Sport-related concussions (SRCs) play a very debilitative role in athletics with between 1.7 – 3 million SRCs occurring annually (Clay et al., 2013;​ Zuckerman et al., 2015). This equates to upwards of 21.5 SRCs per 1000 athletic exposures. Research pertaining to SRC has received intense focus because of the substantial financial impact on the US healthcare system as well as the negative impact that SRCs have on the quality of life of athletes. Much of the research concerning SRC has been centered around management and ensuring adequate recovery from SRC-related symptoms. The current return to play (RTP) criteria from an SRC involves a battery of clinical tests increasing in cognitive and exercise intensity. Athletes typically pass standard RTP criteria and can resume their sport within 1-3 weeks post-SRC. However, recent literature has found that even after being cleared to RTP, long-term cognitive, neuromuscular, and motor impairments may persist. Furthermore, literature examining injury rates has shown that athletes that have a history of SRC are up to 3.7 times more likely to sustain a musculoskeletal injury upon their return to sport compared to athletes who have not suffered an SRC (Brooks et al., 2016;​ Cross et al., 2016;​ Herman et al., 2017 Lynall et al., 2015;​ McPherson et al., 2019;​ Pietrosimone et al., 2015). While there may be a direct link between the cognitive, neuromuscular, and motor control deficits and injury risk in the SRC population, the research to support this phenomenon is currently inconclusive and unable to establish a causal relationship. Prior to expanding upon the current SRC literature, it is important for readers to have a full understanding of SRC, current RTP guidelines, and the possible shortcomings in these guidelines that can place athletes with SRC history at elevated injury risk. A literature review (Chapter 1) precedes reporting of the successive experiments that were undertaken to more fully understand lower extremity function pertaining to SRC history and injury risk.

The overall purpose of this dissertation was to assess the influence of SRC as well as SRC-related mechanisms on lower extremity function during landing tasks. It has been suggested in the literature that athletes with a history of SRC perform high-impact tasks (e.g., jump-landing, jump-cutting) with decreased lower extremity (LE) neuromuscular control compared to control athletes thus increasing injury risk. Further research to support this statement and its applicability with different sports populations is warranted. Current literature comparing LE landing mechanics between SRC and control athletes have neglected to examine the influence of sex as well as limb symmetry during landing. Thus, our first two studies (Chapters 2 and 3) examine the influence of sex on LE mechanics and limb symmetry between athletes with and without a history of SRC. The first study (Chapter 2) assesses the influence of sex on lower extremity limb symmetry in athletes with and without a history of SRC. The second study (Chapter 3) assesses the influence of sex on LE biomechanics in athletes with and without a history of SRC. Both experiments utilized a cross sectional study design and implement identical experimental methods. Athletes from various sports with a history of SRC were recruited for these studies and comprised the SRC group. The healthy control group was comprised of athletes without a history of SRC and were matched by age, sex, and sport. Two-way ANOVAs (α =0.05) were conducted in each study comparing sex and concussion history on selected lower extremity biomechanics parameters that have been previously associated with LE injury risk. For the first study a symmetry index was computed to compare these LE biomechanics between limbs. It was determined that female athletes with a history of SRC had significantly greater limb symmetry in vertical ground reaction force than female athletes in the control group potentially placing them at the greatest risk for LE injury. For the second study, peak discrete biomechanical parameters during landing were obtained and comparisons were made between SRC and control groups. Results showed that males in the SRC group landed with a lesser knee flexion angle than males in the control group, thus placing them at greater risk for LE injury. The third study (Chapter 4) utilized a cross-sectional design on a population of healthy individuals. The purpose of the third study was to compare different directions of induced head acceleration on LE mechanics during landing. One group of individuals was assigned to do a continuous lateral hopping task while the other group was assigned to do a continuous vertical hopping task. Both groups performed a landing task before and after their assigned hopping intervention. Comparisons were made within each group pre- and post-intervention as well as between groups. The group that performed the continuous vertical hopping task exhibited greater vertical head acceleration during the hopping task compared to the lateral hopping task group. Along with greater head acceleration during the hopping intervention, the vertical hopping task group also demonstrated greater knee abduction during landing post-intervention compared to the lateral hopping group. This suggests the utility in assessing head acceleration and its influence on landing mechanics.

In summary, this dissertation consisted of three progressive studies to contribute to the understanding of the mechanisms behind the ongoing issue of athletes with a history of SRC demonstrating high impact landing strategies conducive to increased LE injury risk. The findings of our studies suggest that SRC history as well as induced head acceleration can have an impact on LE landing mechanics. A major limitation of these studies is their cross-sectional nature. This study design limits the conclusions drawn from our first two studies since it cannot be ascertained whether the mechanics observed occurred prior to the SRC being suffered. While our third study was not conducted on a population of athletes with a history of SRC, the cross-sectional design limits the conclusions that can be drawn. Future studies should adopt a study design that allows the investigator to examine the influence of SRC history and/or SRC mechanisms before and after an SRC has been sustained.