A new technique to analyse threshold-intensities based on time dependent change-points in the ratio of minute ventilation and end-tidal partial pressure of carbon-dioxide production
Introduction
Gas exchange threshold (GET) (Beaver et al., 1986) and ventilatory threshold (VT) (Caiozzo et al., 1982) have been typically used in order to identify valid and reliable indexes of physiological boundaries (Amann et al., 2006; Beaver et al., 1986; Binder et al., 2008; Takano, 2000). The GET technique is based on the evaluation of oxygen utilization (VO2) and carbon-dioxide production (VCO2) (Pettitt et al., 2013). During an incremental exercise, the first breakpoint of VCO2 relative to VO2 (GET1) demarcates the transition between isotonic buffering phase and isocapnic buffering phase, while the second breakpoint (GET2) separates from isocapnic buffering to hypercapnic response phase (Kouwijzer et al., 2019). Once the GET1 is exceeded, during isocapnic buffering phase, a non-linear CO2 response occurs against the VO2 as a function of aerobic metabolism plus excess CO2 (non-oxidative CO2) derived from bicarbonate () buffering of hydrogen ion () in response to a systematic increase in lactic acidosis above resting values. When the GET2 is exceeded due to worsening acidosis as response to a more severe exercise intensity, hypercapnia, characterized by an increase in CO2 level in blood that results a decrease in end-tidal partial pressure of CO2 (PETCO2), occurs (Wasserman et al., 1975).
The VT technique relies on evaluating the minute ventilation (VE) and VO2 (Pettitt et al., 2013). Since nonlinear increases in VCO2 enhance extra increases in VE responses, the first (VT1) and the second (VT2) breakpoints in VE relative to VO2 occur (Deruelle et al., 2007; Schöffl et al., 2018; Weston and Gabbett, 2001). The VT1 and VT2 are, therefore, identical for the GET1 and GET2 (Kouwijzer et al., 2019). Moreover, the breakpoint of the VE relative to VCO2, called as respiratory compensation point (RCP), is also used as a valid surrogate of the upper boundary of isocapnic buffering phase (Reinhard et al., 1979). Central and peripheral neurogenic stimuli and metabolites are implicated in the hyperventilatory response observed at the RCP (Rausch et al., 1991), but distinguish in VE relative to VCO2 is principally attributed to the stimulation of carotid-bodies than others, and thus, it is found more related to an increase in respiratory frequency rather than the tidal volume (hyperpnea) (Forster et al., 2012; Nicolò et al., 2020).
Overall, the first transition is commonly determined by GET1 or VT1 as the work rate that results an increase in lactic acidosis or minute ventilation above resting values, while the second threshold-intensity is primarily determined by the RCP as the work rate that induces the hot ventilatory response to a more severe lactic acidosis (Beaver et al., 1986; Wasserman et al., 1981). However, it has been claimed that the RCP is not a valid parameter that defines the upper boundary of isocapnic buffering phase, or in other words, the upper boundary of heavy exercise domain (Caen et al., 2018; Leo et al., 2017; Wang et al., 2017). The RCP is accepted as a function of incremental ramp exercise, and the external power output associated to the RCP is found closely related with the ramp-slope (Leo et al., 2017; Whipp et al., 1981). Furthermore, the RCP typically overestimates maximal lactate steady-state (MLSS) (Dekerle et al., 2003; Meyer et al., 2005; Ozkaya et al., 2020; Pallarés et al., 2016), anaerobic threshold (AT) determined by 2 mmol⋅L−1 lactate response above lactate threshold (i.e., LT +2 mmol⋅L−1) (Pallarés et al., 2016), critical power (CP) (Caen et al., 2018; Ozkaya et al., 2020; Wang et al., 2017), breakpoint of fall in (Meyer et al., 2004), breakpoint in local muscle or brain oxygenation (Caen et al., 2018), and VT2 that is obtained by the same (standard) incremental ramp-slope (30 W⋅min−1) with the RCP (Ozkaya et al., 2020).
As a consequence, the breakpoint of VE-VCO2 can overestimate the second breakpoint of the VCO2-VO2 or VE-VO2 in especially well-trained and elite athletes. Indeed, results of our pilot studies showed that the work rate corresponded to breakpoint of VE-VCO2 is not equal to the work rate giving an initial fall in time dependent PETCO2 in well-trained or elite athletes. In that case, it may be said that there is a gap between the upper boundary of isocapnic buffering phase and the lower boundary of hyperventilatory response phase in well-trained and elites. Therefore, once the GET2 or VT2 is exceeded, VE and VCO2 continue to increase linearly. However, once the work rate giving the RCP is exceeded, there is mainly respiratory rate dependent nonlinear VE increase relative to the VCO2, with a concomitant plateau in tidal volume response (Legrand et al., 2007). Moreover, according to results of our pilot study, although there is only one decrease in time dependent PETCO2 response at where the work rate that is corresponded to the GET2 or VT2 in less trained athletes, there is one more slope that leads a more dramatic fall in PETCO2 over time following its initial decrease in well-trained or elite athletes. Those pre-findings reminded that the rate of VE divided by the PETCO2 (VE/PETCO2), called as respiratory threshold (RT) in this study, can successfully provide clear multiple change-points, which are corresponded to traditional threshold intensities, over time. Moreover, a single criterion based on the same respiratory principle, rather than the different techniques (i.e., breakpoints of VCO2-VO2, VE-VO2 or VE-VCO2), can give a better information about whether the RCP overestimates GET2 in well-trained and elite athletes, or not.
The aim of this study was to test the utility and effectiveness of this alternative computational approach to threshold-intensities based on time dependent change-points in VE/PETCO2 (i.e., RT) to reveal whether the RCP is a third ventilatory threshold. It was hypothesized that a single criterion based on the same respiratory principle (i.e., RT) would clearly show whether the RCP overestimates the other threshold indices, or not.
Section snippets
Participants
The study was approved by the university ethics committee (17-1/2). Experimental procedures were designed according to the rules and principles of the Helsinki Declaration. Prior to the research, a written informed consent was obtained from each participant after clarifying the procedures of the study, potential risks and benefits of being involved in this experiment. A total of 20 participants volunteered to take part in this study. 10 of the participants were recreationally active young
Results
The highest 30-second VO2 mean obtained from incremental ramp tests was 42.6 ± 3.39 mL⋅min−1⋅kg−1 and corresponding power output was 241 ± 62.4 W for recreationally active individuals, while those values indicated to a greater VO2 response (65.8 ± 7.58 mL⋅min−1⋅kg−1) and an external power (390 ± 32.6 W) in well-trained athletes (p < 0.001). When considering recreationally active individuals, the power output and VO2 responses at GET1 and GET2 were not significantly different than those of RT1
Discussion
There have been many conflict results or speculative information that indicate the RCP overestimates other anaerobic threshold indexes such as MLSS (Dekerle et al., 2003; Meyer et al., 2005; Ozkaya et al., 2020; Pallarés et al., 2016), AT (Pallarés et al., 2016), CP (Caen et al., 2018; Ozkaya et al., 2020; Wang et al., 2017), breakpoint of fall in (Meyer et al., 2004), breakpoint in local muscle or brain oxygenation (Caen et al., 2018), and VT2 that is obtained by the same (standard)
Conclusion
It was demonstrated that the time dependent VE/PETCO2 relation is a practical technique to identify intersection points that are obtained from a single incremental test. It successfully determines all change-points in time dependent respiratory responses of both recreationally active (i.e., RT1 and RT2) and well-trained individuals (i.e., RT1, RT2, and RT3). Athletes, coaches and sports scientists should consider that there is a third respiratory threshold in well-trained athletes. Indeed,
Acknowledgements
The authors thank the support of coaches, athletes, and all those who were involved in this study. The experiment complies with the current laws of the country in which they were performed. This work was supported by the Ege University, scientific research projects fund under grant (17.BESYO.002). The authors do not have any conflict of interest.
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