Linear and non‐linear contributions to oxygen transport and utilization during moderate random exercise in humans

Abstract

What is the central question of this study? The pulmonary oxygen uptake (pV̇O2) data used to study the muscle aerobic system dynamics during moderate-exercise transitions is classically described as a mono-exponential function controlled by a complex interaction of the oxygen delivery-utilization balance. This elevated complexity complicates the acquisition of relevant information regarding aerobic system dynamics based on pV̇O2 data during a varying exercise stimulus. What is the main finding and its importance? The elevated complexity of pV̇O2 dynamics is a consequence of a multiple-order interaction between muscle oxygen uptake and circulatory distortion. Our findings challenge the use of a first-order function to study the influences of the oxygen delivery-utilization balance over the pV̇O2 dynamics. The assumption of aerobic system linearity implies that the pulmonary oxygen uptake (pV̇O2) dynamics during exercise transitions present a first-order characteristic. The main objective of this study was to test the linearity of the oxygen delivery-utilization balance during random moderate exercise. The cardiac output (Q̇) and deoxygenated haemoglobin concentration ([HHb]) were measured to infer the central and local O availability, respectively. Thirteen healthy men performed two consecutive pseudorandom binary sequence cycling exercises followed by an incremental protocol. The system input and the outputs pV̇O2, [HHb] and Q̇ were submitted to frequency-domain analysis. The linearity of the variables was tested by computing the ability of the response at a specific frequency to predict the response at another frequency. The predictability levels were assessed by the coefficient of determination. In a first-order system, a participant who presents faster dynamics at a specific frequency should also present faster dynamics at any other frequency. All experimentally obtained variables (pV̇O2, [HHb] and Q̇) presented a certainly degree of non-linearity. The local O availability, evaluated by the ratio pV̇O2/[HHb], presented the most irregular behaviour. The overall [HHb] kinetics were faster than pV̇O2 and Q̇ kinetics. In conclusion, the oxygen delivery-utilization balance behaved as a non-linear phenomenon. Therefore, the elevated complexity of the pulmonary oxygen uptake dynamics is governed by a complex multiple-order interaction between the oxygen delivery and utilization systems.

Mean Normalized Gain: A New Method for the Assessment of the Aerobic System Temporal Dynamics during Randomly Varying Exercise in Humans

“…In support of recent findings (Beltrame and Hughson, 2017a,b; Beltrame et al, 2017a), the purpose of this study was to describe in detail the computation of the mean normalized gain ( MNG ) and to test its consistency to characterize the $\dot{V}{O}_2$ kinetics during random exercise in humans. The MNG will be validated against the time-domain approach, and checked for intra-subject consistency by applying multiple PRBS protocols on different days and times of the day.…”

“…The PRBS protocol (Figure 1B) was generated by a digital shift register with an adder module feedback (Bennett et al, 1981; Hughson et al, 1990; Beltrame and Hughson, 2017b) (Figure 1A). The target Ẇ (reached after <1.5 s of transition following a modification of the ergometer controller) was 25 or 100 W, and the cadence was maintained at ≈ 1 Hz.…”

“…As described in Figure 1, the PRBS protocol comprised 15 units (25 or 100 W) for 30 s (total of 450 s for each PRBS). According to previous studies (Beltrame and Hughson, 2017a,b), the highest Ẇ used in the current study (i.e., 100 W) is constrained to moderate intensity exercise thus avoiding the presence of system distortions that might influence the system temporal analysis.…”

“…An attractive alternative to multiple repetitions of step transitions for evaluating the kinetic behavior of the aerobic energy supply system is the pseudo-random binary sequence (PRBS) in which Ẇ varies between two levels which are normally constrained to the light to moderate intensity exercise domains (Eßfeld et al, 1987; Hughson et al, 1990; Beltrame and Hughson, 2017a). The $\dot{V}{O}_2$ response to PRBS protocols is evaluated in the frequency-domain filtering out non-periodic signals associated with white noise, improving the extraction of parameters related to the aerobic system dynamics.…”

The Complex Reality of VO2 Kinetics to Steady State: Reassessment of the Models Used to Quantify and Interpret VO2 Kinetics, Steady State, and Time to Steady State

“…Intramuscular phosphocreatine kinetics (Rossiter et al, 1999), de-oxygenated haemoglobin kinetics (DeLeroy et al, 2003;MacPhee et al, 2005), and muscle blood flow kinetics (MacPhee et al, 2005) have all been processed using the monoexponential model. However, a recent publication by Beltrame and Hughson (2017) has indicated that the de-oxygenated haemoglobin signal is nonlinear during dynamic exercise. It therefore seems relevant to research that the kinetic response of these metabolic and physiological functions should also be reinvestigated.…”