, 1994) assessed the extent of inhibition of central activation with the twitch-interpolation technique, which is technically demanding during loading. Not surprisingly, Eastwood et al. (1994) were able to record interpolated twitches in only two out of three subjects undergoing inspiratory threshold loading. With this technique, it is difficult to determine whether PLX3397 mouse small interpolated twitches at the conclusion of loading are the result of near maximal diaphragmatic recruitment or the result of submaximal phrenic-nerve stimulation, limited signal resolution (caused by the use of single, as opposed to paired, stimulations) (McKenzie et al., 1992), disproportionate load-induced decrease in the Pdi signal elicited
by single twitches as compared to paired twitches (McKenzie et al., 1992), antidromic collision (Gandevia, 2001), or axonal refractoriness (Gandevia, 2001). In addition, the amplitude of interpolated twitches is affected by the extent of diaphragmatic motor-unit recruitment and it is not affected by diaphragmatic motor-unit firing rate (Beck et al., 1998). That is, the interpolation technique provides
one part of the information related to diaphragmatic activation (Beck et al., 1998). Recordings of EAdi, as in the current investigation, overcome the above limitations. On this basis, we feel confident that the submaximal EAdi at task failure was indeed evidence of load-induced GSI-IX order inhibition of central activation, which, in turn, was at least one of the mechanisms responsible for task failure (Fig. 4). The central role of alveolar hypoventilation in determining task failure is supported by several considerations. CO2 at task failure is an independent
predictor of time to task failure in healthy subjects exposed to various inspiratory resistive loads (Gorman et al., 1999). When healthy subjects breathe through a resistive load, time to task failure is shorter when rebreathing 5% CO2 than when breathing room air (McKenzie et al., 1997). Compared with our subjects, Mador et al. (1996) reported longer time to task failure (22.6 ± 2.2 vs. 7.8 ± 0.7 min, p = 0.0001) and lower PETCO2 (36 ± 1 vs. 46 ± 2 mm Thiamet G Hg, p = 0.002) when healthy subjects sustained a constant threshold load set at 60% of maximal inspiratory esophageal pressure. That is, the time to task failure is prolonged when threshold loading is not sufficient to produce a rise in CO2 and when the load is “constant” and not “incremental”. Activation of bronchopulmonary and respiratory muscles C-fibers is an additional upstream mechanism for the intolerable breathing discomfort at task failure. Activation of bronchopulmonary C-fibers could have been triggered by the intense intrathoracic pressures developed during loading ( Morelot-Panzini et al., 2007). Activation of respiratory muscle C-fibers could have been triggered by the load-associated increase in muscle tension ( Morelot-Panzini et al., 2007).