Inertial measurement units characterise load, terrain, and sex-specific biomechanics of human performance in a military context

<p dir="ltr">Quantifying performance and biomechanics in military personnel may allow practitioners to identify high performers or those resilient to injury in military training. Optical motion capture (OMC) is the gold standard to quantify movements that are associated with performance and/or indicative of injury risk. However, laboratory-based projects occur in a setting that may fail to elicit physical and cognitive demands common in real world scenarios. Advancements in inertial measurement units (IMUs) may enable their use outside of a laboratory to quantify movement patterns in dynamic environments. An array of IMUs can estimate joint kinematics, while acceleration signals can be surrogate measures for deleterious kinetic variables that are traditionally captured in a laboratory. This PhD explored the use of IMUs and the sensitivity of their outputs to the influence of load, grade (flat, uphill, downhill), and sex. </p><p dir="ltr">Initial validation (Chapter 2) compared IMU to OMC estimated kinematics during a bodyweight squat and countermovement jump, tasks common in clinical and performance testing. Ankle and knee joint range of motion were comparable between systems during a bodyweight squat, while hip and knee range of motion were comparable in a countermovement jump. Peak joint angles were significantly different between systems. </p><p dir="ltr">Validity of IMUs to estimate joint kinematics during locomotion, and if this approach is sensitive to changes in grade (flat, uphill, downhill) and/or body borne load (23 kg), was explored in Chapter 3. This chapter also produced an open-source MatLab application for use by other researchers. While IMUs could estimate walking hip and knee ROM, this was not observed in running. Furthermore, inertial measurement units were not sensitive to altered kinematics that occurred due to load and/or grade. However, acceleration metrics reportedly have a close relationship with laboratory-based variables (e.g., vertical loading rate) and were not explored in this study. IMUderived accelerations are minimally processed and field expedient and may be more sensitive to altered gait due to military-specific demands. </p><p dir="ltr">Chapter 4 explored the influence of body borne load, grade, and locomotor speed on acceleration and attenuation variables during locomotion. The relationship between acceleration metrics and OMC variables that may be indicative of performance were also measured. Load reduced accelerations, but elevated the amount of attenuation, and attenuation had a relationship with estimated physiological cost. Chapter 5 took this notion outdoor in a 5-kilometre ruck march to explore acceleration alterations due to out-of-laboratory responses to physical demands of a simulated ruck march. Similar adaptations to grade and sex-specific differences were observed. </p><p dir="ltr">A novel IMU-based approach can estimate joint range of motion during discrete tasks and walking, but not running. Accelerations derived from IMUs, but not estimated kinematics, characterise altered movement strategies that occur as a consequence of military-specific tasks, as well as sexspecific differences. It may be that biomechanics researchers move away from attempting to use IMUs to quantify lower extremity kinematics with total validity. Readily available data from IMUs are capable of supplying information that is sensitive to speed, load, grade, and sex to make informed decisions regarding physical demands for tactical populations. </p>