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August Krogh Institute
University of Copenhagen Muscle Research Center
Copenhagen, Denmark
e-mail: cjuel{at}aki.ku.dk
To the Editor: It has been stimulating to observe and take part in the extensive discussion about the role of lactate and pH in the development of fatigue. However, it is also with concern that we followed the debate (3, 5), because it seems that few are willing to perceive the complexity of the issue and to focus on the limitations of their own studies. Nielsen and Overgaard (5), who have much experience in studying the role of potassium in fatigue development, have unexpectedly argued that accumulation of potassium is not involved in the fatigue process, based on the findings of different potassium levels at exhaustion in two of our studies (4, 6). As also discussed in the original articles (4, 6), the issue is more complex; rather than looking at the effect of accumulated potassium itself, it is more likely that it is the effect of potassium on the membrane potential that leads to an inability to propagate the action potential over the muscle fibers. In both experiments (4, 6), the reduction in the membrane potential and excitability may have been the same at exhaustion (at the various conditions), despite a slight difference in interstitial potassium. The findings from isolated noncontracting and skinned fibers of a positive role of lowered pH have never been demonstrated to apply to contracting isolated skeletal muscle (activated with physiological stimulation parameters), nor to the conditions of human skeletal muscle. Furthermore, how can it be beneficial for performance that lowered pH leads to a greater extracellular accumulation of potassium as demonstrated in a number of studies (1, 7)? None of the contributions have challenged our finding of a reduced performance when muscle pH was lowered by prior arm exercise (2). Those arguing for an important protective role of lowered muscle pH in humans should answer the question: "Why can the subjects not work as long (or even longer) when the muscle pH is lower than without prior arm exercise?" The easy answer given by Lamb and Stephenson is central fatigue, which was discussed in our original study (1) so as not to exclude any potential factors, and, as argued, there was no sign of central fatigue with the subjects having a heart rate of about 120 beats/min, demonstrating that the stress on the central system was low. It is of great value to highlight this interesting topic and we should all contribute to a further understanding by posing relevant questions that can be attacked using different models. Then, when it comes to the interpretation, we have to understand the limitations in each of the models.
REFERENCES
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  A. Mendez-Villanueva, J. Fernandez-Fernandez, and D. Bishop Exercise-induced homeostatic perturbations provoked by singles tennis match play with reference to development of fatigue Br. J. Sports Med., November 1, 2007; 41(11): 717 - 722. [Abstract] [Full Text] [PDF]
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