Response to Professor Shephard's Letter to the Editor:

We very much welcome the opportunity to respond to Dr. Shephard's comments relating to the methodological issues associated with the identification of incidences of plateau at VO_2max. Dr. Shephard's main concern relates to the criteria used to determine that a plateau in VO₂ has been achieved (<50 ml·min⁻¹) as opposed to the <150 ml·min⁻¹ [7] or <2.0 ml·kg⁻¹·min⁻¹ [6]. We would contend that these proposed recommendations were established when the determination of VO₂ responses was achieved through the collection of exhaled gas into Douglas bags over sampling periods of 60 s, or greater [7] and hence require a more robust criteria for demonstrating a change between successive collections. This approach contrasts with the methodology used by Gordon et al. [3] where a commercially available metabolic-cart, which continuously draws air from the mouth-piece, allowing the measurement of gas concentrations at a rate of 50 Hz. Accordingly we would argue that the improved temporal resolution and sampling rates of the metabolic cart allow for an enhanced sensitivity in plateau determination which is closer to the desired VO₂ response of zero change. Indeed it has previously been argued that the aforementioned criteria of <2.0 ml·kg⁻¹·min⁻¹ may be insufficiently discriminating and lead to an underestimation of VO_2max [4] and hence lead to an increased incidence of plateau responses.

Dr. Shephard also contends that a secondary reason for the variances reported in the literature of plateau response is due to the structure of the protocol which is used, with particular reference being placed on the duration of the test. In this regard Shephard suggests that the protocol adopted by Gordon et al. [3] exceeded in some instances the ‘recommended ceiling’ of duration. Again it has previously been argued that optimal test durations for the assessment of VO_2max using a cycle ergometer, as with Gordon et al. [3] should be in the range of 7–26 min [5]. In the case of Gordon et al. [3] tests were completed within 9–12 min (823.0±120.7 s). Finally this recommended ceiling is based on a single study with a small sample size (5) who were described as being moderately fit and completed 8 different trials (cycle ergometer×3, treadmill×5) [1].

The question of participant motivation was also raised by Dr. Shephard who has postulated that it is possible the incidence of plateau response and larger oxygen deficit scores shown by Gordon et al. [3] may be a reflection of better motivated participants. We would argue that athlete motivation, although important under these test conditions, has little association with the plateau response as shown by the low incidence of plateau responses within a group of elite British athletes [2]. Indeed within this group (n=50) of highly motivated athletes the plateau response was shown to be 39% in males and just 25% in females.

Finally we concur with Dr. Shephard's suggestion that an agreed procedure be adopted for both the protocol and determination of a ‘true’ VO_2max. However we would contend that rather than adopting the WHO/BP international working party criteria, more current guidelines be established which take into account the widespread use of computerised instrumentation such as breath-by-breath metabolic carts and electronically/isokinetic controlled cycles.

References

• 1 Buchfuher MJ, Hansen JE, Robinson TE, Sue DY, Wasserman K, Whipp BJ. Optimising the exercise protocol for cardiopulmonary assessment.  J Appl Physiol. 1983;  55 1558-1564
• 2 Doherty M, Nobbs L, Noakes T. Low frequency of the “plateau phenomenon” during maximal exercise in elite British athletes.  Eur J Appl Physiol. 2003;  89 619-623
• 3 Gordon D, Hopkins S, King C, Keiller D, Barnes RJ. The incidence of the plateau at VO_2max is dependent on the anaerobic capacity.  Int J Sports Med. 2011;  32 1-6
• 4 Howley ET, Bassett DR, Welch HG. Criteria for maximal oxygen uptake: review and commentary.  Med Sci Sports Exerc. 1995;  27 1292-1301
• 5 Midgley AW, Bentley DJ, Luttikholt H, McNaughton LR, Millet GP. Challenging a dogma of exercise physiology: Does an incremental exercise test for valid VO_2max determination really need to last between 8 and 12 min?.  Sports Med. 2008;  38 441-447
• 6 Shephard RJ, Allen C, Benade AJS, Davies CTM, di Prampero PE, Hedman R, Merriman JE, Myhre K, Simmons R. The maximum oxygen intake: An international reference standard of cardiorespiratory fitness.  Bull World Health Organ. 1968;  38 757-764
• 7 Taylor HL, Buskirk E, Henschell A. Maximal oxygen intake as an objective measure of cardio-respiratory performance.  J Appl Physiol. 1955;  8 73-80

Correspondence