Doctor of Philosophy
Kowalchuk, John M
Upon an instantaneous increase in work rate (WR), pulmonary O2 uptake (V̇O2p) increases in an exponential-like manner towards a new steady-state. To support this increase in V̇O2p there is an increase in blood flow (BF; O2 delivery) to the muscle. Previous research has shown that the rate of this increase in V̇O2p (which defines V̇O2p kinetics) becomes progressively slower with increasing baseline WR (WRbl). Given the paucity of research on blood flow dynamics and underlying muscle metabolic rate, this thesis examined the effect of metabolic rate on the dynamics and relationships amongst V̇O2p, BF, and muscle deoxygenation ([HHb]). Using mass spectrometry and volume turbinometry, Doppler ultrasound, and near-infrared spectroscopy (NIRS), the response of V̇O2p, BF, and [HHb] were measured, respectively, in response to step transitions in alternate-leg, knee-extension exercise from: i) a common WRbl (3 W) to increasing ΔWR (21, 30, 42, 51, 63 W); and ii) increasing WRbl (3, 12, 24, 33, 45 W) with a common ΔWR (21 W). As the BF signal is distorted by fluctuations in arterial pressure (from the heart rate (HR) cycle) and intramuscular pressure (from the contraction-relaxation (CR) cycle), it was necessary to first determine an acceptable technique for integrating the BF signal. From integrating BF by the CR or HR cycle over 1, 2, or 5 cycles using either ‘binning’ or ‘rolling’ averages, it was found that averaging the signal over a single CR cycle (CR1) was preferred due to its low variability. In analyzing the kinetic response to the various exercise transitions, it was found that: i) BF kinetics were similar across different ΔWR’s, but became progressively slower with increasing WRbl before significantly speeding at the highest WRbl; ii) V̇O2p kinetics became progressively slower with both increasing ΔWR and WRbl; iii) the increase in BF relative to the increase in metabolic demand (ΔBF/ΔV̇O2p) became progressively smaller with both increasing ΔWR and increasing WRbl; iv) [HHb] kinetics became progressively faster with increasing ΔWR, but became progressively slower with increasing WRbl. These findings suggest that with an increasing ΔWR, the muscle has an attenuated increase in bulk BF and a larger reliance on O2 extraction to meet the O2 requirements of the muscle, but with an increasing WRbl the muscle relies less on O2 extraction and more on BF redistribution within the muscle microvasculature.
Summary for Lay Audience
Whenever you instantaneously increase your activity level (work rate (WR)), your body must also instantly increase energy production to continue working at the higher WR. However, your body’s aerobic (oxidative) system, which requires O2 to make energy, does not immediately meet all of these energy needs, but instead gradually increases towards the higher energy requirements until its contribution to energy production is maximized. The rate of increase in aerobic energy production becomes progressively slower as the starting, baseline, intensity (WRbl) becomes higher. Given that increases in muscle BF and O2 delivery are required to support the higher muscle O2 requirements, this thesis investigated the role that blood flow (BF) and O2 delivery to exercising muscle plays in the response of the aerobic system by measuring the rate of adjustment of O2 uptake (V̇O2) and BF. A mass spectrometer and volume turbine were used to measure V̇O2, Doppler ultrasound was used to measure BF, and near-infrared spectroscopy was used to infer muscle O2 extraction from the blood. Subjects performed an instantaneous increase in WR on a knee-extension ergometer from: i) a WRbl of 3 W to 24, 33, 45, 54, or 66 W; or ii) a WR increase (ΔWR) of 21 W from a WRbl of 3, 12, 24, 33, or 45 W. Due to fluctuations in the BF signal caused by the heart pumping, as well as the muscle contracting and relaxing, it was necessary to determine an appropriate technique to analyze BF. By averaging BF over either a single, or 2 or 5 ‘binned’ or ‘rolling’ averages of a heart contraction cycle (HR), or a muscle contraction cycle (CR), it was determined that a single CR cycle was the best method for analyzing BF. When looking at how quickly V̇O2 and BF increased to the new WR for each trial, it was found that: i) BF increases at a similar speed with different ΔWR’s, but the increase becomes slower as WRbl increases, except for the highest WRbl in which BF increased much faster; ii) V̇O2 increases at a slower speed with larger ΔWRs and higher WRbls; iii) the increase in BF relative to the increase in V̇O2 becomes less as ΔWR and WRbl become greater; iv) O2 extraction by the muscle increased faster when ΔWR was greater, but increased slower at higher WRbls. These findings suggest that to meet the requirements for O2 and energy, the muscle relies less on bulk BF and more on O2 extraction at larger ΔWRs, but at higher WRbls it relies less on O2 extraction and more on BF in the microvasculature to meet its V̇O2 needs.
Love, Lorenzo, "Quantifying The Inter-Relationships Amongst Muscle Blood Flow, O2 Uptake And Muscle Deoxygenation Kinetics From Elevated Baseline Metabolic Rates And Increasing Work Rate Amplitudes" (2020). Electronic Thesis and Dissertation Repository. 7576.
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