Why is there still so much confusion about VO2 plateau? A re-examination of the work of A.V. Hill

Maximal oxygen uptake (VO_2max) is regarded as the gold standard for assessing aerobic fitness. In 1923, Hill et al. proposed that VO_2max represents the maximal ability of the body to take in and consume O₂ during strenuous exercise. Recently, however, controversy has arisen over the issue of whether a leveling off, or "plateau" in VO₂ is necessary to verify attainment of VO_2max. Purpose: To compare two different VO_2max protocols and determine if both protocols show direct evidence of an upper limit on VO₂. Methods: Nine runners (18-35 years old) completed a continuous graded exercise test (CGXT), followed by a discontinuous graded exercise test (DGXT). The CGXT consisted of gradually increasing treadmill running speed to the point of volitional exhaustion; the highest speed attained was labeled the peak treadmill speed. Over the next several days, participants ran at 80%, 90%, 100%, 105%, and 110% of peak treadmill speed for 10 minutes, or until volitional exhaustion was reached. Results: All participants (n=9) achieved a "VO₂ ceiling" (or upper limit) on the DGXT, while only 44% (n=4) achieved a "VO₂ plateau" on the CGXT. There was no significant difference between the VO_2max obtained from a CGXT (57.4 ± 2.6 mL*kg^-1min^-1) and DGXT (60.0 ± 3.1 mL*kg^-1min^-1). There was no difference between oxygen uptake measured at 90%, 100%, 105%, and 110% of PTV (p>0.05). However, the highest VO₂ recorded at 80% PTV was significantly lower than that recorded at all other velocities (p<0.05). Conclusion: The VO₂ ceiling effect on a DGXT is inherently different than the VO₂ plateau effect on a CGXT. In this study, a ceiling was always seen on the DGXT, but a plateau was not always seen on the CGXT.