*2.1. Ethical Approval*

The study protocol was approved by the Institutional Review Board of the Bioethical Committee at the Medical University of Warsaw (AKBE/32/2021) and met the necessary regulations of the Declaration of Helsinki. Mandatory written consents to undergo incremental CPET were obtained from each EA before participating in the study. Written informed consent for participation was not required for this study, in accordance with the national legislation and the institutional requirements

#### *2.2. Study Design*

Participants were endurance runners who underwent CPET on the treadmill (TE). The tests were performed according to individual requests of the EA as an element of the training program prescription. Preliminary inclusion criteria were (1) age ≥ 18 years, and (2) ≤±3 SD from mean for all of the tested variables included in Table 1 (only the lowest or the weakest extreme outliers were excluded). Exclusion criteria were (1) CPET was not performed on the TE, (2) any acute or chronic medical condition (including the musculoskeletal system, or addictions), (3) ongoing pharmacological treatment, and (4) smoking. To ensure that each subject reached maximum effort during the CPET, we applied the additional selection protocol consisting of the fulfillment ≥ 6 of the following criteria: (1) plateau in VO2 (growth < 100 mL·min−<sup>1</sup> in VO2 with exercise workload increase), (2) respiratory exchange ratio (RER) ≥ 1.10, (3) respiratory frequency (fR) ≥ 45 breaths·min<sup>−</sup>1, (4) reported exertion during CPET ≥ 18, according to the Borg scale, (5) [La−]b ≥ 8 mmol·L−1, (6) increase in speed ≥ 10% of its RCP value post-exceeding the RCP, and (7) peak HR ≥ 15 beats·min−<sup>1</sup> under predicted maximal HR [21]. For the entire selection procedure, along with the exclusion data, see Figure 1.

**Table 1.** Participants' basic anthropometric characteristics.


Abbreviations: BM, body mass; BMI, body mass index; BF, body fat; FM, fat mass; FFM, fat-free mass. The continuous value is presented as mean (SD), while the categorical value is shown as numbers (%), when appropriate. Comparisons between subgroups (*p*-value) were obtained by Student's *t*-test for independent variables.

**Figure 1.** Flowchart of the preliminary inclusion and exclusion process. Abbreviations: EA, endurance athlete; CPET, cardiopulmonary exercise testing; SD, standard deviation; TE, treadmill; RER, respiratory exchange ratio; VO2, oxygen uptake (mL·min−1·kg−1); [La−]b, lactate concentration (mmol·L<sup>−</sup>1); fR, breathing frequency (breaths·min<sup>−</sup>1); RCP, respiratory compensation point; HRpeak, peak heart rate (beats·min<sup>−</sup>1); HRmax, maximal heart rate (bpm). At both stages of the selection, some participants met several (>1) exclusion criteria.
