An examination of the inspiratory muscle metaboreflex: Functional implications and the influence of inspiratory muscle training

• Main Author: Lomax, Michelle Ellen
• Published: Brunel University 2007
• Subjects: 612.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487015

Inspiratory muscle fatigue (IMF) is knovm to attenuate exercise performance. One way in which this may arise is via a reduction of blood flow to other working skeletal muscle. Previous reports have shovm that IMF causes a reflexive reduction in resting limb blood flow, secondary to an h'1spiratory muscle metaboreflex, which reduces leg vascular conductance (LVC). '''hether this reflexive decline in leg blood flow is overridden in the presence of functional hyperaemic stimuli was unknown. Therefore, the purpose of the thesis was to assess the impact of inspiratory muscle work history upon limb exercise. Results: In the presence of pre-existing IMF, time to calf fatigue (calf Tlim) was significantly curtailed (9.93 ± 1.95 and 6.28 ± 2.24 min, control and IMP + PF, respectively; p < 0.01). As LVC declined during IMP, we propose that the exacerbated time course ofcalf fatigue was due to a decrease in Lve, secondary to the inspiratory muscle metaboreflex, which was elicited before calf exercise. Conversely, when the inspiratory muscles were allowed to recover prior to calf exercise, or the relative inspiratory muscle load was reduced following 4-weeks of inspiratory muscle training (IMT), calf THm was restored to its control value (calf THm = 9.55 ± 1.88 min and 9.52 ± 1.88 min, respectively; p > 0.05). However, when the inspiratory muscle load was increased after IMT to reflect the improvements in inspiratory muscle strength (19%; P < 0.01), calfTlim was attenuated (6.33 ± 1.67 min; p < 0.01). Importantly, the metaboreflex was not elicited when a significantly lower level of lMF was induced. This suggests that fatigue per se is not the trigger, rather, that a critical level of fatigue is required. These results demonstrate that; 1) calf THm is curtailed with pre-existing IMF, probably because of increased sympathetic outflow to the limbs; 2) IMT abolishes this response when an identical inspiratory muscle load is used; and 3) a critical level ofIMF is required to evoke an inspiratory muscle metaboreflex. Thus, our data suggests that the metaboreflex is potent enough to overwhelm the functional hyperaemia.