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Sprint acceleration and ground reaction forces

Sprint acceleration and ground reaction forces

In this article, we will address common misconceptions about the mechanical characteristics of sprint acceleration. We’ll analyze concrete data based on measurements with force platforms, which are the gold standard in biomechanics. By comparing three different types of exercises, we can better understand how ground reaction forces affect acceleration and sprint performance.

Analysis of Force Data in Different Exercises

Let’s start by examining the data obtained from three types of exercises: high acceleration (maximum sprint start), low acceleration (jogging start), and no displacement (in-place jumps). These data will allow us to observe how ground reaction force components vary in each case.

High Acceleration: Maximum Sprint Start

During a maximum sprint start, two main components of ground reaction forces are observed in the sagittal plane:

  • Vertical Component: This is the upward force, including body weight and gravity.
  • Horizontal Anteroposterior Component: This is the force directed forward and backward.

In this exercise, the vertical force exceeds 1100 Newtons, indicating significant vertical impulse. Impulse is the product of force and contact time. Although the magnitude of the horizontal impulse is smaller, it is crucial for forward acceleration.

  • Maximum vertical force: >1100 Newtons
  • Significant vertical impulse
  • Average horizontal force: Lower compared to vertical
  • Proportion of horizontal force: 24% of ground reaction forces
  • Average center of mass acceleration: 3.9 m/s²
  • Final speed after three steps: 2.12 m/s
Low Acceleration: Jogging Start

During a jogging start, the final speed after three steps is 1.5 m/s, with significantly lower average acceleration. In this exercise, the proportion of horizontal force is also lower, indicating a more vertically oriented push.

Despite having similar or even higher vertical force and impulse compared to high-acceleration sprints, performance is lower due to less horizontal force. This demonstrates that it’s not only the magnitude of the force that matters, but also the direction in which it is applied.

  • Final speed after three steps: 1.5 m/s
  • Average center of mass acceleration: Much lower than in maximum sprint starts
  • Proportion of horizontal force: Lower, more vertical push
  • Horizontal force: Much lower than in maximum sprint starts
  • Horizontal impulse: Much lower than in maximum sprint starts
  • Average vertical force: Similar or even higher than in maximum sprint starts
  • Vertical impulse: Similar or even higher than in maximum sprint starts
No Displacement: In-Place Jumps

During in-place jumps, no forward speed or acceleration is generated. The data shows that the average horizontal force is practically zero, while the vertical component is extremely high. This exercise produces the highest vertical component of ground reaction forces but does not generate forward movement.

This underscores that the magnitude of vertical force does not determine its importance for acceleration tasks. The absence of horizontal force explains the lack of forward displacement.

  • Final speed: 0 m/s
  • Acceleration: 0 m/s²
  • Average horizontal force: Practically zero
  • Vertical force: Extremely high, the highest of the three exercises
  • Vertical impulse: High, but lower than in other exercises due to shorter contact time
Magnitude vs. Importance of Force

It is crucial not to confuse the magnitude of a force with its importance for a specific task. In the context of sprint acceleration, horizontal force is much more critical than vertical force. This is because horizontal force is what drives forward movement.

Extreme Example: Jumping Backwards

To illustrate this point, consider an extreme example: jumping backwards. In this case, although the vertical force component is high, the horizontal component is negative, resulting in backward movement. This example highlights that the direction of force is crucial for the type of movement generated.

Analogy in Team Sports

In team sports such as soccer, players walk 80% of the time. Despite the high volume of walking, it does not indicate key performance. What matters in these sports is acceleration and speed, not the amount of walking.

  • Volume of walking: High, 80% of the time
  • Importance for performance: Low
  • Key performance indicators: Acceleration and speed
Conclusions

In summary, achieving high sprint speeds depends on several interrelated factors:

  • Horizontal Force: Crucial for forward acceleration.
  • Force Direction: The direction in which force is applied is as critical as its magnitude.
  • Specific Training: Training programs should focus on improving horizontal force and the ability to apply it rapidly.

Training to enhance horizontal force and rapid force application can be pivotal for sprinters. Additionally, focusing on developing more effective ground contacts from the foot through the kinetic chain is essential for efficient force transmission. Training programs should aim to enhance the force and speed of horizontal force application to optimize sprint performance.

For those interested in exploring effective methods and exercises to improve sprint acceleration and speed, we invite you to subscribe to our exclusive membership. With this subscription, you’ll gain access to detailed training programs, expert advice, and additional resources to help elevate your performance. Join us and transform your speed training approach today!

 

Author

Carlos Wheeler

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