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  • Writer's picturemyegge

Ground Reaction Force (GRF)


When teaching speed, we emphasize two major concepts. The first is Ground Contact Time (GCT), which we discussed in detail in our previous movement blog on "Ground Contact Time". In a nutshell, GCT can be defined as the period of time between the moment an athlete’s foot touches the ground to the moment the athlete’s foot leaves the ground. Naturally, the shorter the GCT, the faster the coordination and implementation of the next movement can occur. The second major concept, and the focus of this blog is Ground Reaction Force (GRF). When teaching GRF, we emphasize a two major points: amplitude of force into the ground AND the direction in which the force is applied.


Newton’s Third Law of Momentum states that for every action there is an equal and opposite reaction. In terms of movement, this applies as follows:

  1. The more force we put into the ground, the more potential force we can get out of the ground into our next movement. To elaborate on this concept, we will create a simple analogy: If I had a bouncy ball and I wanted it to bounce as high as possible, would I drop it from knee height or would I drop it from overhead? For most, this is obvious…the ball dropped from a higher point will have the higher bounce height. Furthermore, if I were to throw this ball down into the ground from overhead, the bounce would be significantly higher. The amount of force imposed into the ground is proportionally reflected into the bounce height of the bouncy ball. Now picture the same experiment performed with a hacky sack. Regardless of the amount of force applied into the ground, the hacky sack is absorptive and will not translate into much (if any) bounce height. This simple analogy directly correlates to our movement. The more force we put into the ground, the more force the ground can potentially give back. To maximize GRF into effective and efficient movements:

    • We need to attack the ground with as much force as possible. Within acceleration and sprint mechanics, we often cue for increased knee drive. The knee drive itself provides minimal momentum in and of itself…however, the higher we drive our knees, the more opportunity we have to apply force into the ground. A more advanced description is as follows: Knee drive is performed by the hip flexors. Increased hip flexion angles yield increased opportunity for hip extension (and triple extension of hips/knees and ankles). Our goal in any athletic movement should be to control the ground utilizing as much force as possible.

    • We need to be reflective of the force applied. To be efficient in movement, we need to embrace the concept of a bouncy ball and be reflective of force, vs absorptive like the hacky sack. This has to do with ground contact time and all that is entailed (Again see previous post on GCT!)

    • The direction in which the force is applied into the ground matters significantly! We need to effectively and efficiently push the ground away from our intended movement.

      • If I want to go up, I need to effectively and efficiently push down into the ground.

      • If I want to go forward, I need to effectively and efficiently push back into the ground.

      • If I want to move laterally to the right, I need to effectively and efficiently push the ground away to the left (and vice versa)

      • If I want to move backwards, I need to effectively and efficiently push the ground forward.

And I need to do all of these with purpose and intent in order to maximize my next movement. Two major tendencies that we often see and address are athletic positioning and shin angles.

  • Athletic positioning and base of support: (See previous "athletic position" blog for more detail!). In order to control the ground, our feet need to be under our center of gravity (in most cases, this means under our hips). Often times, we initially see a base of support that is too wide within our athlete's “athletic positioning” or jump position. Though this wide base does provide stability, it limits the opportunity for power production in any direction because the athlete cannot truly control the ground. If we were to ask our athlete to jump from this wide-base, the ability to produce force down into the ground will be inefficient to explode into a vertical jump. The vectors of force are equal and opposite to the force applied.


Again, this is a very common occurrence where we combat by cueing for a base of support that is no wider than hip to shoulder width. Within a proper athletic position and base of support, the athlete now has the ability to control and exert force into the ground efficiently in terms of both direction and magnitude to maximize ground reaction force into her next movement. In this case, the athlete wants to move up into a vertical jump. She can directly exert force down into the ground and GRF will effectively push her up.




This athletic position and proper base of support is not limited to vertical jump, however. A good athletic position provides an athlete the opportunity to control the ground in any direction. Let's consider lateral movement from the same wide-based position compared to the hip-width base of support in the videos above. Which position allows the most opportunity to exert force into the ground via hip abduction? Look at the first video. How much can she actually push the ground away via hip abduction to perform a lateral movement? Not much...she is already in an abducted angle. Now consider the second vid. The athlete now has the ability to exert force through a hip abduction full range of motion.

  • Shin angles: Shin angles play a major role in our ability to control the ground directionally. For efficient movement, our shin angle should be pointing in the direction of our next intended movement point. Many of our athletes initially over-stride while running. An over-stride (or obtuse knee flexion angle) in which the foot lands in front of the hips is detrimental to linear speed. If our initial ground contact lands in front of our center of gravity (under the hips), ground reactive force is negative vs. positive. In this scenario, we are essentially trying to run while the emergency brake is engaged.

In the above video, the athete is demo'ing an obtuse shin angle. In regard to Acceleration, this will be detrimental to speed. Her foot will land in front of her hips and center of gravity and effectively be a braking mechanism and GRF will push her back vs push her forward.

In the above video, the athete is demo'ing an acute shin angle. In regard to Acceleration, this will be detrimental to speed. Though the athlete may be pushing back into the ground, the amount of force into the ground is quite limited, implying that GRF will also be limited.

In the video above, the athlete is demonstrating parallel shin angles. Her ability to accelerate by "attacking the ground" under her center of gravity (under her hips) is maximized, thus optimizing the opportunity for GRF to propel her forward.


Summary:

  1. GRF is always present. Even standing, we are exerting the force of our bodyweight down into the ground and the ground is matching this force in an equal and opposite direction (upwards).

  2. Increased force exerted into the ground yields increased GRF potential.

  3. For explosive and powerful movements, we need to reflect vs absorb the force applied into the ground.

  4. Direction matters. We can utilize GRF in such a way that it can be for us, against us, or neutral in relation to efficient and explosive movements pending the angle (and force) in which we contact the ground.


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