On the head: the true impact of routine head strikes in sport

Abstract

Published research suggests a potential link between repetitive subconcussive head impacts and alterations in brain function. The results, however, are ambiguous due to the lack of direct evidence for this relationship. A non-invasive brain stimulation technique (transcranial magnetic stimulation, TMS) can detect changes to brain function following mild, moderate and severe brain trauma; yet, no data exist on its effectiveness in detecting subconcussion-related alterations. As such, the overall aim of this thesis is to determine whether TMS is able to detect acute changes in brain function as a result of repetitive subconcussive head impacts (RSHIs). Chapter 1 highlights techniques with potential to detect brain alterations following subconcussive, and concussive head impacts. TMS was identified as a possible tool for detecting such changes. Chapter 2 demonstrated primary and secondary outcome measures corticomotor inhibition and corticospinal excitability, respectively, to have good overall day-to-day reliability. Subsequently, corticomotor inhibition was transiently increased following two separate RSHI models (soccer heading in chapter 3, and sparring in chapter 4). Further, motor unit recruitment was also altered following sparring. Corticospinal excitability and postural control were unchanged in both studies, whilst parameters of cognitive function appeared altered in the immediate follow-ups. These data indicate that RSHIs are associated with measurable, albeit transient alterations to brain function. Chapter 5 further corroborates the notion that brain alterations in chapters 3 and 4 were due to RSHIs, by showing that corticomotor inhibition and corticospinal excitability are largely unaffected by the sole act of performing exercise. No alterations were also observed in the last experimental chapter; the pilot study explored the feasibility of using TMS in detecting alterations following cumulative exposure to RSHIs. This thesis is the first to provide direct evidence for a relationship between RSHIs and brain alterations. The data suggest that subconcussive head impacts, routine in a number of sports, may impair the brain’s ability to control the body (in turn increasing the risk of injury) as well as affecting overall brain health.