Inspiratory muscle training improves exercise capacity with thoracic load carriage

Ren-Jay Shei, Robert F Chapman, Allison H Gruber, Timothy D Mickleborough, Ren-Jay Shei, Robert F Chapman, Allison H Gruber, Timothy D Mickleborough

Abstract

Thoracic load carriage (LC) exercise impairs exercise performance compared to unloaded exercise, partially due to impaired respiratory mechanics. We investigated the effects of LC on exercise and diaphragmatic fatigue in a constant-load exercise task; and whether inspiratory muscle training (IMT) improved exercise capacity and diaphragmatic fatigue with LC. Twelve recreationally active males completed three separate running trials to exhaustion (Tlim ) at a fixed speed eliciting 70% of their V˙O2max . The first two trials were completed either unloaded (UL) or while carrying a 10 kg backpack (LC). Subjects then completed 6 weeks of either true IMT or placebo-IMT. Posttraining, subjects completed an additional LC trial identical to the pretraining LC trial. Exercise metabolic and ventilatory measures were recorded. Diaphragm fatigue was assessed as the difference between preexercise and postexercise twitch diaphragmatic pressure (Pdi, tw ), assessed by bilateral stimulation of the phrenic nerve with esophageal balloon-tipped catheters measuring intrathoracic pressures. Tlim was significantly shorter (P < 0.001) with LC compared with UL by 42.9 (29.1)% (1626.5 (866.7) sec and 2311.6 (1246.5) sec, respectively). The change in Pdi, tw from pre- to postexercise was significantly greater (P = 0.001) in LC (-13.9 (5.3)%) compared with UL (3.8 (6.5)%). Six weeks of IMT significantly improved Tlim compared to pretraining (P = 0.029, %Δ +29.3 (15.7)% IMT, -8.8 (27.2)% Placebo), but did not alter the magnitude of diaphragmatic fatigue following a run to exhaustion (P > 0.05). Minute ventilation and breathing mechanics were unchanged post-IMT (P > 0.05). Six weeks of flow-resistive IMT improved exercise capacity, but did not mitigate diaphragmatic fatigue following submaximal, constant-load running to volitional exhaustion with LC.

Keywords: Diaphragm fatigue; flow limitation; performance; respiratory muscles; ventilation.

© 2018 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.

Figures

Figure 1
Figure 1
Tlim (Panel A) and Pdi, tw (Panel B) data for LC and UL conditions at baseline. Data are given as mean(SD). *significantly different between LC and UL. Tlim was 42.9(29.1)% shorter (P < 0.001, 95% CI: −990.4 to −379.9 sec) with thoracic LC (1626.5 (866.7) sec) compared to UL (2311.6 (1246.5) sec). The postexercise reduction in Pdi, tw was significantly greater in LC (−13.9 (5.3)%), but not in UL (−3.8 (6.5)%); P = 0.001, %Δ, +261.3; 95% CI: −14.7 to −5.3%.
Figure 2
Figure 2
Pre‐ and Posttraining Tlim for each group (open circles, IMT, open squares PLA). Tlim (Panel B) and Pdi, tw (Panel C) data from pre‐ to postIMT. *significantly different from pre‐ to posttraining. Tlim in the IMT group improved significantly more (median change 20.0%, range +9.9 to +45%; P = 0.046, 95% CI +3.7 to +65.1%) compared to the PLA group (median change −3.5%, range −56.6 to +20.6%).

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