Danilo Iannetta

The near-infrared spectroscopy-derived deoxygenated haemoglobin breaking-point is a repeatable measure that demarcates exercise intensity domains

OBJECTIVES A breaking-point in the near-infrared spectroscopy (NIRS)-derived deoxygenated haemoglobin ([HHb]) profile towards the end of a ramp incremental (RI) cycling test has been associated to the respiratory compensation point (RCP). Despite the physiological value of this measure, its repeatability remains unknown. The aim was to examine the repeatability of the [HHb] breaking-point ([HHb]BP) and its association to RCP during a RI cycling test. DESIGN A repeated measures design was performed on 11 males (30.5±8.4 year; 76.5±8.4kg) and 4 females (30.5±5.9 year; 61.9±4.4 Kg). METHODS Gas exchange and NIRS [HHb] data were collected during RI tests performed on two different days separated by 48h. The [HHb]BP and the RCP were determined and compared for each trial. RESULTS The [HHb]BP and the respiratory compensation point (RCP) occurred at the same VO2 in test 1 and test 2 ([HHb]BP: 3.49±0.52Lmin-1 test 1; 3.48±0.45Lmin-1 test 2; RCP: 3.38±0.40Lmin-1 test 1; 3.38±0.44Lmin-1 test 2) (P>0.05). The VO2 associated with the [HHb]BP and the VO2 at RCP were not significantly different from each other either in test 1 as well as in test 2 (P>0.05). Neither test 1 nor test 2 showed significant mean average error between the VO2 at the [HHb]BP and RCP using Bland & Altman plots. CONCLUSIONS The [HHb]BP is a repeatable measure that consistently occurs towards the end of a RI test. The association between the [HHb]BP and the RCP reinforces the idea that these parameters may share similar mechanistic basis.

The plateau in the NIRS-derived [HHb] signal near the end of a ramp incremental test does not indicate the upper limit of O2 extraction in the vastus lateralis

This study aimed to examine, at the level of the active muscles, whether the plateau in oxygen (O2) extraction normally observed near the end of a ramp incremental (RI) exercise test to exhaustion is caused by the achievement of an upper limit in O2 extraction. Eleven healthy men (27.3 ± 3.0 yr, 81.6 ± 8.1 kg, 183.9 ± 6.3 cm) performed a RI cycling test to exhaustion. O2 extraction of the vastus lateralis (VL) was measured continuously throughout the test using the near-infrared spectroscopy (NIRS)-derived deoxygenated hemoglobin [HHb] signal. A leg blood flow occlusion was performed at rest (LBFOCC1) and immediately after the RI test (LBFOCC2). The [HHb] values during the resting occlusion (108.1 ± 21.7%; LBFOCC1) and the peak values during exercise (100 ± 0%; [HHb]plateau) were significantly greater than those observed at baseline (0.84 ± 10.6% at baseline 1 and 0 ± 0% at baseline 2) (P < 0.05). No significant difference was found between LBFOCC1 and [HHb]plateau (P > 0.05) or between the baseline measurements (P > 0.05). [HHb] values at LBFOCC2 (130.5 ± 19.7%) were significantly greater than all other time points (P < 0.05). These results support the existence of an O2 extraction reserve in the VL muscle at the end of a RI cycling test and suggest that the observed plateau in the [HHb] signal toward the end of a RI test is not representative of an upper limit in O2 extraction.

Blood flow occlusion-related O2 extraction "reserve" is present in different muscles of the quadriceps but greater in deeper regions after ramp-incremental test

It was recently demonstrated that an O2 extraction reserve, as assessed by the near-infrared spectroscopy (NIRS)-derived deoxygenation signal ([HHb]), exists in the superficial region of vastus lateralis (VL) muscle during an occlusion performed at the end of a ramp-incremental test. However, it is unknown whether this reserve is present and/or different in magnitude in other portions and depths of the quadriceps muscles. We tested the hypothesis that an O2 extraction reserve would exist in other regions of this muscle but is greater in deep compared with more superficial portions. Superficial (VL-s) and deep VL (VL-d) as well as superficial rectus femoris (RF-s) were monitored by a combination of low- and high-power time-resolved (TRS) NIRS. During the occlusion immediately post-ramp-incremental test there was a significant overshoot in the [HHb] signal ( P < 0.05). However, the magnitude of this increase was greater in VL-d (93.2 ± 42.9%) compared with VL-s (55.0 ± 19.6%) and RF-s (47.8 ± 14.0%) ( P < 0.05). The present study demonstrated that an O2 extraction reserve exists in different pools of active muscle fibers of the quadriceps at the end of a ramp exercise to exhaustion. The greater magnitude in the reserve observed in the deeper portion of VL, however, suggests that this portion of muscle may present a greater surplus of oxygenated blood, which is likely due to a greater population of slow-twitch fibers. These findings add to the notion that the plateau in the [HHb] signal toward the end of a ramp-incremental exercise does not indicate the upper limit of O2 extraction. NEW & NOTEWORTHY Different portions of the quadriceps muscles exhibited an untapped O2 extraction reserve during a blood flow occlusion performed at the end of a ramp-incremental exercise. In the deeper portion of the vastus lateralis muscle, this reserve was greater compared with superficial vastus lateralis and rectus femoris. These data suggest that the O2 extraction reserve may be dependent on the vascular and/or oxidative capacities of the muscles.

Quadriceps Muscles O2 Extraction and EMG Breakpoints during a Ramp Incremental Test

Muscle deoxygenated breakpoint ([HHb]BP) has been found to be associated with other indices of exercise tolerance in the vastus lateralis (VL) muscle but not in the vastus medialis (VM) and rectus femoris (RF). Purpose: To investigate whether the [HHb]BP occurs also in the VM and RF muscles and whether or not it is associated with other physiological indices of exercise tolerance, such as the EMG threshold (EMGt) and the respiratory compensation point (RCP). Methods: Twelve young endurance trained participants performed maximal ramp incremental (RI) cycling tests (25–30 W·min−1 increments). Muscle oxygen extraction and activity as well as ventilatory and gas exchange parameters were measured. After accounting for the mean response time, the oxygen uptake (V·O2) corresponding to the RCP, [HHb]BP, and the EMGt was determined. Results: Peak power output (POpeak) was 359 ± 48 W. Maximal oxygen consumption (V·O2max) was 3.87 ± 0.46 L·min−1. The V·O2 at the RCP was 3.39 ± 0.41 L·min−1. The V·O2 (L·min−1) corresponding to the [HHb]BP and EMGt were: 3.49 ± 0.46 and 3.40 ± 0.44; 3.44 ± 0.61 and 3.43 ± 0.49; 3.59 ± 0.52, and 3.48 ± 0.46 for VL, VM, and RF, respectively. Pearsons correlation between these thresholds ranged from 0.90 to 0.97 (P < 0.05). No difference was found for the absolute V·O2 and the normalized PO (%) at which the thresholds occurred in all three muscles investigated (P > 0.05). Although in eight out of 12 participants, the [HHb]BP in the RF led to a steeper increase instead of leading to a plateau-like response as observed in the VL and VM, the V·O2 at the breakpoints still coincided with that at the RCP. Conclusions: This study demonstrated that local indices of exercise tolerance derived from different portions of the quadriceps are not different to the systemic index of the RCP.

An equation to predict the maximal lactate steady state from ramp-incremental exercise test data in cycling

OBJECTIVES The maximal lactate steady state (MLSS) represents the highest exercise intensity at which an elevated blood lactate concentration ([Lac]b) is stabilized above resting values. MLSS quantifies the boundary between the heavy-to-very-heavy intensity domains but its determination is not widely performed due to the number of trials required. DESIGN This study aimed to: (i) develop a mathematical equation capable of predicting MLSS using variables measured during a single ramp-incremental cycling test and (ii) test the accuracy of the optimized mathematical equation. METHODS The predictive MLSS equation was determined by stepwise backward regression analysis of twelve independent variables measured in sixty individuals who had previously performed ramp-incremental exercise and in whom MLSS was known (MLSSobs). Next, twenty-nine different individuals were prospectively recruited to test the accuracy of the equation. These participants performed ramp-incremental exercise to exhaustion and two-to-three 30-min constant-power output cycling bouts with [Lac]b sampled at regular intervals for determination of MLSSobs. Predicted MLSS (MLSSpred) and MLSSobs in both phases of the study were compared by paired t-test, major-axis regression and Bland-Altman analysis. RESULTS The predictor variables of MLSS were: respiratory compensation point (Wkg-1), peak oxygen uptake (V˙O2peak) (mlkg-1min-1) and body mass (kg). MLSSpred was highly correlated with MLSSobs (r=0.93; p<0.01). When this equation was tested on the independent group, MLSSpred was not different from MLSSobs (234±43 vs. 234±44W; SEE 4.8W; r=0.99; p<0.01). CONCLUSIONS These data support the validity of the predictive MLSS equation. We advocate its use as a time-efficient alternative to traditional MLSS testing in cycling.

Metabolic and performance-related consequences of exercising at and slightly above MLSS

Exercising at the maximal lactate steady state (MLSS) results in increased but stable metabolic responses. We tested the hypothesis that even a slight increase above MLSS (10 W), by altering the metabolic steady state, would reduce exercise performance capacity. Eleven trained men in our study performed: one ramp‐incremental tests; two to four 30‐minute constant‐load cycling exercise trials to determine the PO at MLSS (MLSSp), and ten watts above MLSS (MLSSp+10), which were immediately followed by a time‐to‐exhaustion test; and a time‐to‐exhaustion test with no‐prior exercise. Pulmonary O2 uptake V.O2) and blood lactate concentration ([La−]b) as well as local muscle O2 extraction ([HHb]) and muscle activity (EMG) of the vastus lateralis (VL) and rectus femoris (RF) muscles were measured during the testing sessions. When exercising at MLSSp+10, although V.O2 was stable, there was an increase in ventilatory responses and EMG activity, along with a non‐stable [La−]b response (P < 0.05). The [HHb] of VL muscle achieved its apex at MLSSp with no additional increase above this intensity, whereas the [HHb] of RF progressively increased during MLSSp+10 and achieved its apex during the time‐to‐exhaustion trials. Time‐to‐exhaustion performance was decreased after exercising at MLSSp (37.3 ± 16.4%) compared to the no‐prior exercise condition, and further decreased after exercising at MLSSp+10 (64.6 ± 6.3%) (P < 0.05). In summary, exercising for 30 min slightly above MLSS led to significant alterations of metabolic responses which disproportionately compromised subsequent exercise performance. Furthermore, the [HHb] signal of VL seemed to achieve a “ceiling” at the intensity of exercise associated with MLSS.

Reliability of microvascular responsiveness measures derived from near-infrared spectroscopy across a variety of ischemic periods in young and older individuals

BACKGROUND Cardiovascular disease (CVD) is associated with impairments in microvascular responsiveness. Therefore, reliably assessing microvascular function is clinically relevant. Thus, this study aimed to examine the reliability of the near-infrared spectroscopy (NIRS)-derived oxygen saturation (StO2) reperfusion slope, a measure of microvascular responsiveness, to four different vascular occlusion tests (VOT) of different durations in young and older participants. METHODS Eight healthy young (29 ± 5 yr) and seven older (67 ± 4 yr) men participated in four NIRS combined with VOT (NIRS-VOT; 30 s, 1, 3, and 5 min) in the leg microvasculature on two visits separated by 1-2 weeks. Vascular responsiveness was determined by the StO2 reperfusion slope. The coefficient of variation (CV), repeatability, reliability (ICC), and the limits of agreement (LOA) were calculated for the NIRS-derived reperfusion slopes for each occlusion duration and visit. RESULTS CV for the StO2 reperfusion slope following 30 s, 1, 3 and 5 min of occlusion were 33 ± 29%, 19 ± 21%, 14 ± 12%, and 12 ± 10%, respectively. Repeatability values following 30 s, 1, 3 and 5 min occlusions were 20%, 1%, 4% and 21%, respectively. The ICC for the StO2 reperfusion slopes for each occlusion duration were 0.29, 0.42, 0.84, and 0.88 following 30 s, 1, 3 and 5 min of occlusion, respectively. LOA values between visit 1 and 2 for occlusions were not different from zero. There were no age-related differences for all variables of the study. CONCLUSION NIRS-derived StO2 reperfusion slope, has good reliability across a range of occlusion durations with the strongest reliability during longer occlusion durations.

Reply to Dr. Grassi

TO THE EDITOR: We appreciate Dr. Grassi’s insights (1) on our study demonstrating an O2 extraction reserve in different portions of the quadriceps muscles (2). Dr. Grassi commented that the existence of a “reserve” in O2 extraction is a wellestablished concept demonstrated by previous studies in which, differently from our approach, blood flow occlusion occurred a “few minutes” (rather than immediately) after exercise cessation. Dr. Grassi contended that differences in procedures are “irrelevant” to the interpretation of the results. We disagree with this position. Our study determined whether a potential to further increase microvascular O2 extraction in the vastus lateralis muscle existed in the presence of a plateau in the near-infrared (NIRS)-derived deoxy-hemoglobin ([HHb]) signal, with pulmonary V̇O2 still increasing. In this context, occluding blood flow immediately at the end of the ramp exercise (i.e., V̇O2max) allowed us to evaluate the [HHb] response at a metabolic rate and O2 tension associated with maximal exercise intensity for the proposed exercise. Had we occluded blood flow “after a few minutes of recovery,” this procedure would have not allowed us to answer our experimental question, because during this time muscle V̇O2 and vascular dynamics (e.g., hematocrit, capillary recruitment, O2 diffusive capacity) would have been substantially different, thus affecting the [HHb] response. This is clear in a previous study showing that occlusions of blood flow before the ramp exercise resulted in lower amplitudes in the [HHb] signal compared with those performed immediately after the ramp exercise (3). Thus, although we are aware of Dr. Grassi’s studies, it is difficult to understand how they apply to our experimental conditions. Regarding the plateau in the [HHb] signal, we reasoned that, based on the Fick equation, the absence of a further increase in O2 extraction in the presence of increasing V̇O2 implies that a greater O2 availability is supporting the increased metabolic rate. Dr. Grassi highlighted that data from different groups indicated that there are “. . . substantial amounts of O2 in the venous blood draining from maximally working muscles” and that “muscle O2 extraction does not reach 100% during incremental exercise,” thus the plateau reflects a limitation of the muscle to extract O2. It should be noted that substantial amounts of O2 are to be expected in the venous circulation when samples are collected from a vessel that is draining blood from both highly active and less active muscles. For this reason, the arterial-venous O2 difference is never 20 ml/dl of blood. However, by evaluating the [HHb] signal of the active tissues, this limitation can be somewhat circumvented. Although critical O2 tensions will eventually affect the O2 diffusion gradient and 100% of the O2 will never be extracted, it is highly probable that the NIRS-derived estimation of O2 extraction is more likely to approximate the highest level of O2 extraction in the active fibers compared with other more “systemic” measures. To conclude, this exchange with Dr. Grassi is certainly a positive stimulus for science to progress. For this reason, and in line with the title of this letter, we believe that wellestablished concepts can only benefit from new ideas.