INTRODUCTION: Thoracic loading decreases compliance of the total respiratory system in a mass dependent manner. These perturbations of the tissue elastic properties are likely to impact on the work of breathing. Therefore, we assessed that impact the combined effects of the restrictive and inertial forces accompanying thoracic loading, using a body armour and backpack ensemble, during standing rest and sub-maximal exercise.
METHODS: Nine males (age: 27 [SD 5] y, height: 182 [SD 7] cm; mass: 79 [SD 8] kg) participated in three treatments: a control (clothing only) and two loads with a 25:75 mass distribution (35 and 54 kg) during two conditions: standing rest (n=9) and treadmill walking at 4.8 km.h-1 (n=6) each lasting 15 minutes to obtain steady state. An oesophageal balloon (10 cm) was used to attain the static tissue compliance for the lung tissue, chest wall and total respiratory system. Elastic work of breathing for all three tissues were then calculated via integration between the respective static compliance curves and the zero pressure axis over the end-expiratory to end-inspiratory lung volumes during both standing rest and walking conditions. Values are reported as mean (± SEM).
RESULTS: Increasing thoracic loading with the body armour and backpack ensemble (25:75 mass distribution) did not significantly modify the elastic work of breathing for the total respiratory system during standing rest (control: 0.361 ± 0.108, 35 kg: 0.619 ± 0.156, 54 kg: 0.841 ± 0.235 J, p > 0.05). However, when walking at 4.8 km.h-1 the elastic work of breathing for the total respiratory system significantly increased in a mass dependant manner (control: 0.299 ± 0.185, 35 kg: 0.838 ± 0.277, 54 kg: 1.105 ± 0.365 J, p < 0.05).
CONCLUSIONS: These observations demonstrate that the elastic work of breathing for the total respiratory system was increased by the body armour and backpack ensemble and this was most evident during submaximal exercise. A modified breathing pattern during standing rest likely protected against this physiological strain. The elevated elastic work of breathing during sub-maximal exercise, under conditions in which load carriage already elicits significant increases in ventilation, was unable to be avoided. This potentially predisposes the individual to respiratory-muscle fatigue during occupational load carriage tasks and thereafter decreased exercise tolerance.