Using V̇o_2max as a marker of training status in athletes—can we do better?

INTRODUCTION

One of the fundamental premises of research is that findings from a sample of a population can be extrapolated to the population at large. Therefore, correct classification of the sample population is of paramount importance, especially as it has been shown that there is a nonuniform response to the same intervention in athletes of differing training statuses (1, 2); for example, nitrate supplementation was thought to be beneficial based on research in nonelite populations; however, these findings have not been replicated in elite populations (3). If a strategy described in a research study is to be applied in real-world athletes, the expected outcomes should be similar. However, as demonstrated, this may not be the case if the research study participants are not correctly classified. In sports science studies, participants are most commonly classified based on their maximal oxygen uptake (V̇o_2max) and the maximal power at the end of a laboratory-based graded exercise test (Wmax). The aim of this viewpoint is to present the argument that the training status of participants could be better defined. To this end, we suggest that the power/speed at the boundary of the heavy/severe exercise intensity domain should be reported as the main descriptor of training status in studies where the readership may be interested in the performance implications of a given intervention (Fig. 1).

Figure 1.

Classification of research study participants based on maximal oxygen uptake can lead to questionable translation of research results into practice given that maximal oxygen uptake is not a good predictor of elite endurance performance.

Limitations of Current Classification Practices

Measuring and reporting relative and absolute V̇o_2max values has a long history in the field of exercise sciences because it not only offers a prognosis of health outcomes and mortality but also is more pertinent to the present article; it is believed that normalization of V̇o_2max to body mass is a good predictor of performance, and thus a broad descriptor of the training status. As a result, V̇o_2max is used to describe study participants. There is no doubt that V̇o_2max is a solid predictor of endurance performance in a heterogeneous group of participants (4). However, using V̇o_2max as the primary determinant of participant classification has led to some common issues within the literature (5).

First, there can be a mismatch between the actual performance level of athletes and their classification based on their V̇o_2max values. For instance, participants have been classified as elite despite not even competing at the lowest international level (6, 7). Second, V̇o_2max alone does not predict differences in performance in a relatively homogenous group (13). This is elegantly demonstrated by the nonsignificant differences in V̇o_2max in a group of U23 professional cyclists (9) despite differences in their level of performance. Third, reported V̇o_2max in Olympians, professional athletes, and world record holders would have them classified into inferior categories based purely on V̇o_2max (8, 10, 11). Finally, there can be large discrepancies in V̇o_2max between athletes with similar performance capabilities, for example, V̇o_2max in world-class marathon runners can differ by up to 22 mL·kg⁻¹·min⁻¹ (10).

Differences in actual performance between athletes may therefore be related to additional factors (12). First, exercise economy/efficiency; this parameter describes how well oxygen is converted into locomotion at submaximal intensities and has been shown to be significantly different between groups with nonsignificant differences in V̇o_2max (13). It has also been shown that V̇o_2max is inversely associated with running economy (10), indicating that V̇o_2max per se cannot independently predict performance. Second, interindividual differences in the maximal sustainable fractional utilization of V̇o_2max (%V̇o_2max) (14); even though alone this variable does not always account for differences in performance (15).

Combined these findings show that V̇o_2max can only be used as a descriptor and a predictor of performance when other factors are also reported (16, 41). This is well demonstrated as athletes have been shown to improve their performance irrespective of an increase in V̇o_2max (17, 18).

Recently, a new framework for classification of study participants has been published (2). This work highlights similar drawbacks in current practice to those presented here. It proposes a new classification system based primarily on training norms and competition results. Although we are supportive of the ideas presented in this article, the advantage and disadvantage of this approach is that competitive results are an aggregate of various factors (e.g., psychology, tactical skills) and not necessarily just physiology. We believe that in addition to describing competitive status, classifying participants based on their performance physiology provides an additional layer of information that is useful from both an academic and applied perspective.

A pertinent solution could be the application of an external measure that predicts performance and is a product of the aforementioned underlying physiological parameters (18). The suitability of external measures to discriminate between athletes can be demonstrated using two cycling case studies, one of a multiple grand tour winner (20) and one of an athlete with the highest ever recorded V̇O_2max (21). Comparison reveals that even having the highest V̇o_2max is no guarantee of success. It also highlights that describing participants according to an external measure, in this case, power at a blood lactate concentration of 4 mmol·L⁻¹, is more revealing than V̇o_2max. Namely, the multiple grand tour winner had a lower V̇o_2max but displayed higher power at a lactate concentration of 4 mmol·L⁻¹.

Power/Speed at the Boundary of the Heavy and Severe Exercise-Intensity Domains

An enticing option to classifying study participants (in endurance sports) would be to use the power/speed at the boundary of the heavy and severe exercise-intensity domains. This approach demonstrated a high practical utility in predicting endurance performance (22) and can differentiate between performance in athletes with similar V̇o_2max (23). The demarcating intensity between the two domains has been described as critical power (CP), critical speed (CS), maximal lactate steady state (MLSS), or the second ventilatory threshold (VT2) (24). Although all three represent physiological landmarks occurring at a similar exercise intensity, the current weight of evidence points toward the CP/CS model offering the most comprehensive explanation of performance over various exercise durations (25–29). It has also been suggested that the CP/CS best represents the threshold between steady and nonsteady exercise (24, 30, 31); however, the arguments surrounding this topic are outside the scope of this viewpoint (32, 33).

We, therefore, propose that CP/CS rather than V̇o_2max should be used as the primary descriptor of participants’ training status.

CP/CS was first described as the asymptote of the curvilinear relationship between power/speed and time to task failure (34). Subsequent developments in the understanding of the mechanistic basis of the CP/CS means it is currently understood to be the maximum power or speed at which there is no metabolite-induced progressive derangement of muscle cell homeostasis (35). By using the CP/CS concept, one can also calculate the fixed work capacity above the CP/CS (W’ or D’). W’/D’ represents a fixed work capacity above the CP/CS that can be utilized within the severe exercise-intensity domain (30). Using the CP/CS and W’/D’ together it is possible to predict performance in shorter events (28, 36).

Thus, CP/CS, accompanied by the W’/D’, arguably gives an insight into performance capacity across a wider range of durations and exercise modalities than either V̇o_2max or Wmax (28), or indeed any other measure of the heavy/severe exercise-intensity domain border (37). Indeed, the CP concept has been applied to predict performance across exercise durations from single repetition maximum (29) to marathon performance (27).

Additional Benefits of Using Critical Power/Speed to Determine Participant Status

Although there are methodical issues associated with deriving CP/CS (38), the authors believe that if recognized guidelines are applied, valid CP/CS estimates can be easily obtained. CP/CS estimates can easily be derived in both formal laboratory and field-based testing (9, 39) without the use of specialized equipment. Due to the ease of determination, practitioners can easily derive CP/CS in their own athlete populations and compare these values with those in a given study to judge whether an intervention is warranted and allow a better prediction of the magnitude of potential performance improvements.

V̇o_2max and Wmax are also often used in studies to determine exercise intensity in subsequent interventions. However, this approach is flawed, as there are interindividual differences in the percentage of V̇o_2max and Wmax at which boundaries between different exercise intensity domains occur. Thus, different physiological responses between participants can be observed when anchoring exercise intensity to fractions of V̇o_2max or Wmax (40). If the CP/CS is determined as part of the classification process, these values can also be used to anchor exercise intensity in any subsequent intervention.

CONCLUSIONS

Based on the arguments above, it is the authors’ opinion that researchers should be encouraged to describe study participants based on the physiological parameters capable of best predicting performance across a wide range of intensities and to move away from reporting solely V̇o_2max. It is our belief that application of the CP/CS concept would provide the most appropriate way to do this.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

T.P., P.L., and J.S. drafted manuscript; T.P., P.L., and J.S. edited and revised manuscript; T.P., P.L., and J.S. approved final version of manuscript.