1Center for the Advanced Study of Hominin Paleobiology, The George Washington University, 2Department of Human Evolutionary Biology, Harvard University
Saturday 1:30-1:45, Ballroom C
High-speed throwing is a distinctive human behavior and may have been an important mode of early hunting. However, the biomechanical bases of how humans produce high-speed throws are not completely understood. Using inverse dynamics analyses of kinematic data from 20 baseball players fitted with 4 different restrictive braces, each of which inhibits specific motions, we test a model for how power is generated at key joints during the throwing motion.
We show that achieving high projectile speeds during throwing requires a combination of elastic energy storage at the shoulder, as well as the transfer of kinetic energy from proximal body segments to distal segments. We find that the largest power contributions during the throwing motion come from rotation of the torso. However, intervertebral brace restriction data suggest that this rapid, torso rotation motion (average peak ~850 deg/sec) is powered almost exclusively (90%) by the large, hip rotator muscles. Further analyses show that this hip-driven, torso rotation motion accounts for between 30-50% of the power needed to internally rotate the humerus, extend the elbow, and flex the wrist during throwing.
These data suggest that evolutionary shifts in the hip rotator muscles, most notably the gluteals, would have significant effects on throwing performance. Fossil evidence for the expansion of the gluteals and for the rotational mobility of the spinal column suggest incremental shifts in torso mobility and power generation began in the australopiths and likely reached modern levels in Homo erectus. The adaptive contexts of these anatomical shifts are also considered.
This study was generously funded by: National Science Foundation, BCS - 0961943 (Roach & Lieberman)