What is biomechanics?
Biomechanics is the field of study that combines physics with biology to describe the movement of animals. People have been wondering how animals move since Aristotle (384 BC-322 BC). Leonardo da Vinci (1452-1519) and Galileo Galilei (1564-1642) made large contributions to the field by studying how human and animal muscles, joints and bones work. Giovanni Alfonso Borelli (1608-1679) is known as the father of biomechanics and made many contributions to the field. Eadweard Muybridge (1830-1904) and Étienne-Jules Marey (1830-1904) later advanced the field by creating detailed photographs of animals (including humans) in motion. Studying biomechanics typically involves a lot of math. However, in this tutorial we will keep the math to a minimum and only cover concepts.
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Movement On Land
Let's begin our journey with how animals move on land. There is a major obstacle that animals face when on land: gravity. Gravity is the force that pulls all of us towards the ground (or, rather, towards the center of the Earth). Even standing up can be difficult when there is a force pulling you down. Different animals have solved this problem in different ways. Some have stayed close to the ground, like snakes and worms and minimized the amount of energy they need to expend counteracting gravity. Many other animals use some number of legs to prop themselves up. Birds use two feet, horses use four, insects use six feet and spiders use eight.
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The arrangement of these legs is important to keeping the animal upright. The legs need to be situated correctly around the center of mass of the animal. Imagine adding up the weight that of the hammer to the right and finding the average. The center of mass is the point on the hammer where this average lies. The head of the hammer is really heavy, while the handle of the hammer is much lighter. The center of mass ends up being closer to the head of the hammer to make up for the difference in weights. Everything has a center of mass, including animals. In animals with legs, their legs need to be situated around this center of mass to keep the animal upright. Where is the center of mass in humans?
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Gravity is just one of many forces that act on animals moving on land. A force is a push or a pull on an object. When studying biomechanics, it is important to keep all of the forces on an animal in mind. Scientists use free-body diagrams to keep track of these forces. A free-body diagram is a cartoon that illustrates the forces acting on an animal. The animal is usually drawn very simply (for example, the circle to the right could represent a hamster) and the forces are drawn as arrows. The direction the area is pointing shows which direction the force is acting in and the length of the arrow shows the relative strength of the force. For example, in the free-body diagram to the left, the arrows on the top and bottom of the circle are the same length, so the forces pulling the hamster up and down are equal. But the force on the left is longer than the force on the right, meaning the force pulling the hamster to the left is stronger. Assuming nothing changes, we might assume the hamster will accelerate to the left.
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Animals use muscles, tendons, and joints to move around. Let's consider three ways a muscle-tendon unit (group of muscles and tendons attached to each other) can be used. First, we can use muscle-tendon units as a motor. This is the most intuitive use of muscle-tendon units. If you move your arm up, you are contracting the muscles in your arm and using that energy to lift your arm. The boy in the first picture is using his arm muscles as motors to lift the girl. Secondly, you can use muscle-tendon units as brakes. You do this when you walk downhill. Your legs stop you from rapidly tumbling downhill. The man in the second picture is about to use his arm muscles as brakes to catch his daughter. Thirdly, you can use your muscle-tendon units as struts. This means that you use them the same way a pole vaulter uses a pole. The limb stays straight and taut and you use it to support some weight. The man in the third picture is using his arm muscles as struts to keep the woman in the air.
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When thinking about muscle systems that create movement, it can be useful to think of them as levers. The video to the left gives a brief overview of the physics of movement in muscle groups that act as levers. |
Now that we have a basic understanding of how muscles and bones can be used, let's consider a walking on land. We'll use humans to understand how walking on two legs can be accomplished. The motion of walking in humans can be thought of as an inverted pendulum. To understand what that is, imagine a grandfather clock. In the clock is a metal ball swinging back and forth on a pole. This is a pendulum. If you flip the pendulum upside down and keep the movement the same, you have created an inverted pendulum. This is a useful way of thinking about walking because humans use their legs like inverted pendulums as we walk. We set our leg down, vault over it, and then swing it out to set it down again. If you follow the center of mass of a human while walking, it follows a pattern similar to the ball in the clock, but upside-down. A key characteristic of walking in humans is we always have one of our feet on the ground.
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We've looked at how walking on land works. But what about running? Running can be thought of as a mass-spring system. Instead of a pendulum, your leg can can be thought of as a spring and the rest of your body is a mass, or weight, held on top of the spring. Imagine bouncing on a pogo stick. The pogo stick has a spring in it and it bounces you (the mass) up and down. When running, you use both of your legs like pogo sticks. A key characteristic of running in humans is the flight phase of running, where both of your feet are off the ground. Check out the video to the left to see how Usain Bolt, the world's fastest sprinter, runs using his leg like a mass-spring system.
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Other animals use different gaits, or types of movement, like walking and running too. Horses have several different known gaits, such as trotting, galloping, and cantering. The differences between these gaits has to do with footfall pattern, how much time each foot is touching the ground, and if the animal is ever completely off the ground. There is also a difference in speed, like walking versus running in humans. It turns out, there are different speeds that different gaits are optimized for. |
Let's apply what we know about moving on land to horse racing. If you've ever watched a horse race, you'll know that jockeys don't sit on the horse while it's running. Rather, they stand up in the saddle in a crouched position while the horses are running. This is a way to decrease the force of the jockey on the horse. By standing up in the saddle, the jockey can move like a spring on the horse's back translating some of the downward force laterally. The jockey's center of mass moves out of sync with the horse's center of mass. This essentially means the horse feels less of the jockey's weight. And without the extra weight pushing down on the horse, it can run a little bit faster.
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Lastly, let's consider some man-made land movement: cars. Cars move very differently from legged animals. Most notably, cars have wheels and move forward without bouncing up and down too much. Wheels are a very efficient way of moving on land. So why don't we see animals with wheels? Why aren't there rabbits and deer that have wheels instead of feet? Wheels are not often seen in nature for two main reasons. 1.) Wheels need fairly flat surfaces to roll on in order to be most efficient, which is why we build roads everywhere. 2.) Wheels need parts that aren't connected to each other the way muscles and tendons are. Muscles and tendons work like pulleys and levers, pushing and pulling on each other while firmly connected. Wheels, do not have firm connections. They work by rotating completely around an axis. It's almost impossible to do that with muscles and tendons.
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Next, let's get away from land and delve into the sea. In Part Two, we'll explore how animals move in water.