Purpose We investigated the accuracy of the Moxus Modular Metabolic System (MOXUS) against the Douglas Bag Method (DBM) during high-intensity exercise, and whether the two methods agreed when detecting small changes in VO2 between two consecutive workloads (VO2 ). Methods Twelve trained male runners performed two maximal incremental running tests while gas exchange was analyzed simultaneously by the two systems using a serial setup for four consecutive intervals of 30 s on each test. Comparisons between methods were performed for VO2 , VE , fractions of expired O2 (FeO2) and CO2 (FeCO2) and VO2 . Results The MOXUS produced significant higher (mean ± SD, n = 54) readings for VO2 (80 ± 200 mL/min, p = 0.005) and V?E (2.9 ± 4.2 L/min, p 2 (-0.01 ± 0.09). Log-transformed 95 % limits of agreement for readings between methods were 94-110 % for VO2 , 97-108 % for V?E and 99-101 % for FeO2. VO2 for two consecutive measurements was not different between systems (120 ± 110 vs. 90 ± 190 mL/min for MOXUS and DBM, respectively, p = 0.26), but agreement between methods was very low (r = 0.25, p = 0.12). Discussion Although it was tested during high-intensity exercise and short sampling intervals, the MOXUS performed within the acceptable range of accuracy reported for automated analyzers. Most of the differences between equipments were due to differences in VE . Detecting small changes in VO2 during an incremental test with small changes in workload, however, might be beyond the equipment`s accuracy.
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