Under the context of assessing the effects of mitochondrial stress on skeletal muscles, consideration for the ratio of inorganic phosphate to phosphocreatine (Pi:PCr) may be used as a surrogate marker. Furthermore, assessing the systemic Pi:PCr, under states of high metabolic and respiratory demands with reduced oxygen supply (VO2), such as strenuous exercise while only using nasal breathing (NB), might provide valuable information regarding respiratory strain to a given exercise bout. PURPOSE: To determine if the systemic assessment of the Pi:PCr ratio can serve as an indicator of respiratory stress. METHODS: Fourteen young males (age = 20.57 ± 1.22 yrs) completed a graded maximal exercise test (GXT) on a recumbent bike using either NB (n = 8) or combined (CB, n = 6) breathing. Plasma Pi and PCr were measured pre- and immediately post-GXT. Pi was analyzed using the malachite green colorimetric method, and PCr was quantified via sandwich enzyme-linked immunosorbent assay. The Pi:PCr was calculated by Pi ÷ PCr. A paired-sample t-test examined the Pi: PCr change following a GXT. A Pearson correlational analysis examined the relationship between the Pi:PCr and the respiratory performance during preset intensities (40%, 55%, 70%, 85%, and 100% VO2max). Data are presented as mean ± SD. RESULTS: The Pi:PCr ratio significantly decreased in the NB group (0.589 ± 0.313 to 0.530 ± 0.304, p = 0.034) mainly due to the elevated PCr (601.68 ± 318.88 mmol/L to 689.40 ± 410.60 mmol/L, p = 0.033). The Pi:PCR ratio was negatively correlated with VO2 only in the NB group throughout all preset intensities of the GXT. After controlling for the pre-GXT Pi:PCr as a covariate, the NB group showed a non-significant (p = 0.340) but higher mean concentration for post-exercise Pi:PCr compared to the CB group [NB: 0.451 ± 0.027 mmol/L versus CB: 0.407 ± 0.320 mmol/L. CONCLUSION: The results from the present study suggest that the assessment of the Pi:PCr ratio could serve as a valuable marker of physical strain pertaining to high respiratory demands. Moreover, the assignment of NB during exercise is becoming a novel method due to its postulated benefits towards respiratory function, which may induce a more significant cellular stress as denoted by an increase in the post-exercise Pi:PCr ratio.



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