Effects of Body Armor on Respiratory Muscle Fatigue and Muscle Blood Flow During Progressive Treadmill Marching

Introduction: Torso-borne loads such as body armor are essential for modern military operations but impose mechanical and metabolic strain. These physiological stressors can trigger the inspiratory metaboreflex, which redirects blood flow from locomotor to respiratory muscles, further reducing muscle oxygen and endurance. While aerobic fitness has been shown to alleviate these effects, the interaction between sex, aerobic capacity, and oxygenation under load is not well understood. This study examined the physiological effects of torso-borne load carriage on respiratory muscle fatigue, skeletal muscle oxygenation, and exercise performance to volitional exhaustion and the degree to which sex and aerobic capacity influence these effects.Methods: Nineteen physically active participants (9 males, 10 females; 33±8 years) with prior load carriage experience completed a maximal graded VO2max treadmill test and two randomized treadmill marching trials to volitional exhaustion: unloaded and loaded with a standardized 28 kg military fighting load (body armor, helmet, and rifle). Each trial included four 10-minute stages which progressively increased in speed and grade until volitional fatigue. Maximal inspiratory pressure (MIP), ratings of perceived exertion (RPE), and blood pressure (BP) were assessed between intervals, with blood lactate collected pre- and post-exercise. Near-infrared spectroscopy (NIRS) sensors were placed on the vastus lateralis, triceps surae, and intercostal muscles to monitor changes in skeletal muscle oxygenation (SmO2) and total hemoglobin (THb). Results: Time to exhaustion was significantly reduced in the loaded condition (2642±589 s) compared to unloaded (3833±636 s; p < 0.001; d = 1.8), with males completing more stages under both conditions (loaded: 6.4±0.5 vs. 4.5±0.5 stages; p < 0.001). Despite carrying a greater relative load (42.0±6.7% vs. 34.4±4.5% body mass; p < 0.01), females exhibited smaller declines in quadriceps SmO2 under load (ΔSmO2: -15.9±13.2%) compared to males (ΔSmO2: -22.4±15.1%) over the first four stages, with a dramatic, significant decline in quadriceps SmO2 in males at volitional exhaustion (male ΔSmO2: -19.3±2.7%; female ΔSmO2: +6.4±4.5%). Post hoc analysis showed a medium-to-large sex difference in intercostal SmO2 under load (d = 0.6), favoring females, while differences were negligible in the unloaded condition, suggesting load-dependent sex disparities in respiratory muscle oxygenation. Higher VO2max was weakly associated with preserved quadriceps oxygenation under load (r = 0.33), although aerobic fitness explained only 11% of the variance in SmO2 responses. Conclusion: Torso-borne load carriage reduces exercise tolerance by impairing ventilatory mechanics and muscular oxygenation responses and accelerating the onset of respiratory muscle fatigue. These effects are consistent with activation of the respiratory muscle metaboreflex and greater cardiovascular strain. Notably, females demonstrated relative fatigue resistance under load even when carrying a higher percentage of their body mass. This suggests that sex-specific physiological differences, such as less absolute force production and decreased vascular compression, may preserve muscle perfusion. While aerobic fitness offers some protection, it does not fully explain inter-individual variability in muscle oxygenation responses. These findings identify the need for optimized load carriage strategies and appropriate conditioning programs among mixed-sex tactical populations.