Review of early development of near-infrared spectroscopy and recent advancement of studies on muscle oxygenation and oxidative metabolism

Sourcebook update: using near-infrared spectroscopy to assess skeletal muscle oxygen uptake

Monitoring the metabolic cost or oxygen consumption associated with rest and exercise is crucial to understanding the impact of disease or physical training on the health of individuals. Traditionally, measuring the skeletal muscle oxygen cost associated with exercise/muscle contractions can be rather expensive or invasive (i.e., muscle biopsies). More recently, specific protocols designed around the use of near-infrared spectroscopy (NIRS) have been shown to provide a quick, noninvasive easy-to-use tool to measure skeletal muscle oxygen consumption ([Formula: see text]). However, the data and results from NIRS devices are often misunderstood. Thus the primary purpose of this sourcebook update is to provide several experimental protocols students can utilize to improve their understanding of NIRS technology, learn how to analyze results from NIRS devices, and better understand how muscle contraction intensity and type (isometric, concentric, or eccentric) influence the oxygen cost of muscle contractions.NEW & NOTEWORTHY Compared to traditional methods, near-infrared spectroscopy (NIRS) provides a relatively cheap and easy-to-use noninvasive technique to measure skeletal muscle oxygen uptake following exercise. This laboratory not only enables students to learn about the basics of NIRS and muscle energetics but also addresses more complex questions regarding skeletal muscle physiology.

Feasibility of tongue image detection for coronary artery disease: based on deep learning

Aim

Clarify the potential diagnostic value of tongue images for coronary artery disease (CAD), develop a CAD diagnostic model that enhances performance by incorporating tongue image inputs, and provide more reliable evidence for the clinical diagnosis of CAD, offering new biological characterization evidence.

Methods

We recruited 684 patients from four hospitals in China for a cross-sectional study, collecting their baseline information and standardized tongue images to train and validate our CAD diagnostic algorithm. We used DeepLabV3 + for segmentation of the tongue body and employed Resnet-18, pretrained on ImageNet, to extract features from the tongue images. We applied DT (Decision Trees), RF (Random Forest), LR (Logistic Regression), SVM (Support Vector Machine), and XGBoost models, developing CAD diagnostic models with inputs of risk factors alone and then with the additional inclusion of tongue image features. We compared the diagnostic performance of different algorithms using accuracy, precision, recall, F1-score, AUPR, and AUC.

Results

We classified patients with CAD using tongue images and found that this classification criterion was effective (ACC = 0.670, AUC = 0.690, Recall = 0.666). After comparing algorithms such as Decision Tree (DT), Random Forest (RF), Logistic Regression (LR), Support Vector Machine (SVM), and XGBoost, we ultimately chose XGBoost to develop the CAD diagnosis algorithm. The performance of the CAD diagnosis algorithm developed solely based on risk factors was ACC = 0.730, Precision = 0.811, AUC = 0.763. When tongue features were integrated, the performance of the CAD diagnosis algorithm improved to ACC = 0.760, Precision = 0.773, AUC = 0.786, Recall = 0.850, indicating an enhancement in performance.

Conclusion

The use of tongue images in the diagnosis of CAD is feasible, and the inclusion of these features can enhance the performance of existing CAD diagnosis algorithms. We have customized this novel CAD diagnosis algorithm, which offers the advantages of being noninvasive, simple, and cost-effective. It is suitable for large-scale screening of CAD among hypertensive populations. Tongue image features may emerge as potential biomarkers and new risk indicators for CAD.

Muscle reoxygenation is slower after higher cycling intensity, and is faster and more reliable in locomotor than in accessory muscle sites

Introduction

Wearable near-infrared spectroscopy (NIRS) can be used during dynamic exercise to reflect the balance of muscle oxygen delivery and uptake. This study describes the behaviour and reliability of postexercise reoxygenation with NIRS as a function of exercise intensity at four muscle sites during an incremental cycling test. We discuss physiological components of faster and slower reoxygenation kinetics in the context of sport science and clinical applications. We hypothesised that reoxygenation would be slower at higher intensity, and that locomotor muscles would be faster than accessory muscles. We quantified test-retest reliability and agreement for each site.

Methods

Twenty-one trained cyclists performed two trials of an incremental cycling protocol with 5-min work stages and 1-min rest between stages. NIRS was recorded from the locomotor vastus lateralis and rectus femoris muscles, and accessory lumbar paraspinal and lateral deltoid muscles. Reoxygenation time course was analysed as the half-recovery time (HRT) from the end of work to half of the peak reoxygenation amplitude during rest. Coefficient of variability (CV) between participants, standard error of the measurement (SEM) within participants, and intraclass correlation coefficient (ICC) for test-retest reliability were evaluated at 50%, 75%, and 100% peak workloads. A linear mixed-effects model was used to compare differences between workloads and muscle sites.

Results

HRT was slower with increasing workload in the VL, RF, and PS, but not DL. VL had the fastest reoxygenation (lowest HRT) across muscle sites at all workloads (HRT = 8, 12, 17 s at 50%, 75%, 100% workload, respectively). VL also had the greatest reliability and agreement. HRT was sequentially slower between muscle sites in the order of VL < RF < PS < DL, and reliability was lower than for the VL.

Discussion

This study highlights the potential for using wearable NIRS on multiple muscle sites during exercise. Reoxygenation kinetics differ between local muscle sites with increasing intensity. Moderate-to-good reliability in the VL support its increasing use in sport science and clinical applications. Lower reliability in other muscle sites suggest they are not appropriate to be used alone, but may add information when combined to better reflect systemic intensity and fatigue during exercise at different intensities.

Cerebral and muscle tissue oxygenation during exercise in healthy adults: A systematic review

BACKGROUND

Near-infrared spectroscopy (NIRS) technology has allowed for the measurement of cerebral and skeletal muscle oxygenation simultaneously during exercise. Since this technology has been growing and is now successfully used in laboratory and sports settings, this systematic review aimed to synthesize the evidence and enhance an integrative understanding of blood flow adjustments and oxygen (O2) changes (i.e., the balance between O2 delivery and O2 consumption) within the cerebral and muscle systems during exercise.

METHODS

A systematic review was conducted using PubMed, Embase, Scopus, and Web of Science databases to search for relevant studies that simultaneously investigated cerebral and muscle hemodynamic changes using the near-infrared spectroscopy system during exercise. This review considered manuscripts written in English and available before February 9, 2023. Each step of screening involved evaluation by 2 independent authors, with disagreements resolved by a third author. The Joanna Briggs Institute Critical Appraisal Checklist was used to assess the methodological quality of the studies.

RESULTS

Twenty studies were included, of which 80% had good methodological quality, and involved 290 young or middle-aged adults. Different types of exercises were used to assess cerebral and muscle hemodynamic changes, such as cycling (n = 11), treadmill (n = 1), knee extension (n = 5), isometric contraction of biceps brachii (n = 3), and duet swim routines (n = 1). The cerebral hemodynamics analysis was focused on the frontal cortex (n = 20), while in the muscle, the analysis involved vastus lateralis (n = 18), gastrocnemius (n = 3), biceps brachii (n = 5), deltoid (n = 1), and intercostal muscle (n = 1). Overall, muscle deoxygenation increases during exercise, reaching a plateau in voluntary exhaustion, while in the brain, oxyhemoglobin concentration increases with exercise intensity, reaching a plateau or declining at the exhaustion point.

CONCLUSION

Muscle and cerebral oxygenation respond differently to exercise, with muscle increasing O2 utilization and cerebral tissue increasing O2 delivery during exercise. However, at the exhaustion point, both muscle and cerebral oxygenation become compromised. This is characterized by a reduction in blood flow and a decrease in O2 extraction in the muscle, while in the brain, oxygenation reaches a plateau or decline, potentially resulting in motor failure during exercise.

Analysis of Muscle Oxygenation after a Normobaric Hypoxia Tolerance Test

The aim of this work was to analyze the influence of acute normobaric hypoxia on quadricep oxygenation. Muscle oxygen saturation (SmO2) was measured using near-infrared spectrometry (NIRS) technology during a normobaric hypoxia tolerance test (NHTT). SmO2 was measured with a Humon Hex® device. In total, 54 healthy subjects participated, 68.5 of which were males and 31.5% of which were females. They performed an NHTT with the IAltitude® simulator, breathing air with an FiO2 level of 11% (equivalent to 5050 m). The maximum duration of the NHTT was set at 10 min, stopping if it reached 83% SpO2. The initial values (PRE) were compared with those obtained at the end of the test (POST) and after 10 min of recovery. The participants were divided into two groups based on whether (G1) they completed the ten minutes or not (G2). In total, 35.1% of men and 41.2% of women completed the 10 min. In both groups, significant differences were observed in the decrease in SmO2 values (p < 0.0001) (G1: PRE = 59.5 ± 12.48%; POST = 55.95 ± 14.30%; G2: PRE = 60.06 ± 13.46%; POST = 57.2 ± 12.3%). There were no differences between groups in any of the three periods. Exposure to normobaric hypoxia produces a decrease in quadricep levels of SmO2 in both sexes, regardless of whether the test is completed. Two patterns appeared: A.-less time and more hypoxia; B. a longer duration and less hypoxia.

Muscle Oximetry in Sports Science: An Updated Systematic Review

BACKGROUND

In the last 5 years since our last systematic review, a significant number of articles have been published on the technical aspects of muscle near-infrared spectroscopy (NIRS), the interpretation of the signals and the benefits of using the NIRS technique to measure the physiological status of muscles and to determine the workload of working muscles.

OBJECTIVES

Considering the consistent number of studies on the application of muscle oximetry in sports science published over the last 5 years, the objectives of this updated systematic review were to highlight the applications of muscle oximetry in the assessment of skeletal muscle oxidative performance in sports activities and to emphasize how this technology has been applied to exercise and training over the last 5 years. In addition, some recent instrumental developments will be briefly summarized.

METHODS

Preferred Reporting Items for Systematic Reviews guidelines were followed in a systematic fashion to search, appraise and synthesize existing literature on this topic. Electronic databases such as Scopus, MEDLINE/PubMed and SPORTDiscus were searched from March 2017 up to March 2023. Potential inclusions were screened against eligibility criteria relating to recreationally trained to elite athletes, with or without training programmes, who must have assessed physiological variables monitored by commercial oximeters or NIRS instrumentation.

RESULTS

Of the identified records, 191 studies regrouping 3435 participants, met the eligibility criteria. This systematic review highlighted a number of key findings in 37 domains of sport activities. Overall, NIRS information can be used as a meaningful marker of skeletal muscle oxidative capacity and can become one of the primary monitoring tools in practice in conjunction with, or in comparison with, heart rate or mechanical power indices in diverse exercise contexts and across different types of training and interventions.

CONCLUSIONS

Although the feasibility and success of the use of muscle oximetry in sports science is well documented, there is still a need for further instrumental development to overcome current instrumental limitations. Longitudinal studies are urgently needed to strengthen the benefits of using muscle oximetry in sports science.

The Improvement in Exercise Performance during Reduced Muscle Mass Exercise is Associated with an Increase in Femoral Blood Flow in Older and Younger Endurance-Trained Athletes

This study investigated whether the improved performance observed with maximal self-paced single-leg (SL), compared with double-leg (DL) cycling, is associated with enhanced femoral blood flow and/or altered tissue oxygenation. The hyperaemic response to exercise was assessed in younger and older athletes. Power output was measured in 12 older (65 ± 4 y) and 12 younger (35 ± 5 y) endurance-trained individuals performing 2 x 3 min maximal self-paced exercise using SL and DL cycling. Blood flow (BF) in the femoral artery was assessed using Doppler ultrasound and muscle oxygenation was measured using near-infrared spectroscopy on the vastus lateralis. SL cycling elicited a greater power output (295 ± 83 vs 265 ± 70 W, P < 0.001) and peak femoral BF (1749.1 ± 533.3 vs 1329.7 ± 391.7 ml/min, P < 0.001) compared with DL cycling. Older individuals had a lower peak BF in response to exercise (1355.4 ± 385.8 vs 1765.2 ± 559.6 ml/min, P = 0.019) compared with younger individuals. Peak BF in response to exercise was correlated with power output during SL (r = 0.655, P = 0.002) and DL (r = 0.666, P = 0.001) cycling. The greater exercise performance during SL compared with DL cycling may be partly explained by a greater hyperaemic response when reducing active muscle mass. Despite regular endurance training, older athletes had a lower femoral BF in response to maximal self-paced exercise compared with younger athletes.

NIRS-Derived Muscle-Deoxygenation and Microvascular Reactivity During Occlusion-Reperfusion at Rest Are Associated With Whole-Body Aerobic Fitness

Purpose: Near-infrared spectroscopy (NIRS) indices during arterial occlusion-reperfusion maneuver have been used to examine the muscle's oxidative metabolism and microvascular function-important determinants of whole-body aerobic-fitness. The association of NIRS-derived parameters with whole-body VO2max was previously examined using a method requiring exercise (or electrical stimulation) followed by multiple arterial occlusions. We examined whether NIRS-derived indices of muscle deoxygenation and microvascular reactivity assessed during a single occlusion-reperfusion at rest are (a) associated with maximal/submaximal indices of whole-body aerobic-fitness and (b) could discriminate individuals with different VO2max. We, also, investigated which NIRS-parameter during occlusion-reperfusion correlates best with whole-body aerobic-fitness. Methods: Twenty-five young individuals performed an arterial occlusion-reperfusion at rest. Changes in oxygenated- and deoxygenated-hemoglobin (O2Hb and HHb, respectively) in vastus-lateralis were monitored; adipose tissue thickness (ATT) at NIRS-application was assessed. Participants also underwent a maximal incremental exercise test for VO2max, maximal aerobic velocity (MAV), and ventilatory-thresholds (VTs) assessments. Results: The HHbslope and HHbmagnitude of increase (occlusion-phase) and O2Hbmagnitude of increase (reperfusion-phase) were strongly correlated with VO2max (r = .695-.763, p < .001) and moderately with MAV (r = .468-.530; p < .05). O2Hbmagnitude was moderately correlated with VTs (r = .399-.414; p < .05). After controlling for ATT, the correlations remained significant for VO2max (r = .672-.704; p < .001) and MAV (r = .407; p < .05). Individuals in the high percentiles after median and tritile splits for HHbslope and O2Hbmagnitude had significantly greater VO2max vs. those in low percentiles (p < .01-.05). The HHbslope during occlusion was the best predictor of VO2max. Conclusion: NIRS-derived muscle deoxygenation/reoxygenation indices during a single arterial occlusion-reperfusion maneuver are strongly associated with whole-body maximal indices of aerobic-fitness (VO2max, MAV) and may discriminate individuals with different VO2max.

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.

Using multi-channel near-infrared spectroscopy to assess the effect of cupping therapy on the spatial hemodynamic response of the biceps muscle: A preliminary study

BACKGROUND

The local hemodynamic response after cupping therapy has been considered as a contributing factor for improving muscle tissue health; however, the effects of cupping pressure and duration on the spatial hemodynamic response have not been investigated.

OBJECTIVE

The objective of this study was to investigate the hemodynamic response inside and outside the cupping cup under various pressures and durations of cupping therapy.

METHODS

A 3-way factorial design with repeated measures was used to investigate the main and interaction effects of the location (areas inside and outside the cup), pressure (-225 and -300 mmHg) and duration (5 and 10 min) on the hemodynamic response of the biceps muscle. A functional near-infrared spectroscopy was used to assess hemodynamic changes in 18 participants.

RESULTS

A significant three-way interaction of the location, pressure, and duration factors was observed in oxyhemoglobin (p= 0.023), deoxy-hemoglobin (p= 0.013), and blood volume (p= 0.013). A significant increase was observed in oxyhemoglobin, blood volume, and oxygenation compared to pre-cupping (p< 0.05) in the area outside the cup.

CONCLUSION

Our findings indicate that an appropriate combination of cupping pressure and duration can effectively affect the spatial hemodynamic response of the biceps.

Non-invasive estimation of in vivo optical properties and hemodynamic parameters of domestic animals: a preliminary study on horses, dogs, and sheep

Biosensors applied in veterinary medicine serve as a noninvasive method to determine the health status of animals and, indirectly, their level of welfare. Near infrared spectroscopy (NIRS) has been suggested as a technology with this application. This study presents preliminary in vivo time domain NIRS measurements of optical properties (absorption coefficient, reduced scattering coefficient, and differential pathlength factor) and hemodynamic parameters (concentration of oxygenated hemoglobin, deoxygenated hemoglobin, total hemoglobin, and tissue oxygen saturation) of tissue domestic animals, specifically of skeletal muscle (4 dogs and 6 horses) and head (4 dogs and 19 sheep). The results suggest that TD NIRS in vivo measurements on domestic animals are feasible, and reveal significant variations in the optical and hemodynamic properties among tissue types and species. In horses the different optical and hemodynamic properties of the measured muscles can be attributed to the presence of a thicker adipose layer over the muscle in the Longissimus Dorsi and in the Gluteus Superficialis as compared to the Triceps Brachii. In dogs the absorption coefficient is higher in the head (temporalis musculature) than in skeletal muscles. The smaller absorption coefficient for the head of the sheep as compared to the head of dogs may suggest that in sheep we are indeed reaching the brain cortex while in dog light penetration can be hindered by the strongly absorbing muscle covering the cranium.

Analysis of electrical stimulation and voluntary muscle contraction on skeletal muscle oxygen uptake and mitochondrial recovery using near-infrared spectroscopy

PURPOSE

This investigation was to compare differences in skeletal muscle oxygen consumption ([Formula: see text]) and mitochondrial recovery between voluntary (VOL) and electrically stimulated (ES) plantarflexion contractions.

METHODS

Twelve men and women (26 ± 4.0 years; 171.8 ± 5.1 cm; 74.0 ± 13.7 kg) were seated in a chair with their right knee fully extended and right foot secured to a force transducer. ES electrodes and a near-infrared spectroscopy device were placed on the gastrocnemius. Participants performed ES plantarflexion contractions across a range of stimulation intensities at frequencies of 1 and 2 Hz and similar VOL contractions. Cuff occlusion occurred immediately following each series of contractions to measure [Formula: see text]. A standardized mitochondrial function assessment protocol was also performed to calculate K-constants between work-matched ES and VOL contractions.

RESULTS

For mitochondrial assessments, there were no significant differences between ES and VOL rate constants (2.03 ± 0.98 vs. 1.25 ± 1.35 min-1, p = 0.266). ES resulted in a significantly greater workrate-[Formula: see text] slope at 1 Hz (0.007 ± 0.007 vs. 0.001 ± 0.002% [Formula: see text]/s/N, p = 0.014) and 2 Hz (0.010 ± 0.010 vs. 0.001 ± 0.001% [Formula: see text]/s/N, p = 0.012), as well as a significantly greater workrate-[Formula: see text] Y-intercept at 2 Hz (1.603 ± 1.513 vs. 0.556 ± 0.564% [Formula: see text]/s, p = 0.035) but not 1 Hz (0.579 ± 0.448 vs. 0.442 ± 0.357% mV̇O2/s, p = 0.535) when compared to VOL.

CONCLUSION

ES results in a significantly greater [Formula: see text] at similar work rates compared to VOL, however, the mitochondrial recovery rate constants were similar. The greater mVO2 with ES may partially contribute to the increased rate of fatigue during ES exercise in individuals with muscle paralysis.

Using near-infrared spectroscopy to investigate the effects of pressures and durations of cupping therapy on muscle blood volume and oxygenation

Cupping therapy has been widely used to manage musculoskeletal impairment. However, the effects of pressure and duration of cupping therapy on the hemodynamic activity of the muscle have not been investigated. A 2 × 2 repeated measures factorial design was used to examine the main effect and interaction of pressure (-225 and -300 mmHg) and duration (5 and 10 min) on biceps muscle blood flow using near-infrared spectroscopy in 18 participants. The results showed that a significant interaction is between pressure and duration on deoxy-hemoglobin (p = 0.045). A significant main effect of pressure is on oxyhemoglobin (p = 0.005) and a significant main effect of duration is on oxyhemoglobin (p = 0.005). Cupping therapy at -300 mmHg for 10 min results in a higher oxyhemoglobin (6.75 ± 2.08 μM) and deoxy-hemoglobin (1.71 ± 0.78 μM) compared to other three combinations. Our study provides first evidence that the pressure and duration factors of cupping therapy can significantly affect muscle blood volume and oxygenation.

Evaluation of inter-rater and test-retest reliability for near-infrared spectroscopy reactive hyperemia measures

BACKGROUND

Near-infrared spectroscopy (NIRS) is a non-invasive tool used to measure blood flow in peripheral tissues. More information on inter-rater agreement and test-retest reliability of NIRS-based reperfusion assessments is needed.

PURPOSE

To assess inter-rater agreement for NIRS based data analysis, and evaluate the measurement's reliability across days.

METHODS

On three separate days (average days between visits 1 and 3: 19.4 ± 6.9 days), participants' (N = 15 males, 22 ± 2 yr.) post-occlusion reactive hyperemia (PORH) was measured in the left gastrocnemius muscle using Continuous-Wave NIRS (CW-NIRS). A blood pressure cuff was placed proximal to the knee and inflated to occlude lower leg blood flow for 5 min. The following CW-NIRS parameters were selected: (1) percent saturation in HbO₂ (StO₂%) at baseline; (2) the O₂Hb range used to normalize the NIRS signal; (3) the time for the O₂Hb signal to reach 50 % peak post-occlusion hyperemia (T_1/2), and (4) the post peak hyperemic O₂Hb recovery slope (O₂REC-SLP). Absolute agreement between the two analysts was calculated using two-way random effects Intraclass Correlation Coefficients (ICC_2,1). Consistency between analysts and across days was calculated using two-way mixed models (ICC_3,1). Mean and 95 % confidence intervals (CI) of ICCs are reported. Coefficient of variation (CV) and standard error of the measurement (SEM) are reported.

RESULTS

The ICC_2,1 data indicated "adequate" to "excellent" absolute agreement between the two NIRS analysts. ICC_2,3 data indicated "adequate" to "good" reliability across visits. The CV and SEM for rater 1 and rater 2 across visit were StO₂ (CV: 3.79 % ± 2.71 % and 4.50 % ± 2.37 %; SEM: 3.42 and 3.82), O₂Hb range (CV: 10.50 ± 5.93 and 12.79 ± 12.41; SEM: 3.26 and 4.71), T_1/2 (CV: 11.15 % ± 5.52 % and 10.96 % ± 4.50; SEM: 1.22 and 1.11), and O₂REC-SLP (CV: 19.49 % ± 9.99 % and 18.45 % ± 9.48 %; SEM: 0.04 and 0.04).

CONCLUSION

It is concluded that NIRS parameters assessed show adequate reliability between analysts and across three visits. It is recommended, when feasible and because of the absence of 100 % reliability, that investigators employ more than one rater for scoring at least a portion of the data across each trial in a study's control condition in order to have the ability to estimate the magnitude of error attributable to imperfect reliability.

In Vivo Analysis of a Biodegradable Magnesium Alloy Implant in an Animal Model Using Near-Infrared Spectroscopy

Biodegradable magnesium-based implants offer mechanical properties similar to natural bone, making them advantageous over nonbiodegradable metallic implants. However, monitoring the interaction between magnesium and tissue over time without interference is difficult. A noninvasive method, optical near-infrared spectroscopy, can be used to monitor tissue's functional and structural properties. In this paper, we collected optical data from an in vitro cell culture medium and in vivo studies using a specialized optical probe. Spectroscopic data were acquired over two weeks to study the combined effect of biodegradable Mg-based implant disks on the cell culture medium in vivo. Principal component analysis (PCA) was used for data analysis. In the in vivo study, we evaluated the feasibility of using the near-infrared (NIR) spectra to understand physiological events in response to magnesium alloy implantation at specific time points (Day 0, 3, 7, and 14) after surgery. Our results show that the optical probe can detect variations in vivo from biological tissues of rats with biodegradable magnesium alloy "WE43" implants, and the analysis identified a trend in the optical data over two weeks. The primary challenge of in vivo data analysis is the complexity of the implant interaction near the interface with the biological medium.

New Methodology of Human Health Express Diagnostics Based on Pulse Wave Measurements and Occlusion Test

Nowadays, with the increase in the rhythm of life, the relevance of using express diagnostics methods for human health state estimation has significantly increased. We present a new express diagnostics method based on non-invasive measurements (the pulse wave shape, heart rate, blood pressure, and oxygen saturation of blood vessels and tissues). A feature of these measurements is that they can be carried out both in the hospital and at home. The new compact and portable optical hardware–software complex has been developed to measure tissue oxygen saturation. This complex makes it possible to reduce the measurement time from 60 min to 7–8 min, which reduces the likelihood of artifacts in the measurement process and increases its reliability. A new technique has been developed to carry out these measurements. A new optical sensor based on a line of charge-coupled devices has been developed to register a pulse wave in the far peripheral zone. The developed new technique for processing the pulse waveform and data on the oxygen saturation of hemoglobin in the blood and tissues allows a person to obtain additional information about their state of health independently. It will help to make conclusions about taking the necessary measures. This additional information allows the attending physician to provide more effective control over the course of treatment of the patient at any time since the methods of express diagnostics proposed by us have no restrictions on the number of applications. The functional state of more than 300 patients was studied. The results of various measurements are presented.

Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

Age-Related Changes in Skeletal Muscle Oxygen Utilization

The cardiovascular and skeletal muscle systems are intrinsically interconnected, sharing the goal of delivering oxygen to metabolically active tissue. Deficiencies within those systems that affect oxygen delivery to working tissues are a hallmark of advancing age. Oxygen delivery and utilization are reflected as muscle oxygen saturation (SmO2) and are assessed using near-infrared resonance spectroscopy (NIRS). SmO2 has been observed to be reduced by ~38% at rest, ~24% during submaximal exercise, and ~59% during maximal exercise with aging (>65 y). Furthermore, aging prolongs restoration of SmO2 back to baseline by >50% after intense exercise. Regulatory factors that contribute to reduced SmO2 with age include blood flow, capillarization, endothelial cells, nitric oxide, and mitochondrial function. These mechanisms are governed by reactive oxygen species (ROS) at the cellular level. However, mishandling of ROS with age ultimately leads to alterations in structure and function of the regulatory factors tasked with maintaining SmO2. The purpose of this review is to provide an update on the current state of the literature regarding age-related effects in SmO2. Furthermore, we attempt to bridge the gap between SmO2 and associated underlying mechanisms affected by aging.

Optical bone densitometry insensitive to skin thickness

Skin thickness, including the adipose layer, which varies from individual to individual, affects the bone density measurement using light. In this study, we proposed a method to measure skin thickness using light and to correct the bias caused by differences in skin thickness and verified the proposed method by experiments using a phantom. We measured simulated skin of different thicknesses and bovine trabecular bone of different bone mineral densities (BMDs) using an optical system consisting of lasers of 850 and 515 nm wavelengths, lenses, and slits. Although the slope of the light intensity distribution formed on the surface of the material when irradiated by the 850 nm laser is affected by the thickness of the skin phantom. The difference of the intensity distribution peaks (δy) between the 850 and 515 nm lasers was strongly correlated with the thickness of the skin phantom. The coefficient of determination between the measurements and the BMD was improved by correcting the 850 nm laser measurements with δy. This result suggests that the method is applicable to optical bone densitometry, which is insensitive to differences in skin thickness.

Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O2 diffusion capacity in humans

The final steps of the O2 cascade during exercise depend on the product of the microvascular-to-intramyocyteP O 2 ${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O2 diffusing capacity (D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non-invasive methods to determineD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) recovery rate constant (k), measured by near-infrared spectroscopy (NIRS) using intermittent arterial occlusions, is associated with muscle oxidative capacity in vivo. We reasoned that k would be limited byD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ when muscle oxygenation is low (kLOW ), and hypothesized that: (i) k in well oxygenated muscle (kHIGH ) is associated with maximal O2 flux in fibre bundles; and (ii) ∆k (kHIGH  - kLOW ) is associated with capillary density (CD). Vastus lateralis k was measured in 12 participants using NIRS after moderate exercise. The timing and duration of arterial occlusions were manipulated to maintain tissue saturation index within a 10% range either below (LOW) or above (HIGH) half-maximal desaturation, assessed during sustained arterial occlusion. Maximal O2 flux in phosphorylating state was 37.7 ± 10.6 pmol s-1  mg-1 (∼5.8 ml min-1  100 g-1 ). CD ranged 348 to 586 mm-2 . kHIGH was greater than kLOW (3.15 ± 0.45 vs. 1.56 ± 0.79 min-1 , P < 0.001). Maximal O2 flux was correlated with kHIGH (r = 0.80, P = 0.002) but not kLOW (r = -0.10, P = 0.755). Δk ranged -0.26 to -2.55 min-1 , and correlated with CD (r = -0.68, P = 0.015). mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ k reflects muscle oxidative capacity only in well oxygenated muscle. ∆k, the difference in k between well and poorly oxygenated muscle, was associated with CD, a mediator ofD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O2 diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O2 availability, for comparison with biopsy variables. The mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ recovery rate constant in HIGH O2 availability was hypothesized to reflect muscle oxidative capacity (kHIGH ) and the difference in k between HIGH and LOW O2 availability (∆k) was hypothesized to reflect muscle O2 diffusing capacity. kHIGH was correlated with phosphorylating oxidative capacity of permeabilized muscle fibre bundles (r = 0.80). ∆k was negatively correlated with capillary density (r = -0.68) of biopsy samples. NIRS provides non-invasive means of assessing both muscle oxidative and oxygen diffusing capacity in vivo.

Diffuse optical spectroscopic method for tissue and body composition assessment

SIGNIFICANCE

Growing levels of obesity and metabolic syndrome have driven demand for more advanced forms of body composition assessment. While various techniques exist to measure body composition, devices are typically expensive and not portable, involve radiation [in the case of dual-energy x-ray absorptiometry (DXA)], and are limited to analysis of adiposity while metabolic information from blood supply and oxygenation are not considered.

AIM

We evaluate whether diffuse optical spectroscopic imaging (DOSI) can be used to predict site-specific adiposity and percent fat (whole body) while simultaneously providing information about local tissue hemoglobin levels and oxygenation.

APPROACH

DOSI measures of tissue composition in gastrocnemius, quadriceps, abdomen, and biceps, DXA whole-body composition, and ultrasound-derived skin and adipose tissue thickness (SATT) in the quadriceps were obtained from 99 individuals aged 7 to 34 years old.

RESULTS

Various DOSI-derived parameters were correlated with SATT and an optical method is proposed for estimating SATT using a newly defined parameter, the optical fat fraction (OFF), which considers all parameters that correlate with SATT. Broadband absorption and scattering spectra from study participants with the thinnest (SATT  ≈  0.25  ±  0.02  cm) and thickest SATT (SATT  ≈  1.55  ±  0.14  cm), representing best estimates for pure in vivo lean and fatty tissue, respectively, are reported. Finally, a trained prediction model is developed which allows DOSI assessment of OFF to predict DXA body-fat percentage, demonstrating that DOSI can be used to quantify body composition.

CONCLUSIONS

This study shows that DOSI can be used to assess the adiposity of specific tissues or the entire human body, and the OFF parameter is defined for corroboration and further evaluation in future research.

No alteration of back muscle oxygenation during isometric exercise in individuals with non-specific low back pain

The aim of our study was (I) To compare back muscle oxygenation and perfusion as well as Biering–Sorensen muscle endurance (BSME) test holding times between chronic non-specific low back pain (CNSLBP) patients and asymptomatic controls matched for age, body mass index (BMI), sex and physical activity, and (II) to investigate factors associated with BSME holding times. Muscle perfusion (tHb) and oxygenation (SmO₂) were measured by near-infrared spectroscopy (NIRS) based oximetry in three back muscles during the BSME. Reliability of tHb and SmO₂ was assessed in a separate sample. BSME holding time and SmO₂ were compared between patients (n = 45) and controls (n = 45) and factors associated with BSME holding time were assessed using multiple linear regression. Reliability for SmO₂ was excellent (ICC = 0.87–0.99). THb showed poor to moderate reliability and was not further used. Groups differed for BSME holding time (P = 0.03), pain intensity (P ≤ 0.0005) and subcutaneous tissue thickness (P = 0.01) but not for NIRS measures. Physical activity and BMI were associated with BSME holding times. Insufficient muscle oxygenation does not seem to be a major factor contributing to CNSLBP. Future investigation should evaluate other determinants of BSME holding times, such as motivation and recruitment of auxiliary muscles.

Evaluating Arterial Blood Flow Limitation Using Muscle Oxygenation and Cycling Power

OBJECTIVE

To explore the combination of measuring muscle oxygenation with near-infrared spectroscopy (NIRS) and cycling power during provocative incremental exercise for the detection of iliac arterial blood flow limitation (IAFL) in an otherwise healthy, well-trained cyclist.

DESIGN

Case report and methodological pilot study.

SETTING

University research setting.

PATIENT

A well-trained amateur competitive male cyclist, aged 31 years, presenting with symptoms consistent with IAFL, but in whom diagnostic imaging was equivocal.

INTERVENTIONS

Four ramp incremental cycling tests performed on separate days to exercise intolerance, in a randomized order, in either typical race position (RP) or modified upright position (UP).

MAIN OUTCOME

A novel ratio of unilateral cycling power to NIRS-derived muscle oxygenation termed "power-deoxygenation factor" was measured during provocative incremental exercise and compared with other NIRS-derived measures of vascular responsiveness and performance outcomes across the 2 body position conditions.

RESULTS

The power-deoxygenation factor was able to show clinically important, progressive differences between the affected and unaffected limbs, coinciding with worsening performance impairments related to the body position that were not detected with traditional measures of vascular responsiveness taken after exercise.

CONCLUSIONS

This method was used to detect bilateral differences consistent with IAFL in a cyclist where traditional diagnostic criteria were equivocal, but subsequent intraoperative findings confirmed the diagnosis. A similar screening test could be performed noninvasively and without requiring specialized medical care. Future work should investigate the validity and sensitivity of this methodology to improve the ability to identify and monitor athletes with IAFL.

In-Line Estimation of Fat Marbling in Whole Beef Striploins (Longissimus lumborum) by NIR Hyperspectral Imaging. A Closer Look at the Role of Myoglobin

Fat marbling, the amount, and distribution of intramuscular fat, is an important quality trait for beef loin (Longissimus lumborum) and is closely connected to sensory properties such as tenderness, juiciness, and flavor. For meat producers, it would be of value to grade and sort whole loins according to marbling on the production line. The main goal of this study was to evaluate high-speed NIR hyperspectral imaging in interaction mode (760–1047 nm) for in-line measurement of sensory assessed marbling in both intact loins and loin slices. The NIR system was calibrated based on 28 whole striploins and 412 slices. Marbling scores were assessed for all slices on a scale from 1 to 9 by a trained sensory panel. The calibrated NIR system was tested for in-line measurements on 30 loins and 60 slices at a commercial meat producer. Satisfactory accuracy for prediction of marbling was obtained by partial least squares regression for both slices and whole loins (R² = 0.81 & 0.82, RMSEP = 0.95 & 0.88, respectively). The concentration of myoglobin in the meat and its state of oxygenation has a strong impact on the NIR spectra and can give deviations in the estimated marbling scores. This must be carefully considered in industrial implementation.

Calf Muscle Oxygenation is Impaired and May Decline with Age in Young Patients with Total Cavopulmonary Connection

Patients palliated with Total Cavopulmonary Connection have a lower muscle mass and a lower exercise capacity. We assessed calf muscle oxidative metabolism during and after heel raise exercise to exhaustion in young patients with TCPC compared to healthy peers. Near-infrared spectroscopy was used for measuring oxygen metabolism in the medial portion of the gastrocnemius muscle. Forty-three patients with TCPC, aged 6-18 years, were compared with 43 age and sex-matched healthy control subjects. Subgroups were formed to include children (6-12 years) and adolescents (13-18 years) to determine if these age groups influenced the results. During exercise, for the patients compared to controls there was a lower increase in deoxygenated hemoglobin (oxygen extraction) (5.13 ± 2.99au vs. 7.75 ± 4.15au, p = 0.001) and a slower rate of change in total hemoglobin (blood volume) (0.004 ± 0.015au vs 0.016 ± 0.01au, p = 0.001). Following exercise, patients exhibited a slower initial increase in tissue oxygenation saturation index (0.144 ± 0.11au vs 0.249 ± 0.226au, p = 0.007) and a longer half-time to maximum hyperemia (23.7 ± 11.4 s vs 16.8 ± 7.5 s, p = 0.001). On the subgroup level, the adolescents differed compared to healthy peers, whereas the children did not. Young patients with TCPC had impaired oxidative metabolism during exercise and required a longer time to recover. In that the differences were seen in the adolescent group and not in the children group may indicate a declining function with age.

Time-domain NIRS system based on supercontinuum light source and multi-wavelength detection: validation for tissue oxygenation studies

We present and validate a multi-wavelength time-domain near-infrared spectroscopy (TD-NIRS) system that avoids switching wavelengths and instead exploits the full capability of a supercontinuum light source by emitting and acquiring signals for the whole chosen range of wavelengths. The system was designed for muscle and brain oxygenation monitoring in a clinical environment. A pulsed supercontinuum laser emits broadband light and each of two detection modules acquires the distributions of times of flight of photons (DTOFs) for 16 spectral channels (used width 12.5 nm / channel), providing a total of 32 DTOFs at up to 3 Hz. Two emitting fibers and two detection fiber bundles allow simultaneous measurements at two positions on the tissue or at two source-detector separations. Three established protocols (BIP, MEDPHOT, and nEUROPt) were used to quantitatively assess the system's performance, including linearity, coupling, accuracy, and depth sensitivity. Measurements were performed on 32 homogeneous phantoms and two inhomogeneous phantoms (solid and liquid). Furthermore, measurements on two blood-lipid phantoms with a varied amount of blood and Intralipid provide the strongest validation for accurate tissue oximetry. The retrieved hemoglobin concentrations and oxygen saturation match well with the reference values that were obtained using a commercially available NIRS system (OxiplexTS) and a blood gas analyzer (ABL90 FLEX), except a discrepancy occurs for the lowest amount of Intralipid. In-vivo measurements on the forearm of three healthy volunteers during arterial (250 mmHg) and venous (60 mmHg) cuff occlusions provide an example of tissue monitoring during the expected hemodynamic changes that follow previously well-described physiologies. All results, including quantitative parameters, can be compared to other systems that report similar tests. Overall, the presented TD-NIRS system has an exemplary performance evaluated with state-of-the-art performance assessment methods.

Forearm muscle mitochondrial capacity and resting oxygen uptake: Relationship to symptomatic fatigue in persons with multiple sclerosis

Background

Mitochondrial dysfunction has been implicated in the pathogenesis of multiple sclerosis (MS). Whether mitochondrial alterations are a function of ambulatory dysfunction or are of a non-ambulatory systemic nature is unclear.

Objective

To compare oxidative capacity, and rest muscle oxygen consumption (mVO₂) in the upper limb of persons with multiple sclerosis (PwMS) to a control group (CON), whereby an upper limb would be comparatively independent of ambulation or deconditioning.

Methods

Near infra-red spectroscopy was used to measure oxidative capacity of the wrist flexors in PwMS (n = 16) and CON (n = 13). Oxidative capacity was indicated by the time constant (TC) of mVO₂ recovery following brief wrist flexion contractions. Measurements included well-being, depression, symptomatic fatigue, disability, handgrip strength, cognition, and functional endurance. Analysis was by T-tests and Pearson correlations with p ≤ 0.05. Data are mean (SD).

Results

TC of mVO₂ recovery was slower in PwMS (MS = 47(14) sec, CON = 36(11) sec; p = 0.03). No significant correlations were found between oxidative capacity and any other measures. Rest mVO₂ was not different between groups, but correlated with symptomatic fatigue (r = 0.694, p = 0.003) and strength (0.585, p = 0.017) in PwMS.

Conclusion

Oxidative capacity was lower in the wrist flexors of PwMS, possibly indicating a systemic component of the disease. Within PwMS, rest mVO₂ was associated with symptomatic fatigue.

Measuring tibial hemodynamics and metabolism at rest and after exercise using near-infrared spectroscopy

The bone vascular system is important, yet evaluation of bone hemodynamics is difficult and expensive. This study evaluated the utility and reliability of near-infrared spectroscopy (NIRS), a portable and relatively inexpensive device, in measuring tibial hemodynamics and metabolic rate. Eleven participants were tested twice using post-occlusive reactive hyperemia technique with the NIRS probes placed on the tibia and the medial gastrocnemius (MG) muscle. Measurements were made at rest and after 2 levels of plantarflexion exercise. The difference between oxygenated and deoxygenated hemoglobin signal could be reliably measured with small coefficients of variation (CV; range 5.7-9.8%) and high intraclass correlation coefficients (ICC; range 0.73-0.91). Deoxygenated hemoglobin rate of change, a potential marker for bone metabolism, also showed good reliability (CV range 7.5-9.8%, ICC range 0.90-0.93). The tibia was characterized with a much slower metabolic rate compared with MG (p < 0.001). While exercise significantly increased MG metabolic rate in a dose-dependent manner (all p < 0.05), no changes were observed for the tibia after exercise compared with rest (all p > 0.05). NIRS is a suitable tool for monitoring hemodynamics and metabolism in the tibia. However, the local muscle exercise protocol utilized in the current study did not influence bone hemodynamics or metabolic rate. Novelty: NIRS can be used to monitor tibial hemodynamics and metabolism with good reliability. Short-duration local muscle exercise increased metabolic rate in muscle but not in bone. High level of loading and exercise volume may be needed to elicit measurable metabolic changes in bone.

Changes in Optical Path Length Reveal Significant Potential Errors of Muscle Oxygenation Evaluation during Exercise in Humans

PURPOSE

Near-infrared spectroscopy (NIRS), performed with a commonly available noninvasive tissue oxygenation monitoring device, is based on the modified Beer-Lambert law (MBLL). Although NIRS based on MBLL (NIRSMBLL) assumes that the optical path length (PL) is constant, the effects of changes in PL during exercise on muscle oxygenation calculated by MBLL are still incompletely understood. Thus, the purposes of this study were to examine the changes in optical properties during ramp incremental exercise and to compare muscle oxygen dynamics measured by time-resolved NIRS with those calculated based on MBLL.

METHODS

Twenty-two healthy young men performed ramp incremental cycling exercise until exhaustion. Optical properties (reduced scattering coefficient and PL) and absolute oxygenated, deoxygenated, and total hemoglobin and myoglobin concentrations (oxy[Hb + Mb], deoxy[Hb + Mb], and total[Hb + Mb], respectively) at the vastus lateralis were continuously monitored by a three-wavelength (763, 801, and 836) time-resolved NIRS device. The values of oxy-, deoxy-, and total[Hb + Mb] were then recalculated by assuming constant PL.

RESULTS

PL at all wavelengths statistically significantly shortened during exercise. In particular, PL at 763 nm was greatly shortened, and the average changes during exercise were a 9.8% ± 3.1% reduction. In addition, significant differences in the kinetics of oxy-, deoxy-, and total[Hb + Mb] between directly measuring PL and assuming constant PL were found. The average changes in measured PL and assuming constant PL-deoxy[Hb + Mb] were increases of 28.8 ± 16.0 μM and increases of 16.4 ± 9.3 μM, respectively.

CONCLUSION

Assuming constant PL in NIRSMBLL significantly underestimated actual muscle oxy/deoxygenation as compared with measurements obtained by real-time PL determination. The percent degree of the underestimated oxy/deoxygenation was greater than the percent degree of the changes in PL.

Using a Portable Near-infrared Spectroscopy Device to Estimate The Second Ventilatory Threshold

A breakpoint in a portable near-infrared spectroscopy (NIRS) derived deoxygenated haemoglobin (deoxy[Hb]) signal during an incremental VO2max running test has been associated with the second ventilatory threshold (VT2) in healthy participants. Thus, the aim was to examine the association between this breakpoint (NIRS) and VT₂ in well-trained runners. Gas exchange and NIRS data were collected during an incremental VO_2max running test for 10 well-trained runners. The breakpoint calculated in oxygen saturation (StO₂) and the VT₂ were determined and compared in terms relative to %VO_2max, absolute speed, VO₂, and maximum heart rate (HRmax). There were no significant differences (p>0.05) between the breakpoint in StO₂ and VT₂ relative to %VO_2max (87.00±6.14 and 88.28 ± 3.98 %), absolute speed (15.70±1.42 and 16.10±1.66 km·h^-1), VO₂ (53.71±15.17 and 54.66±15.57 ml·kg^-1·min^-1), and%HRmax (90.90±4.17 and 91.84±3.70%). There were large and significant correlations between instruments relative to%VO_2max (r=0.68, p<0.05), absolute speed (r=0.86, p<0.001), VO₂ (r=0.86, p<0.001), and %HR_max (r=0.69; p<0.05). A Bland and Altman analysis of agreement between instruments resulted in a mean difference of - 1.27±4.49%, -0.40±0.84 km·h^-1,-0.90±3.07 ml·kg^-1·min^-1, and - 0.94±3.14 for %VO_2max, absolute speed, VO_2, and %HR_max, respectively. We conclude that a portable NIRS determination of the StO₂ breakpoint is comparable with VT₂ using gas exchange and therefore appropriate for use in determining exercise training above VT₂ intensity. This is the first study to analyze the validity with the running mode using a NIRS portable device.

Reduced Scattering Coefficient During Incremental Exercise Is Constant Without Being Affected by Changes in Muscle Oxygenation or Hemodynamics

Previous studies have reported that the reduced scattering coefficient (μs') in the vastus lateralis changes during ramp-incremental exercise due to blood volume changes or accumulation of metabolic by-products. We aimed to clarify the influences of deoxygenation and blood volume changes during exercise on μs' dynamics in subjects with various aerobic capacities. Twenty-three healthy young men participated in this study. All subjects performed a ramp-incremental cycling exercise until exhaustion and were divided into two groups: lower (Low: n = 12; peak pulmonary oxygen uptake per kg of fat-free mass (VO2peak), 54.2 ± 5.3 mL/kg/min) and higher aerobic capacity group (High: n = 11; VO2peak, 69.7 ± 5.2 mL/kg/min) by median of VO2peak. Deoxygenated hemoglobin and myoglobin concentrations (deoxy[Hb + Mb]) and total [Hb + Mb] (total[Hb + Mb]) in the vastus lateralis were monitored during the exercise by three-wavelength (760, 800, and 830 nm) time-resolved NIRS. Similarly, μs' at each wavelength was continuously monitored. With increasing exercise intensity, deoxy[Hb + Mb] and total[Hb + Mb] significantly increased in both groups, and the average values of the peak amplitudes of deoxy[Hb + Mb] and total[Hb + Mb] during exercise showed a 106.4% increase and a 17.9% increase from the start of the exercise, respectively. Furthermore, the peak amplitude of total[Hb + Mb] was significantly greater in High. Conversely, there were no changes in μs' at any wavelength during exercise and no differences between two groups, suggesting that the great deoxygenation and blood volume changes during incremental exercise have little effect on μs' dynamics.

The Case for Measuring Long Bone Hemodynamics With Near-Infrared Spectroscopy

Diseases and associated fragility of bone is an important medical issue. There is increasing evidence that bone health is related to blood flow and oxygen delivery. The development of non-invasive methods to evaluate bone blood flow and oxygen delivery promise to improve the detection and treatment of bone health in human. Near-infrared spectroscopy (NIRS) has been used to evaluate oxygen levels, blood flow, and metabolism in skeletal muscle and brain. While the limited penetration depth of NIRS restricts its application, NIRS studies have been performed on the medial aspect of the tibia and some other prominent bone sites. Two approaches using NIRS to evaluate bone health are discussed: (1) the rate of re-oxygenation of bone after a short bout of ischemia, and (2) the dynamics of oxygen levels during an intervention such as resistance exercise. Early studies have shown these approaches to have the potential to evaluate bone vascular health as well as the predicted efficacy of an intervention before changes in bone composition are detectable. Future studies are needed to fully develop and exploit the use of NIRS technology for the study of bone health.

Sustained fatigue assessment during isometric exercises with time-domain near infrared spectroscopy and surface electromyography signals

The effect of sustained fatigue during an upper limb isometric exercise is presented to investigate a group of healthy subjects with simultaneous time-domain (TD) NIRS and surface electromyography (sEMG) recordings on the deltoid lateralis muscle. The aim of the work was to understand which TD-NIRS parameters can be used as descriptors for sustained muscular fatigue, focusing on the slow phase of this process and using median frequency (MF) computed from sEMG as gold standard measure. It was found that oxygen saturation and deoxy-hemoglobin are slightly better descriptors of sustained fatigue, than oxy-hemoglobin, since they showed a higher correlation with MF, while total-hemoglobin correlation with MF was lower.

Improved Exercise Tolerance, Oxygen Delivery, and Oxygen Utilization After Transcatheter Aortic Valve Implantation for Severe Aortic Stenosis

Background

Transcatheter aortic valve implantation (TAVI) represents an effective therapeutic procedure, particularly in patients with severe aortic stenosis. We hypothesized that the decreased afterload induced by TAVI would improve exercise capacity by enhancing oxygen uptake in working muscles.

Methods

A standardized exercise test was performed in patients with severe aortic stenosis the day before TAVI and within 5 days thereafter. The main study endpoint was the workload achieved during a 5-minute standardized exercise test. Using electrical cardiometry and near-infrared spectroscopy, we explored and compared the changes in cardiac index (CI), as well as muscular and cerebral tissue oximetry, during the 2 exercise tests.

Results

Thirty patients completed the study protocol. Compared with the pre-TAVI period, patients achieved a higher median workload after TAVI (316 Joules [interquartile range {IQR}: 169–494] vs 190 Joules [IQR: 131–301], P = 0.002). Baseline CI increased from 2.5 l/min per m² (IQR: 2.1–2.9) to 2.9 l/min per m² (IQR: 2.5–3.2; P = 0.009), whereas CI at the end of the exercise test increased from 4.5 l/min per m² (IQR: 3.4–5.3) to 4.7 l/min per m² (3.4–6.4; P = 0.019). At the end of the exercise test, cerebral tissue oximetry increased from 70% (IQR: 65–72) to 74% (IQR: 66–78), and muscle tissue oximetry increased from 62% (IQR: 58–65) to 71% (65–74; P = 0.046 and P < 0.001, respectively).

Conclusions

Early improvement of exercise capacity after TAVI is associated with increased CI and better oxygen utilization in the brain and skeletal muscles.

Effects of Capsinoid Intake on Brown Adipose Tissue Vascular Density and Resting Energy Expenditure in Healthy, Middle-Aged Adults: A Randomized, Double-Blind, Placebo-Controlled Study

Capsinoids are some of the most promising ingredients to increase energy expenditure (EE) due to brown adipose tissue (BAT) activation. However, there is limited information regarding the effect of prolonged capsinoid ingestion (CI) on BAT activity and resting EE (REE) in healthy, middle-aged, normal to overweight subjects (Sub_healthy) with distinct BAT characteristics. We examined the changes in BAT density (BAT-d), using near-infrared time-resolved spectroscopy, and REE/kg induced by daily CI. Forty Sub_healthy [age, 43.8 (mean) years; BMI, 25.4 kg/m²] received either capsinoid (9 mg/day) or a placebo daily for 6 weeks in a double-blind design. Total hemoglobin concentration in the supraclavicular region ([total-Hb]_sup), an indicator of BAT-d, and REE/kg were measured. The changes in post-intervention [total-Hb]_sup were greater in the capsinoid group (CA-G) than in the placebo group (PL-G) [5.8 µM (+12.4%) versus 1.0 µM (+2.1%); p = 0.017]. There was a significant relationship between BAT-d and REE/kg; however, post-supplementation REE/kg was not significantly different between the two groups (p = 0.228). In the overweight subgroup, changes in REE/kg were greater in the CA-G than in the PL-G [0.6 cal/kg/min (+4.3%) versus -0.3 cal/kg/min (-2.1%); p = 0.021]. CI enhanced [total-Hb]_sup, a reflection of BAT-d, showing a good correlation with REE in Sub_healthy.

Performance of measurands in time-domain optical brain imaging: depth selectivity versus contrast-to-noise ratio

Time-domain optical brain imaging techniques introduce a number of different measurands for analyzing absorption changes located deep in the tissue, complicated by superficial absorption changes and noise. We implement a method that allows analysis, quantitative comparison and performance ranking of measurands under various conditions - including different values of reduced scattering coefficient, thickness of the superficial layer, and source-detector separation. Liquid phantom measurements and Monte Carlo simulations were carried out in two-layered geometry to acquire distributions of times of flight of photons and to calculate the total photon count, mean time of flight, variance, photon counts in time windows and ratios of photon counts in different time windows. Quantitative comparison of performance was based on objective metrics: relative contrast, contrast-to-noise ratio (CNR) and depth selectivity. Moreover, the product of CNR and depth selectivity was used to rank the overall performance and to determine the optimal source-detector separation for each measurand. Variance ranks the highest under all considered conditions.

New Directions in Exercise Prescription: Is There a Role for Brain-Derived Parameters Obtained by Functional Near-Infrared Spectroscopy?

In the literature, it is well established that regular physical exercise is a powerful strategy to promote brain health and to improve cognitive performance. However, exact knowledge about which exercise prescription would be optimal in the setting of exercise–cognition science is lacking. While there is a strong theoretical rationale for using indicators of internal load (e.g., heart rate) in exercise prescription, the most suitable parameters have yet to be determined. In this perspective article, we discuss the role of brain-derived parameters (e.g., brain activity) as valuable indicators of internal load which can be beneficial for individualizing the exercise prescription in exercise–cognition research. Therefore, we focus on the application of functional near-infrared spectroscopy (fNIRS), since this neuroimaging modality provides specific advantages, making it well suited for monitoring cortical hemodynamics as a proxy of brain activity during physical exercise.

Near Infrared Spectroscopy Measurements of Mitochondrial Capacity Using Partial Recovery Curves

Background

Near-infrared spectroscopy (NIRS) has been used to measure muscle mitochondrial capacity (mVO₂max) as the recovery rate constant of muscle metabolism after exercise. The current method requires as many as 50 short ischemic occlusions to generate two recovery rate constants.

Purpose

To determine the validity and repeatability of using a 6-occlusion protocol versus one with 22 occlusions to measure muscle mitochondrial capacity. The order effect of performing multiple Mito6 test was also evaluated.

Method

In two independent data sets (bicep n = 7, forearm A n = 23), recovery curves were analyzed independently using both the 6 and 22 occlusion methods. A third data set (forearm B n = 16) was generated on the forearm muscles of healthy subjects using four 6-occlusion tests performed in succession. Recovery rate constants were generated using a MATLAB routine.

Results

When calculated from the same data set, the recovery rate constants were not significantly different between the 22 occlusion and 6 occlusion methods for the bicep (1.43 ± 0.33 min^–1, 1.43 ± 0.35 min^–1, p = 0.81) and the forearm A (1.97 ± 0.40 min^–1, 1.97 ± 0.43 min^–1, p = 0.90). Equivalence testing showed that the mean difference was not different than zero and the 90% confidence intervals were within 5% of the average rate constant. This was true for the Mito6 and the Mito5^∗ approaches. Bland–Altman analysis showed a slope of 0.21 min^–1 and an r of 0.045 for the bicep dataset and a slope of −0.01 min^–1 and an r of 0.045 for the forearm A dataset. When performing the four 6-occlusion tests; recovery rate constants showed no order effects (1.50 ± 0.51 min^–1, 1.42 ± 0.54 min^–1, 1.26 ± 0.41 min^–1, 1.29 ± 0.47 min^–1, P > 0.05).

Conclusion

The Mito6 analysis is a valid and repeatable approach to measure mitochondrial capacity. The Mito6 protocol used fewer ischemic occlusion periods and multiple tests could be performed in succession in less time, increasing the practicality of the NIRS mitochondrial capacity test. There were no order effects for the rate constants of four repeated 6-occlusion tests of mitochondrial capacity, supporting the use of multiple tests to improve accuracy.

Near-Infrared Time-Resolved Spectroscopy for Assessing Brown Adipose Tissue Density in Humans: A Review

Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. ¹⁸F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) with computed tomography (CT) (¹⁸FDG-PET/CT) is a standard method for assessing BAT activity and volume in humans. ¹⁸FDG-PET/CT has several limitations, including high device cost and ionizing radiation and acute cold exposure necessary to maximally stimulate BAT activity. In contrast, near-infrared spectroscopy (NIRS) has been used for measuring changes in O₂-dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIR_TRS) can quantify the concentrations of oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb], respectively) by emitting ultrashort (100 ps) light pulses and counts photons, which are scattered and absorbed in the tissue. The basis for assessing BAT density (BAT-d) using NIR_TRS is that the vascular density in the supraclavicular region, as estimated using Hb concentration, is higher in BAT than in white adipose tissue. In contrast, relatively low-cost continuous wavelength NIRS (NIR_CWS) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIR_TRS and NIR_CWS in estimating human BAT characteristics. The indicators (Ind_NIRS) examined were [oxy-Hb]_sup, [deoxy-Hb]_sup, total hemoglobin [total-Hb]_sup, Hb O₂ saturation (StO_2sup), and reduced scattering coefficient ( μs sup' ) in the supraclavicular region, as determined by NIR_TRS, and relative changes in corresponding parameters, as determined by NIR_CWS. The evidence comprises the relationships between the Ind_NIRS investigated and those determined by ¹⁸FDG-PET/CT; the correlation between the Ind_NIRS and cold-induced thermogenesis; the relationship of the Ind_NIRS to parameters measured by ¹⁸FDG-PET/CT, which responded to seasonal temperature fluctuations; the relationship of the Ind_NIRS and plasma lipid metabolites; the analogy of the Ind_NIRS to chronological and anthropometric data; and changes in the Ind_NIRS following thermogenic food supplementation. The [total-Hb]_sup and [oxy-Hb]_sup determined by NIR_TRS, but not parameters determined by NIR_CWS, exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to ¹⁸FDG-PET. We conclude that NIR_TRS can provide useful information for assessing BAT-d in a simple, rapid, non-invasive way, although further validation study is still needed.