- Maximal aerobic capacity (V̇O₂max) is the most widely used measure for assessing cardiorespiratory fitness. It is a strong contributor to performance in endurance sports, predicts functional capacity in older adults, and is related to overall disease risk. As such, it is important that testing methodologies for determining V̇O₂max provide valid and reliable measurements. Despite the widespread use of V̇O₂max, challenges exist whether using field or laboratory tests. With field testing, other physiologic variables have the potential to influence performance independent of V̇O₂max. During lab testing, despite direct gas exchange measurement, many individuals fail to achieve a plateau in ventilatory oxygen consumption. The intensity-time relationship [critical speed (CS), and the curvature constant (W’)] has the potential to influence performance in both of these testing environments, as its components effectively predict exercise tolerance in the severe intensity domain. To this end, this study examined the influence of the CS-W’ relationship on performance during a commonly used field test, the Cooper 12-Minute Run (12MR), and on the measurement of V̇O₂max during a traditional graded exercise test. Thirty individuals (15 male, 15 female) with varied levels of running experience volunteered for and completed this study. Each subject performed the following tests over four visits in a randomized order: one maximal graded treadmill exercise test, six runs to volitional exhaustion (90 seconds – 12 minutes duration, split over two lab visits), and a maximal 12-minute run. For the 12MR, multiple regression analyses showed a significant positive correlation between 12MR distance and V̇O₂max (r = 0.888; p < 0.05), but was not statistically significant for W’ (r = 0.160; p = 0.845). Bland-Altman plots showed no apparent bias for either prediction method by distance, but was different from zero (p < 0.05) for both V̇O₂max (mean difference = -163.52 m) and CS-W’ (mean difference = -143.53 m), with wider limits of agreement for V̇O₂max than CS-W’ (LoA: -819.42 to +492.38 m vs -403.86 to +116.80 m). For V̇O₂max, both CS and W’ were found to contribute significantly to variance observed in V̇O₂max (sp² = 0.803 and 0.095, respectively; p < 0.01), with overall R2 = 0.828. These data provide additional evidence of the link between 12MR performance and aerobic capacity, while supporting its use to track V̇O₂max over time without concern for changes in W’ influencing results. They do, however, suggest that W’ may influence the measurement of V̇O₂max during lab testing, although it is unclear whether this is due to the V̇O2 slow component or differences in anaerobic capacity. Nonetheless, when testing individuals who test administrators anticipate to have a low anaerobic capacity, adjusting test protocols to minimize time in the severe intensity domain may be warranted. Given the large proportion of V̇O₂max variation that the CS-W’ model accounted for though, using these variables as a proxy for V̇O₂max may be another alternative for assessing aerobic fitness.