Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion

JB MorinJB Morin is a man on a mission to unravel the science and discover the practices behind whatbest makes athletes sprint faster. He has amassed an incredible team of researchers who are equally as interested in advancing speed training science and methodology. I urge athletes and coaches to read through this interview carefully, as the information is cutting edge andhighly insightful.Hi JB (follow JB on TwitterHERE), thanks for agreeing to do another interview. YourLAST INTERVIEWwas very well received in the strength coaching and track & field communities. Youve been very busy, and your BRAND NEW PAPERis getting some great attention. But lets back up a bit. Over the past several years, your lab has published some incredible research on sprint mechanics. Why should we care about sprinting forces how can it help us improve upon our training methods?Thanks Bret, its always a pleasure to share views and research conclusions with you and your followers. Initially, the labs and collaborators involved in these studies were performed by the research group at the University of Saint-Etienne on thesprint instrumented treadmill, but then we extended and basically confirmed our results in the recent paper using track embedded force plates at the French National Institute of SportsINSEP. Even though I personally moved to a new position at the University of Nice, our multi-center research group still has projects under review, and in preparation.As to your question, our approach is considering the overall mechanical output of the neuro-musculo-tendinous system and the resulting forces applied onto the ground to propel the center of mass (CoM), and in turn the entire body. So in this dynamics context, ground reaction forces (GRF) in relation to subjects body mass are the main determinants of the motion of the athletes body, and in turn of his sprint performance.This is a macroscopic approach in that we only measure and study the overall output of the system, i.e. the magnitude of resultant GRF produced, and the orientation of its application, from its horizontal, vertical and lateral components. So here is one limit of this approach in terms of training, it tells us how much force is produced and how effectively it is oriented onto the ground but it does not go into further details as to the muscles involved, their respective contributions to the total (resultant) GRF, and their timing of activation (coordination). Still, these two basic elements: magnitude of GRF and orientation of the GRF vector are very interesting from a training perspective to quantify both the muscular capabilities of the sprinter, and the effectiveness of GRF application. I usually refer to technique for the latter concept, but it is also a matter of muscle action.In the2011 paper, and then in a2012 paperinvolving top level individuals, both using the instrumented treadmill technique, and now with this force-plate 2015 study, we basically show that the sprinters who are able to produce the highest acceleration and 40-m or 100-m performance are not those who produce the highest amounts of resultant GRFduring the acceleration, but those who are able to orient their push with the highest horizontal component step after step. To make things simple, if I train and produce a resultant GRF of 100 arbitrary units per body mass, and orient this push with a high versus low effectiveness, the horizontal component of this GRF, i.e. the component that will propel my CoM forward, will be high or low, and my forward acceleration of the CoM will directly follow. So in terms of training, our studies consistently show that for the acceleration phase performance, horizontally-orienting the GRF produced is of higher importance than trying to develop a higher GRF production capability. Of course, training to both produce more forceANDorient it more forward will be the ideal, but for sure at a given same level of force output capability, the highest acceleration will be produced by the athlete able to orient his push the most horizontallyThe main training challenges now are to find out how to best train this ability. As your readers know, training science has gone into much detail as to how to increase the lower limbs total force, now the idea is to increase the athletes ability to apply this force with effectiveness. Our aim in this series of studies was to investigate what mechanical factors differ between non-specialists, good and world-class sprinters during their acceleration. Now our challenge (ours and coaches) is to investigate and test the best solutions to develop these factors through practice. Training implies time, money, sometimes pressure for performance, and risk of injury, so you want to know what to work on and how.I am excited for future research to emerge elucidating the best methods for improving the horizontal orientation of GRF production. Now I ask you to please discuss the pros and cons of your previous labs methods.Our previous methods were based on having the subjects sprint on amotorized treadmillthat accurately and instantaneously responds to the subjects motor actions. Every Newton of propulsive or braking ground reaction force was associated with an acceleration or a deceleration of the device. To see this in detail, watchthis video. The obvious limit of this device is that subjects do not sprint over ground, and a serious familiarization session is needed before undertaking any sound measurements. Furthermore, the top speed reached and the overall sprint performance is around 20% lower than what subjects could typically do on the field. However, we showed in a2011 paperthat treadmill and field performances were correlated and the fastest sprinters on the treadmill were also the fastest sprinters on the track, and thus inter-individual comparisons were possible.For the pros, first and foremost, this device allows measurement of the three components of the GRF, at the sampling rate of 1000 Hz, and for any type of duration (for instance we simulated400-m efforts for a fatigue study, or repeated sprints in another one). It is totally safe, and even highly skilled sprinters told us that the acceleration feeling was very close to what they experienced on the track. So its virtually like having a force plate under the feet for the entire sprint. In addition, this device allows athletes to perform a typical acceleration phase (from zero to high speed), which was hitherto not possible with other devices on which subjects typically had to drop themselves onto the fast rolling belt. Last, the motor torque is adjustable so we can do resisted or assisted sprints (data in process). Only 2 motorized treadmills of this kind exist in the world; one is at the Laboratory of Exercise Physiology at the University of Saint-Etienne in France, and the other one is at ASPETAR Sports Medicine Hospital in Doha, Qatar.Sounds like the pros of using the motorized treadmill for research far outweigh the cons. To your knowledge, how many published studies on sprinting forces (or impulse or power) to date examine elite sprinters?If by elite sprinters you mean sub-10 second guys, very few scientists were lucky enough to specifically and directly study such a population, and publish the results in peer-review Journals. Peter Weyand and Ken Clark didrecently,Bezodis et al., and Japanese colleagues who published a congress proceedings abstract showing the GRF traces of top Jamaican sprinters. I like to remember that Archibald V Hill, the famous physiologist, also did field experiments on top-level sprinters in the 1920s (see this fascinatingtribute paper):

My favorite sport science picture. Archibald Hill during field sprint measurements.By the way, Hill and others showed the exponential decay of sprint velocity as a function of time in papers from1927and1928(see below).Of the published papers that currently examine sprinting forces (or impulse or power) what is the major drawback of every single one of the papers?Studying sprint mechanics is a pick your poison game: either researchers like Peter Weyand or our group used instrumented force treadmill and studied treadmill sprinting rather than overground sprinting, or researchers used force plates and studied real sprinting mechanics, but over a very limited number of steps (typically 1 or 2). The treadmill modality allows you to measure GRF continuously but either at realistictopspeeds only (Weyands studies) or the entire acceleration phase but until top speeds that are lower than reality (our studies).The main drawback of the force-plate approach is that you get accurate but really narrow pictures of the sprinting mechanics, since you have data measured over one or two steps in the blocks, at 8, 12, 25 or 40-m of the sprint. It does not allow you to have a broad knowledge of the GRF production over the entire performance, and there is a risk that the data you collect do not represent the entire sprint.Well put JB. When I was in New Zealand at AUT University, I told my supervisor John Cronin that we should have built an entire 100 meter track lined with force plates embedded into the ground. I dont think it would be that hard to do, and it would have allowed for some incredible research. There currently arent any Universities to my knowledge that have anything like this. What was your solution to overcome this problem?Should money and logistics allow you to build it, this kind of track is possible, however, empowering the plates, setting filters, gains in the signals, time-synchronizing the different plates, digitizing the data and eventually getting smooth and treatable GRF data will be a huge challenge. In our recent paper, the INSEP engineer, Antoine Couturier, had a really hard time getting final data for us to process and study, and the force plate setting in Paris is only about 7-m long! In fact, Figure 1 in our recent paper (see image below) is one of the most impressive Figures Ive ever inserted in a publication, and there were many hours of work behind these data.

Vertical, horizontal, and lateral GRF traces over time and the corresponding positions of the force-plate system.So it might take a little while until longer installations than the currently existing 10-m to 15-m force plate systems are available for human performance research. But at the time of Hills research on sprint running in the 1920s they used magnets and coils along the track, so we can definitely anticipate that such instrumented tracks will exist in the future.In our most recent study, we virtually re-constructed a complete 40-m by having athletes perform several 10, 15, 20, 30 and 40-m sprints, and placing the starting blocks at various distances from the force plates, so that we collected data for almost all the steps of a 40-m sprint in each of the subjects. Because these highly skilled sprinters have very reproducible kinetic and kinematic patterns, we assume that, should this have been possible, the data collected over a continuous 40-m measurement would not have been different from those we present here.Below is a schematic representation of the protocol design:

Schematic representation of the protocol, by Pierre Samozino.This is a great step up from previous research! Does this mean its perfect?Perfection does not exist in science, and we always struggle to limit drawbacks and biases, and use the least worst solution, method, design, device, etc. Sometimes it is frustrating, but my view is that when nothing better exists, you must do with whats available to you, and just be conscious of the limits floating above your results and conclusions. Refutability and debate is the basis of the scientific game, and what is accepted as true now will be adjusted/refuted/confirmed in the future. You have to be humble and accept the inherent limitations of the current methods, and try to put sound knowledge forward. Otherwise, you just stay at your desk waiting for perfection. This is for sure easier as there is no risk of having to say, we were wrong, but it is unproductive and hopeless.Amen JB! Lets talk about what we learned from this latest research. Please discuss the relationship with stride rate and stride frequency throughout the entire 40 meter sprint.Our results show that while step length increases regularly over the acceleration phase (from the blocks push-off to the 40-m line), step frequency is almost instantaneously leveled at the maximal value athletes can reach: 80% at the first step and about 90% after the third step, and then remained constant thereafter. This is basically what Sofie Debaere et al. also reported in2013: high-level sprinters develop in the 0-to-10-m phase a step frequency higher than 95% of the step frequency they reach at maximal speed. As a consequence, the speed increase of high-level athletes is afterwards quasi-exclusively caused by the increase in step length.Fascinating! Do faster sprinters produce more total force into the ground in the blocks and during the 40 meter sprint, or is vertical or horizontal force more important? What about the ratio of forces?In the blocks, elite sprinters produce much more (about 25% more) horizontal force and power than less skilled sprinters.When averaging data from all the steps of the 40-m sprints after the blocks push-off, we observe that faster sprinters produce 10% higher horizontal force (Effect Size of 1.75). Contrastingly, they produce less vertical force (2% on average, Effect Size of 0.59). Note that our comparison is based on the data of 4 elite sprinters (best 100-m times ranging from 9.95 to 10.29) and 5 sub-elite sprinters (10.4 to 10.6 s). Sorry we could not recruit more athletes of the same level to perform more detailed stats than just % difference and Effect Size analyses.Ratio of force data (the ratio of force in our approach is the ratio, for each contact phase, of the horizontal component to the resultant GRF) show that elite athletes are more efficient both in the blocks (+13%, ES of 1.46) and on average over the 40-m (+10%, ES of 2.31). In other words, since elite sprinters produce around 10% more horizontal force and around 2% less vertical force than their sub-elite counterparts, their ratio of horizontal to total force ends up being around 13% greater.Does this validate your previous findings using the sprint treadmill?Overall, yes, clearly. The horizontal force production during the acceleration is significantly and highly related to field sprint performance, whereas the vertical and resultant GRF production are not. This is better expressed through the paramount importance of the ratio of force, and in summary, the fastest sprinters at the end of the acceleration phase are not those who produce the highest amounts of total force with their lower limbs, but those who orient their push the most effectively, and in turn produce the highest horizontal force. In both our treadmill and overground studies, acceleration and 40-m or 100-m performances are significantly and highly correlated to the average horizontal force produced per unit body mass, and not to the average vertical force produced. Note that the vertical force produced per unit of body mass was only related to the value of top speed reached, as previously observed byWeyand et al.Our new study supports the fact that the importance of horizontal versus vertical force production to sprint performance basically depends on what phase of the sprint you are interested in: whole acceleration versus top-speed phase only. However, even in top speed sprinting, horizontal force is critical.If we look at Figure 1 from the study (shown above), we see that throughout the 40 meter sprint, vertical force rises and horizontal propulsive force diminishes. Many track & field specialists infer this to mean that vertical force is more important than horizontal force. Why is this outlook unacceptable?Yes, this is true if you look at the force peak only, but what is really important is to look at impulses (i.e. force-time integral). The vertical impulse increases and the horizontal net impulse decrease over the acceleration, respectively because the CoM height increases from the starting-blocks to the classical running position and because as you know (muscles force-velocity relationship), our bodies cant help producing lower and lower amounts of force as moving velocity increases.We recently submitted a study focusing specifically on impulses in which the vertical impulse per unit body mass is not correlated to 40-m performance, whereas horizontal net impulse is. So definitely, to accelerate in the early phase of a sprint (typically a 20 or a 40-m sprint), what is a key determinant is the amount of net horizontal, not vertical impulse you produce. So although speed and vertical impulse increase concomitantly over the acceleration, the amount of speed produced (i.e. sprint performance) is not correlated to the amount of vertical impulse produced.That said, our data (treadmill and track) clearly and consistently show that only the average vertical force producedAT TOP SPEEDis well correlated to the amount of top speed reached, and not to the amount of acceleration produced beforehand. So maybe here is the main confusion made, you should not extrapolate what is important for top speed runningonlyto what is important for acceleration. Our paper, currently in review, shows that vertical impulse tends to be negatively related to 40-m acceleration performance in very high-level sprinters (when its influence on performance is tested independently from that of the horizontal impulse, by means or multiple regression analyses).Many coaches (such as myself) seem more interested in maximum speed sprint mechanics. Why should we be more interested in acceleration sprint mechanics for sports?To me this is paradoxical, if you look at the descriptive time-motion data published in various sports, you see that in fact, only in track and field 100 and 200-m is top speed running a key factor. In all other sports where sprinting is a major physical ability (soccer, football, rugby, etc.), only much shorter distances represent most of the demands of the game, and very rarely do players actually reach their individual top speed. A simple reason for this is that in order to run at your top speed, you need an acceleration of 30 to 50-m depending on your level, in a straight line, with no opposition (tackle, change of direction, etc.). This almost never happens, and when it does, it occurs in situations that are not key situations for a defensive or offensive action. Take a rugby player (except maybe in Rugby 7s) who is able to slalom between defenders and sprint towards the try line, the decisive action will be his ability to accelerate and change direction over the first 5-20-m, afterwards, his top speed (once again, reached after 30 to 50-m) might not be a determinant anymore. Last, studies show that insoccerorrugby, most sprints and decisive sprints are shorter than 20 or even 10-m, so no top speed is involved here just top acceleration capability.So to me, if you list all sports involving sprinting and sub-list only those involving pure top speed (reached after a complete acceleration) versus those involving maximal acceleration over distances too short to allow players to reach top speed, there might be only 100-m and 200-m events concerned versus a plethora of other sports. Not to mention that if you think, from a training perspective, about the number of athletes concerned in the world (i.e. 100/200-m sprinters versus all the other sports), then top speed capability might be considered as a really minor capability compared to shorter distances acceleration capability. Take the famous NFL combine 40-yard dash. It is clearly an acceleration test. Even ifin fine, a high acceleration on the field will result in a high running speed, the typical constraints of the rugby, soccer, and football codes do not often allow players enough time and space to reach their absolute maximal speed. From the tenths of sprint acceleration files weve processed in elite athletes and soccer and rugby (all codes) players, I think 90% of all players reach their top speed between 30 and 40-m. However, we observed that in young players (below 18), top speed is reached before 30-m. So these might be exceptions to the aforementioned general observation. Some authors observed very short distances to reach top speed incollege football players, during specific short sprint tests, but we may wonder whether this top speed could have been actually higher in a longer sprint acceleration context. This is the difference between the absolute top speed of an athlete or a team sport player and his top speed in the context of the shorter-than-necessary distances that the demands of his sport allow (see discussionhere). So when talking about top speed in sports, we should definitely clarify between absolute top speed and top speed in a specific sport context.Last, even though some acceleration sprints are flying start sprints, the durations still rarely allow players to reach their absolute top speeds. Nota: for numerous and detailed references in all the above-mentioned sports, look for time-motion analysis or physical demands of these sports in PubMed.I see that you were also able to calculate horizontal power overground. How did you pull this off? Previously we needed a specialized treadmill in order to enable us to do this?Yes, one of our recent aims was to propose and validate a field method to get horizontal net force, and mechanical power output in sprinting from either time-distance (photocells split times) or time-speed (radar or laser) measurements. The idea was to allow many more coaches, scientists and athletes to assess and monitor the mechanical power, force-velocity profile and even effectiveness of force application (ratio of forces), and use these variables in their practice (training load, follow-ups, effects of fatigue, etc.).The theoretical basis of this simple method, first authored byPierre Samozino, is that when humans (whatever their level) accelerate from null to maximal speed, the change in speed over time superbly follows an exponential decay. So we fit the speed-time curve with this simple exponential function, and then derivate the speed-time curve to get the acceleration-time curve. Then, using the laws of motion in the forward direction and estimating the air friction on the runner from body mass, height, wind speed and air temperature and pressure, we obtain the net horizontal force output, and when multiplying by speed (since power equals force times velocity), we obtain the mechanical power output produced in the horizontal direction. The validation was made by comparing our computations to the force plate data weve mentioned earlier, and basically, it works very well. The validation results werefirst presented in an abstractat the International Society of Biomechanics in Brazil in 2013, and the full paper is currently under review. Many coaches in various sports currently use this approach, in collaboration with us, and weve already published a couple of studies aboutsoccer players acceleration versus top speed performance, hamstring injuries andhow injured players differ in their mechanical output from non-injured players, at the return to sport and after 2 months of full practice, and acomparison between elite players from different rugby codes.See below for instance the computations we did of Usain Bolts force-velocity profile and power-velocity relationships over his World Record 100-m sprint. The cool thing is that our computations are based on the published 10-m splits of the World Record, but should force plates have been installed under the track in Berlin, our validation results show that we would have obtained values within 2-3% of what Bolt exactly producedFor a detailed explanation, see thisvideoof a workshop I did at Auckland University of Technology SPRINZ institute in 2013 (second part, sprint force-velocity-power profile).

Force-Velocity-Power Profile of Usain Bolts World Record PerformanceThis is incredible! Thanks JB. Moving on, horizontal power was the most highly correlated marker of maximum sprint speed and 40 meter performance in your study. Do we know how to best increase horizontal power in training? What are your ideas?I think that for historical reasons (use of weights and weightlifting for S&C) and for obvious mechanical reasons (gravity and loads apply a vertical constraint), it has been overall easier to work on vertical than horizontal power development. And if I agree that there might be a transfer between gains in vertical power through training and gains in horizontal power, I am convinced it is the case in lower level and heterogeneous populations, but not above a certain level. In collaboration with researchers Matt Brughelli (see an interview with MattHERE) and Pedro Jimenez-Reyes, we recently collected a ton of data in various sports and for levels ranging from leisure to world-class, and our first results (paper writing in process) show that jumping power in elite rugby players for instance is not correlated at all to sprint power as we measure it with our method.

In pro rugby players, sprint power and 30-m sprint time are highly correlated (bottom left), but not sprint FV profile and jump FV profile (top left), not sprint max power and jump max power (top right), and not jump max power and 30-m sprint time (bottom right)I like to illustrate this by the toothpaste tube theory: when the toothpaste tube is full (low skill level), no matter how you press the tube (no matter how you train) youll get some toothpaste (performance improvements). So any kind of force training might result in sprint performance gains. But after a certain skill level, when the toothpaste is almost empty (very narrow margins for improvement), youll have to press the tube with very specific and well-designed gestures. In a sprint acceleration training context, this might relate to the horizontal force training, when vertical force does not transfer to sprint performance anymore.So I really think there is an interest in specifically targeting horizontal force and power development to train for sprint acceleration especially with highly skilled athletes. One of the best ways is probably resisted sprinting using sled and additional loads, or elastic bands, or uphill sprinting. As to sled training, some studies recently confirmed the higher effectiveness of high loads compared to low loads.Kawamori showedthat heavy load sled training resulted in a higher sprint acceleration performance, not because more resultant force was applied after training, but becauseheavy loads led subjects to push with more effectivenessand apply similar amounts of resultant GRF, but with a more horizontal orientation (and thus higher horizontal component of the GRF and in turn forward acceleration). So the key is in coaches hands, I think they should systematically think about finding exercises and training modalities that stimulate the forward push, in addition to or instead of the vertical push, whenever possible.I concur! Thanks again JB, one last question. Do you have any other interesting research in the works?Yep, our main current project is a one-season follow-up of tenths of well-trained athletes, soccer, rugby players force-velocity and power profiles in sprinting, using our simple radar method. Our focus is to seek whether power output and force-velocity profile in sprinting (or an unexpected change in these variables) is related to a higher risk of injury, and could be an objective variable to focus on in the prevention, rehabilitation and return to sport process. Other projects are in the works about the role of hip extensors in the horizontal GRF production, the so-called transfer between vertical and horizontal power (now that we can assess the latter precisely and specifically in sprinting), and the best training methods to develop this horizontal power output in many sports such as athletics, rugby or soccer. So I guess well have new and exciting things to share for the next interview.