Mild traumatic brain injury (mTBI) is recognized as a major public health threat. Concern for mTBI is heightened due to mounting evidence that it may produce debilitating long-term effects, particularly in athletes. In sports, health measures to better detect and prevent mTBI are limited because we haven’t figured out how and why certain brain structures are injured during head impacts. In this thesis, I present a mechanism of trauma to the corpus callosum, a critical structure for communication in the brain, and its implications for injury detection and prevention. From novel measurements of human head motion during sports impacts, including the first direct and complete rotation measurements of a human mild traumatic brain injury, I present four key findings. First, I observe that head rotation measurements are actually more predictive of injury risk than the translational counterparts used in government and other regulatory safety standards. Second, I show that particular directions of head rotation can cause corpus callosum trauma by driving motion of a rigid membrane above it. Third, I characterize the brain’s tolerance to rotational velocity and show high levels can be sustained in a low-acceleration regime. Finally, I show how current helmet safety standards use a model of head rotation that cannot reproduce the conditions likeliest to cause mTBI. These findings can motivate more targeted, and thereby more effective, approaches to detecting, preventing, treating and ultimately mitigating the societal burden of mTBI.