THESIS ABSTRACT This thesis work was undertaken to investigate the effects of internal (i.e. morphology, physiological capacity, stress and inflammatory cytokines) and external (i.e. heat exposure) factors on military physical performance in members of the Canadian Armed Forces (CAF). By gaining insight into how these factors affect military physical performance, training and intervention strategies could be better tailored for optimizing performance. Both morphology and physiological capacity have previously been recognized to affect performance on certain military physical performance tasks. However, the effect of such factors on the newly implemented Common Military Task Fitness Evaluation (CMTFE), the current physical employment standard for the CAF, has not been determined. Stress and inflammatory cytokines have also been shown to affect physical performance in the general population, but there is no data available on levels of stress and inflammatory cytokines in the CAF, or their potential effect on military physical performance. Recently, the CAF have also implemented a loaded march, followed by an assessment of military physical performance (FORCEcombat), as a part of the physical performance testing for all members of the Canadian Army. However, there is a lack of research currently available on how factors like heat exposure can affect thermoregulatory and cardiovascular response, as well as performance on a loaded march, and the following FORCEcombat test. In order to provide key information about the requirements and delivery of the military physical performance tests currently used in the CAF, my thesis focused on four main parts to better understand the importance of some of the internal and external factors known to affect physical performance. Firstly, my thesis assessed CAF members’ morphological and physiological characteristics that may affect overall performance on the CMTFE. In Chapter 2, results showed that both characteristics of morphology and physiological capacity separated the top and bottom performers. Even though a difference in morphology was observed between top and bottom performers, performance on the CMTFE was mainly dependent on aerobic capacity and measures of strength, rather than morphology. Aerobic capacity explained ~36% of variability in performance among women, and ~32% variability in performance among men. Core strength also had a significant effect on performance in both groups, however, men relied more on upper body strength than did women. Apart from showing that physiological capacity, rather than morphology was the main component affecting performance outcome on the CMTFE, it was also concluded that, unlike physical performance tests used by the U.S. Armed Forces (i.e. push-ups, sit-ups, mile run), no body mass bias exists against larger individuals performing the CMTFE. Chapter 3 and 4 focused on describing levels of stress and inflammatory cytokines among CAF members, and their effect on military physical performance. Stress exposure is known to induce an increase in the production of stress and inflammatory cytokines, and an increase in levels of inflammatory cytokines have been shown to be associated with a decrease in physical performance in general population. Members of the armed forces are susceptible to high stress exposure, but no current data exist on basal levels of stress and inflammatory cytokines in a military population. We therefore performed a descriptive analysis of levels of stress and inflammatory cytokines among CAF members. The results from this analysis showed a generally low detection rate of most of the inflammatory cytokines measured in our military population. However, we did observe a higher detection rate for IFN-γ, TNF-α, IL-2, IL-5, IL-8, IL-17a, IL-23 and IL-31 with increasing age. Adiponectin levels were higher in women compared to men (5.81 (3.52-13.19) µg/ml vs 16.71 (7.68-25.32) µg/ml), whereas IL-18 levels were higher in men compared to women (89.25 (84.03-94.48) vs 75.91 (69.70-82.13) pg/ml). Increasing age was associated with higher basal levels of C-Reactive Protein (CRP), Adiponectin, IL-18 and IL-2, and we also found a significant positive correlation between body fat percentage (BF%) and CRP levels. Following the outcomes of the descriptive study, the associations between levels of stress and inflammatory cytokines and military physical performance were elucidated. Using multiple linear regression, controlling for covariates such as age, sex and BF%, a significant negative association was observed between CRP levels and Total Performance on the CMTFE (p=0.01), picking and digging performance (p=0.04), aerobic capacity (p=0.05) and plank time (p<0.01) among CAF members. Finally, in Chapter 5, the effect of heat exposure on the capacity to perform a task oriented military test was quantified. Many CAF members are subject to a significant heat exposure on a daily basis, and studies from the general population have shown that heat exposure can have a detrimental effect on physical performance. As mentioned previously, The Canadian Army have recently implemented a test of loaded march performance, as a part of their occupational physical performance testing. The loaded march will be performed while wearing military personal protective equipment (PPE) and a daypack (~35 kg), and all members of the Canadian Army will be required to perform this test. Due to this need for mass testing, a large part of the loaded march performance assessments will be required to take place during the summer months. Temperature and relative humidity (RH) can reach high levels on several of the Canadian Army bases located around Canada, during the summer months. However, there is currently limited research available assessing the thermoregulatory and cardiovascular responses to performing a loaded march in the heat while wearing military PPE. Consequently, a study was designed to determine the thermoregulatory and cardiovascular responses to a loaded march (60 min, 5.17 km*h-1, ~35 kg external load) at normal temperature (21°C, 50% RH) and in the heat (30°C, 50% RH). This study also aimed to quantify the effect of heat exposure and previous experience on loaded march and military physical performance (FORCEcombat). Ten participants experienced with loaded march (military reservists), and ten participants inexperienced with loaded march (civilians) were recruited for this study. The results showed that whereas nine out of ten participants in the experienced group completed the loaded march in the heat, only five of the ten participants in the inexperienced group were able to do the same. Performing the loaded march in the heat while wearing military PPE led to a state of uncompensable heat stress, for both the experienced and the inexperienced group. Both groups showed a continuous increase in core temperature and heat storage (0.025°C/min and 0.02°C/min mean increase in core temp, 8.7 kj/min and 6.7 kj/min mean increase in heat storage, for the inexperienced and experienced group respectively) throughout the heat trial. Apart from the difference in completion rate on the loaded march, experienced participants also had a lower heart rate (134.2±11.9 vs 143.1±8.9 bpm, p≤0.05), perceived exertion (10.2±1.4 vs 13.0±0.9, p≤0.05), thermal comfort (1.9±0.5 vs 2.4±0.4, p≤0.05), and FORCEcombat completion time (662±133 vs 530±49 sec, p≤0.05) compared to the inexperienced participants. The overall results from this thesis show that physiological capacity, inflammatory cytokines, heat exposure and previous experience, all have an effect on military physical performance. It was found that physiological capacity rather than morphology, was the superior predictor of performance on the CMTFE: inflammatory cytokines are present in CAF members and CRP levels increased with increasing age, CRP levels were negatively associated with military physical performance, performing a loaded march in the heat while wearing military PPE exposed both experienced and inexperienced participants to a state of uncompensable heat stress and decreased performance on the FORCEcombat test, and that previous experience has a positive effect on loaded march completion rate and FORCEcombat performance.