The maximum oxygen uptake (Ω_O2,max), reflecting the upper limit of the body’s aerobic functioning, is the most widely used parameter characterising the effective integration of the cardiopulmonary and metabolic systems. As such it is used to characterise function in subjects ranging from elite athletes to patients with various pathophysiological conditions. Its quantification requires Ω_O2 to reach a value such that further increases in work rate do not result in further (or trivially small) increases in Ω_O2 (i.e. a plateau is attained). This is consistently achieved, over a range of work rates, when each Ω_O2 datum is derived from a discrete constant-load test. However, the advent of rapidly incrementing work rate tests (IWTs) has largely replaced this progressive, constant-load format. It has been suggested that during such tests, plateaus of Ω_O2 are rarely attained (e.g. Roca & Whipp, 1997) despite ‘good effort’ from the subjects (i.e. resulting in a peak Ω_O2).

We were therefore interested in: (a) whether a Ω_O2 plateau was a consistent manifestation of ramp incremental cycle ergometry (15-25 W min^-1) performed to the limit of tolerance in a large group of normal subjects (71 healthy males, aged 19-61 years), and (b) the relationship between the peak Ω_O2 from this test and that determined from a maximal constant-load test (n = 38) and also that from a series of progressive constant-load tests (n = 6). Ventilatory and pulmonary gas exchange variables were measured breath-by-breath using a turbine and mass spectrometer. Each test was performed on a separate day, informed consent having been provided as approved by the Local Research Ethics Committee.

The Ω_O2,peak (3.51 ± 0.8, mean ± S.D.), as determined from the average of the last 30 s of the incremental test in the 71 subjects, did not differ significantly from that extrapolated from linear regression of the response – from 4 min after the ramp onset to 1 min from the end; the mean difference was only 0.017 ± 0.15 l min^-1. In 12 of these subjects the Ω_O2,peak was less than the extrapolated value by 0.1 to 0.4 l min^-1 (i.e. a ‘plateau’): in 19, however, the Ω_O2,peak was actually higher by 0.05 to 0.4 l min^-1: in the remaining 40 subjects we could not discriminate a difference. The Ω_O2,peak from the IWT (3.64 ± 0.7 l min^-1) did not significantly differ (P > 0.05, paired t test) from that of the constant load test in the 38 subjects (3.64 ± 0.7 l min^-1). The Ω_O2,peak also did not differ from the Ω_O2,max (ANOVA, P > 0.05; in the 6 subjects), i.e. as determined from the ‘traditional criteria’ from constant-load testing.

We therefore conclude, that while a plateau in Ω_O2 (i.e. Ω_O2,max) is an obligatory consequence of a series of discrete tests, a plateau in the actual Ω_O2 response from the IWT is not. Consequently, the Ω_O2,peak attained on an IWT is likely to be a valid index of Ω_O2,max – despite no evidence of plateauing in subjects exercising to the limit of tolerance. However, without evidence from additional tests one cannot be certain.