Transcription
The ground reaction force, what was that exactly? What the floor has to do with it, why the term reaction is in it, and why it is so important for orthopedic technology—that's what this video is about now!
First of all, let's take a look at what a force actually is. A force is always described and defined using three properties: the direction, the amount, and the point of attack. Basically, it's very simple and maybe even self-explanatory. The direction describes in which direction a force acts—very simple. In this case, in which direction the arrow is pointing, namely to the right. The amount shows the size or strength of a force. This is expressed here by the length of the arrow. Longer arrow, greater strength. And the point of application is the point at which the effects of a force become noticeable or where the force acts directly on a body. Don't get confused! In biomechanics, it is not the tip of the arrow but the other end, thus showing where and how strongly a body is being pressed by a force.
In the literature it says: "Forces always occur in pairs. If a body A exerts a force on another body B, the same, but oppositely directed force from body B acts on body A." Uh, what?? Let's take the whole thing apart a bit. Ok, let's pull out the most important information. No. 1: When one force occurs, a second is at the start. Second, the first force acts on some object. And thirdly: This object also has a force acting in the same magnitude, but in exactly the opposite direction. If you abbreviate the whole thing, it is also called the "actio reactio principle." Or so that the whole thing sounds a little smarter, "actio = reactio." So if you slap your colleague, it is an action on which a corresponding and opposite reaction follows.
Let's take a look at some examples from the well... everyday life. The karate zen master and his apprentice, breaking through boards. Collect a little more Ki, and the apprentice gathers all his strength in this one shot, hits the board; the board is more blatant and hits back. And not too little. The action-reaction principle here: the karate student also exerts a force on the board with his strength, but as soon as he touches the board, the reaction force from the board counteracts it.
Another example. Here we see Markus Rehm, long jumper, gold medal winner, and right leg amputee, while writing history at the Paralympics. And without a ground reaction force, a long jump wouldn't really be feasible. Let's take a look at the whole thing slowly. Rehm runs off and braces himself with his entire body mass over the prosthesis into the ground. The ground, in turn, has an opposing force on Markus Rehm, catapulting him into the air and giving him what feels like 100 gold medals.
The action-reaction principle goes back to this guy here: Sir Isaac Newton. This guy was pretty crazy. He discovered the laws of optics, found out that white light is made up of the mixture of rainbow colors, and he discovered the laws of dynamics and gravity. When a friend asked why the planets move in ellipsoidal and not circular orbits, Newton went home, came back to his buddy a few months later and said, "Hey, I got it, but I just had to invent differential and integral calculus first." And then... Newton turned 26. That alone should prove what a genius Newton was.
Our rocket can only take off here because the action-reaction principle makes it possible. The rocket's thrust acts downwards. This principle pushes the rocket in the opposite direction and can leave our orbit at a very high speed. This shows that this principle forms the basis for the technology used in several areas. If you understand this theory behind it, you can make use of it.
OK, let's finally get to the precise definition of the floor reaction force. "The ground reaction force is the force between the ground and the body that acts in the direction of the body." But as always, don't panic! We will discuss it bit by bit and link that to what we already know.
This is Heinrich. He likes to be outdoors and is forced to make himself available for this video. Like all of us, he has body mass. Since we are looking at the ground reaction force, we must first address the cause of this, the weight force, which in turn is caused by gravity. As before, we'll go through this using the three qualities a power has. Point no. 1 is the direction as before. In which direction does gravity work? Well, we'll find out easily. Heinrich is drawn by the earth's gravity, perpendicular to the earth's center. It thus has a force in the direction of the center of the earth. So we already know the direction of the force, namely perpendicular to the ground and below. On the moon, gravity would also act towards the center of the moon, but less strongly. The heavier a planet, the greater its gravitational pull. We would have clarified the direction. Gravity acts towards the center of the earth. Based on the three properties of a force, we can get a good picture of how these forces work and even make use of them. We'll use the same procedure for the ground reaction force, but now continue with our simpler example, gravity.
What about the amount? Sure, Heinrich has a certain body weight, and the force he exerts on the earth is correspondingly great. The amount of a force is determined with this formula. No fear! This looks more complicated at first glance than it is. A force results from a mass multiplied by an acceleration. Don't get confused! The designations for force, mass, and acceleration are always given in the international system of units as SI units. Therefore, F stands for Force, M for Mass, and a for acceleration, the acceleration. Let's do an example calculation. Let's say Heinrich weighs 80 kg. That is the value of our crowd. The gravity is our acceleration and has a certain value on earth of 9.81 m/s2. It might look a bit strange, but that's the unit for acceleration. If you multiply the whole thing, we get a value of 784.8 Newtons—the unit for the force. Well, whoever invents it will be named after it! OK. The amount is also ticked.
What about the point of attack? Gravity acts on the entire body, so by our definition the point of attack is everywhere, right? Strictly speaking, yes; to make it easier for us, we press the entire mass of Heinrich into a tiny point, which we call the center of gravity. This is where gravity attacks. In humans, the center of gravity is usually at the level of the belly button. Point of attack, check!
What is so special about the ground reaction force? As before, let's go through the three properties of a force on the ground reaction force. Ok, let's look for clues to the direction... ahhh, that's right there! The ground reaction force is, as the name suggests, the reaction to our action, namely our weight. We act on the floor with our weight force; this reacts in the opposite direction with the floor reaction force. The direction of the force thus rotates once by 180 degrees. The amount corresponds to Heinrich's weight, since the counterforce, in this case the floor reaction force, is equally strong. As already mentioned, Actio is like Reactio. Don't get confused here either. We turn the sign from plus to minus because the direction of the force has reversed. Then we do the math again—don't worry, it's super easy, there is just a minus before our result. This means that the floor reaction force corresponds exactly to the force that Heinrich exerts on the floor with his weight—only with a negative sign. Hmm, ok, two of three properties have been clarified, wasn't that difficult, was it? But what about the point of attack? Where does the ground reaction force directly affect Heinrich? At the contact point between Heinrich and the ground, or rather contact points. The points of application of the ground reaction force are at Heinrich's feet! Heinrich stands with both feet on the ground, and the strength also works through his legs. Thus we have also clarified the point of attack or the points of attack.
What if the ground reaction force were now smaller or larger than our weight? Does that actually happen? What would that do? Here is a thought experiment: This is Margarethe; she likes to take photos, likes to travel, preferably to distant deserts, just as everyone likes... preferably in distant deserts, as everyone likes. Okay, let's imagine our dear Margarethe is standing on a sand hill, and her physical strength is greater than the ground reaction force. What is happening? She sinks into the ground, but not with shame. In this case, gravity is greater than the force of the soil, in this example consisting of the sand. This is not an unlikely scenario when you think of quicksand or, even more mundane, of water, which you can't walk over, so not all of us. This imbalance of forces occurs again and again when walking. A lower ground reaction force only means that our body's center of gravity is lowering. This is quite normal when walking; the body's center of gravity moves in a wave-like shape from top to bottom.
Then let's clear up the other case! We assume that the ground reaction force would be larger than Margarethe's body weight force. Can you guess what is happening? Right, she's taking off. Exaggerated. In everyday life, that just means that the focus is raised. Which is quite normal in the gait cycle. The body's center of gravity rises and always sinks a little. And just now we have also clarified what is behind it.
Let's come back to our Heinrich. As we found out earlier, the point of attack is on our feet. Both feet together form a surface on which the point of attack can move. When standing normally, this point of attack moves back and forth. If you want to know more about the effects of floor reaction force on our bodies, we'll go over it in detail in this video. Back to Heinrich. Depending on where we lean, our center of gravity also shifts, and the ground reaction force moves with it accordingly. If we move with our body's center of gravity outside of our support area and do not enlarge it through compensatory steps or the like, we fall over! You can enlarge your support area with a few simple means. Can you think of any? One or the other of you may know it from your own skiing experience. You can lean very far forwards and backwards without falling over. However, if you lean to one side, you will quickly find yourself in the snow. The skis give us a larger ground support surface forwards and backwards, and the ground reaction force can move further away from us on this enlarged contact surface, which increases stability. Standing wider or using a cane follows the same principle and helps us to keep our balance better.
So, and why do you need this knowledge in orthopedic technology? This knowledge should be part of the basics of anyone working in orthopedic technology. The static structure of prostheses and orthotics is based on such principles; if you know what stresses prevail in healthy people, you try to approximate them in patients. Illnesses can be explained by incorrect stress; these basic rules also derive guidelines in orthopedic technology that lay the basis for good care. So if you understand all of this, it makes your everyday work easier, and you can solve problems faster and better.
Thank you for being there again this time and for taking a closer look at the ground reaction force with me! I hope with this video I was able to expand your understanding a little and make it easier for you to get started with the biomechanics of orthopedic technology. With this in mind, I say goodbye and hopefully I can see you again next time, for another video on... got it!