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J Appl Physiol 103: 1911, 2007; doi:10.1152/japplphysiol.00784.2007
8750-7587/07 $8.00
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LETTER TO THE EDITOR

Is supramaximal work capacity negatively correlated with the rate of decrease in muscle pH?

TO THE EDITOR: The recent article by Messonnier et al. (3) contains a number of interesting findings. Their findings regarding the potential role of cytosolic carbonic anhydrases during supramaximal exercise is particularly novel. However, their other main finding that supramaximal work capacity is negatively correlated with the rate of decrease in muscle pH is more controversial (2). In support of their conclusion, the authors reported a strong, negative correlation between supramaximal work (WSUP) and {Delta}pH/WSUP-se (where "se" indicates a standardized event where time was fixed to time performed in the control condition minus 20 s). Their raw data were estimated from their figure [Fig. 1A from published paper (3)] and redrawn below (Fig. 1).


Figure 1
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  		Fig. 1. Relationship between supramaximal work performed (WSUP; kJ) and the decrease in muscle pH ({Delta}pH/WSUP-se) [Redrawn from Fig. 1A in Messonnier et al. (3)].

 
The first observation is that despite the very good correspondence between the redrawn figure and their figure (and the similar correlation coefficients; r = 0.80 vs. 0.81), the estimated pH change (0.75 ± 0.05 pH units) is quite different from that reported in their paper (0.67 ± 0.05 pH units). Regardless of which value is considered, this pH change is much larger than previously reported following exercise to exhaustion [~0.50 pH units (1, 6)]. This becomes even more apparent when one considers that despite recording a mean pH of 6.51 at the end of the standardized event, subjects in the alkalosis group exercised, on average, for an additional 53 s when the task was performed until fatigue.

A more important concern is that the reported negative correlation between supramaximal work (WSUP) and {Delta}pH/WSUP-se may actually represent a spurious correlation. A spurious correlation can be defined as one that could occur in the absence of a real physiological relationship between the two variables of interest (4). A spurious correlation may arise whenever x is involved in the calculation of y (or vice versa). For example, despite any obvious physiological explanation, there is a very strong correlation between peak VO2 and height/peak VO2 (r = 0.96). The variables that were correlated by Messonnier et al. (3) are not independent; i.e., it is expected that there is a very strong correlation between WSUP-se (used to calculate the x variable) and WSUP (the y variable).

The possible spurious nature of the correlation reported by Messonnier et al. (3) may be explored by generating random numbers for change in pH, recalculating the x variable ({Delta}pH/WSUP-se) and determining if the correlation persists (5). When this was done 100 times (using random pH values within the same physiological range), the mean correlation coefficient was r = 0.71 (range 0.53 < r < 0.98). Thus these results suggest that when there is a small variance in pH change [as in the study by Messonnier et al. (3)], it is a mathematical certainty that those who do more work will have a smaller change in pH per kiloJoule of supramaximal work performed, leading to a possible spurious correlation between WSUP and {Delta}pH/WSUP-se.

In conclusion, while currently an area of considerable debate (2), there may indeed be a real physiological relationship between supramaximal work capacity and the rate of decrease in muscle pH. However, it needs to be considered that the relationship used by Messonnier et al. (3) to support this conclusion may in fact be spurious.

FOOTNOTES


Address for reprint requests and other correspondence: D. Bishop, Facoltà di Scienze Motorie, Università di Verona, via Casorati 43, Verona 37131, Italy (e-mail: bishop{at}motorie.univr.it)

REFERENCES

  1. Hermansen L, Osnes J. Blood and muscle pH after maximal exercise in man. J Appl Physiol 32: 304–308, 1972.[Free Full Text]
  2. Lamb GD, Stephenson DG, Bangsbo J, Juel C. Point:Counterpoint: Lactic acid accumulation is an advantage/disadvantage during muscle activity. J Appl Physiol 100: 1410–1412, 2006.[Free Full Text]
  3. Messonnier L, Kristensen M, Juel C, Denis C. Importance of pH regulation and lactate/H+ transport capacity for work production during supramaximal exercise in humans. J Appl Physiol 102: 1936–1944, 2007.[Abstract/Free Full Text]
  4. Pearson K. On a form of spurious correlation that may arise when indices are used in the measurement of organs. Proc Royal Soc Lond Series A 60: 489–498, 1897.
  5. Pendleton B, Newman I, Marshall R. A Monte Carlo approach to correlational spuriousness and ratio variables. J Stat Comp Sim 18: 93–124, 1983.[CrossRef]
  6. Sahlin K, Harris RC, Nylind B, Hultman E. Lactate content and pH in muscle samples obtained after dynamic exercise. Pflügers Arch 367: 143–149, 1976.[CrossRef][ISI][Medline]

David Bishop
 Team Sport Research Group, Facoltà di Scienze Motorie, Università di Verona, Verona, Italy




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Corrigendum
J Appl Physiol, January 1, 2008; 104(1): 318 - 318.
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