THE EFFECTS OF TRAINING HABITS ON CUMULATIVE LOAD AND TIBIAL STRESS FRACTURE INJURY RISK IN RUNNERS
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Running for exercise is beneficial for preventing chronic diseases, but the incidence and prevalence of running related injuries are high, creating a barrier to participation. Traditional research paradigms attribute high running injury rates to anatomical factors, training habits, and high peak loads resulting from gait mechanics. However, the specific mechanisms of tibial stress fracture injuries, a serious running-related injury, and why females are at such high risk for these injuries, are largely unknown. Runners often train at variable running speeds and durations that can affect the accumulation of potentially injurious loads, but until recently, studies on running injuries have mostly considered training habits and mechanical loads separately. Therefore, the purpose of this dissertation was to identify how training factors of running speed, volume, and duration contribute to the loads accumulated by the body in relation to tibial stress fracture injury risk. Specifically, this dissertation consists of three studies which determine i) the cumulative load of two proportions of running speed over a constant distance and average pace of running, ii) how fatigue-related gait adjustments affect the loads accumulated per-kilometer within a single prolonged run, and if there is a relationship between gait adjustments and physiological or cognitive fatigue outcomes; and iii) if fatigue-related changes in running gait affect the model-predicted cumulative damage and probability of tibial stress fracture. In study 1, a combination of slow and fast speeds led to greater estimated cumulative load compared to running at all normal speed. The greater cumulative load resulted from greater loading during slow running compared to fast running. In study 2, runners maintained gait mechanics and cumulative loads throughout an easy run to fatigue. In study 3, the model-predicted cumulative damage and probability of tibial stress fracture injury were similar between hypothetically maintained gait and measured fatigue-adjusted gait conditions. These results suggest running volume and average pace are not sufficient metrics for tracking cumulative load, and fatigue during running is not likely a major injury risk factor. Further, these results suggest that other training factors or individual factors may play a greater role in injury development than running speed, volume, or fatigue.