The Human Body and Flight : Why We Can’t Fly
A 1779 engraving by Jacques Gamelin of a winged skeleton, originally from Gamelin’s book Nouveau Recueil d’Osteologie et de Myologie, or New Collection of Osteology and Mycology.
The ability to fly is something that’s been on the human mind since time immemorial. We see birds and other creatures flying through the air, and we seek to fly as they do. We have dreams about flying, and we’ve thought up myriad examples of human-like characters throughout our legends and myths who can fly. For all our hoping, however, it’s impossible for us to fly under our own strength. Let’s take a closer look at why this is true.
Our dreams about flying began in our myths, legends and folk-lore. Our ancestors told myriad stories about angels, seraphs, cherubs, fairies, and many other winged creatures with the power of flight. These characters usually had human-like bodies with wings attached to their backs, similar to the above illustration. These added wings have no apparent bearing on the creature’s arm or leg muscles, which is a luxury we real humans don’t have. If we want to fly under our own strength, we need to use our pectoral and arm muscles to do so.
In order to fly under our own strength, our muscles need to generate enough power to lift our bodies off the ground. Enter the field of biomechanics, which is the study of how our bodies function and move. Also enter the father of biomechanics, Giovanni Alfonso Borelli. Born in Naples in 1608, Borelli was a Renaissance scientist whose work focused on physics and physiology. His major work, De Motu Animalium, or On the Movement of Animals, was the first major investigation into biomechanics. Throughout the work, Borelli studied the flight of birds and made many discoveries relating to animal muscles and how they function. He also considered human flight, and he calculated that our bodies are too heavy and our muscles are too weak to fly. Put simply, he concluded that it is impossible that men should be able to fly craftily by their own strength.
Plate from Giovanni Alfonso Borelli’s 1680 De Motu Animalium, exploring the physiology of animal movements, including humans, birds and horses. Borelli saw animal bodies as the same as machines, and he applied mechanical principles to animal movement throughout the work.
In order to reach his conclusion, Borelli used what he learned about bird flight and applied it to the human body. His hypothesis was based on three points. First, the strength and size of our muscles. Second, the design of the wings. Third, the weight of the human body. In order to demonstrate this hypothesis, he compares the human body to the body of a bird.
Birds are able to fly in part because their pectoral muscles are very dense and quite large in proportion to their body weight. According to Borelli, the muscles that generate the forces needed to fly in a bird are no less than one sixth of the bird’s entire body weight. This allows a bird’s wings to generate a force ten-thousand times greater than the resistance of their weight to gravity. Therefore, in order for a human to fly, we’d need to generate a similar force related to our own body weight.
The problem is, our pectoral muscles are much smaller than one-sixth of our body weight, and they aren’t nearly as dense as a bird. We also don’t have built-in wings, so we’d need to account for that added weight as well. Therefore, if we want to fly, either our muscle mass must increase substantially or our body weight must decrease substantially. Borelli concludes by declaring that wing flapping by the contraction of muscles cannot give out enough power to carry up the heavy body of a man.
Illustration of human arm muscles, drawn by Jean-Baptiste Marc Bourgery for his Traité complet de l’anatomie de l’homme, or Complete Treatise on the Human Anatomy. The treatise was published between 1831 and 1854, with the above illustration from 1834.
The question of scaling up our muscles is an interesting one to ponder, but it’s a dead-end when it comes to human flight. This is because of the strength-to-weight ratio of our muscles. Put simply, as our muscles grow, their weight increases faster than their strength. Therefore, an increase in muscle mass doesn’t equal the same gain in strength. This is because the cells that make up our muscles have a fixed size, which makes a true scaling of size impossible: you can’t increase the size of your cells, you can only add more of them to the muscle. In addition, as our muscles grow, they also get heavier, which means they must generate more and more power in order to lift the extra weight, and so on and so on. Successful flight is a balancing act, and our bodies aren’t balanced enough to achieve it. Birds are able to fly because their body weight is in balance with their muscle strength and wingspan. They are also much smaller and lighter than us, which helps too.
We’ve known for quite some time that human muscles alone aren’t enough to produce flight, but that hasn’t stopped us from trying. The early history of human flight is filled with wing-like contraptions, inspired by myths such as the Greek myth of Daedalus and Icarus. Since then, we abandoned this idea and went on to develop an amazing history of various ideas for flying machines, and we’ve ultimately achieved flight through mechanical means. There’s still something primal and freeing about the idea of spreading a pair of wings and taking flight under your own strength, though.
Read about flying machines throughout history here.
Quotes from Borelli taken from: Borelli, Giovanni Alfonso. “De Motu Animalium” in A Source Book in Animal Biology. Edited by Thomas S. Hall. Cambridge: Harvard University Press, 1970. 158-164. Digital transcript available at http://www.originalsources.com.