because I'm already I feel like I went over time. We're starting to look at trajectories a little bit and the variability and trajectory that we that we observe from trial to trial. And we're trying to parse that out between right meaningful variability versus just noise. And so it's an interesting subject because I think it gets a 30 of right. What variability, right, is there variability in this in the barbell trajectory and does that actually matter to some extent right. What do coaches try to fix. If there's variability that's not related to a performance outcome then. Right. Let that variability run wild I say so. Yeah, I think it leads to the whole functional variability. Yeah, and the second question is around the horizontal displacements I know early on in the talk you had the kind of vertical reference lines and then saying an early viewer of efficiency was essentially any horizontal movement or force was inefficient. So the question is, do you think there should be a specific or a specific recommended range that's maybe acceptable or unacceptable, or should it be as close to vertical as possible, or how would you, I guess, give some recommendations around that. Yeah, so I mean in the, in some of the original, again, I had another side and some of the original papers, I think Garhama has a range, right, so there's a range for backward displacement during the first poll should be between three to five centimeters maybe right forward displacement should be between right so there are ranges. And of course, right, I'm not. It's probably not a linear relationship either. There's probably a non linear or a curvilinear relationship where you need some horizontal movement to properly do the lift. To be able to turn the barbell over to produce that horizontal acceleration to counter to actually turn the barbell over. So the bar going all over the place in the horizontal direction, obviously no so. I think short answer yes there are ranges. And maybe we should look at those with a right not just with a not with a linear lens but maybe a curvilinear type of analysis, rather than linear regressions. Um, yeah I think I've got a couple of questions I could ask well potentially save those for another time I think if, if anyone wants to find out more about any of your research or just yeah any of the ideas you've got is there a best way of even getting in touch or checking that out. Um, yeah either Twitter or email. My most of my research articles on research gates and if somebody can find it quickly and they will send me an email about be happy to be happy to share the stuff that we have. Brilliant and the link to Christoph's research gate pages or it should be below the video in the description as well. So that should happen I know when I looked at it quite a lot for them were open access as well so you should be able to access those on there. So yes brilliant thanks ever so much again Christoph and thanks everyone for listening and hopefully see you again next week.