background: firefighters experience high rates of injury – both physically and to their mental health. firefighting is a unique context to study the interaction of these factors due to the cyclical nature of the work and interaction of physiological, biomechanical and mental health demands. measuring these constructs over multiple time-points will provide insights into the cumulative demands associated with firefighting. thesis objectives: to quantify the biomechanical, physiological, and psychological loads during two simulated firefighting tasks, at three time-points over one year and assess variability in these factors over a one-year period, among thunder bay fire rescue firefighters. the overarching goal of this thesis is to identify prevalent multifactorial firefighting injury risk factors for the development of effective injury prevention strategies. methods: biomechanical load was inferred from measurement of musculoskeletal (msk) injury risk using the ovako working posture analyzing system (owas) applied to video of firefighters performing two simulated firefighting tasks. physiological load was determined from metrics extracted from the zephyr bioharness physiological monitoring device worn by firefighters during both tasks. the critical incident inventory (cii) questionnaire was used to determine critical incident exposure and infer impacts on firefighters’ mental health. data was collected at three time points over one year (november 2017, may 2018, november 2018) to gain insights on cumulative loads associated with firefighting. results: analysis of variance over time indicated that heart rate (f(2, 48) = 4.685, p < .05, ηp2 = 0.163) and working intensity (f(2, 48) = 5.598, p < .05 ηp2 = 0.189) significantly increased from baseline to one-year during the performance of the hose drag task. heart rate variability significantly increased from baseline to one-year during the performance of the patient transfer task while lifting at the head (f(2, 26) = 5.2030, p < .05, ηp2 = .287) and lifting at the feet (f(2, 28) = 3.807, p < .05, ηp2 = .214). there were no statistical differences in msk injury risk over time. critical incident exposure remained consistently high (xg = 6.3 incidents) across all timepoints within the one-year period (x2(2) = 0.977, p = .614) with highest exposures to incidents involving direct exposure to blood and body fluids as well as incidents involving one or two deaths. conclusion: thunder bay fire rescue firefighters’ exposures to biomechanical, physiological, and psychological loads appear to be high and remain high over the one-year period. musculoskeletal injury risk during task performance is also high and remains unchanged over time. results indicate that there is a need for ergonomic intervention targeting biomechanical, physiological, and psychological loads associated with firefighting. future studies should consider the interaction of these factors on firefighter work health including msk injury risk.