• helenbayne

Getting Up To Speed

For many, the 100-metres in athletics is the pinnacle of the Olympic Games. I was in the stands for the 2016 men’s final in Rio to watch Usain Bolt claim his third consecutive title and that evening is going to be hard to beat as my number one live sports experience (also because Akani Simbine ran a 5th place, and Wayde van Niekerk broke the 400-m world record in the same session!). Since 2013, I have worked with sprint programmes at the TuksAthletics Club, the hub of South African speed on the track, and this has helped to shape the way I think about analysing performance. As athletes prepare to race in Tokyo, here is a brief look at some of the biomechanical aspects of top level sprinting.


Sports performance can be viewed on different levels – I describe these very simply as the “what, why and how”. The “what” is a description of what the athlete did (e.g. they ran 100 m in 10 seconds). The “why” refers to mechanical determinants that explain the performance – the application of the laws of physics that enables the athlete to move from the starting blocks to the finish line. The “how” relates to the actions of the athlete that enable them to produce these mechanical parameters, and these are the trainable elements that would be targeted by the coach.


What?

Thanks to measurements taken during a number of major competitions, we know precisely what a world class 100-m performance looks like. Below is a graph showing the velocity profile of a 100-m race based on the average of 50 male athletes (black line) and 50 female athletes (grey line) measured during the final of eight World Championships and two Olympics between 1987 and 2012. The dashed line shows the data from Bolt’s world record-setting run of 9.58 s in 2009. This dataset shows us that athletes who finish with better times (men vs women and Bolt vs the rest of the men) reach a higher top speed (“Vmax”, where the vertical line is) later in the race. In other words, they continue to accelerate for longer even while running very fast.


Velocity over a 100-m race [Slawinksi, J., et al. 2015]

Why?

The science of sprinting has been the topic of a huge amount of research. In the interest of keeping this Olympic-themed series to short and easily digestible reads, I will limit the rest of this post to the biomechanics of initial acceleration, which was also the primary focus of our analysis and intervention when I started working with sprint coaches.


The sharp rise in the velocity curve at the start in the figure above indicates the high acceleration seen in the first couple of seconds of the race. Essentially, the sprinter’s main objective as the gun goes off is to accelerate their body forwards and to increase their speed with each subsequent step. One of the most widely recognised principles of physics, Newton’s second law of motion, applies here: the acceleration of an object is directly proportional to the magnitude of the net force, occurs in the direction of the net force, and is inversely proportional to the object’s mass. We can write this mathematically as: Force (F) = mass (m) x acceleration (a). Because the athlete’s body mass doesn’t change during a sprint, we can simply say that the larger the net force applied (meaning the sum total of all forces acting in all directions on the athlete’s centre of mass), the greater their acceleration will be in that direction.


During the ground contact phase of each step, the athlete transmits forces produced by their muscles to the ground via the foot, and the ground reaction force is the “equal and opposite” force exerted by the earth on the athlete that will cause their acceleration (or deceleration).

Force is a vector – it has a magnitude and direction. When we analyse force, we normally split it up into its vertical and horizontal components (there is also a mediolateral or “side-to-side” component, but it is much smaller than the vertical or horizontal forces in sprinting).


During acceleration, the vertical component needs to be large enough to overcome gravity and gradually raise the athlete’s centre of mass with each step until they are in a fully upright running posture. The horizontal component is the key to good acceleration because it’s aligned with the overall direction they want to travel. In fact, non-sprinters may apply the same amount, if not more, total force to the ground as elite 100-m athletes but the differentiating factor is that the proportion of horizontal force is higher in top sprinters. In other words, being able to generate a lot of force is not going to help your acceleration unless it’s applied in the right direction.


How?

The technical work that athletes do on their starts is all aimed at optimising the direction of this ground reaction force and there are a couple of key visual markers that coaches look for. Firstly, there is a relationship between the projection angle at the end of each step and the orientation of the force vector – the lower the angle, the greater the proportion of horizontal force. But, there is a thin line between a low angle than maximises horizontal force and one that might lead an athlete to stumble. Secondly, landing with the foot behind the centre of mass (a negative touchdown distance) is preferred over a positive touchdown distance (foot ahead of the centre of mass) because the latter would apply the brakes more during contact and would lead to reduced net horizontal force.


The period of time where the athlete is in contact with the ground is obviously crucial, because this is when the ground reaction force is produced. However, the way that the athlete moves while they are in flight sets up the position that they’ll be in and how their limbs will be moving when the foot lands. During these initial steps, the flight phase lasts less than a tenth of a second, and in this time the athlete has to rapidly switch their limbs, so that the lead leg is actively driving back into the ground on contact and the trail leg is underneath the athlete and swinging forwards. The amount of time spent in the air versus on the ground can be manipulated – a fraction of a second longer in the air might be needed to get into a good position at contact, but too long in the air takes away from the time where the athlete is applying the force needed to accelerate. It’s a fine balance.


The sprint events in Tokyo are lined up for drama and intrigue! One of the coaches I work with, Hennie Kriel, once said to me that “the whole world pauses to watch the Olympic 100-m final”. I will be pausing with him and the other coaches at the Games to see who the new champion will be in the men’s race and whether Shelly-Ann Fraser-Pryce can return to the top of the podium. I’ll also have a close eye on the Tuks athletes (Gift Leotlela, Clarence Munyai and Akani Simbine) in their individual events and when they combine in the 4 x 100 m for South Africa.


References

Bezodis NE, Trewartha G, Salo AI. Understanding the effect of touchdown distance and ankle joint kinematics on sprint acceleration performance through computer simulation. Sports Biomech. 2015 Jun;14(2):232-45. doi: 10.1080/14763141.2015.1052748.

Kugler F, Janshen L. Body position determines propulsive forces in accelerated running. J Biomech. 2010 Jan 19;43(2):343-8. doi: 10.1016/j.jbiomech.2009.07.041.

Morin JB, Edouard P, Samozino P. Technical ability of force application as a determinant factor of sprint performance. Med Sci Sports Exerc. 2011 Sep;43(9):1680-8. doi: 10.1249/MSS.0b013e318216ea37.

Slawinski J, et al. How 100-m event analyses improve our understanding of world-class men's and women's sprint performance. Scand J Med Sci Sports. 2017 Jan;27(1):45-54. doi: 10.1111/sms.12627.

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