The determination of MIP is simple, and is typically obtained by measuring mouth pressure during a maximal isometric inspiratory maneuver performed at residual lung volume (i.e., Mueller maneuver).
While the POWERbreathe[R] S-index cannot measure an individual's true MIP, we hypothesize that the POWERbreathe[R] S-index can offer a reliable alternate measure of respiratory muscle fatigue to the gold-standard technique, the Mueller maneuver.
The chief investigator described and demonstrated the correct techniques for the measurement of MIP during the Mueller maneuver, and the S-index using a POWERbreathe[R] inspiratory muscle trainer (Ironman K5, HaB Ltd, UK).
MIP was assessed on a custom-made Manometer by performing the Mueller maneuver immediately before and after the repeated-sprint protocol.
A fully factorial ANOVA with repeated-measures for trial and day was used to compare S-Index values across all six trials for the POWERbreathe[R] while a simple 1-way repeated measures ANOVA was used to determine differences in MIP values across three trials for the Mueller maneuver. When differences were detected, pairwise comparisons were performed to determine their source.
Figure 2a illustrates the relationship between peak MIP measured during a maximal isometric inspiration using the Manometer (i.e., Mueller maneuver) and the S-Index determined during a maximal dynamic inspiration using the POWERbreathe[R].
However, the POWERbreathe[R] S-index is not an equivalent measure of MIP when obtained during the gold-standard test, the Mueller maneuver. In addition to these findings, it is reported that repeated-sprint cycling does not induce globalized inspiratory muscle fatigue in recreationally-active men and women.
The measurement of MIP during the Mueller maneuver yielded an ICC of 0.95 indicating "almost perfect" reliability.
Nonetheless, maximal static respiratory pressures generated at the mouth after full expiration (i.e., Mueller maneuver) is the most widely used method of measuring MIP (McConnell and Copesake, 1999; Sclauser et al., 2013) and has been considered as a useful voluntary test of respiratory muscle strength (Green et al., 2002).
Before concluding that the S-Index determined by the POWERbreathe[R] is invalid, one should consider the distinct difference between the muscle function qualities being tested during the Mueller maneuver compared to the dynamic contraction of the respiratory muscle using the POWERbreathe[R] (Baker et al., 1994).