Ground Reaction Forces
The foundation for all movement resides in the knowledge that movement is completely dependent on the ability of the athlete to utilize and manipulate forces, most importantly the forces that interact with the ground (ground reaction forces GRF).
The ability to powerfully apply force to the ground is crucial to movement, but is only part of the equation. We must also efficiently deal with the force the ground supplies back into the body. These are the primary forces that propel and stop the body.
Thanks to Isaac Newton's Third Law of Motion (1687), we know that for every action there is an equal and opposite reaction. With respect to movement, this means that when we push against the ground with X pounds of force, the ground is going to push back with that same (X) amount of force (dependent upon the surface).
I call this the FIFO (force in = force out) response. When you further examine the FIFO response you will see that the harder you push into the ground, the harder the ground pushes back. This concept will become increasingly important later when we start to discuss acceleration and deceleration.
During movement we must not only be concerned with the magnitude or amount of force we produce, but also with the direction of application of the force vector. For the purposes of this article, a force vector will be defined as an imaginary line that defines the direction of application of a force. Understanding force vectors allows us to visualize the forces we apply and receive.
Note: When you push against the ground with your foot, you create a force vector dependent on the direction of push. In response, the ground is going to directly oppose that force vector with its own. It is important to visualize both vectors when we start to analyze the movement.
We must know how to appropriately direct forces to create clean movement. If we misdirect the force we apply to the ground, the resultant reaction will not efficiently help us create or deviate movement.
Although the foot is the point of contact with the ground, it does not determine the force vector created by the push. Rather, one must consider the segment of the leg from the knee down to the foot. It is this segment that determines the direction of that vector.
One must also consider the center of gravity (COG). The relationship between the location of the COG and the angle of the GRF becomes very important during movement.
Once again we are reminded of Newton's laws of motion. Newton's First Law of Motion (1687) stated that a body will stay in a constant state of motion (or motionless) until it's acted upon by an outside force. Typically what we see (when the body is motion) is that when the force vector created by the GRF opposes the direction of travel of the COG we create a breaking or decelerating moment which disrupts the current state of movement.
This becomes apparent when an athlete tries to rapidly decelerate and they sound like a clydesdale stomping. They create substantial breaking forces to rapidly decelerate their COG. These forces directly oppose the COGs direction of travel.
Likewise, when the force vector angles the same direction as the COG, we produce propelling or accelerative forces. If you were to analyze a sprinter coming out of the blocks you would notice that they maintain a significant forward lean for 8-12 yards (some more some less).
This forward lean puts the lower shank of the leg at a close angle to the ground. This angle creates a force vector that angles the same direction as the direction of travel of the COG. This means that they are propelling, or pushing the COG forward.
If we examine jumping from a stand still we see that the GRF's are parallel to the COG and perpendicular to the ground. This creates lift or vertical displacement since you are pushing the COG up.
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With this discussion on how GRF's affect our direction of travel, we can start to see how minor changes in our GRF's can cause small deviations in the path of or COG. This becomes an issue when this path is not consistently directed toward our goal.
The next step of the process lies in understanding how we must configure the body to appropriately apply and utilize ground reaction forces.
I have divided cutting into 5 parts. This is a particularly remedial breakdown of movement where I will be discussing the FUNDIMENTALS in an extremely controlled environment. Once these fundamentals are ingrained we will begin to work on how they apply to a more realistic chaotic environment.
Start by setting 6-8 cones up in a zig zag pattern so the cones allow 6-8 steps to be taken and form an approximate 90-degree angle to the next cone.
The terminal stance is the bodily configuration used to optimize the biomechanics of the body with respect to cutting. There are 8 key elements to this stance.
- The Feet: You should be on the front half of the foot on both feet with the heel slightly elevated. I refer to this as up on the platform. Both feet should be facing the cone with the foot opposite of the direction of the cut in front.
- Lower Leg Angle: This is somewhat dependant on your anatomy. Generally we want the back leg angle to be much lower than the front (with respect to the ground). This will allow you to get your hips low and actively use them when you push. The front leg should be angled slightly back toward the body.
This leg will act like a shock absorber, and will create a breaking moment to help absorb the deceleration of the COG (center of gravity). Furthermore, this will allow the musculature of that leg and the hips to preload and reactively respond during the push phase. It should also be noted that the lower legs should have a perpendicular medial lateral reference.
- Upper Leg Angle: You should approximate a 90-degree angle between the front and back thigh. This will help determine foot spacing front to back.
- Hips Low: Lowering the center of gravity will help in the manipulation of the momentum of the body. In general, when we lower the COG, we want to use the hips, not the upper body (flexion at the waist). I like to use the analogy of what corners better, a sports car or a school bus? The sports cars COG sits much lower, which allows it to corner much more efficiently.
- Balance: The body must be balanced. The COG should be centered between the feet front to back and side to side. This will allow for the effective manipulation of momentum and ground reaction forces (GRF).
Note: Do not allow the hip of the plant leg to push out lateral.
- Upper Body Angle: The upper body should create an angle slightly less than 90 degrees with the front leg. When closed or opened, the hips will not be positioned in an advantageous position for the expression of force.
- Spinal Alignment: The spine should stay in a neutral alignment. Avoid rounding the back.
- Head: The head should stay up. Lowering the head can cause rounding of the back and over flexion at the waist. Do not focus on the cone with your eyes; look toward the horizon.
The approach is vital since it leads into the terminal stance.
- Approximately 2-4 steps before hitting the terminal stance, you need to begin lowering your center of gravity (lower using the hips not the upper body).
- Use minor stride adjustments to hit the terminal stance stride length. Avoid jumping or reaching to get to the cone and the stance.
- Decelerate as you lower your center of gravity. The amount of deceleration is dependent on your mechanics and leg strength. Weaker athletes may not be able to decelerate as quick. Furthermore, when the COG resides to high, momentum may make deceleration more difficult.
Assume the terminal stance. The body should be configured as described above. At this point in time there should be no medial or lateral tilt to the lower leg. If we were to maintain this position, hip and knee extension would cause elevation of the COG or posterior displacement. Since our goal is to move the COG horizontally toward a cone that resides in front and to the side of your current position, we must change this bodily configuration somewhat.
The front leg must be tilted medially toward our destination (typically 15-20 degrees, but who is measuring). Do not rotate the foot, rather keep it pointed toward the cone, and rotate the leg in and toward the next cone. This will allow the push to project the COG toward the next cone.
Practice the approach into the terminal stance. On cue, drop the knee and hold the position. Once balance can be maintained, approach the cone and hit the terminal stance with the leg rotated in.
Triple Extension Push
Just as it is better to absorb force through multiple joints, it is better to express force through multiple joints. Since we know that extension means to straighten or increase the angle at a joint structure, we can realize that triple extension must refer to 3 joints.
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In this situation these joints are the hip, knee and ankle.
When we are in the terminal stance, we have flexed and loaded these 3 joint structures. With the knee tilted, simultaneous triple extension will effectively push the COG toward the next cone.
When correctly orchestrated, this triple extension can be the most powerful motion the body can produce. This has been demonstrated by the huge power outputs measured by lifters performing Olympic lifts (which are based on this triple extension). This motion needs to be practiced and perfected.
Here are some ways to practice this motion.
Start in the terminal stance (with the front leg rotated in). Explode through triple extension of the front leg and jump toward the next cone. Land on the opposite foot and regain your balance. Focus on the simultaneous triple extension and complete extension at the hip (which is many times not fully completed). Also work on rapidly regaining balance on the landing (which will be on one foot).
Stand on one foot. Jump diagonally landing on the opposite foot. Absorb the landing through the hip, knee, and ankle and make sure you land with your hips pushed back and your knee behind your toes. As soon as the decent terminates, explosively triple extend and jump back to the starting position. Practice this off both feet. The intensity of this exercise is determined by the distance of the jump.
Younger or weaker athletes should not leap as far forward as more conditioned athletes. If there is a problem controlling the landing, then the initial jump was too great.
The first step is made by the back foot coming out of the terminal stance and should be directly toward the destination, which in this case is the next cone. Since we know the shortest distance between two points is a straight line, the location of this step could potentially add distance to our path if misplaced.
Although we practiced jumping off the cut to execute triple extension, we truly do not want to lose contact with the ground when we cut. This is so for two reasons. First, when you lose contact with the ground you lose the ability to change direction (since you are in the air, you are going to go the way you jumped). Second, when we talked about moving the COG in a straight line toward the next cone to decrease the distance traveled, we need to also consider vertical oscillation.
The first step needs to be a comfortable stride. Overstriding will cause a breaking action and will hinder forward momentum. Likewise, understriding will not allow for full triple extension off of the cut. This size of this step needs to be worked with. It will be different for each person.
In addition the first step, should initiate from the lead shoulder. By this I mean that when coming out of the terminal stance, the lead shoulder should push toward the destination as the lead leg executes triple extension. If this is not performed properly, the shoulders will lag behind the hips on the push causing a momentary lag as they catch up.
Take time learning these techniques as they are fundamental to multidirectional movement. These mechanics need to be reflexive in nature to truly have on impact on confrontation chaotic movement.
Time spent learning the basics will pay off tenfold in the future.