This project studies how changes in the body that occurred during human evolution affect the ability to throw well. Humans are unique among primates in being able to throw objects with both great precision and speed. This ability may have helped our ancestors hunt and defend themselves. To better understand the evolution and biomechanical bases for human throwing capabilities, this study investigates how changes in the shape of the bones in the arms, shoulders and back affect throwing performance. The researchers are collecting data on how the body moves during a throw using a high speed, 3D camera system. They will then use a custom written computer program to break down and examine the individual motions of each body part during that throw. These data will be collected from a large sample of individuals who vary in the anatomy of their arms, shoulders and back. In addition, restrictive braces will be used to alter the throwing motion in controlled ways. These manipulations will allow us to assess how different components of the trunk and arms contribute to speed and accuracy.
This project uses an interdisciplinary approach combining aspects of anthropology, biology, physics and sports medicine to address how different aspects of human anatomy relate to throwing performance. A better understanding of this relationship will improve not only our understanding of human evolution, but will also contribute to our knowledge of how injury occurs in throwing athletes.
How and why do humans throw projectiles with both considerable accuracy and high velocities? Human throwing ability is unique among animals and universal to all human cultures. High-speed throwing was likely important during human evolution, enabling our ancestors (hominins) to hunt more effectively and safely, to drive off predators, and to interact aggressively with other individuals from a safer distance than more direct forms of fighting. Despite the potential adaptive importance of high performance throwing, little is known about the evolution of this unique and critical behavior. This project investigated how humans produce high-speed throws and examines how differences in skeletal and soft tissue anatomy affect throwing performance. The results of these analyses are applied to the hominin fossil record in order to provide insight into the evolutionary history and antiquity of high-speed throwing. We collected 3D movement data on humans as they threw. In order to test how differences in anatomy affected how well individuals threw, we looked at both anatomical features in which our subjects varied naturally and also created variation in other features using restrictive braces. We then analyzed the resulting movement data using Newtonian physics to determine the forces and motions occurring at each joint. We found that humans produce very fast throws by storing energy in the ligaments and tendons of the shoulder and using this energy to power rapid movements in the arm. This energy is stored as the arm is cocked back during the throwing motion. When this energy is released the arm recoils in a whip-like manner, producing the fastest motion that has been observed in the human body (internal rotation of the humerus). This is the first documented instance of elastic energy being used in the human upper body. We also found that the largest contributions to throwing speed come from rotation at the hips, accounting for more than half of the energy needed to power high-speed throws. Additionally, our data suggest important roles for the wrist and elbow during throwing, despite motions at these joints being produced mostly passively. When the data we collected were used to interpret the hominin fossil record, we found that the ability to produce high-speed throws likely dates back almost 2 million years, to Homo erectus. Given that the earliest evidence of hunting dates to approximately this time, we propose that the high-speed throwing ability was probably first used to help our hominin ancestors hunt using wooden spears and stones. In addition to providing insight into the course of our evolution and help us better understand the amazing human ability to throw, this project also makes significant contributions to our understanding of the mechanical workings of the body. By better understanding the human throwing motion, we can help to address why certainly individuals (such as some throwing athletes, like baseball players) are more likely to become injured. Our data also provide information that can be used to better help train young throwers to throw effectively and try to avoid injury. We have also developed and tested new techniques to examine skeletal anatomy in living humans without subjecting those individuals to radiation from x-rays or CT scans. These techniques can be used by coaches and physical therapists to determine which individuals may be at heightened risk of injury. In order to reach the broadest possible audience for our results, we have and will continue to publish and present our finding in both anthropological and medical journals and at professional meetings. This project represents the doctoral dissertation training and work of N. Roach, but also provided an opportunity for many undergraduate students (including minority students) to participate in research and learn about human anatomy, the physics of human motion, and our evolutionary history.
