Factors Affecting V̇O₂ and Fat Oxidation Responses During Step Incremental Exercise

Step incremental exercise tests are widely used to assess endurance performance determinants; however, to what extent step duration influences oxygen uptake (V̇O₂) and carbon dioxide production (V̇CO₂) is unclear. This study assessed the influence of sampling duration on V̇O₂, V̇CO₂, and fat oxidation (Fat_ox) across different exercise intensities and modalities, and participant age and biological sex during graded exercise tests. A total of 169 participants (41 females; peak V̇O₂ [V̇O₂peak]: 54.7 ± 8.6 mL/kg/min) completed a running (n = 96) or cycling (n = 73) submaximal step test with V̇O₂ and V̇CO₂ measured continuously. Each stage was 6 min, and three 30 s averaging windows (2.5-3, 3.5-4, and 4.5-5 min) were compared to the 5.5-6 min reference. Linear mixed models assessed the effects of averaging window, exercise intensity domain, biological sex, age, fitness status, and exercise modality. The analysis included 1031 valid stages (588 running; 443 cycling). In running, V̇O₂ differences from the 6-min averaging window were observed at 3 (-28 mL/min), 4 (-15 mL/min), and 5-min averaging windows (-8 mL/min) (all p < 0.001), with similar results for V̇CO₂. No differences in V̇O₂ or Fat_ox between averaging windows were observed in cycling. These findings were consistent across exercise intensity, fitness status, biological sex, and age. Averaging window differences were < 2% for all conditions. Despite subtle differences, 3-, 4-, and 5-min averaging windows do not meaningfully impact V̇O₂, V̇CO₂, or Fat_ox compared to a 6-min stage duration. In conclusion, stage durations of ≥ 3 min may be sufficient to estimate these variables in running and cycling in participants with a wide range of characteristics.