Comparison of Two-Minute Incremental Threshold Loading and Maximal Loading as Measures of Respiratory Muscle Endurance

doi:10.1378/chest.96.3.557

Chest

Volume 96, Issue 3, September 1989, Pages 557-563

https://doi.org/10.1378/chest.96.3.557 Get rights and content

We performed a two-minute incremental threshold loading test (incremental test) in ten normal subjects on three occasions, and having ascertained the maximum load (max load) against which they could inspire for two minutes, measured how long this load could be tolerated by these same subjects on three further occasions (tlim test). We compared the reproducibility of the two tests. There were no significant differences found in the mean max loads in the three incremental tests, or in the endurance times in the three tlim tests. However, the intraindividual coefficients of variation of max load in the incremental test (0 to 14 percent) were much smaller than the intraindividual coefficients of variation of endurance time in the tlim test (20 to 65 percent). We found that the large variability in endurance time in our tlim tests was most likely accounted for by variability in breathing pattern, inspiratory flow rate and breath-by-breath mouth pressure generation. Differences in these parameters did not, however, explain why in the tlim test a given subject could tolerate for 19 minutes a load only 100 g less than that which he was unable to tolerate for two minutes in the incremental test. These findings emphasize the differences between these two tests of respiratory muscle endurance. Since there was less intraindividual variability in the two-minute incremental threshold loading test, we suggest that this test may be more useful than the tlim test.

Section snippets

METHODS

The study group consisted of ten subjects, five men and five women, with no history of pulmonary, cardiac or neuromuscular disease. Ages ranged from 21 to 39 years and all had previous experience with the weighted plunger apparatus (see Table 1).

In each subject, maximal inspiratory pressure (MIP) at the mouth was measured using the protocol of Black and Hyatt.¹⁰ The determination was repeated until three measurements, with less than 5 percent variability and sustained for at least one second

RESULTS

In the two-minute incremental test, the mean max loads achieved by the ten subjects in three consecutive tests were not different (Table 2). The intraindividual coefficients of variation of max load ranged from 0 to 14 percent. No significant intertest difference was found for Pmean, Ppeak/MIP, TI/TT or VT/TI (all p>0.05) measured in the final two minutes of the incremental tests (Table 2 and Fig 1). Mean PETCO₂ in the final two minutes of the three incremental tests was 40 ±3, 39 ±5, 40 ±5 mm

DISCUSSION

We found that both the two-minute incremental threshold loading test and the tlim test were reproducible over three measurements at least 24 hours apart. The statistical test used to reach this conclusion, a repeated measures ANOVA, while an appropriate test to apply, disguises a more important observation. This is that while the coefficients of variation of max load in the incremental test were small (range 0 to 14 percent), the coefficients of variation of tlim in the tlim test were large

ACKNOWLEDGMENT:

The authors wish to thank Barry Wiggs for his help with statistical analysis.

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Thereafter, PImax being influenced by hyperinflation, we calculated the decrease on PImax by liter of FRC increase above predicted [14]. Inspiratory muscle endurance (IME) was assessed by the incremental threshold loading (ITL) test as previously described with a modified commercial Threshold IMT® (Respironics) device with a spring loaded valve [15–17]. The spring was modified to produce inspiratory pressure loads up to 120 cm H2O.
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Recent studies have shown that using inspiratory pressure load training can prevent these defects. The potential advantage of this kind of device is that the inspiratory pressure is independent of the inspiratory flow rate.23-28 Nevertheless, the relative benefits of RIMT are that it is an inexpensive training procedure that may be self-administrated over the long term, and that the DHD inspiratory resistive muscle trainer is simple, light, and portable.
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Supported by the B.C. Lung Association and Medical Research Council, Canada.

Manuscript received August 16; revision accepted December 9.

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Chest

Clinical InvestigationsComparison of Two-Minute Incremental Threshold Loading and Maximal Loading as Measures of Respiratory Muscle Endurance

Section snippets

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGMENT:

Respir Physiol

Chest

Measuring ventilatory muscle endurance in humans as sustainable inspiratory pressure

J Appl Physiol

Fatigue of inspiratory muscles and their synergic behaviour

J Appl Physiol

Ventilatory function tests: I. Voluntary ventilation capacity

J Clin Invest

Ventilatory muscle strength and endurance training

J Appl Physiol

Inspiratory muscle training in patients with chronic obstructive pulmonary disease

Am Rev Respir Dis

Diaphragmatic fatigue in man

J Appl Physiol

Measurement of inspiratory muscle performance with incremental threshold loading

Am Rev Respir Dis

Clinical Investigations
Comparison of Two-Minute Incremental Threshold Loading and Maximal Loading as Measures of Respiratory Muscle Endurance