Assessing Aerobic and Anaerobic Thresholds with Emphasis on Isocapnic Buffering in Endurance Runners

Accurate determination of metabolic thresholds is essential for designing effective endurance training. This study aimed to apply a multi-visit Step-Ramp-Step (SRS) protocol to identify aerobic (VT1) and anaerobic (VT2/RCP) thresholds in trained endurance runners, with a particular focus on delineating the isocapnic buffering region-the ventilatory phase between VT1 and VT2 where carbon dioxide (PetCO₂) remains stable despite rising ventilation. Twelve trained male runners (mean age: 27.1 ± 1.9 years; VO₂max: 60.5 ± 2.1 ml·kg^-1·min^-1) completed the SRS protocol across separate lab visits. Each session included a 4-minute moderate-intensity phase, a progressive ramp to volitional exhaustion, and a 4-minute heavy-intensity step following a 30-minute recovery. Breath-by-breath gas exchange data (VO₂, VCO₂, VE, RER, PetO₂, PetCO₂) were analyzed using 20-second smoothing. Results showed that VT1 and RCP occurred at 73.2 ± 4.1% and 89.6 ± 3.8% of VO₂max, respectively. The isocapnic buffering zone spanned ~16.4% of the VO₂max range. Unlike previous SRS studies focused on cycling, this study uniquely applies the protocol to running and specifically quantifies the buffering region. These findings support the use of SRS running protocols for efficient, individualized assessment of metabolic transitions in endurance athletes.