Muscle oxygen saturation rates coincide with lactate-based exercise thresholds.

INTRODUCTION: Monitoring muscle metabolic activity via blood lactate is a useful tool for understanding the physiological response to a given exercise intensity. Recent indications suggest that skeletal muscle oxygen saturation (SmO2), an index of the balance between local O2 supply and demand, may describe and predict endurance performance outcomes. PURPOSE: We tested the hypothesis that SmO2 rate is tightly related to blood lactate concentration across exercise intensities, and that deflections in SmO2 rate would coincide with established blood lactate thresholds (i.e., lactate thresholds 1 and 2). METHODS: Ten elite male soccer players completed an incremental running protocol to exhaustion using 3-min work to 30 s rest intervals. Blood lactate samples were collected during rest and SmO2 was collected continuously via near-infrared spectroscopy from the right and left vastus lateralis, left biceps femoris and the left gastrocnemius. RESULTS: Muscle O2 saturation rate (%/min) was quantified after the initial 60 s of each 3-min segment. The SmO2 rate was significantly correlated with blood lactate concentrations for all muscle sites; RVL, r = - 0.974; LVL, r = - 0.969; LG, r = - 0.942; LHAM, r = - 0.907. Breakpoints in SmO2 rate were not significantly different from LT1 or LT2 at any muscle sites (P > 0.05). Bland-Altman analysis showed speed threshold estimates via SmO2 rate and lactate are similar at LT2, but slightly greater for SmO2 rate at LT1. CONCLUSIONS: Muscle O2 saturation rate appears to provide actionable information about maximal metabolic steady state and is consistent with bioenergetic reliance on oxygen and its involvement in the attainment of metabolic steady state.

Applications of near-infrared spectroscopy in "anaerobic" diagnostics - SmO2 kinetics reflect PCr dephosphorylation and correlate with maximal lactate accumulation and maximal pedalling rate.

We investigated the relationship of the time-dependent behaviour of muscle oxygen saturation SmO2(t), phosphagen energy supply WPCr(t) and blood lactate accumulation ΔBLC(t) during a 60-s all-out cycling sprint and tested SmO2(t) for correlations with the end of the fatigue-free state tFf, maximal pedalling rate PRmax and maximal blood lactate accumulation rate v̇Lamax. Nine male elite track cyclists performed four maximal sprints (3, 8, 12, 60 s) on a cycle ergometer. Crank force and cadence were monitored continuously to determine PRmax and tFf based on force-velocity profiles. SmO2 of the vastus lateralis muscle and respiratory gases were measured until the 30th minute after exercise. WPCr was calculated based on the fast component of the post-exercise oxygen uptake for each sprint. Before and for 30 minutes after each sprint, capillary blood samples were taken to determine the associated ΔBLC. Temporal changes of SmO2, WPCr and ΔBLC were analysed via non-linear regression analysis. v̇Lamax was calculated based on ΔBLC(t) as the highest blood lactate accumulation rate. All models showed excellent quality (R2 > 0.95). The time constant of SmO2(t) τSmO2 = 2.93 ± 0.65 s was correlated with the time constant of WPCr(t) τPCr = 3.23 ± 0.67 s (r = 0.790, p < 0.012), v̇Lamax = 0.95 ± 0.18 mmol · l-1 · s-1 (r = 0.768, p < 0.017) and PRmax = 299.51 ± 14.70 rpm (r = -0.670, p < 0.049). tFf was correlated with τSmO2 (r = 0.885, p < 0.001). Our results show a time-dependent reflection of SmO2 kinetics and phosphagen energy contribution during a 60-s maximal cycling sprint. A high v̇Lamax results in a reduction, a high PRmax in an increase of the desaturation rate. The half-life of SmO2 desaturation indicates the end of the fatigue-free state.

Isolated finger flexor vs. exhaustive whole-body climbing tests? How to assess endurance in sport climbers?

PURPOSE: Sport climbing requires high-intensity finger flexor contractions, along with a substantial whole-body systemic oxygen uptake ([Formula: see text]O2) contribution. Although fatigue is often localised to the finger flexors, the role of systemic ̇[Formula: see text]O2 and local aerobic mechanisms in climbing performance remains unclear. As such, the primary purpose of this study was to determine systemic and local muscle oxygen responses during both isolated finger flexion and incremental exhaustive whole-body climbing tests. The secondary aim was to determine the relationship of isolated and whole-body climbing endurance tests to climbing ability. METHODS: Twenty-two male sport climbers completed a series of isometric sustained and intermittent forearm flexor contractions, and an exhaustive climbing test with progressive steepening of the wall angle on a motorised climbing ergometer. Systemic [Formula: see text]O2 and flexor digitorum profundus oxygen saturation (StO2) were recorded using portable metabolic analyser and near-infra red spectroscopy, respectively. RESULTS: Muscle oxygenation breakpoint (MOB) was identifiable during an incremental exhaustive climbing test with progressive increases in angle (82 ± 8% and 88 ± 8% [Formula: see text]O2 and heart rate climbing peak). The peak angle from whole-body treadwall test and impulse from isolated hangboard endurance tests were interrelated (R2 = 0.58-0.64). Peak climbing angle together with mean [Formula: see text]O2 and StO2 from submaximal climbing explained 83% of variance in self-reported climbing ability. CONCLUSIONS: Both systemic and muscle oxygen kinetics determine climbing-specific endurance. Exhaustive climbing and isolated finger flexion endurance tests are interrelated and suitable to assess climbing-specific endurance. An exhaustive climbing test with progressive wall angle allows determination of the MOB.

A systematic review of the relationship between muscle oxygen dynamics and energy rich phosphates. Can NIRS help?

BACKGROUND: Phosphocreatine dynamics provide the gold standard evaluation of in-vivo mitochondrial function and is tightly coupled with oxygen availability. Low mitochondrial oxidative capacity has been associated with health issues and low exercise performance. METHODS: To evaluate the relationship between near-infrared spectroscopy-based muscle oxygen dynamics and magnetic resonance spectroscopy-based energy-rich phosphates, a systematic review of the literature related to muscle oxygen dynamics and energy-rich phosphates was conducted. PRISMA guidelines were followed to perform a comprehensive and systematic search of four databases on 02-11-2021 (PubMed, MEDLINE, Scopus and Web of Science). Beforehand pre-registration with the Open Science Framework was performed. Studies had to include healthy humans aged 18-55, measures related to NIRS-based muscle oxygen measures in combination with energy-rich phosphates. Exclusion criteria were clinical populations, laboratory animals, acutely injured subjects, data that only assessed oxygen dynamics or energy-rich phosphates, or grey literature. The Effective Public Health Practice Project Quality Assessment Tool was used to assess methodological quality, and data extraction was presented in a table. RESULTS: Out of 1483 records, 28 were eligible. All included studies were rated moderate. The studies suggest muscle oxygen dynamics could indicate energy-rich phosphates under appropriate protocol settings. CONCLUSION: Arterial occlusion and exercise intensity might be important factors to control if NIRS application should be used to examine energetics. However, more research needs to be conducted without arterial occlusion and with high-intensity exercises to support the applicability of NIRS and provide an agreement level in the concurrent course of muscle oxygen kinetics and muscle energetics. TRIAL REGISTRATION: https://osf.io/py32n/ . KEY POINTS: 1. NIRS derived measures of muscle oxygenation agree with gold-standard measures of high energy phosphates when assessed in an appropriate protocol setting. 2. At rest when applying the AO protocol, in the absence of muscle activity, an initial disjunction between the NIRS signal and high energy phosphates can been seen, suggesting a cascading relationship. 3. During exercise and recovery a disruption of oxygen delivery is required to provide the appropriate setting for evaluation through either an AO protocol or high intensity contractions.

Forearm muscle oxygenation and blood volume parameters during sustained contraction performance in youth sport climbers.

BACKGROUND: Forearm muscle strength and endurance are essential determinants of sports climbing success. This study aimed to investigate whether delayed rates of muscle oxygen saturation and total hemoglobin correlate to sustained contraction performance of youth climbers. METHODS: Twelve recreational and competitive youth sport climbers (six females, six males) participated in the study. Variables included finger flexors muscle maximal voluntary contraction, sustained contraction test (SCT), muscle oxygen dynamics (SmO2), and blood volume (tHb) parameters. Pearson's correlation coefficients were calculated to determine the correlation between physiological and performance variables. RESULTS: SCT had a significant positive relationship to SmO2 delayed rate (r=0.728, P=0.007), and a significant negative relationship to tHb delayed rate (r=-0.690, P=0.013). SmO2 delayed rate and tHb delayed rate also had a significant negative correlation (r=-0.760, P=0.004). CONCLUSIONS: According to the results of this study, it can be suggested that delayed rates of SmO2 and tHb could be used in determining and predicting sustainable finger flexors performance in youth climbers. However, future studies investigating delayed rates of SmO2 and tHb in climbers of different ability levels are warranted to investigate this issue in more detail.

A Novel Approach to Determining the Alactic Time Span in Connection with Assessment of the Maximal Rate of Lactate Accumulation in Elite Track Cyclists.

PURPOSE: Following short-term all-out exercise, the maximal rate of glycolysis is frequently assessed on the basis of the maximal rate of lactate accumulation in the blood. Since the end of the interval without significant accumulation (talac) is 1 of 2 denominators in the calculation employed, accurate determination of this parameter is crucial. Although the very existence and definition of talac, as well as the validity of its determination as time-to-peak power (tPpeak), remain controversial, this parameter plays a key role in anaerobic diagnostics. Here, we describe a novel approach to determination of talac and compare it to the current standard. METHODS: Twelve elite track cyclists performed 3 maximal sprints (3, 8, and 12 s) and a high-rate, low-resistance pedaling test on an ergometer with monitoring of crank force and pedaling rate. Before and after each sprint, capillary blood samples were taken for determination of lactate accumulation. Fatigue-free force-velocity and power-velocity profiles were generated. talac was determined as tPpeak and as the time point of the first systematic deviation from the force-velocity profile (tFf). RESULTS: Accumulation of lactate after the 3-second sprint was significant (0.58 [0.19] mmol L-1; P < .001, d = 1.982). tFf was <3 seconds and tPpeak was ≥3 seconds during all sprints (P < .001, d = - 2.111). Peak power output was lower than maximal power output (P < .001, d = -0.937). Blood lactate accumulation increased linearly with increasing duration of exercise (R2 ≥ .99) and intercepted the x-axis at ∼tFf. CONCLUSION: Definition of talac as tPpeak can lead to incorrect conclusions. We propose determination of talac based on tFf, the end of the fatigue-free state that may reflect the beginning of blood lactate accumulation.

Muscle Oxygen Saturation Breakpoints Reflect Ventilatory Thresholds in Both Cycling and Running.

Pulmonary gas exchange analysis was compared to changes in muscle oxygen saturation as measured by near-infrared spectroscopy. First, ventilatory thresholds determined by common gas exchange analysis and breakpoints in muscle oxygen saturation were assessed for agreement during exercise with increasing intensity. Secondly, the relationship between muscle oxygen saturation as a surrogate for local oxygen extraction and peak oxygen uptake was assessed. In order to lend robustness to future NIRS testing on a broader scale, considering its potential for simple and cost-effective application, the question of a running versus a cycling modality was integrated into the design. Ten participants, of whom five were recreationally trained cyclists and five recreationally trained runners, were tested; each during a cycling test and a running test with increasing intensity to voluntary exhaustion. Muscle oxygen saturation and pulmonary gas exchange measurements were conducted. Bland-Altman analysis showed a moderate degree of agreement between both muscle oxygen saturation breakpoint 1 and muscle oxygen saturation breakpoint 2 and corresponding ventilatory threshold 1 and ventilatory threshold 2, for both cycling and running disciplines; generally speaking, muscle oxygen saturation breakpoints underestimated ventilatory thresholds. Additionally, a strong relationship could be seen between peak oxygen uptake and the minimally attained muscle oxygen saturation during cycling exercise. Muscle oxygen saturation measured using NIRS was determined to be a suitable method to assess ventilatory thresholds by finding breakpoints in muscle oxygen saturation, and muscle oxygen saturation minimum was linked to peak oxygen uptake.

Critical oxygenation: Can muscle oxygenation inform us about critical power?

The power-duration relationship is well documented for athletic performance and is formulated out mathematically in the critical power (CP) model. The CP model, when applied properly, has great predictive power, e.g. pedaling at a specific power output on an ergometer the model precisely calculates the time over which an athlete can sustain this power. However, CP presents physiological inconsistencies and process-oriented problems. The rapid development of near-infrared spectroscopy (NIRS) to measure muscle oxygenation (SmO2) dynamics provides a physiological exploration of the CP model on a conceptual and empirical level. Conceptually, the CP model provides two components: first CP is defined as the highest metabolic rate that can be achieved through oxidative means. And second, work capacity above CP named W'. SmO2 presents a steady-state in oxygen supply and demand and thereby represents CP specifically at a local level of analysis. Empirically, exploratory data quickly illustrates the relationship between performance and SmO2, as shown during 3-min all-out cycling tests to assess CP. During these tests, performance and SmO2 essentially mirror each other, and both CP and W' generate solid correlation with what would be deemed their SmO2 counterparts: first, the steady-state of SmO2 correlates with CP. And second, the tissue oxygen reserve represented in SmO2, when calculated as an integral corresponds to W'. While the empirical data presented is preliminary, the proposition of a concurring physiological model to the current CP model is a plausible inference. Here we propose that SmO2 steady-state representing CP as critical oxygenation or CO. And the tissue oxygen reserve above CO would then be identified as O'. This new CO model could fill in the physiological gap between the highly predictive CP model and at times its inability to track human physiology consistently. For simplicity's sake, this would include acute changes in physiology as a result of changing climate or elevation with travel, which can affect performance. These types of acute fluctuations, but not limited to, would be manageable when applying a CO model in conjunction with the CP model. Further, modeling is needed to investigate the true potential of NIRS to model CP, with a focus on repeatability, recovery, and systemic vs local workloads.

Muscle oxygen dynamics in elite climbers during finger-hang tests at varying intensities.

The aim of this study was to measure muscle oxygen saturation (SmO2) dynamics during a climbing specific task until failure in varying conditions. Our prediction was that SmO2 should be a good marker to predict task failure. Eleven elite level climbers performed a finger-hang test on a 23 mm wooden rung under four different weighted conditions, 1. body weight (BW), 2. body weight +20% (BW +20), 3. body weight -20% (BW -20) and 4. body weight -40% (BW -40), maintaining half crimp grip until voluntary exhaustion. During each trial SmO2 and time to task failure (TTF) were measured. TTF was then compared to the minimally attainable value of SmO2 (SmO2min) and time to SmO2min (TTmin). There is a considerable degree of agreement between attainable SmO2min at high intensity conditions (MBW = 21.6% ± 6.4; MBW+20 = 24.0% ± 7.0; MBW-20 = 23.0% ± 7.3). Bland-Altman plot with an a priori set equivalency interval of ±5% indicate that these conditions are statistically not different (MBW-BW + 20 = -2.4%, 95% CI [1.4, -6.2]; MBW-Bw-20 = -1.3, 95% CI [2.5, -5.1]). The fourth and lowest intensity condition (MBW -40 = 32.4% ± 8.8) was statistically different and not equivalent (MBW-BW -40 = -8.8%, 95% CI [-5.0, -12.6]). The same agreement was found between TTF and TTmin for the high intensity conditions plotted via Bland-Altman. While the rate with which oxygen was extracted and utilised changed with the conditions, the attainable SmO2min remained constant at high intensity conditions and was related to TTF.

Near-infrared spectroscopy-derived muscle oxygen saturation on a 0% to 100% scale: reliability and validity of the Moxy Monitor.

Near-infrared spectroscopy (NIRS) to monitor muscle oxygen saturation (SmO2) is rapidly expanding into applied sports settings. However, the technology is limited due to its inability to convey quantifiable values. A test battery to assess reliability and validity of a 0% to 100% scale modeled by a commercially available NIRS device was established. This test battery applies a commonly used technique, the arterial occlusion method (AOM) to assess repeatability, reproducibility, and face validity. A total of 22 participants completed the test battery to scrutinize the 0% to 100% scale provided by the device. All participants underwent repeated AOM tests in passive and active conditions. The SmO2 minimum and SmO2 maximum values were obtained from the AOM and were used in the subsequent analysis. Repeatability and reproducibility were tested for equivalency and Bland-Altman plots were generated. Face validity was assessed by testing SmO2 values against an a priori; defined threshold for mixed venous blood during AOM response. The device exhibits an appropriately functional 0% to 100% scale that is reliable in terms of repeatability and reproducibility. Under the conditions applied in the test battery design, the device is considered valid for application in sports.