Article Text

Download PDFPDF
On issues of confidence in determining the time constant for oxygen uptake kinetics
  1. G H Markovitz1,
  2. J W Sayre2,
  3. T W Storer3,
  4. C B Cooper1
  1. 1Departments of Medicine and Physiology, UCLA School of Medicine, Los Angeles, CA 90095, USA
  2. 2Departments of Biostatistics and Radiological Sciences, UCLA School of Medicine
  3. 3Exercise Science Laboratory, El Camino College, Torrance, CA 90506, USA
  1. Correspondence to:
 Dr Cooper
 37-131 CHS, UCLA Medical Center, 10833 Le Conte Avenue, Los Angeles, CA 90095, USA; ccooper{at}


Background: τV˙o2 at the onset of constant work rate (CWR) exercise is a variable of aerobic fitness that shortens with physical training and lengthens with cardiopulmonary disease. Determination of τV˙o2 with sufficiently high confidence has typically required multiple exercise transitions limiting its clinical application.

Objectives: To design a protocol to determine τV˙o2 reliably but simply.

Methods: On each of three days, five healthy men performed two CWR tests on a cycle ergometer below the metabolic threshold (V˙o2θ) for blood lactate accumulation as determined by gas exchange measurements followed by an incremental work rate (IWR) test. τV˙o2 was determined (a) from the on-transit (on-τV˙o2) and off-transit (off-τV˙o2) of six CWR tests both individually and superimposed, using non-linear regression with a monoexponential model, and (b) by geometric analysis of the IWR tests (ramp-τV˙o2).

Results: Group means (SD) were: V˙o2max 3.84 (0.44) litres/min, V˙o2θ 1.88 (0.23) litres/min, steady state exercise V˙o2 1.67 (0.07) litres/min, on-τV˙o2 38.0 (5.3) seconds, off-τV˙o2 39.0 (4.3) seconds, and ramp-τV˙o2 60.8 (15.4) seconds. On-τV˙o2 correlated with off-τV˙o2 (r  =  0.87), V˙o2max (r  =  −0.73), and V˙o2θ (r  =  0.89). The pooled mean τV˙o2 from six superimposed tests agreed with the arithmetic grand mean of the six tests.

Conclusions: The average of on-τV˙o2 and off-τV˙o2 fell within the 95% confidence interval of the pooled mean by the second test. Ramp-τV˙o2 was longer and less reproducible. These findings support the use of both on- and off-transit data for the determination of τV˙o2, an approach that reduces the number of transitions necessary for accurate determination of τV˙o2, potentially enhancing its clinical application.

  • CWR, constant work rate
  • IWR, incremental work rate
  • oxygen uptake kinetics
  • time constant
  • accuracy
  • reproducibility

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.