Muscle oxygenation and pulmonary gas exchange kinetics during cycling exercise on-transitions in humans

Abstract

Near-infrared spectroscopy (NIRS) was utilized to gain insights into the kinetics of oxidative metabolism during exercise transitions. Ten untrained young men were tested on a cycle ergometer during transitions from unloaded pedaling to 5 min of constant-load exercise below (<VT) or above (>VT) the ventilatory threshold. Vastus lateralis oxygenation was determined by NIRS, and pulmonary O2 uptake (Vo --> Vo2) was determined breath-by-breath. Changes in deoxygenated hemoglobin + myoglobin concentration Delta[deoxy(Hb + Mb)] were taken as a muscle oxygenation index. At the transition, [Delta[deoxy(Hb + Mb)]] was unmodified [time delay (TD)] for 8.9 +/- 0.5 s at <VT or 6.4 +/- 0.9 s at >VT (both significantly different from 0) and then increased, following a monoexponential function [time constant (tau) = 8.5 +/- 0.9 s for <VT and 7.2 +/- 0.7 s for >VT]. For >VT a slow component of Delta[deoxy(Hb + Mb)] on-kinetics was observed in 9 of 10 subjects after 75.0 +/- 14.0 s of exercise. A significant correlation was described between the mean response time (MRT = TD + tau) of the primary component of Delta[deoxy(Hb + Mb)] on-kinetics and the tau of the primary component of the pulmonary Vo2 on-kinetics. The constant muscle oxygenation during the initial phase of the on-transition indicates a tight coupling between increases in O2 delivery and O2 utilization. The lack of a drop in muscle oxygenation at the transition suggests adequacy of O2 availability in relation to needs.

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

“…In the present study, despite a slower LBF response in S2 (and similar Amp of LBF increase), the TD before an increase in muscle deoxygenation (HHb) was reduced in S2 (ϳ14 s) compared with S1 (ϳ20 s), and the HHb-Amp was greater (S2, 5.2 ⌬OD units; S1, 2.7 ⌬OD units), suggesting a greater mismatch between local blood flow distribution and O 2 utilization earlier in the S2 transition, with muscle O 2 utilization increasing by means of a greater O 2 extraction, as local blood flow was slowed. The TD of the NIRS-derived muscle HHb during the initial phase of the exercise transition reported in the present investigation is consistent with the findings from previous studies (9,19). The higher steady-state G (⌬V O 2 /⌬WR) in S2 (18.1 ml⅐min Ϫ1 ⅐W Ϫ1 ) than S1 (13.5 ml⅐min Ϫ1 ⅐ W Ϫ1 ) and lower steady-state LBF-to-V O 2 increase in S2 supports the contention that the activation of muscle O 2 consumption in the upper region of the moderate-intensity domain was greater than that of blood flow, requiring increased O 2 extraction to meet the O 2 demands of the exercising muscle.…”

“…The NIRS-derived HHb signal reflects the balance between local muscle O 2 delivery and muscle O 2 utilization within the region of NIRS interrogation and, when used in combination with pulmonary V O 2 and muscle blood flow measurements, provides information on the adaptation of local muscle O 2 utilization (9,19).…”

Influence of blood flow occlusion on muscle oxygenation characteristics and the parameters of the power-duration relationship

“…Consistent with these muscle physiological responses being independent of O 2 delivery, similar skeletal muscle electromyography (EMG) characteristics are expressed between normoxic and hypoxic exercise (15,41). In contrast, fractional O 2 extraction {indicated by deoxygenated hemoglobin ϩ myoglobin concentration, (deoxy[HbϩMb]) (10,14,19,20,22)} at T lim appears to be scaled to O 2 delivery, such that it is greater with hypoxia (41) and less with hyperoxia (51) than normoxia, respectively. However, it remains to be determined whether similar EMG characteristics and fractional O 2 extractions are attained at T lim for severe-intensity domain power outputs within a given O 2 delivery.…”

“…The LED-detector fiber bundle separation distances are 2.0, 2.5, 3.0, and 3.5 cm. This NIRS device measures and incorporates the dynamic reduced scattering coefficients to provide absolute concentrations ( M) for deoxy [HbϩMb] (10,19,22) and has been used to reliably estimate the fractional oxygen extraction (10,13,14,19,20,22,34). The NIRS probe was calibrated before each test, according to the manufacturer's recommendations.…”

Estimated contribution of hemoglobin and myoglobin to near infrared spectroscopy

“…(Ferreira et al, 2005). However, while temporal data on Mb desaturation during exercise is scant, Richardson et al (1995) noted that within 20 sec they could detect desaturation of Mb by 1 H-MRS, which is the time course noted by many labs for the kinetics of increase in the NIRS deoxy signal (Ferreira, 2005;Grassi, 2003;DeLorey, 2003).…”