The topic of how various variables in the cardiopulmonary system are linked to the overall capacity of the human body to transport oxygen to the tissues, with a focus on skeletal muscle during exercise, has been a key issue for a century. The developments and views were closely related to the enhancement of methods to precisely determine the variable of interest (1). With the introduction of CO to determine total amount of hemoglobin (t-Hb), it was demonstrated that t-Hb is as closely related to aerobic exercise capacity – maximal oxygen uptake (VO_2max) as e.g. heart size (2). The role of t-Hb became further underscored by studies where t-Hb was varied by acute changes in red cell mass with and without alteration in blood volume or Hb concentration ([Hb]) (3). These data also opened for evaluating the role of total blood volume and [Hb] relative to t-Hb. The latter variable is still considered to be the most critical variable, but at a given t-Hb a higher [Hb] results in a higher VO_2max. This is most clearly demonstrated in experiments where EPO has been used to vary both t-Hb and [Hb] (4, 5). With blood manipulation, still being quite common in sports, the CO-method to measure t-Hb has had a renaissance, also bringing a multitude of t-Hb data on many groups of athletes. In addition to confirming the close relationship between t-Hb and VO_2max it is clear that it holds for many ethnic groups under study (6). References 1. Sjöstrand T. Physiol Rev 1953, 33; 202-28. 2. Holmgren A, Åstrand PO. J Appl Physiol 1966, 21; 1463-70. 3. Calbet JA, Lundby C, Koskolou M, Boushel R. Respir Physiol Neurobiol 2006, 151; 132-40. 4. Lundby C, Thomsen JJ, Boushel R, Koskolou M, Warberg J, Calbet JA, Robach P. J Physiol 2007, 578; 309-14. 5. Ekblom B. Am J Sports Med 1996, 24; S40-42. 6. Prommer N, Thoma S, Quecke L, Gutekunst T, Völzke C, Wachsmuth N, Niess AM, Schmidt W. Med Sci Sports Exerc 2009, Nov 27 [E-pub ahead of print].