Sex-specific biomechanical and neuromuscular adaptations to a targeted physical training program for military load carriage

Tactical occupations within military organisations require soldiers to routinely undertake physically demanding tasks such as load carriage (i.e., carrying load externally fixed on the body). Carried loads are typically comprises essential equipment military personnel require during training and operational tasks, and therefore cannot be easily reduced without compromising operational capabilities. Recurrent exposure to excessive external loads adversely impacts on soldiers' physical capabilities (i.e., mobility, task sustainment, and strength), and are highly associated with increased risks of musculoskeletal injury and/or impaired performance. Importantly, the capacity for soldiers to tolerate external loading can be improved through physical training to help mitigate  otential performance implications and injury risk. However, current physical training sessions for military personnel are not tailored to meet specific occupational task demands, or individuals' physical capacity. This issue is of relevance for females who are now eligible to apply for combat-related roles in most countries, meaning they will be required to complete the same physical training and job-specific tasks as males. As females typically have lower physical capabilities compared to males, they may be at a disadvantage in these physically demanding roles. Identifying and understanding sex-specific responses to load carriage tasks and associated physical training will inform the optimisation of training programs that better prepare both males and females for operational readiness. Therefore, the purpose of this thesis was to quantify the neuromuscular, physical, and biomechanical adaptations to load carriage over time, and in response to an evidence-based physical training program. 

The first research paper, Chapter 3, examines the implementation of a 10-week physical training program which was designed using findings from previous research to target the neuromuscular demands of load carriage in a male civilian population. Current approaches in the military do not tailor training to match load carriage task demands, even though this method has been shown to improve individuals' physical capacities and mitigate associated injury risks. Fifteen male civilians completed a load carriage task equivalent to the minimum Australian Army All Corps physical employment standard (i.e., walking for 5 km at 5.5 km·h-1, wearing a 23 kg torso-borne vest) before and after 10 weeks of training. Measures of physical capacities (i.e., maximal jumps, push-ups, sit-ups, and beep test) were conducted before, during, and after the 10 weeks of training to identify neuromuscular adaptions, as evidenced by improvements in performance. Psychophysical responses were assessed during the load carriage task before and after training, which showed reductions in task perceptions, indicating an improved task tolerance as a direct result of training. These positive findings could serve as an alternative approach to training soldiers in roles that regularly undertake load carriage tasks. The manuscript highlighting physical and psychophysical performance improvements was published as Wills, J. A., Saxby, D. J., Glassbrook, D. J., & Doyle, T. L. A. (2019). Load Carriage Conditioning Elicits Task-Specific Physical and Psychophysical Improvements in Males. Journal of Strength and Conditioning Research, 33(9), 2338-2343. 

The second research paper, Chapter 4, examines the same 10-week physical training program completed by a female civilian population. These results were comparatively analysed to the male data presented in chapter 3 to examine if sex-specific responses exist between males and females in response to the same physical training. Since the opening of combat-related roles, females have seemingly been placed at a disadvantage due to inherent differences in physical capacities (e.g., strength) compared to males, despite undertaking the same training. Favourable main effects of training were found for both sexes, however, males outperformed females in all tests other than sit-ups. Surprisingly, females' push-up performance improved at mid- and post-testing, compared to the start; while male improvements were only realised at the end of the training period. Male cardiovascular responses as per beep test scores improved after training, but females did not. Irrespective of overall positive responses in performance, sex-specific differences were still evident after completing the same physical training, suggesting that females should be trained differently to males in order to optimise performance on load carriage tasks and physical conditioning in general. The manuscript describing these results has been submitted as Wills, J. A., Saxby, D. J., Glassbrook, D. J., & Doyle, T. L. A. (2019). Sex-Specific Physical Performance Adaptive Responses are Elicited after 10 weeks of Load Carriage Conditioning, Journal of Science and Medicine in Sport.

The third research paper, Chapter 5, investigates lower limb biomechanical adaptations during a load carriage task in a male civilian population. Load carriage tasks are known to alter lower limb biomechanics. However, limited research has examined time-course changes of loaded walking or detailed changes in external joint-level mechanics (i.e., joint moments, power, and work) in response to a physical training program for load carriage. Whole-body marker kinematics and ground reaction forces were acquired in over-ground walking trials before and after the 5 km load carriage task. Subsequently, individual scaled anatomic models were created in OpenSim and experimental data were used to calculate inverse kinematics and inverse dynamics to determine lower limb joint angles, net joint moments, powers, and work, respectively. Primary mechanical changes were observed at the knee and ankle (p < 0.05). Knee moments were maintained for longer after training compared to before. Positive power contribution shifted distally after training, increasing at the 5 km measure from 39.9% to 43.6% at the ankle joint (p < 0.05). Findings suggests that 10 weeks of periodised training may reduce injury risk through favourable ankle and knee joint adaptations and can enable individuals to sustain performance during a load carriage task. The manuscript detailing these results was accepted as Wills, J. A., Saxby, D. J., Lenton, G. K., & Doyle, T. L.A. (2019). Ankle and Knee Moment and Power Adaptations are Elicited through Load Carriage Conditioning in Males. Journal of Biomechanics, 97.

The fourth research paper, Chapter 6, is a continuation of research conducted in Chapter 5 and comparatively analysed male and female lower limb biomechanical data. This study aimed to identify and quantify potential sex-specific differences over the duration of a standardised load carriage marching task, and in response to the same 10 weeks of training. Training resulted in females generating significantly greater hip power during loaded walking compared to males, whereas ankle joint power contributions increased for both sexes. Over the duration of the 5 km march, most mechanical adaptive changes were observed in females. Findings strongly indicate sex-specific differences and highlights how physical training should be tailored to the requirements of each sex in order to maximise the benefits of training. The manuscript describing these results has been submitted as Wills, J. A., Saxby, D. J., Lenton, G. K., & Doyle, T. L. A. (2019). Lower Limb Biomechanical Responses during a Standardised Load Carriage Task are Sex-Specific. Journal of Biomechanics. 

The fifth research paper, Chapter 7, examined female-only physiological and psychophysical responses during the 5 km load carriage task before and after the 10-week training program. Reductions in maximal oxygen uptake (V 02) requirements during the load carriage task after training (p < 0.05) suggests participants improved their mechanical efficiency through neural and muscular responses to the strength aspect of training. Furthermore, a shift towards fat utilisation was observed after training, with other physiological responses demonstrating an increased ability to sustain the metabolic demands of the load carriage task. Results indicate the 10 weeks of training reduced the physiological demands of the load carriage task, and improved task tolerance. The manuscript describing these findings has been submitted as Wills, J. A., Drain, J., Fuller, J. T., & Doyle, T. L.A. (2019).  Physiological Responses of Female Load Carriage Improves after 10 weeks of Training, Medicine and Science in Sports and Exercise.

In conclusion, physical training that targets the specific neuromuscular demands of load carriage tasks helps to enhance individuals' physical capacities and load carriage performance, whilst potentially mitigating injury risks. The data from this thesis provides important insights into sex-specific neuromuscular, psychophysical, physiological, and biomechanical responses to physical training targeting load carriage. Importantly, these world-first findings provide evidence that can be used to inform future physical training programs that meet the specific requirements of each sex and load carriage tasks.