In discussions surrounding sprint performance, coaches and researchers have debated whether vertical force or horizontal force is more important for running fast.
While many will pick a side and claim one is superior to the other, this debate presents a false dichotomy. In reality, the ability to produce horizontal and vertical force are both crucial for sprinting performance.
This article will dive into the research on the subject, using my experience as a sprint coach and competitive sprinter to explore these concepts in an understandable manner.
What This Article Will Cover:
- Vertical Force Production In Sprinting
- Flawed Reasoning Regarding Vertical Force Production
- Horizontal Force Production In Sprinting
- How Forces Vary In Acceleration & At Maximal Velocity
Much of the information here is derived from studies from researchers such as JB Morin, Pierre Samozino, Rya Nagahara, Ralph Mann, and others. All of the conclusions we draw here are due to their hard work, so make sure to check out any linked studies or articles and follow them for future developments in the world of sprinting research.
The Forces In Sprinting - Key Takeaways
Sprinting performance results from one's ability to apply large amounts of force to the ground, in short periods of time, in the proper direction.
- Vertical force production is required to support the athlete's body weight throughout the sprint and is correlated strongly with maximal velocity sprinting performance when applied to the ground in a short time.
- Acceleration performance is more dependent on orienting forces horizontally rather than producing the largest ground reaction forces and total forces, regardless of orientation.
- Maintaining a greater ratio of force (percentage of force which is directed horizontally) strongly correlates with acceleration and 100-meter dash performance.
- Vertical force demands must be satisfied for an athlete to apply horizontal propulsive forces.
- Elite sprinters maintain a higher ratio of force at higher velocities than sub-elite sprinters.
- Sub-elite sprinters often exhibit higher total force production than elite sprinters at high velocities, but their ability to direct force in the proper direction is below that of elite sprinters.
Important Terms For Discussing Force Production In Sprinting
To understand what we will be discussing today, there are some terms we should be familiar with:
- Ground Reaction Forces (GRF): Ground reaction forces are the forces exerted by the ground on the body of the athlete, which result from the athlete striking the ground.
- Resultant Force: Describes the total force that produces an acceleration of an object when combining forces acting in multiple vectors or directions.
- Ratio Of Force (RF): Horizontal GRF relative to Resultant GRF, or the proportion of resultant force which is directed horizontally.
- Decrease In Ratio Of Force (Drf): Drf is the linear decrease in the ratio of force which all athletes exhibit as running velocities increase. This describes the reduction in the proportion of resultant forces which are directed horizontally as athletes accelerate.
- Vertical Force: Force that is applied to the ground by the athlete and returned to the athlete as GRF, which is directed vertically.
- Horizontal Force: Force that is applied to the ground by the athlete and returned to the athlete as a GRF, which is directed in a horizontal or forward direction.
- Total Force: The total amount of force produced in each stride regardless of the direction in which force is applied.
To sprint fast, athletes produce large ground reaction forces in order to overcome gravity and air resistance to lift the athlete off of the ground and propel them forward through the air.
The athlete who can produce the largest forces in the shortest amount of time and direct these forces in the most appropriate direction will run the fastest.
Vertical Force Production In Sprinting
It is common knowledge that gravity is the most prominent force an athlete has to overcome in sprinting. Because of this, vertical force production is higher than horizontal force throughout most of the sprint. Even when standing on the ground with zero horizontal velocity, your body will experience a GRF equivalent to your body mass.
As the athlete lands on the ground while sprinting, they must be able to stop the body's downward movement and project themselves back up into the air into the next stride. This requires ample vertical force production, a high eccentric rate of force development, and the ability to contract muscles rapidly at the right time.
To do this for up to 50 strides in a 100-meter dash sprint, the athlete's peak vertical force production abilities must significantly exceed the average vertical force requirements of any single stride. Sprinters need to be very strong in the positions in which they land during sprinting so they can handle the impacts in short periods of time.
High Vertical Force Production Is Correlated With Top Speed Sprinting Performance
As an athlete reaches maximal velocity, vertical force production will be very high, and net horizontal force production will approach zero. As maximal velocity is achieved, vertical force production can be as high as ten times the horizontal force produced at that point in the sprint.
Maximal velocity describes the point at which horizontal velocity is no longer increasing. During this phase of the sprint, braking and horizontal propulsive forces equal one another, leading to a net horizontal force of zero once top speed has been reached.
As braking forces overcome one's ability to produce horizontal propulsive forces, or as the athlete's vertical force production fails to satisfy the demands of sprinting, the athlete will decelerate.
Peter Weyand found that high top speeds seen in elite sprinters are a result of applying larger vertical forces to the ground at top speed, not by them moving their legs more quickly.
Further research by Ryu Nagahara shows that increases in sprinting speed are associated with higher levels of vertical and ankle joint stiffness, suggesting that vertical stiffness likely plays a role in being able to apply large vertical forces rapidly during ground contact in sprinting.
Another factor identified by research from Ken Clark & his colleagues is the importance of thigh angular velocity in producing vertical force. Essentially, the faster a sprinter can accelerate their thigh onto the ground, the higher the angular velocity of the lower leg and the greater the vertical force that is applied to the ground.
With all of this in mind, coaches and athletes who want to improve their maximal velocity sprinting abilities should work to improve vertical leg stiffness, ankle joint stiffness, and thigh angular velocity in both hip extension and hip flexion. Collectively, these improvements should enhance the sprinter's ability to apply vertical force during sprinting.
Faulty Assumptions About Vertical Force
People tend to assume that more is better and will look at the fact that vertical force production is higher than horizontal force production in sprinting as justification for vertical force being the main determinant of sprinting performance.
Similarly, they will state that net horizontal force production at top speed is zero, which it is, and then assume that horizontal force production is thus unimportant for reaching a high maximal velocity.
This reasoning is flawed.
First, as stated by JB Morin, one must understand that the magnitude of a force in sprinting does not dictate its importance to the movement. For example, if you were to run in-place on a force plate, you would see large vertical forces and virtually zero horizontal forces, and exhibit zero horizontal velocity. High vertical forces are meaningless to forward acceleration unless horizontal propulsive forces are being produced.
Instead, let's look at the difference in forces between slower and faster sprints or slower and faster sprinters. We see that faster sprinters produce more horizontal force as a proportion of their total force production than slower sprinters as they accelerate.
The simple fact that horizontal force production is less than vertical force is unimportant. What is important is the difference in force production between faster and slower sprinters, and the difference we see is that faster sprinters produce more horizontal force.
Second, while it is true that at top speed net horizontal force production is zero, and producing a large vertical force at top speed is crucial for running fast, looking only at the forces at this point in the sprint ignores what led to reaching maximal velocity in the first place.
Acceleration is what allows us to reach maximal velocity, and many races are won and lost during the acceleration phase. If an athlete can reach a higher velocity sooner in the race and continues to accelerate at the same rate as everyone else, this athlete will win the race.
Acceleration ability is strongly influenced by an athlete's ability to apply horizontal force and create large horizontal propulsive impulses. It is beneficial to the athlete during acceleration to minimize vertical impulse and to only apply the vertical force needed to support their own body weight and lift the body off of the ground so they can be launched into the next stride.
All together, it should be clear that vertical force is in-fact not the holy grail, as has been suggested by some coaches. Instead, vertical force production should be looked at as playing a supportive role throughout acceleration, while contributing more to the ability to attain and maintain high velocities later in the sprint.
Vertical Force Production is Required To Produce Horizontal Force
Every athlete's force production capacity is limited. If the total force an athlete produces cannot satisfy the vertical force demands of sprinting, they will not have the capacity to apply horizontal force in that stride.
If an athlete cannot produce the vertical force needed to support their body at ground contact and send their body back up into the air, they will begin to decelerate. This can result from being weak, fatigued, or needing more skill to attack the ground properly.
In The Biomechanics of Sprinting And Hurdling, authors Ralph Mann and Amber Murphy discuss the forces generated during sprinting.
In this book, the authors state:
"A sprinter's ability to produce horizontal velocity is directly related to their ability to exceed the vertical force demands of the activity."
Additionally, the authors state that:
"To produce the greatest Maximum Velocity, the goal must be to minimize Forces in all other directions (primarily the Vertical) so that some productive Horizontal Force can still be produced. As long as the TOTAL Horizontal Force created during Ground Contact is positive, Horizontal Velocity will increase."
The authors allude to the fact that to produce net-positive horizontal forces, which will cause the athlete to continue to accelerate and eventually reach a higher top speed, the vertical force demands of sprinting must first be satisfied.
If the athlete's total force production capability is below that of the vertical force demands of sprinting at a particular velocity, the athlete will fail to accelerate. The only way for an athlete to accelerate is to apply a net positive horizontal force to the ground, but this force cannot be applied if the athlete cannot meet the vertical force demands of sprinting at that speed.
It is only when the athlete's force production capabilities exceed the vertical force requirements they will be able to use this excess force to apply horizontal force. In this case, excess force refers to the amount of force a sprinter can produce that goes beyond what is demanded by the vertical force component of the stride.
Horizontal Force Production In Sprinting
Horizontal force production is highest at the start of the sprint when athletes need to launch themselves off of the blocks, overcome inertia, and accelerate down the track.
As the athlete accelerates, horizontal force production will decrease until the maximal velocity is reached, where braking forces and horizontal propulsive forces become equalized.
Direction Of Force Application Is A Determinant Of Sprinting Performance
While everybody talks about how important force production is in sprinting, the most important factor determining how well you accelerate is the direction in which force is applied to the ground.
According to research by Morin, Edouard, and Samozino:
"...the orientation of the total force applied onto the supporting ground during sprint acceleration is more important to performance than its amount."
This makes sense intuitively that we're better off applying a little less force but directing it in the right direction versus applying more force in the wrong direction. This is where the concept of the ratio of force comes into play.
Other research from JB Morin's group showed that an athlete's ratio of force was significantly correlated to 100-meter dash performance and acceleration sprinting performance, while the resultant GRF and vertical force production was not.
Morin's group also found that sub-elite sprinters will often produce more total force than elite sprinters at top speed, but they are unable to direct this force in an effective manner. Elite sprinters have the skill to be more productive with lower total force outputs.
They went on to say the following:
"It seems important to notice that the vertical GRF has a major influence on the sprint mechanics as the athletes have to produce the vertical force needed to overcome the negative vertical acceleration because of gravity. However, our results support the argument that the vertical component of the GRF is not by itself a determinant of performance in high-level athletes during the sprint acceleration phase."
Essentially, what these researchers concluded was that the athletes who accelerate the fastest and run the fastest 100-meter dash times are better at generating and maintaining a higher ratio of force throughout the sprint, while slower sprinters exhibit a lower ratio of force throughout the sprint. This means that faster sprinters use horizontal force more effectively and produce it at higher velocities than slower sprinters.
Remember that ratio of force is the ratio of Horizontal Ground Reaction Forces relative to the Resultant Force of each step. Maintaining a higher ratio of force as speeds increase means that an athlete can continue to apply net-positive horizontal forces to the ground as speeds increase.
The sprinters who can orient more of their forces in a horizontal direction for longer while also satisfying the vertical force demands of each step will accelerate for longer and reach higher top speeds as they sprint.
Slower sprinters will fail to do this, either because they cannot satisfy the vertical force demands or they lack the skill and specific strength needed to apply horizontally oriented forces at high velocities.
Importance Of Forces In Acceleration And Maximal Velocity Sprinting
With all of this in mind, we can paint a picture of how vertical and horizontal forces play different roles throughout the sprint.
Since the only way we can achieve a high top speed is to accelerate up to that point, we cannot assume that the factors correlated with maximal velocity sprinting can be extrapolated backward into the acceleration which got the athlete to maximal velocity in the first place.
JB Morin's research groups have shown that in order to accelerate effectively, athletes need to produce and maintain a high ratio of force (proportion of total force that is directed horizontally) as their speed increases. This is not easy to do, but it is required if you want to accelerate faster and longer into the sprint.
Similarly, research from the National Institute of Fitness and Sports in Kanoya states that to accelerate well, athletes need to maximize propulsive forces (in a horizontal direction) while minimizing braking forces and vertical forces. Force application needs to be directed horizontally during acceleration, with vertical force application being limited to the vertical force needed to support the body during ground contact. Excessive vertical forces during acceleration would lead to less horizontal acceleration and more vertical acceleration, which would cause the athlete to go up rather than out as they build up speed.
In contrast, once the athlete has attained maximal velocity, high levels of vertical force production in short time frames are crucial for maintaining top speed. To minimize deceleration, athletes need to minimize braking forces by attacking the ground in the proper direction, while the high vertical forces propel the athlete up and off the ground quickly to maintain momentum and velocity.
- Morin JB, Bourdin M, Edouard P, Peyrot N, Samozino P, Lacour JR. Mechanical determinants of 100-m sprint running performance. Eur J Appl Physiol. 2012 Nov;112(11):3921-30. doi: 10.1007/s00421-012-2379-8. Epub 2012 Mar 16. PMID: 22422028.
- 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. PMID: 21364480.
- Rabita G, Dorel S, Slawinski J, Sàez-de-Villarreal E, Couturier A, Samozino P, Morin JB. Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion. Scand J Med Sci Sports. 2015 Oct;25(5):583-94. doi: 10.1111/sms.12389. Epub 2015 Jan 31. PMID: 25640466.
- Weyand, P. G., Sternlight, D. B., Bellizzi, M. J., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89(5), 1991–1999. https://doi.org/10.1152/jappl.2000.89.5.1991
- Nagahara R, Zushi K. Development of maximal speed sprinting performance with changes in vertical, leg and joint stiffness. J Sports Med Phys Fitness. 2017 Dec;57(12):1572-1578. doi: 10.23736/S0022-4707.16.06622-6. Epub 2016 Jul 13. PMID: 27406013.