In this issue of Medicine & Science in Sports & Exercise ®, Hopker et al. (4) report on the reliability of gross efficiency as measured using the Douglas bag technique (3) (often miscited), finding a total within-subject variation of 1.5%. This excellent reliability is considerably better than previously reported results in which breath-by-breath systems were used (e.g., Moseley and Jeukendrup  and Noordhof et al. ). The article of Hopker et al. (4) focuses largely on a meticulous investigation of the Douglas bag technique itself, particularly sampling reliability, residual volume of the Douglas bag, gas exchange between the Douglas bag and ambient air, and measurement of the gas volume. To be sure, they could have been even more complete by detailing exact methods of gas analyzer calibration via zero and span gases plus a midrange gas standard. Nevertheless, to their credit, they have previously reported on the equally important issue of accuracy and reliability of power measurement during cycling (5).
In their current article (4), the overall attention to detail is impressive, and the methods and data are of the highest quality. However, one could argue that the results are neither original nor likely to be of high impact. In fact, as one reviewer noted, “These techniques have been ‘common practice’ in our lab for the past 40 years.” So why publish this paper in MSSE ® when acceptance rates are less than 25%?
Before the invention of metabolic carts and breath-by-breath systems, scientists regularly collected gas samples during periods of 1 min or longer in large “Douglas” bags followed by later gas analysis and volume determination. As noted by Hopker et al. (4), accurate and reliable measurement of oxygen consumption (V˙O2) in this manner is based largely on first principles and requires a comprehensive understanding of the details of the setup and calculations. Careful researchers go through procedures similar to those described by Hopker et al. (4), checking accuracy and reliability at every link in the chain of V˙O2 and carbon dioxide production (V˙CO2) measurement (8). Final results are sometimes not available for a considerable period after the test is over.
Recently, the ease of use and rapid feedback provided by computerized metabolic systems have tempted scientists and practitioners alike to use them “as is” with little concern for validation or accuracy. Megabytes of data showing results that “look like they should” can be collected shortly after the system arrives in the laboratory. However, although metabolic carts, including breath-by-breath systems, have become more sophisticated and reliable over time, attention to validation remains a necessity. Examples are available showing how to use the Douglas bag technique to validate such systems, whether ventilation is measured on the inspired or expired side of the system (1). Further, metabolic systems that have been previously validated can produce erroneous results when the user fails to pay attention to details such as adjusting sampling pump flow rates and using water vapor drying systems properly. Nevertheless, for scientists who are interested in gas exchange on-kinetics (e.g., Berger et al. ), breath-by-breath systems are not only state-of-the-art but also essential for the description of the time course of gas exchange.
However, what seems to be often forgotten is that, for measurement of small differences in V˙O2 and V˙CO2 under steady-state conditions, Douglas’s 100-year-old method (3) remains the state-of-the-art gold standard. Therefore, we consider the article of Hopker et al. (4) to be a cogent reminder of the constant need to establish accuracy and reliability, even in one of the most basic techniques in the history of exercise physiology. Accordingly, conscientious researchers are urged to pick up a bag and step away from the automated systems for accurately assessing potentially small, but important changes in efficiency and even V˙O2max. The Douglas bag approach should also be used for validation of automated systems on a routine basis. Students are urged to learn the details of the Douglas bag technique to have a better understanding of the underlying basis of the V˙O2 and V˙CO2 measurements.
L. Bruce Gladden, PhD
Department of Kinesiology
James W. Yates, PhD
Department of Kinesiology
and Health Promotion
University of Kentucky
Edward T. Howley, PhD
Department of Kinesiology
Recreation, and Sport Studies
University of Tennessee
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