Functional Inspiratory Muscle Training Improves The Strength of Inspiratory Muscles During Load Carriage In Cold-hypoxia

Abstract

PURPOSE: Load carriage (LC) exceeding 20kg elicits respiratory muscle fatigue (RMF) in sea level thermo-neutral conditions. Sub-maximal physical activity in cold-hypoxia has shown to elicit RMF. Inspiratory muscle training (IMT) combined with LC has failed to reduce RMF. Functional IMT (FIMT) may activate non-respiratory roles of the diaphragm and respiratory muscles resulting in adaptations beyond that of static IMT. METHODS: Loaded (18.2kg) walking trials were completed pre, mid and post training (trial 1, 2 and 3, respectively). Participants (n=15) performed a 6km loaded walk at 50%VO2max over 4 stages 0 - 6km in 0.5 increments at 0, 5, and 10% gradient in cold-hypoxia (4300m in -10°C). Following trial 1, participants were randomly assigned to control (Con=7) or experimental (Exp=8) to undertake 4 weeks of IMT using a pressure threshold training device. The Exp performed 2 x 30 breaths daily at 50% maximal inspiratory pressure (Pimax) and Con performed 60 daily breaths at 15%Pimax. FIMT (5 exercises designed to engage core muscles, 3 involved LC) was then performed 3 times weekly at the same intensities as IMT. RESULTS: Inspiratory muscle fatigue was prevalent following trial 1 (p <.001). Relative to baseline (126.9 ± 15.7 cmH2O) trends were identified for greater Pimax in Exp post-IMT (145.5 ± 20.5 cmH2O, p=.066) with no changes in Con. FIMT showed no further significant increase in Pimax (p=.104). Pimax values post-6km in Exp were significantly greater than Con and higher than pre-6km pre-intervention values (p=.007). However, ΔPimax was unchanged in Exp (p ≥.214). No significant relationships were observed between ΔPimax vs baseline Pimax and VO2max vs ΔPimax. CONCLUSION: Four weeks of IMT and FIMT strengthened inspiratory muscles by 23%. Despite no reduction in RMF, Pimax values were significantly greater post-6km in Exp and higher than pre-6km pre-intervention values. Protocols employing more progressive training loads may reduce RMF. Due to the low intensity and prolonged nature of LC in cold-hypoxia, respiratory muscle endurance may be more dominant than strength. Thus, requiring further investigation.