Letters to the Editor-in-Chief Considerations for Identifying the Boundaries of Sustainable Performance Dear Editor-in-Chief: We applaud Keir et al. (4) for their undertaking of a thorough study that investigated the association between several physiological phenomena and the threshold of sus- tainable exercise (i.e., critical power (CP)). This demarcation of sustainable exercise holds critical significance to applied and clinical exercise physiology, and the conclusion of the authors that several physiological phenomena coincide with CP is tempting but dangerous because of possible misinter- pretation and overstatement of association between these phenomena. The authors leave the reader with the impres- sion that their findings are the last, final word on the asso- ciation among these various responses. It is unfortunate that the authors did disservice to the readership of the journal by ignoring recent work that contradicts the authors_ con- clusion that all of these responses occur at the same V ˙ O 2 , suggesting these are ‘‘equivalent’’ and ‘‘interchangeable’’ phenomena. Consistent with previous findings, the current study dem- onstrated no significant differences between the means of these physiological phenomena when expressed as metabolic rates. However, it should not be lost that a lack of significant difference does not necessitate equivalence. It is paramount to consider the high degree of intrasubject variability and lack of universal expression, which preclude the ‘‘interchange- ability’’ of these phenomena within a given subject (1–3). Moreover, a clear temporal separation among these phenom- ena suggests a sequence or cascade rather than a single event (1). These overlooked results highlight the likelihood of distinct physiological mechanisms that determine these phe- nomena and warrant consideration when interpreting the re- sults of the current study. The authors acknowledged the observational nature of the current study but frequently alluded to equivalent deter- mining mechanisms of these phenomena. Awareness must be raised with regard to the physiological interpretation of these data. Without rigorous investigation of the mechanisms un- derlying these phenomena or attempting to dissociate these parameters, one must take care to not overstate the ‘‘equiva- lence’’ and ‘‘interchangeability’’ of these phenomena. Thus, we believe that the association of the phenomena that occur in the metabolic and work rate region of CP is not settled but must await future investigations that seek to mechanistically link or uncouple these phenomena. Jesse C. Craig Department of Kinesiology Kansas State University Manhattan, KS Ryan M. Broxterman Department of Kinesiology and Department of Anatomy and Physiology Kansas State University Manhattan, KS Thomas J. Barstow Department of Kinesiology Kansas State University Manhattan, KS DOI: 10.1249/MSS.0000000000000676 REFERENCES 1. Boone J, Barstow TJ, Celie B, Prieur F, Bourgois J. The impact of pedal rate on muscle oxygenation, muscle activation and whole- body V ˙ O 2 during ramp exercise in healthy subjects. Eur J Appl Physiol. 2015;115:57–70. 2. Broxterman RM, Ade CJ, Craig JC, Wilcox SL, Schlup SJ, Barstow TJ. The relationship between critical speed and the respiratory compensation point: coincidence or equivalence. Eur J Sport Sci. 2014;13:1–9. 3. Broxterman RM, Ade CJ, Barker T, Barstow TJ. Influence of pedal cadence on the respiratory compensation point and its relation to critical power. Respir Physiol Neurobiol. 2015;208:1–7. 4. Keir DA, Fontana FY, Roberston TC, et al. Exercise intensity thresholds: identifying the boundaries of sustainable performance. Med Sci Sports Exerc. 2015;47(9):1932–40. 1997 SPECIAL COMMUNICATIONS Copyright © 2015 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.