Ground Reaction Force and its Relation to Speed
Newton’s Third Law dictates virtually all movement in life and training is no different. For every action there is an equal and opposite reaction, or more properly stated in Newton’s own words “the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.” This is incredibly important when it comes both to locomotion and training in general and we’re going to discuss that here.
Ground Reaction Force Explained
Now that we’ve got a proper definition for Newton’s Third Law of Motion we can take a look at the specific force called normal force. In order to understand ground reaction force (GRF) we much first have an understanding of normal force. Simply put normal force is a display of Newton’s Third Law of Motion where two objects are in contact with each other and exert a perpendicular force vector upon the contact surfaces. For example, when you’re standing still you are exerting your force (your bodyweight) onto the ground and the ground is exerting that same force back into you in the opposite direction. The reason we know and can understand this principle is simply because we don’t fall straight through to the center of the Earth when we’re on the ground. When we’re not moving, we can calculate normal force (Fn) by multiplying your mass (m) by the acceleration of gravity (g) or:
Fn= m*g
Since we’ve defined normal force, we can dive into GRF. In the above scenario, as well as squatting, jumping and even benching GRF by would mostly coincide with normal force, however GRF differs in the sense that there are sometimes parallel forces in addition to perpendicular force vectors. These forces come in the form of friction. The easiest way to think about this would be to imagine sprinting on a track, and then in the sand, and then on the ice. The friction coefficients of all of these surfaces differ greatly in that some are substantially lower leading to inefficient force transfer, while others are higher leading to more efficient force transfer. It’s the same reason we wear skates on the ice and cleats on the field, we’re able to mitigate friction issues and increase GFR. Simply put, GRF is the amount of force the ground can redeliver back through you when you deliver force into it. We act by exerting force into the ground, and the ground reacts by applying equal force into the point of contact.
Why it Matters
All of this would be great and training would be super easy if the ground only had to react and deliver force through the point of contact. If that were the case, we could just live in the leg press and call it a day. The problem comes in the fact that we’ve typically got a body attached to that point of contact that also needs to be propelled in the same direction. Athletes not only need to be able to deliver that force, they’ve got to be able to accept and react off of it as well. In order to increase GRF we have to be selective with both exercises as well as training surfaces and modalities. An easy exercise to think about is the leg press, while having the ability to increase leg strength does not do an adequate job of increasing GRF. The reason being is this exercise ends at the hip and does not require forces to transfer throughout the entire body. In essence no real force transfer happens outside of the lower extremity which is not going to allow those GRF to transfer through the body to propel up forwards/backwards/upwards. If we’re really looking to develop athletic qualities such as speed and strength, we need to make sure we’re prescribing proper movements and surfaces for training. Understanding GRF will allow you as a coach to program the proper exercises and movements to increase transfer of those skills and carryover from the gym onto the field of play.
How Not to Increase Ground Reaction Force
The fact is that there are some exercises that transfer to the field of play, and some that really don’t. We talked about the leg press in the last section but there’s many modalities and exercises that won’t help increase GFR but get utilized in an effort to increase speed by many coaches. I’m sure I sound like every other coach who’s complaining about agility ladders but it’s absolutely one of the most improperly utilized pieces of equipment in the strength coach’s arsenal. Back in 2017 Paul J Fabritz conducted a study dealing with peak force production in the Ickey Shuffle as compared to a simple two step cut using a force plate. What he found was that the peak force in the Ickey Shuffle was, on average about 570 Newtons while a simple two step cut was over 1200 Newtons. What does this mean? Well for starters it shows the GRF aren’t high enough in agility ladder movements to even match the force production of a two step cut. With this movement we’re not even talking about the forces associated with accelerating, decelerating, rapid change of direction or landing, it’s just a quick 45 degree cut. If GRF aren’t high enough within an exercise its incredibly difficult to make the argument that it will have any meaningful carryover to sport. While there are good reasons to utilize them in some populations and in some settings, don’t fool yourself into believing there will be much carryover to speed and agility.
Another modality used by some coaches is training in sand. While training in sand can create higher muscle activation in the lower extremity it creates an environment where forces get displaced to too high of a degree to have any real positive carryover to speed. You can see this and even feel it when in the sand, but there was a study published in the Equine Veterinarian Journal by Crevier-Denoix, et al comparing varying deepness of beach sand and GRF production in horses (yes I know it’s horses, however force production and force transfer doesn’t tend to change much across species so stay with me). They found the deeper the sand, the less maximal GRF was produced. If you train in sand and do it often enough, you’ll more than likely see a reduction in overall speed due to a decrease in GRF when compared to harder surfaces. While sand training can be effective for some populations, such as beach volleyball and early return to play protocols, I believe it has very minimal application in the pursuit of athletic qualities due to the low GRF being produced.
How to Increase Ground Reaction Force
We’ve looked at a few training modalities that aren’t very conducive to increasing GRF, and now we’ll talk about some ways we can train to help increase them. First thing that tends to come to mind is the squat. And the squat probably gets too much undue praise from this prospective because in reality all loaded ground-based movements are going to have positive impact on GRF. The squat seems to be the king of these movements simply because we tend to be able to load this movement much more than others. A higher load is going to mean more output, which means more force production on the field of play. For example a 350lb squatter is going to have to produce roughly 1559N just to hold the bar on their back, which is much higher than a 150lb lunge which yields 668N of force. Even if you multiply it by 2, it’s still lower than the back squat. Math is math and more force is more force. That being said outside of traditional barbell movements, weighted jumps, medicine ball throws, jumping/landing/depth drops, and sprinting can all have a positive effect on GRF if effort level is high enough and there is already a base level of strength to be able to display the power and explosive speed needed to complete the movements. I won’t be going over programming parameters as it is outside the scope of this article, but here are some examples of how these movements can help:
Weighted Jumps: Weighted jump variations can work in two ways. First, they can force you to deliver more force into the ground in an attempt to achieve maximal height. During the eccentric acceleration portion of the movement, the added load makes us deliver more force into the ground, and thus creates more GRF than simply jumping. Secondly the force of the landing is now going to be considerably higher helping create more GRF, otherwise we would collapse and fall to the ground. We’ll go over jumping and landing forces more in the depth drop jump section ahead.
Medicine Ball Throws: Any exercise that includes a transfer of force is going to help in increasing GRF production and can help in relation to speed. Vertical throws, horizontal throws and rotational throws all include delivering force into the ground and transferring it into the ball so they will all have fairly good GRF production within the movement. One exercise in particular that I like to utilize is a medicine ball slam straight into a jump. This exercise has many benefits including the utilization of overspeed eccentrics leading to more efficient utilization of the stretch shortening cycle (SSC), but we’re going to talk about the throw itself. Donald Chu wrote about arm action and its role in jump height in Jumping into Plyometrics. He concluded that arm action can account for upwards of 18% of jump height and even sprint speed. The reason for this is the increase in GRF through aggressive arm action. Taking this into account we can conclude that to a certain degree, modalities that increase the speed and aggressiveness of arm action can help to increase GRF when jumping and sprinting. This exercise is simple and can be done with a variety of different weights but I like to err on the side of a higher load (18-22lbs in higher training ages, 12-16lbs in medium trained individuals, 6-10 in lower training ages) to increase the aggressiveness of the arm action. To perform this exercise, you’ll have an athlete hold a med ball over their head while standing on their toes. Then they’ll aggressively throw the ball towards the ground (preferably a no bounce ball so it doesn’t hit them in the face) as they drop their hips in a jump ready position with their hands clearing their hips. They’ll then quickly accelerate out of the jump ready position (should shoot for a short amortization phase of roughly 0.2s) and jump as high as they can. There’s no real way for me to quantify the amount of force produced as I haven’t been able to do this on a force plate, it’s more than likely higher than a simple jump due to the necessity to become much more aggressive with arm action.
Jumping/Landing/Depth Drops: There’s a ton of studies out there dealing with ground reaction forces with jumping and landing so I won’t go over all of them, but every single one shows two things: 1) Ground reaction forces in a jump are minimally 3.5-5 times body weight in adolescents who have a low training age by default, and 2) landings on average from a 15 inch drop are about 5-7 times body weight in athletes with a higher training age. If we take the same 100kg athlete from our weighted jumps it’s going to take him a minimum of 3,430N to achieve maximal jump height, and on his landing he will be overcoming 6,860N. As you can see the forces being produced and overcome are incredibly high, and can absolutely have carryover onto the field of play. In addition to that jumping/landing and accelerating/decelerating are all characteristics of play, so this is a win/win from a training perspective.
Sprinting: I once got the pleasure of hearing Buddy Morris speak and I remember three things he said more than any other. First was a quote about conditioning from Charlie Francis where he said “it doesn’t matter how many times you can repeat a task if the output isn’t there to begin with.” The second was how sprinting is one of the most unnatural things we can do from a force production standpoint. And it is absolutely true. At maximum velocity we’re overcoming a peak vertical force of 3-6 times our body weight. Now in relation to landing from moderate heights it doesn’t sound that intense, however it’s happening every single stride. We’ll take our 100kg athlete again in this example, but this time we’re going to make him run a 40-yard sprint. Mike Boyle always liked to say the 40 should take about 17 steps so we will use that for the sake of brevity. We can also assume that in the acceleration phase of the sprint he’s going to be closer to 3 times body weight, and once he’s upright he’ll be closer to 5-6 times body weight so we will use the average of 4 times body weight in this fictitious scenario. Our 100kg athlete is going to be over coming 3,920N per stride and 66,640N over the course of 5 seconds. That’s pretty intense! I never mentioned the third thing that I remembered from Buddy and that was on purpose because I’m going to mention it now. He said “early on the weights will drive up the sprints, and as training age becomes higher the sprints will start to drive up the weights.” Essentially what he’s saying is that with a low training age, getting stronger will help with speed and as your training age increases the sprints will drive your ability to get stronger. And it makes total sense when you look at maximal force produced. Sprinting is an excellent way to help increase GRF.
When you’re programming for speed and carryover to sport (whatever the sport), remember that exercise selection is of upmost importance! Err on the side of helping your athletes, and not eye wash and sex appeal.
Hopefully this article gave you better insight into the how, what, and why when when it comes to ground reaction force. If you have any questions or comments feel free to email the author at Connor@theLDSP.com.
