Sprint Research, Biomechanics, and Practical Implications – An Interview With Matt Brughelli

Okay readers, I’m going to try to pack this interview with as much content as possible, so we’ll dive right into things! Matt Brughelli is a PhD researcher who studies biomechanics, strength training, and sport training. He’s a heck of a smart guy. He recently published a study that examined the effects of running velocity on running kinetics and kinematics. His findings have created quite a stir in the strength & conditioning and track & field worlds. Here we go!

1. Matt, what in the hell is going on? Did sprint researchers and track & field coaches have it all wrong? Your study shows that as running speed increases, vertical oscillation of center of mass decreases and horizontal forces increases at a faster rate than vertical forces. This indicates that horizontal force production is probably more important than vertical force production as speed increases. Doesn’t this fly in the face of the famous Weyand study? Did Weyand do something wrong? What gives?

First I would like to say that everything I’ve written in this interview is my opinion alone. I do not speak for my co-authors or anyone else involved in this study.

Hi Bret, lots of great questions here. I’d like to start with the relationship between vertical ground reaction forces (GRF’s) and maximum running velocity. Then I’ll give my take on Weyand et al. 2000, and will address the questions on horizontal force and “did they have it all wrong” in questions 5 and 7.

I think there is now overwhelming evidence that maximum running velocity is not limited by vertical GRF’s. With the addition of my recent study, there are now three studies that have directly investigated the effects of running velocity on vertical GRF’s over a range of velocities up to maximum (Brughelli et al. 2010; Kuitunen et al. 2002; Nummela et al. 2007). Each study used an athletic population and reported that vertical GRF’s (i.e. peak and average GRF’s) remained constant after reaching ~70% maximum velocity. This is direct evidence against the claim that maximum running velocity is limited by vertical GRF’s.

In addition, take a look at Table 1 in Chang and Kram, 2006. Vertical GRF’s were measured over different running curvatures (i.e. similar to a 200m sprint). Here, maximum running velocities were decreased due to the curvature. But vertical GRF’s did not significantly decrease until running velocities dropped below 60% maximum (i.e. inside leg only). This would also suggest that maximum running velocity is not limited with vertical GRF’s.

In regards to the famous Peter Weyand study (Weyand et al. 2000), I actually like this study very much. I have a lot of respect for Professor Weyand and consider him an expert on sprint mechanics. However, I think they (i.e. Weyand and colleagues) made very strong conclusions based on the quality of their methodology. They had a heterogeneous group of subjects (i.e. 24 men and 9 women; recreationally active; ages 18 to 36; no sprinting background) run at maximum velocities on a motorized treadmill that could measure vertical GRF’s. Then they performed linear regressions between maximum running velocities and ground support forces (i.e. average vertical force during the contact phase, relative to body mass). It should be noted that correlations and linear regressions do not imply “cause and effect”. As far as methodological quality, they rank relatively low.

I am in complete agreement with Karl Zelik (Buckley et al. 2010) that correlations and linear regressions should not be used as a surrogate for fundamental mechanical understanding of speed limitations. Instead of making such strong conclusions, I think Weyand and colleagues should have embraced the shortcomings and limitations of their study in order to motivate further research.

One more study I wanted to mention. Peter Weyand has published a new study in the Journal of Applied Physiology (Weyand et al. 2010) on the same topic. In this study, forward hopping and backward running were compared with maximum running velocity. Weyand and colleagues concluded that maximum running velocity is NOT limited by vertical ground support forces. Instead they propose that maximum running velocity is limited by the time required to produce ground support forces, which they argue is more due to muscle contractile kinetics.

One last point. Look at Figure 3 in Weyand et al. 2010. There are 6 subjects running over a range of velocities. With five of the six subjects (E was the exception, and the slowest runner), vertical ground support forces remain constant above ~7.0 m/s. This is very similar to the literature I have presented above.

Despite all of the above, Weyand et al. 2000 did find that faster runners produce significantly greater ground support forces in comparison with slower runners. So vertical GRF’s most likely do have some minor role in maximum running velocity. My only argument here is that maximum running velocity is not LIMITED by vertical GRF’s or ground support forces.

2. Matt, I’m going to be devil’s advocate here and attempt to cast serious doubt on your research. Please defend yourself. First, your study used a non-motorized Woodway treadmill which required sprinters to exert more horizontal force than regular overground sprinting since the belt slows down due to friction.

I don’t think friction is a problem with the Woodway treadmill. It’s possible. I don’t know of any studies comparing non-motorized treadmills for friction. According to the manufacturer, the Woodway uses a low friction bearing system that uses two bearing rails. Thus the decks do not need to be flipped like a conventional treadmill. I think if horizontal forces were increased (in comparison with overground sprinting) it would more likely be due to the tether as you mention in question 3.

I’d also like to point out that non-motorized treadmills have been shown to be valid in comparison with overground running for maximum running velocity. They have also been shown to have similar running kinetics or kinematics to overground running, and have excellent reliability. In a side note, Weyand et al. 2000 used a “motorized” force treadmill. Motorized force treadmills have been shown to alter running kinematics compared with overground running (McKenna et al. 2007)

You might wonder why researchers use treadmills at all. Why not use force plates mounted in the ground? Well its not easy to use force plates with subjects running at maximum velocity. And you only get a single step with a single force plate (if you are lucky) for each maximum running effort. With force treadmills you can get as many steps as you want. This is a HUGE advantage for researchers. Also, as Weyand et al. 2000 pointed out you do not need to deal with air resistance with treadmills. Faster runners would have greater air resistance in comparison with slower runners. There are always limitations, even when doing research with overground sprinting.

3. Second, you used a cable tether that exerted a rearward pull and therefore required more horizontal force production in comparison to regular overground sprinting.

As mentioned in question 2, it is possible that non-motorized treadmills create greater horizontal forces in comparison with overground running. However, if this was the case then you would expect two things to occur: 1) the studies on overground running would report less horizontal force in comparison with the studies using non-motorized treadmills; and, 2) the percentage of horizontal to vertical force production would be different between overground and non-motorized treadmill studies. This is clearly not the case. The studies using non-motorized treadmills have reported very similar, or lower, values for horizontal force in comparison with overground running (~400 N) at maximum velocity. And the percentage of horizontal to vertical forces is also very similar (~20%). Thus it is not likely that the tether is increasing horizontal force production during sprinting.

4. Third, net horizontal force at constant velocities are zero. I guess now you’re suggesting that Sir Isaac Newton was wrong?

Not at all. As I’ve mentioned in the previous questions, several researchers have used non-motorized force treadmills during running. My study is not novel in this sense, and none of us are breaking any of Newton’s laws.

It’s true that net horizontal forces are zero at constant velocities. This does not mean they are insignificant. Most studies only report peak propulsive forces and not braking forces. Most non-motorized force treadmills do not measure horizontal GRF’s. The horizontal forces are measured from a load cell that is attached to the tether. So braking forces are not measured. I was measuring vertical GRF’s from the force plate (i.e. four strain gauges) under the belt, and horizontal forces from the load cell attached to the tether. Again, this is nothing crazy or novel. Researchers have been using these machines since at least 1984 (Lakomy et al. 1984).

5. Forth, don’t you think you’re making some pretty ballsy claims for a single study!

Actually, I think my conclusions were very conservative. Most conclusions use terminology such as “these findings suggest that” or “we conclude that”. Lets take a look at my conclusion. First I said “it would seem”. This is not ballsy. Then I said “may be more dependent”. “Maybe” is not a term of great ballsiness. I did not say that horizontal forces limit maximum running velocity. In fact, I did not make any claim about any variable limiting maximum running velocity. I simply said that sprinting ability might be more dependent on horizontal forces in comparison with vertical forces. Again, “in comparison with vertical forces”. This is definably not a strong conclusion. It is not likely that vertical GRF’s have a major influence on maximum running velocity. So I feel these conclusions were conservative, but yet appropriate.

My conclusions were based on my own findings, and my understanding of the previous literature. They were not based on a single study. Both Nummela et al. 2007 and Kuitunen et al. 2002 also reported that horizontal forces significantly increase up to maximum running velocities. In addition, Nummela et al. 2007 and Brughelli et al. 2010 reported significant correlations between maximum running velocity and horizontal forces (r = 0.66 and r = 0.47), but not vertical. So my interpretation of these findings is that horizontal forces may be more important than vertical GRF’s for maximum running velocity.

For comparison take a look at the conclusions in Weyand et al. 2000, then look at the conclusions in Weyand et al. 2010. You tell me who makes ballsy claims.

I’d also like to say that I am not the first to make this conclusion about horizontal vs. vertical forces. Nummela et al. 2007 made a stronger conclusion that I did, as well as Randell et al. 2010. In addition, a few researchers have contacted me about my conclusion, and have mentioned that it supports their recent findings. So you will be seeing more papers in the next few years discussing horizontal vs. vertical force.

6. Moving along, based on your expertise, do we know what limits maximal speed production? What are some of the possibilities?

No one currently knows what limits maximum running velocity. I agree more with Weyand et al. 2010 that it could be due to the “time” side of the curve as opposed to the “force” side. Giovanni Cavagna has done some very interesting work in this area as well. I think that the time available to produce high forces is very important. In addition, I think horizontal force production is very important. So maybe some combination of the two.

I have a few ideas for more research on the limitations on maximum running velocity. However, after that I will probably never visit the topic again. I think constant velocity running is not very practical for most athletes. I think we are all missing the boat on this one. Why is there so much time and effort spend on this topic? How often does a soccer player, or rugby or basketball player run at a constant velocity? We need to go in other directions if we want to progress the field.

7. In light of this research, are you wondering if traditional methods of strength & conditioning might “leave something on the table” in terms of maximum acceleration and speed?

This is a very important question. I do think that traditional strength and conditioning (S&C) can improve speed and acceleration. However, I do NOT think the improvements are due to greater vertical GRF’s during running. I think Weyand et al. 2010 has made a very strong case for this point. In their new study they have reported that runners apply sub-maximal forces (i.e. vertical GRF’s and extensor muscle forces) during maximum velocity running. So I doubt if you improve an athletes squat, he/she will produce greater vertical or extensor muscle forces during running. I would like to see someone do a correlation between squat strength and vertical GRF’s during maximum velocity running. I doubt there would be any relationship.

Thus I feel that traditional S&C improves performance through other adaptations than vertical force production during maximum velocity running. I think these adaptations could include favorable changes in rate of force development, being able to maintain high force levels for longer periods, inter and/or intra-muscular coordination, etc.

I think coaches should be excited by the recent findings about vertical forces, and be open to implementing additional training methods for improving speed, acceleration and overall athletic performance. I think coaches should start implementing more horizontal strength and power exercises, hip extension/hyper-extension exercises, proper eccentric exercises, and continue to implement single-leg exercises.

Yuri Verkhoshansky introduced several examples of complex training in the famous Steven Fleck article (Fleck, 1986). Many of these examples complexed traditional vertical exercises with horizontal exercises. It’s an easy way to implement horizontal strength and power exercises into S&C programs. And now we have more exercises to choose from. Why not complex a squat with a horizontal weighted jump, or a squat jump with 30m sprints, or your hip thrust with 10m falling accelerations. The variations are endless. And this training is much more fun for the athletes. AND periodization becomes much more fun for the coach with complex training.

So in conclusion, I do not think traditional S&C exercises should be thrown out. I think they should be complexed with: horizontal strength and power exercises; hip extensor/hyper-extensor exercises; single-leg exercises, etc. I think eccentric exercise should also be considered for improving sprint and acceleration. Especially exercises that eccentrically contract the hamstrings during hip flexion, and eccentrically contract the hip flexors during hip extension/hyper-extension. But that’s another topic for another day.

8. What are some things that you’re excited about in strength & conditioning as well as biomechanics research? Where future research do you believe will positively impact athletic development?

There are a few areas in S&C and biomechanics that are wide open for research. And very practical areas as well. I think acceleration, deceleration and change of direction are very exciting areas. With these movements, the braking and propulsive forces are not equally balanced. Thus the muscle-tendons units act very differently during constant velocity “bouncing” gaits and acceleration/deceleration/COD. I also think a lot of great biomechanics research can be done on horizontal movements, and developing methods of improving horizontal force and power. Eccentric exercise is another very interesting area of research. Not just for muscle injury prevention but also for athletic performance. General injury prevention and analyzing leg asymmetries and deficits is also very interesting. Biomechanics of movement deficiencies (i.e. individual or sport/movement-related deficiencies) is another interesting area. So many different areas to still explore!

9. Matt, talk about the barbell hip thrust, how it differs from the squat, and how it might lead to increases in acceleration sprinting more so than maximal speed.

I love this exercise, and the other variations as well. With the hip thrust, the hip extensors are trained all the way through the end ranges of hip extension and hip hyper-extension. The moment arm of the glutes increases with hip extension/hyper-extension. Thus we need to find ways of training the glutes as the hip extends and hyper-extends. I think it is possible that the hip thrusts could improve hip strength throughout the end ranges of hip extension and into hip hyper-extension.

To me, this is the biggest difference with the squat. With the squat, the greatest benefits occur during the bottom position as the muscles are at longer lengths and switch from eccentric to concentric contractions. At the bottom position, the hips are behind the load increasing the moment arm from the vertical position. As you ascend from the bottom position and the hips extend, this moment arm decreases. Thus the hip extensors are not trained throughout the end ranges of hip extension and hyper-extension with the squat. Bands may help a little with this, but not much due to the position on the hips in relation to the load during the end ranges of hip extension.

Thus I think the hip thrusts would have a much greater effect on horizontal movements (i.e. in comparison with squats). I also think the hip thrusts would have a greater effect on acceleration than constant velocity sprinting. During acceleration, the braking forces are greatly reduced and propulsive forces are increased. The hip extensor muscles are required to produce power during acceleration. So it is important to find methods of improving muscle power of the hip extensors during athletic movements. I think the hip thrusts might be able to accomplish this, along with horizontal strength and power exercises.

10. Thank you very much for the interview! Last question. What does the future hold for Matt Brughelli?

Thanks for the opportunity Bret! The future is going to hold a whole lot more research. I will begin a post-doctorate research position in Belgium in a few months. I have several colleagues in Europe and am very excited about the coming years. There’s still so much to learn and I hope to be successful as a researcher in the future.

References

Brughelli et al. 2010. Effects of running velocity on running kinetics and kinematics. Journal of Strength and Conditioning Research, (Epub).

Buckley et al. 2010. Comments on Point: Counterpoint: Artificial limbs do/do not make artificially fast running speeds possible. J Appl Physiol 108: 1016-1018

Chang & Kram, 2007. Limitations to maximum running speed on flat curves. J Exp Biol 210: 971–982.

Fleck, 1986. Complex Training. NSCA Journal. 8(5): 66-68

Kuitunen et al. 2002. Knee and angle joint stiffness in sprint runners. Med Sci Sports Exerc 34: 166–173.

Lakomy, 1984. An ergometer for measuring the power generated during sprinting. J Physiol 33: 354.

McKenna et al. 2007. A comparison of sprinting kinematics on two types of treadmill and over-ground. Scand J Med Sci Sports 17: 649–655.

Nummela et al. 2007. Factors related to top running speed and economy. Int J Sports Med 28: 655–661

Weyand et al. 2000. Faster top running speeds are achieved with greater ground forces not more rapid leg movements. J Appl Physiol 89: 1991–1999

Weyand et al. 2010. The biological limits to running speed are imposed from the ground up. J Appl Physiol 108: 950 – 961.