Kinetics of O₂uptake, leg blood flow, and muscle deoxygenation are slowed in the upper compared with lower region of the moderate-intensity exercise domain

Abstract

(SD 4)] performed repetitions (6 -8) of twolegged, moderate-intensity, knee-extension exercise during two separate protocols that included step transitions from 3 W to 90% estimated lactate threshold ( L) performed as a single step (S3) and in two equal steps (S1, 3 W to ϳ45% L; S2, ϳ45% L to ϳ90% L). The time constants ( ) of pulmonary oxygen uptake (V O2), leg blood flow (LBF), heart rate (HR), and muscle deoxygenation (HHb) were greater (P Ͻ 0.05) in S2 ( V O2, ϳ52 s; LBF, ϳ 39 s; HR, ϳ42 s; HHb, ϳ33 s) compared with S1 ( V O2, ϳ24 s; LBF, ϳ21 s; HR, ϳ21 s; HHb, ϳ16 s), while the delay before an increase in HHb was reduced (P Ͻ 0.05) in S2 (ϳ14 s) compared with S1 (ϳ20 s). The V O2 and HHb amplitudes were greater (P Ͻ 0.05) in S2 compared with S1, whereas the LBF amplitude was similar in S2 and S1. Thus the slowed V O2 response in S2 compared with S1 is consistent with a mechanism whereby V O2 kinetics is limited, in part, by a slowed adaptation of blood flow and/or O2 transport when exercise was initiated from a baseline of moderate-intensity exercise.oxygen uptake kinetics; femoral arterial blood flow kinetics; Doppler ultrasound; knee-extension exercise; near-infrared spectroscopy ACCOMPANYING A RISE IN EXERCISE intensity, there is a challenge to increase the rate of oxidative phosphorylation to meet the new metabolic demand of the working muscle. To meet this demand, the respiratory and cardiovascular systems must adapt in a coordinated manner to transport O 2 from the atmosphere to the mitochondria of the exercising muscle, thus allowing oxidative phosphorylation to proceed at the required rate. Pulmonary O 2 uptake (V O 2 ) kinetics is an index of the overall efficiency and conditioning of these integrated systems and can provide pertinent information with regard to the various mechanisms regulating O 2 delivery and O 2 utilization by skeletal muscle during exercise.Recently, it was reported (8) that, for a given absolute increase in work rate (WR), the adaptation of V O 2 during leg-cycling exercise was slower and the gain (G) (i.e., ⌬V O 2 / ⌬WR) was greater when exercise was initiated in the upper compared with the lower regions of the moderate-intensity exercise domain. These observations agree with those of Hughson and Morrissey (22,23) and DiPrampero et al. (13), but differ from those of DiPrampero et al. (12) and Diamond et al.(10), who reported either a faster or similar adaptation, respectively, when comparing exercise initiated from either prior moderate-intensity exercise or rest.Brittain et al. (8) attributed the slowing of V O 2 kinetics in the upper region of the moderate-intensity domain to the bioenergetic properties of the newly recruited motor units, which were assumed to be less efficient (i.e., greater O 2 or ATP cost per contraction) with a more slowly adapting V O 2 response than those motor units recruited initially at exercise onset from rest or very light exercise (i.e., lower region of the moderateintensity domain). Hughson and Morrissey (22,23), however, suggested that the sl...

Relating pulmonary oxygen uptake to muscle oxygen consumption at exercise onset: in vivo and in silico studies

“…Data from these studies are comparable to those of others (DeLorey et al 2003;Grassi et al 2003;MacPhee et al 2005). One of the limitations of using NIRS to evaluate StO 2m in skeletal muscle is that the sampling is not restricted to working muscle, but includes-in addition-skin, adipose tissue, capillaries, and small arterioles and venules.…”

“…In particular, near infrared spectroscopy (NIRS) can be used to monitor the relative concentrations of deoxyhemoglobin and deoxymyoglobin or tissue oxygen saturation, StO 2m , during dynamic exercise. In lieu of specific assessments of intracellular oxygenation, NIRS can provide information reflecting the balance between local muscle oxygen delivery and utilization within the interrogated tissue region (DeLorey et al 2003;Grassi et al 2003;MacPhee et al 2005).…”

Effects of Age and Long-Term Endurance Training on V·O2 Kinetics

“…A limitation in O 2 delivery to the exercising muscle has been proposed as a likely mechanism regulating the rate of adaptation of oxidative phosphorylation (Murias et al, 2010(Murias et al, , 2011b(Murias et al, , 2014Poole et al, 2008;. It has been hypothesized (Phillips et al, 1995) that faster femoral artery blood velocity (from endurance training) was responsible for reductions in τVO 2p ; however, measures of muscle conduit artery blood flow kinetics in young healthy adults have shown that the rate of adjustment is similar to or faster than that of VO 2p (duManoir et al, 2010;MacPhee et al, 2005).…”

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

“…There have been several studies that have provided some empirical evidence to oppose the use of τ (Hughson & Morrissey, 1982;Brittain et al, 2001;Koppo et al, 2004;MacPhee et al, 2005;Stirling et al, 2005), however they are limited and there lacks any strong empirical validation of the methods. Therefore, it felt necessary to reassess the pre-defined concepts of τ and 4τ as they apply to two contrasting subject groups, highly-trained and untrained.…”

“…However, even with its versatile use over the past four decades, the mono-exponential model has yet to be explicitly validated. This is all the more problematic when past research exists which questions the validity of using a simple mono-exponential model to explain V O2 kinetics behaviour (Linnarson, 1974;Hughson & Morrissey, 1982;Casaburi et al, 1989;Koppo et al, 2004;MacPhee et al, 2005;Stirling et al, 2005;McNulty et al, 2015).…”