Validity of NIR spectroscopy for quantitatively measuring muscle oxidative metabolic rate in exercise

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.

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.

Beyond VO2: the complex cardiopulmonary exercise test

Cardiopulmonary exercise test (CPET) is a valuable diagnostic tool with a specific application in heart failure (HF) thanks to the strong prognostic value of its parameters. The most important value provided by CPET is the peak oxygen uptake (peak VO2), the maximum rate of oxygen consumption attainable during physical exertion. According to the Fick principle, VO2 equals cardiac output (Qc) times the arteriovenous content difference [C(a-v)O2], where Ca is the arterial oxygen and Cv is the mixed venous oxygen content, respectively; therefore, VO2 can be reduced both by impaired O2 delivery (reduced Qc) or extraction (reduced arteriovenous O2 content). However, standard CPET is not capable of discriminating between these different impairments, leading to the need for 'complex' CPET technologies. Among non-invasive methods for Qc measurement during CPET, inert gas rebreathing and thoracic impedance cardiography are the most used techniques, both validated in healthy subjects and patients with HF, at rest and during exercise. On the other hand, the non-invasive assessment of peripheral muscle perfusion is possible with the application of near-infrared spectroscopy, capable of measuring tissue oxygenation. Measuring Qc allows, by having haemoglobin values available, to discriminate how much any VO2 deficit depends on the muscle, anaemia or heart.

Developing a near-infrared spectroscopy and microwave-induced thermoacoustic tomography-based dual-modality imaging system

Near-infrared spectroscopy (NIRS) techniques can provide noninvasive in vivo hemoglobin oxygenation information but suffer from relatively low resolution in biological tissue imaging. Microwave-induced thermoacoustic tomography (TAT) can produce high-resolution images of the biological tissue anatomy but offer limited physiological information of samples because of the single species of the chromophore it maps. To overcome these drawbacks and take advantage of the merits of the two independent techniques, we built a dual-modality system by combining a NIRS system and a TAT system to image biological tissues. A series of phantom trials were carried out to demonstrate the performance of the new system. The spatial resolution is about 1 mm, with a penetration depth of at least 17.5 mm in the human subject. A cohort of five healthy subjects was recruited to conduct real-time forearm venous and arterial cuff occlusion experiments. Numerous results showed that this dual-modality system could measure oxygen metabolism and simultaneously provide anatomical structure changes of biological tissues. We also found that although the hemoglobin concentration varied consistently with many other published papers, the TAT signal intensity of veins showed an opposite variation tendency in the venous occlusion stage compared with other existing work. A detailed explanation is given to account for the discrepancy, thus, providing another possibility for the forearm experiments using TAT. Furthermore, based on the multiple types of information afforded by this dual-modality system, a pilot clinical application for the diagnosis of anemia is discussed.

Local vibration therapy increases oxygen re-saturation rate and maintains muscle strength following exercise-induced muscle damage

CONTEXT

Exercise induced muscle damage (EIMD) is associated with transient reductions in strength and athletic performance. Studies conclude aetiology is due in part to muscle micro vascular damage and disruption of blood flow. Previous research on vibration therapy reports modulation in muscle blood flow, oxygenation and strength.

OBJECTIVE

The aim of this study was to observe if local vibration therapy (VT) alleviates the impairments and haemodynamic changes associated with EIMD.

DESIGN

Controlled laboratory study.

SETTING

Laboratory and public gymnasium.

PATIENTS OR OTHER PARTICIPANTS

Ten healthy participants (6 males: 4 females; age: 38±15 yrs; height: 1.72±0.48 m; mass 72.0±10.4 kg) were randomized into experimental (VT) and control (CON) groups.

INTERVENTIONS

Both groups performed 10 sets of 10 eccentric wrist flexions at 70% of 1-repetition maximum to induce muscle damage. Subsequent assessment of wrist flexor strength and flexor carpus ulnaris (FCU) muscle oxygen saturation (SmO2) occurred at 1-, 24- and 48 hr-post exercise. VT group underwent 10 min of local VT (45 Hz) starting 1 hr-post exercise and applied twice daily (separated by 8 hrs) for 48 hrs during habitual waking hours. CON group received no local VT.

MAIN OUTCOME MEASURE(S)

Grip strength, resting muscle oxygen (SmO2), muscle oxygen de-saturation and re-saturation rate.

RESULTS

No difference in grip strength observed pre EIMD, but the VT group demonstrated greater strength at 1 hr (P=0.004), 24 hr (P=0.031) and 48 hr (P=0.021) post EIMD compared to controls. No difference in SmO2 re-saturation over time (P>0.05), but the VT group had a greater re-saturation rate compared to controls at 1 hr (P=0.007, d = 2.6), 24 hr (P=0.001 d = 3.1) and 48 hr (P=0.035, d = 1.7) post EIMD.

CONCLUSIONS

Local VT successfully attenuated the effects of EIMD and increased SmO2 re-saturation in FCU muscles. Including local VT as part of a recovery protocol post-EIMD could be beneficial for rehabilitation and athletic training purposes.

Muscle mitochondrial capacity in high- and low-fitness females using near-infrared spectroscopy

The recovery of muscle oxygen consumption (m V ˙ O₂ ) after exercise measured using near-infrared spectroscopy (NIRS) provides a measure of skeletal muscle mitochondrial capacity. Nevertheless, due to sex differences in factors that can influence scattering and thus penetration depth of the NIRS signal in the tissue, e.g., subcutaneous adipose tissue thickness and intramuscular myoglobin and hemoglobin, it is unknown whether results in males can be extrapolated to a female population. Therefore, the aim of this study was to measure skeletal muscle mitochondrial capacity in females at different levels of aerobic fitness to test whether NIRS can measure relevant differences in mitochondrial capacity. Mitochondrial capacity was analyzed in the gastrocnemius muscle and the wrist flexors of 32 young female adults, equally divided in relatively high ( V ˙ O₂ peak ≥ 47 ml/kg/min) and relatively low aerobic fitness group ( V ˙ O₂ peak ≤ 37 ml/kg/min). m V ˙ O₂ recovery was significantly faster in the high- compared to the low-fitness group in the gastrocnemius, but not in the wrist flexors (p = 0.009 and p = 0.0528, respectively). Furthermore, V ˙ O₂ peak was significantly correlated to m V ˙ O₂ recovery in both gastrocnemius (R²  = 0.27, p = 0.0051) and wrist flexors (R²  = 0.13, p = 0.0393). In conclusion, NIRS measurements can be used to assess differences in mitochondrial capacity within a female population and is correlated to V ˙ O₂ peak. This further supports NIRS assessment of muscle mitochondrial capacity providing additional evidence for NIRS as a promising approach to monitor mitochondrial capacity, also in an exclusively female population.

Reliability and reproducibility of a four arterial occlusions protocol for assessing muscle oxidative metabolism at rest and after exercise using near-infrared spectroscopy

OBJECTIVE

To assess the reliability and reproducibility of using a four arterial occlusions protocol and near-infrared spectroscopy (NIRS) to measure resting and post-exercise muscle oxidative metabolism (mVO₂).

APPROACH

mVO₂ was measured on the forearm muscles on two different days (day1 and day2) within one week in 11 healthy young adults (24.2 ± 2.7 years; 5 males). mVO₂ was measured using NIRS during four repeated arterial occlusions at rest, and 5 min after exercise consisting of 90 s of rapid concentric contractions (5 minEPOC).

MAIN RESULTS

Resting mVO₂ with four measurements was 17.88 ± 3.04% min^-1 on day 1 and 19.42 ± 3.03% min^-1 on day 2 (p = 0.171) with a coefficient of variation (CV) of 10.1%. When using only the first measurement, the CV increased to 18.5% (p = 0.039). 5minEPOC was 212.4 ± 142.5% and 177.1 ± 125.8% higher than resting and was not different between days one and two (53.83 ± 21.17% min^-1 and 52.22 ± 22.10% min^-1, respectively, p= 0.199). The CV and intraclass correlation (ICC) for 5minEPOC between days one and two were, 6.5% and 0.98, respectively. Using only the first value for 5minEPOC resulted in slightly higher CV but similar ICC (7.6% and 0.98, respectively; both p > 0.05).

SIGNIFICANCE

Our results suggest that within a single testing session, one arterial occlusion can provide reproducible measurements for both resting and post-exercise mVO₂ similar to that of a four arterial occlusions protocol. While a four arterial occlusion protocol provides similar reliability for post-exercise mVO₂ with one arterial occlusion, it reduces the day-to-day variance for resting mVO₂ and therefore should be employed for longitudinal studies.

The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise

Background

Decreased oxygenation of muscle may be accentuated during exercise at high altitude. Monitoring the oxygen saturation of muscle (SmO₂) during hand grip exercise using near infrared spectroscopy during acute exposure to hypoxia could provide a model for a test of muscle performance without the competing cardiovascular stresses that occur during a cycle ergometer or treadmill test. The purpose of this study was to examine and compare acute exposure to normobaric hypoxia versus normoxia on deoxygenation and recruitment of the flexor digitorum superficialis (FDS) during submaximal intermittent handgrip exercise (HGE) in healthy adults.

Methods

Twenty subjects (11 M/9 F) performed HGE at 50% of maximum voluntary contraction, with a duty cycle of 2 s:1 s until task failure on two occasions one week apart, randomly assigned to normobaric hypoxia (FiO₂ = 12%) or normoxia (FiO₂ = 21%). Near-infrared spectroscopy monitored SmO₂, oxygenated (O₂Hb), deoxygenated (HHb), and total hemoglobin (tHb) over the FDS. Surface electromyography derived root mean square and mean power frequency of the FDS.

Results

Hypoxic compared to normoxic HGE induced a lower FDS SmO₂ (63.8 ± 2.2 vs. 69.0 ± 1.5, p = 0.001) and both protocols decreased FDS SmO₂ from baseline to task failure. FDS mean power frequency was lower during hypoxic compared to normoxic HGE (64.0 ± 1.4 vs. 68.2 ± 2.0 Hz, p = 0.04) and both decreased mean power frequency from the first contractions to task failure (p = 0.000). Under both hypoxia and normoxia, HHb, tHb and root mean square increased from baseline to task failure whereas O₂Hb decreased and then increased during HGE. Arterial oxygen saturation via pulse oximetry (SpO₂) was lower during hypoxia compared to normoxia conditions (p = 0.000) and heart rate and diastolic blood pressure only demonstrated small increases. Task durations and the tension-time index of HGE did not differ between normoxic and hypoxic trials.

Conclusion

Hypoxic compared to normoxic HGE decreased SmO₂ and induced lower mean power frequency in the FDS, during repetitive hand grip exercise however did not result in differences in task durations or tension-time indices. The fiber type composition of FDS, and high duty cycle and intensity may have contributed greater dependence on anaerobiosis.

Mitochondrial capacity using NIRS and incomplete recovery curves: Proximal and Medial Vastus Lateralis muscle

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 2 recovery rate constants.

PURPOSE

To determine the effectiveness of using a 6-occlusion protocol (Mito6) versus one with 22 occlusions (Mito22) to measure muscle mitochondrial capacity.

METHOD

In two independent data sets (bicep n=7, forearm A n=23), recovery curves were analyzed independently using both the Mito6 and Mito22 analyses. A third data set (Forearm B, n=16) was generated on forearm muscles of healthy subjects using four Mito6 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 Mito22 and Mito6 analyses for the bicep (1.43+0.33min-1, 1.43+0.35min-1, p=0.81) and the forearm A (1.97+0.40min-1, 1.97+0.43min-1, p=0.90). The correlation between Mito22 and Mito6 recovery rate constants was y=1.07x-0.09, R2=0.90 for the bicep data and 1.00x+0.01, R2=0.85 for the forearm A data. When performing the four Mito6 tests in the Forearm B study; recovery rate constants were not different between tests (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).

CONCLUSIONS

Muscle mitochondrial capacity was not different between the Mito6 analysis and the longer Mito22 analysis. The Mito6 protocol was considered more practical as it used fewer ischemic occlusion periods, and multiple tests could be performed in succession in less time. There were no order effects for the rate constants of four repeated Mito6 tests of mitochondrial capacity, supporting the use of multiple tests to improve accuracy.

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.

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

Near-infrared spectroscopy (NIRS) has become an increasingly valuable tool to monitor tissue oxygenation (Toxy) in vivo. Observations of changes in the absorption of light with Toxy have been recognized as early as 1876, leading to a milestone NIRS paper by Jöbsis in 1977. Changes in the absorption and scatting of light in the 700-850-nm range has been successfully used to evaluate Toxy. The most practical devices use continuous-wave light providing relative values of Toxy. Phase-modulated or pulsed light can monitor both absorption and scattering providing more accurate signals. NIRS provides excellent time resolution (~ 10 Hz), and multiple source-detector pairs can be used to provide low-resolution imaging. NIRS has been applied to a wide range of populations. Continued development of NIRS devices in terms of lower cost, better detection of both absorption and scattering, and smaller size will lead to a promising future for NIRS studies.

Characterising skeletal muscle haemoglobin saturation during exercise using near-infrared spectroscopy in chronic kidney disease

Background

Chronic kidney disease (CKD) patients have reduced exercise capacity. Possible contributing factors may include impaired muscle O₂ utilisation through reduced mitochondria number and/or function slowing the restoration of muscle ATP concentrations via oxidative phosphorylation. Using near-infrared spectroscopy (NIRS), we explored changes in skeletal muscle haemoglobin/myoglobin O₂ saturation (SMO₂%) during exercise.

Methods

24 CKD patients [58.3 (± 16.5) years, eGFR 56.4 (± 22.3) ml/min/1.73 m²] completed the incremental shuttle walk test (ISWT) as a marker of exercise capacity. Using NIRS, SMO₂% was measured continuously before, during, and after (recovery) exercise. Exploratory differences were investigated between exercise capacity tertiles in CKD, and compared with six healthy controls.

Results

We identified two discrete phases; a decline in SMO₂% during incremental exercise, followed by rapid increase upon cessation (recovery). Compared to patients with low exercise capacity [distance walked during ISWT, 269.0 (± 35.9) m], patients with a higher exercise capacity [727.1 (± 38.1) m] took 45% longer to reach their minimum SMO₂% (P = .038) and recovered (half-time recovery) 79% faster (P = .046). Compared to controls, CKD patients took significantly 56% longer to recover (i.e., restore SMO₂% to baseline, full recovery) (P = .014).

Conclusions

Using NIRS, we have determined for the first time in CKD, that favourable SMO₂% kinetics (slower deoxygenation rate, quicker recovery) are associated with greater exercise capacity. These dysfunctional kinetics may indicate reduced mitochondria capacity to perform oxidative phosphorylation—a process essential for carrying out even simple activities of daily living. Accordingly, NIRS may provide a simple, low cost, and non-invasive means to evaluate muscle O₂ kinetics in CKD.

Time Domain Near Infrared Spectroscopy Device for Monitoring Muscle Oxidative Metabolism: Custom Probe and In Vivo Applications

Measurement of muscle oxidative metabolism is of interest for monitoring the training status in athletes and the rehabilitation process in patients. Time domain near infrared spectroscopy (TD NIRS) is an optical technique that allows the non-invasive measurement of the hemodynamic parameters in muscular tissue: concentrations of oxy- and deoxy-hemoglobin, total hemoglobin content, and tissue oxygen saturation. In this paper, we present a novel TD NIRS medical device for muscle oxidative metabolism. A custom-printed 3D probe, able to host optical elements for signal acquisition from muscle, was develop for TD NIRS in vivo measurements. The system was widely characterized on solid phantoms and during in vivo protocols on healthy subjects. In particular, we tested the in vivo repeatability of the measurements to quantify the error that we can have by repositioning the probe. Furthermore, we considered a series of acquisitions on different muscles that were not yet previously performed with this custom probe: a venous-arterial cuff occlusion of the arm muscle, a cycling exercise, and an isometric contraction of the vastus lateralis.

Assessing muscular oxygenation during incremental exercise using near-infrared spectroscopy: comparison of three different methods

Using continuous-wave near-infrared spectroscopy (NIRS), this study compared three different methods, namely the slope method (SM), the amplitude method (AM), and the area under the curve (AUC) method to determine the variations of intramuscular oxygenation level as a function of workload. Ten right-handed subjects (22+/-4 years) performed one isometric contraction at each of three different workloads (30 %, 50 % and 90 % of maximal voluntary strength) during a period of twenty seconds. Changes in oxyhemoglobin (delta[HbO(2)]) and deoxyhemoglobin (delta[HHb]) concentrations in the superficial flexor of fingers were recorded using continuous-wave NIRS. The results showed a strong consistency between the three methods, with standardized Cronbach alphas of 0.87 for delta[HHb] and 0.95 for delta[HbO(2)]. No significant differences between the three methods were observed concerning delta[HHb] as a function of workload. However, only the SM showed sufficient sensitivity to detect a significant decrease in delta[HbO(2)] between 30 % and 50 % of workload (p<0.01). Among these three methods, the SM appeared to be the only method that was well adapted and sensitive enough to determine slight changes in delta[HbO(2)]. Theoretical and methodological implications of these results are discussed.

Oxygen recovery kinetics in the forearm flexors of multiple ability groups of rock climbers

The purpose of this study was to determine muscle tissue oxidative capacity and recovery in intermediate, advanced, and elite rock climbers. Forty-four male participants performed (a) sustained and (b) intermittent contractions at 40% of maximal volitional contraction (MVC) on a sport-specific fingerboard until volitional fatigue. Near-infrared spectroscopy was used to assess muscle tissue oxygenation during both the exercise and the 5-minutes passive recovery period, in the flexor digitorum profundus (FDP) and flexor carpi radialis (FCR). During the sustained contraction only, muscle tissue deoxygenation (O2 debt) in the FDP and FCR was significantly greater in elite climbers compared with the control, intermediate, and advanced groups (FDP: 32 vs. 15, 19, 22%; FCR: 19 vs. 11, 8, 15%, respectively). However, elite climbers had a significantly quicker time to half recovery (T1/2) than the control and intermediate groups in the FDP (8 vs. 95 and 47 seconds, respectively) and the FCR (7 vs. 30 and 97 seconds, respectively) because the O2% recovered per second being significantly greater (FDP: 4.2 vs. 0.7 and 0.3; FCR: 4.8 vs. 0.1 and 0.2, respectively). Furthermore, during the intermittent contraction, T1/2 in elite climbers was significantly quicker compared with the control and intermediate groups in the FDP (8 vs. 93 and 83 seconds, respectively) and FCR (16 vs. 76 and 50 seconds, respectively). Consequently, lower-level climbers should focus training on specific intermittent fatigue protocols. Competition or elite climbers should make use of appropriate rests on route to aid recovery and increase the chances of reaching the next hold.

Possible Influences on the Interpretation of Functional Domain (FD) Near-Infrared Spectroscopy (NIRS): An Explorative Study

The influence of subcutaneous adipose tissue (ATT) and oxygen (O2) delivery has been poorly defined in frequency domain (FD) near-infrared spectroscopy (NIRS). Therefore, the aim of this study was to investigate the possible influence of these variables on all FD NIRS responses using a reliable protocol. Moreover, these influences were also investigated when using relative oxy- and deoxyhemoglobin and -myoglobin (oxy[Hb + Mb] and deoxy[Hb + Mb]) values (in %). A regression analysis was carried out for ATT and maximal-minimum oxy[Hb + Mb], deoxy[Hb + Mb], oxygen saturation (SmO2), and total hemoglobin (totHb) amplitudes during an incremental cyclic contraction protocol (ICCP) in a group of 45 participants. Moreover, the same analysis was carried out between subcutaneous ATT and the relative oxy- and deoxy[Hb + Mb] values (in %). In the second part of this study, a regression analysis was performed for peak forearm blood flow (FBF) during ICCP and the absolute and relative NIRS values in a group of 37 participants. Significant exponential correlation coefficients were found between ATT and deoxy[Hb + Mb] (r = 0.53; P < 0.001), oxy[Hb + Mb] (r = 0.57; P < 0.001), and SmO2 amplitudes (r = 0.57; P < 0.001). No significant relations were found between ATT and relative oxy[Hb + Mb] (r = 0.37; P = 0.07) and deoxy[Hb + Mb] (r = 0.09; P = 0.82). Significant positive correlation coefficients were found between force at exhaustion and maximal FBF (r = 0.66; P < 0.001), maximal differences in deoxy[Hb + Mb] (r = 0.353; P = 0.032) and totHb (r = 0.512; P = 0.002) while no significant correlation coefficients were found between these maximal force values and maximal differences in oxy[Hb + Mb] (r = -0.267; P = 0.111) and SmO2 (r = -0.267; P = 0.111). Significant linear correlation coefficients were found between FBF and deoxy[Hb + Mb] (r = 0.51; P = 0.001), oxy[Hb + Mb] (r = -0.50; P = 0.001), SmO2 (r = -0.54; P = 0.001), and totHb amplitude (r = 0.61; P < 0.001). No significant correlations were found when using relative oxy[Hb + Mb] (r = -0.01; P = 0.957) and deoxy[Hb + Mb] (r = -0.02; P = 0.895). Based on these findings, caution is advised when using NIRS values, as subcutaneous ATT and O2 delivery significantly influence NIRS measurements. To eliminate these influences, use of relative deoxy[Hb + Mb] is advised, especially in clinical settings or in people with a higher subcutaneous ATT layer.

Once-weekly muscle endurance and strength training prevents deterioration of muscle oxidative function and attenuates the degree of strength decline during 3-week forearm immobilization

PURPOSE

Muscle unloading causes muscle function deterioration, but the extent to which training frequency or volume can be reduced while preserving muscle function during muscle unloading is unknown. We examined the effects of low-volume muscle endurance and strength training on forearm muscle oxidative capacity, endurance, and strength during a 3-week immobilization.

METHODS

Twenty-seven, healthy, male volunteers were divided into four groups: immobilization only (IMM); immobilization with endurance and strength training, once-weekly (IMM + EST1) or twice-weekly (IMM + EST2); and control, without immobilization or training (CNT). Endurance training involved dynamic handgrip exercise, at 30% of maximal voluntary contraction (MVC), until exhaustion (~60 s). Strength training involved intermittent isometric handgrip exercise at 70% MVC (40 s). Muscle oxidative capacity was evaluated after exercise using the phosphocreatine recovery time constant using (31)phosphorus magnetic resonance spectroscopy. Endurance performance was evaluated according to the total work during dynamic handgrip exercise at 30% MVC at 1 Hz until exhaustion.

RESULTS

Muscle oxidative capacity and total work deterioration was restricted to the IMM (P < 0.05) group. MVC decreased in the IMM and IMM + EST1 (P < 0.05) groups. However, the MVC amplitude decrease in the IMM + EST1 group was smaller than that in the IMM (P < 0.05) group. MVC remained unchanged in the other groups.

CONCLUSION

During the 3-week immobilization, twice-weekly low-volume muscle endurance and strength training prevented deterioration in muscle strength, oxidative capacity, and endurance performance. Moreover, once-weekly muscle endurance and strength training prevented the deterioration of muscle oxidative capacity and endurance performance, and attenuated the degree of muscle strength decline.

Forearm oxygenation and blood flow kinetics during a sustained contraction in multiple ability groups of rock climbers

Currently, the physiological mechanisms that allow elite level climbers to maintain intense isometric contractions for prolonged periods of time are unknown. Furthermore, it is unclear whether blood flow or muscle oxidative capacity best governs performance. This study aimed to determine the haemodynamic kinetics of 2 forearm flexor muscles in 3 ability groups of rock climbers. Thirty-eight male participants performed a sustained contraction at 40% of maximal voluntary contraction (MVC) until volitional fatigue. Oxygen saturation and blood flow was assessed using near infrared spectroscopy and Doppler ultrasound. Compared to control, intermediate, and advanced groups, the elite climbers had a significantly (P < 0.05) higher strength-to-weight ratio (MVC/N), de-oxygenated the flexor digitorum profundus significantly (P < 0.05) more (32, 34.3, and 42.8 vs. 63% O2, respectively), and at a greater rate (0.32, 0.27, and 0.34 vs. 0.77 O2%·s(-1), respectively). Furthermore, elite climbers de-oxygenated the flexor carpi radialis significantly (P < 0.05) more and at a greater rate than the intermediate group (36.5 vs. 14.6% O2 and 0.43 vs. 0.1O2%·s(-1), respectively). However, there were no significant differences in total forearm ∆ blood flow. An increased MVC/N is not associated with greater blood flow occlusion in elite climbers; therefore, oxidative capacity may be more important for governing performance.

Reproducibility of near-infrared spectroscopy measurements of oxidative function and postexercise recovery kinetics in the medial gastrocnemius muscle

The purpose of this study was to assess the reproducibility of resting blood flow, resting oxygen consumption, and mitochondrial capacity in skeletal muscle using near-infrared spectroscopy (NIRS). We also determined the influence of 2 exercise modalities (ergometer and rubber exercise bands) on the NIRS measurements. Fifteen young, healthy participants (5 female, 10 male) were tested on 2 nonconsecutive occasions within an 8-day period. The NIRS device was placed on the medial gastrocnemius. Venous and arterial occlusions were performed to obtain blood flow and oxygen consumption. A series of repeated arterial occlusions was used to measure the recovery kinetics of muscle oxygen consumption after ∼7–10 s of voluntary plantar flexion exercise. Resting blood flow had mean coefficients of variation (CV) of 42% and 38% for bands and ergometer, respectively, and resting metabolism had mean CVs of 17% and 12% for bands and ergometer, respectively. The recovery time constant of oxygen consumption (day 1 bands and ergometer: 23.2 ± 3.7 s, 27.6 ± 6.5 s, respectively; day 2 bands and ergometer: 25.5 ± 5.4 s, 25.0 ± 4.9 s, respectively) had mean CVs of 10% and 11% for bands and ergometer, respectively. We conclude that measurements of oxygen consumption and mitochondrial capacity using NIRS can be obtained with good reproducibility.

Differential effect of sympathetic activation on tissue oxygenation in gastrocnemius and soleus muscles during exercise in humans

NEW FINDINGS

What is the central question of this study? The normal ability of sympathetic nerves to cause vasoconstriction is blunted in exercising skeletal muscle, a phenomenon termed 'functional sympatholysis'. Animal studies suggest that functional sympatholysis appears to occur preferentially in fast-twitch type II glycolytic compared with slow-twitch type I oxidative skeletal muscle. We asked whether these findings can be extended to humans. What is the main finding and its importance? We show that skeletal muscles composed largely of fast-twitch type II fibres may also be more sensitive to functional sympatholysis in humans, particularly at lower exercise intensities. Additionally, independent of muscle fibre type composition, the magnitude of sympatholysis is strongly related to exercise-induced increases in metabolic demand. Animal studies suggest that functional sympatholysis appears to occur preferentially in glycolytic (largely type II) compared with oxidative (largely type I) skeletal muscle. Whether these findings can be extended to humans currently remains unclear. In 12 healthy male subjects, vasoconstrictor responses in gastrocnemius (i.e. primarily type II) and soleus muscles (i.e. primarily type I) were measured using near-infrared spectroscopy to detect decreases in muscle oxygenation (HbO(2)) in response to sympathetic activation evoked by a cold pressor test (CPT). The HbO(2) responses to a CPT at rest were compared with responses during steady-state plantar flexion exercise (30 repetitions min(-1)) performed at 10, 20 and 40% maximal voluntary contraction (MVC) for 6 min. In resting conditions, HbO(2) at the gastrocnemius (-14 ± 1%) and soleus muscles (-16 ± 1%) decreased significantly during CPT, with no differences between muscles. During planter flexion at 20% MVC, the change in HbO(2) in response to the CPT was blunted in gastrocnemius but not soleus, whereas during 40% MVC both muscles exhibited a significant attenuation to sympathetic activation. The decreases in HbO(2) in response to the CPT during exercise were significantly correlated with the metabolic demands of exercise (the decreases in HbO(2) in response to steady-state plantar flexion) in both gastrocnemius and soleus muscles. Collectively, these results suggest that skeletal muscles composed mainly of glycolytic type II fibres are more sensitive to functional sympatholysis, particularly at lower intensities of exercise. Moreover, the blunting of sympathetic vasoconstriction during exercise is strongly related to metabolic demand; an effect that appears independent of fibre type composition.

A cross-validation of near-infrared spectroscopy measurements of skeletal muscle oxidative capacity with phosphorus magnetic resonance spectroscopy

The purpose of this study was to cross-validate measurements of skeletal muscle oxidative capacity made with near-infrared spectroscopy (NIRS) measurements to those made with phosphorus magnetic resonance spectroscopy ((31)P-MRS). Sixteen young (age = 22.5 ± 3.0 yr), healthy individuals were tested with both (31)P-MRS and NIRS during a single testing session. The recovery rate of phosphocreatine was measured inside the bore of a 3-Tesla MRI scanner, after short-duration (∼10 s) plantar flexion exercise as an index of skeletal muscle oxidative capacity. Using NIRS, the recovery rate of muscle oxygen consumption was also measured using repeated, transient arterial occlusions outside the MRI scanner, after short-duration (∼10 s) plantar flexion exercise as another index of skeletal muscle oxidative capacity. The average recovery time constant was 31.5 ± 8.5 s for phosphocreatine and 31.5 ± 8.9 s for muscle oxygen consumption for all participants (P = 0.709). (31)P-MRS time constants correlated well with NIRS time constants for both channel 1 (Pearson's r = 0.88, P < 0.0001) and channel 2 (Pearson's r = 0.95, P < 0.0001). Furthermore, both (31)P-MRS and NIRS exhibit good repeatability between trials (coefficient of variation = 8.1, 6.9, and 7.9% for NIRS channel 1, NIRS channel 2, and (31)P-MRS, respectively). The good agreement between NIRS and (31)P-MRS indexes of skeletal muscle oxidative capacity suggest that NIRS is a valid method for assessing mitochondrial function, and that direct comparisons between NIRS and (31)P-MRS measurements may be possible.

A comparison of exercise type and intensity on the noninvasive assessment of skeletal muscle mitochondrial function using near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) can be used to measure muscle oxygen consumption (mVO(2)) using arterial occlusions. The recovery rate of mVO(2) after exercise can provide an index of skeletal muscle mitochondrial function. The purpose of this study was to test the influence of exercise modality and intensity on NIRS measurements of mitochondrial function. Three experiments were performed. Thirty subjects (age: 18-27 yr) were tested. NIRS signals were corrected for blood volume changes. The recovery of mVO(2) after exercise was fit to a monoexponential curve, and a rate constant was calculated (directly related to mitochondrial function). No differences were found in NIRS rate constants for VOL and ES exercises (2.04 ± 0.57 vs. 2.01 ± 0.59 min(-1) for VOL and ES, respectively; P = 0.317). NIRS rate constants were independent of the contraction frequency for both VOL and ES (VOL: P = 0.166 and ES: P = 0.780). ES current intensity resulted in significant changes to the normalized time-tension integral (54 ± 11, 82 ± 7, and 100 ± 0% for low, medium, and high currents, respectively; P < 0.001) but did not influence NIRS rate constants (2.02 ± 0.54, 1.95 ± 0.44, 2.02 ± 0.46 min(-1) for low, medium, and high currents, respectively; P = 0.771). In summary, NIRS measurements of skeletal muscle mitochondrial function can be compared between VOL and ES exercises and were independent of the intensity of exercise. NIRS represents an important new technique that is practical for testing in research and clinical settings.

Noninvasive optical characterization of muscle blood flow, oxygenation, and metabolism in women with fibromyalgia

Introduction

Women with fibromyalgia (FM) have symptoms of increased muscular fatigue and reduced exercise tolerance, which may be associated with alterations in muscle microcirculation and oxygen metabolism. This study used near-infrared diffuse optical spectroscopies to noninvasively evaluate muscle blood flow, blood oxygenation and oxygen metabolism during leg fatiguing exercise and during arm arterial cuff occlusion in post-menopausal women with and without FM.

Methods

Fourteen women with FM and twenty-three well-matched healthy controls participated in this study. For the fatiguing exercise protocol, the subject was instructed to perform 6 sets of 12 isometric contractions of knee extensor muscles with intensity steadily increasing from 20 to 70% maximal voluntary isometric contraction (MVIC). For the cuff occlusion protocol, forearm arterial blood flow was occluded via a tourniquet on the upper arm for 3 minutes. Leg or arm muscle hemodynamics, including relative blood flow (rBF), oxy- and deoxy-hemoglobin concentration ([HbO₂] and [Hb]), total hemoglobin concentration (THC) and blood oxygen saturation (StO₂), were continuously monitored throughout protocols using a custom-built hybrid diffuse optical instrument that combined a commercial near-infrared oximeter for tissue oxygenation measurements and a custom-designed diffuse correlation spectroscopy (DCS) flowmeter for tissue blood flow measurements. Relative oxygen extraction fraction (rOEF) and oxygen consumption rate (rVO₂) were calculated from the measured blood flow and oxygenation data. Post-manipulation (fatiguing exercise or cuff occlusion) recovery in muscle hemodynamics was characterized by the recovery half-time, a time interval from the end of manipulation to the time that tissue hemodynamics reached a half-maximal value.

Results

Subjects with FM had similar hemodynamic and metabolic response/recovery patterns as healthy controls during exercise and during arterial occlusion. However, tissue rOEF during exercise in subjects with FM was significantly lower than in healthy controls, and the half-times of oxygenation recovery (Δ[HbO₂] and Δ[Hb]) were significantly longer following fatiguing exercise and cuff occlusion.

Conclusions

Our results suggest an alteration of muscle oxygen utilization in the FM population. This study demonstrates the potential of using combined diffuse optical spectroscopies (i.e., NIRS/DCS) to comprehensively evaluate tissue oxygen and flow kinetics in skeletal muscle.

Noninvasive optical quantification of absolute blood flow, blood oxygenation, and oxygen consumption rate in exercising skeletal muscle

This study investigates a method using novel hybrid diffuse optical spectroscopies [near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS)] to obtain continuous, noninvasive measurement of absolute blood flow (BF), blood oxygenation, and oxygen consumption rate (V̇O(2)) in exercising skeletal muscle. Healthy subjects (n=9) performed a handgrip exercise to increase BF and V̇O(2) in forearm flexor muscles, while a hybrid optical probe on the skin surface directly monitored oxy-, deoxy-, and total hemoglobin concentrations ([HbO(2)], [Hb], and THC), tissue oxygen saturation (S(t)O(2)), relative BF (rBF), and relative oxygen consumption rate (rV̇O(2)). The rBF and rV̇O(2) signals were calibrated with absolute baseline BF and V̇O(2) obtained through venous and arterial occlusions, respectively. Known problems with muscle-fiber motion artifacts in optical measurements during exercise were mitigated using a novel gating algorithm that determined muscle contraction status based on control signals from a dynamometer. Results were consistent with previous findings in the literature. This study supports the application of NIRS/DCS technology to quantitatively evaluate hemodynamic and metabolic parameters in exercising skeletal muscle and holds promise for improving diagnosis and treatment evaluation for patients suffering from diseases affecting skeletal muscle and advancing fundamental understanding of muscle and exercise physiology.

Effect of exercise intensity on oxygen consumption kinetics in non-exercising muscle during exercise

This study examined the effect of exercise intensity on the kinetics of muscle oxygen consumption in non-exercising forearm flexor muscles (VO(2mf)) during exercise. Seven healthy male subjects performed cycling exercise for 60 min at 30% of maximal oxygen consumption (%VO(2max)) and 30 min at 50% VO(2max) on separate days. The VO(2mf) values at rest and during exercise were measured by near-infrared spectroscopy. The VO(2mf) at 30% VO(2max) significantly increased to 1·2 ± 0·1-fold over resting value at 20 min after the beginning of exercise (P<0·05) and remained constant within 1·2- to 1·3-fold over resting value until 60 min during exercise. The VO(2mf) at 50% VO(2max) significantly increased to 1·2 ± 0·1-fold over resting value at 15 min after the beginning of exercise (P<0·05). Subsequently, the VO(2mf) at 50% VO(2max) increased with time to 1·3 ± 0·1-fold over resting value at 20 min after the beginning of exercise and to 1·5 ± 0·2-fold over resting value at 30 min. The VO(2mf) 15-30 min of exercise at 50% VO(2max) was significantly higher than that at 30% VO(2max) (P<0·05). These data suggest that the increase in VO(2mf) has a time lag from the beginning of exercise, and the kinetics of VO(2mf) during exercise differs with exercise intensity. Therefore, we conclude that the kinetics of VO(2mf) during exercise is dependent on exercise intensity.

Tissue Oxygenation in Men and Women During Repeated-Sprint Exercise

Purpose:

To understand the role of O2 utilization in the sex differences of fatigue during intermittent activity, we compared the cerebral (prefrontal lobe) and muscle (vastus lateralis) oxygenation of men and women during repeated-sprint exercise (RSE).

Methods:

Ten men and 10 women matched for initial-sprint mechanical work performed ten, 10 s cycle sprints (with 30 s of rest) under normoxic (NM: 21% FIO2) and acute hypoxic (HY: 13% FIO2) conditions in a randomized single-blind and crossover design. Mechanical work was calculated and arterial O2 saturation (SpO2) was estimated via pulse oximetry during every sprint. Cerebral and muscle oxy- (O2Hb) and deoxy-hemoglobin (HHb) were monitored continuously by near-infrared spectroscopy.

Results:

Compared with NM, work decrement was accentuated (P = 0.01) in HY for both men (–16.4 ± 10.3%) and women (–16.8 ± 9.0%). This was associated with lower SpO2 and lower cerebral Δ[O2Hb] in both sexes (–13.6 ± 7.5%, P = .008, and –134.5 ± 73.8%, P = .003, respectively). These HY-induced changes were nearly identical in these men and women matched for initial-sprint work. Muscle Δ[HHb] increased 9-fold (P = .009) and 5-fold (P = .02) in men and women, respectively, and plateaued. This muscle deoxygenation was not exacerbated in HY.

Conclusions:

Results indicate that men and women matched for initial-sprint work experience similar levels of fatigue and systemic, cerebral, and peripheral adjustments during RSE performed in NM and HY. These data suggest that cerebral deoxygenation imposes a limitation to repeated-sprint performance.

The use of muscle near-infrared spectroscopy in sport, health and medical sciences: recent developments

Near-infrared spectroscopy (NIRS) has been shown to be one of the tools that can measure oxygenation in muscle and other tissues in vivo. This review paper highlights the progress, specifically in this decade, that has been made for evaluating skeletal muscle oxygenation and oxidative energy metabolism in sport, health and clinical sciences. Development of NIRS technologies has focused on improving quantification of the signal using multiple wavelengths to solve for absorption and scattering coefficients, multiple pathlengths to correct for the influence of superficial skin and fat, and time-resolved and phase-modulated light sources to determine optical pathlengths. In addition, advances in optical imaging with multiple source and detector pairs as well as portability using small wireless detectors have expanded the usefulness of the devices. NIRS measurements have provided information on oxidative metabolism in various athletes during localized exercise and whole-body exercise, as well as training-induced adaptations. Furthermore, NIRS technology has been used in the study of a number of chronic health conditions. Future developments of NIRS technology will include enhancing signal quantification. In addition, advances in NIRS imaging and portability promise to transform how measurements of oxygen utilization are obtained in the future.

What triggers the aerobic dive limit? Patterns of muscle oxygen depletion during dives of emperor penguins

The physiological basis of the aerobic dive limit (ADL), the dive duration associated with the onset of post-dive blood lactate elevation, is hypothesized to be depletion of the muscle oxygen (O(2)) store. A dual wavelength near-infrared spectrophotometer was developed and used to measure myoglobin (Mb) O(2) saturation levels in the locomotory muscle during dives of emperor penguins (Aptenodytes forsteri). Two distinct patterns of muscle O(2) depletion were observed. Type A dives had a monotonic decline, and, in dives near the ADL, the muscle O(2) store was almost completely depleted. This pattern of Mb desaturation was consistent with lack of muscle blood flow and supports the hypothesis that the onset of post-dive blood lactate accumulation is secondary to muscle O(2) depletion during dives. The mean type A Mb desaturation rate allowed for calculation of a mean muscle O(2) consumption of 12.4 ml O(2) kg(-1) muscle min(-1), based on a Mb concentration of 6.4 g 100 g(-1) muscle. Type B desaturation patterns demonstrated a more gradual decline, often reaching a mid-dive plateau in Mb desaturation. This mid-dive plateau suggests maintenance of some muscle perfusion during these dives. At the end of type B dives, Mb desaturation rate increased and, in dives beyond the ADL, Mb saturation often reached near 0%. Thus, although different physiological strategies may be used during emperor penguin diving, both Mb desaturation patterns support the hypothesis that the onset of post-dive lactate accumulation is secondary to muscle O(2) store depletion.

Comparisons of local and systemic aerobic fitness parameters between finswimmers with different athlete grade levels

To study the relationship between the local and systemic aerobic fitness parameters, and between the muscle oxygenation and aerobic performance, 16 female finswimmers were recruited and divided into high-level (HL) group and low-level group. Cardiorespiratory function, blood lactate concentration and near infrared spectroscopy muscle oxygenation in the vastus lateralis (VL) were monitored simultaneously during a maximal incremental exercise. We found that the break point (Bp) of the oxygenation index (OI) in the VL (BpVL) had significant correlations with lactate threshold (LT) and gas exchange threshold (GET), and the appearance sequence of the three thresholds was BpVL ≈ LT ≤ GET. When considering different levels, the [Formula: see text] at BpVL, LT and GET were higher in the HL group. During intensive exercise, there were significantly faster [Formula: see text] increase and evidently slower OI decrease in the HL group, suggesting that faster [Formula: see text] increase in the HL group slowed down the muscle deoxygenation and facilitated subjects to cycle to higher workloads. In conclusion, multi-modality approaches combining local and systemic physiological monitoring technologies might provide better explanations of the relationship between local and systemic aerobic fitness parameters, and might be a novel way to analyze the difference between groups of different levels.

Myoglobin's old and new clothes: from molecular structure to function in living cells

Myoglobin, a mobile carrier of oxygen, is without a doubt an important player central to the physiological function of heart and skeletal muscle. Recently, researchers have surmounted technical challenges to measure Mb diffusion in the living cell. Their observations have stimulated a discussion about the relative contribution made by Mb-facilitated diffusion to the total oxygen flux. The calculation of the relative contribution, however, depends upon assumptions, the cell model and cell architecture, cell bioenergetics, oxygen supply and demand. The analysis suggests that important differences can be observed whether steady-state or transient conditions are considered. This article reviews the current evidence underlying the evaluation of the biophysical parameters of myoglobin-facilitated oxygen diffusion in cells, specifically the intracellular concentration of myoglobin, the intracellular diffusion coefficient of myoglobin and the intracellular myoglobin oxygen saturation. The review considers the role of myoglobin in oxygen transport in vertebrate heart and skeletal muscle, in the diving seal during apnea as well as the role of the analogous leghemoglobin of plants. The possible role of myoglobin in intracellular fatty acid transport is addressed. Finally, the recent measurements of myoglobin diffusion inside muscle cells are discussed in terms of their implications for cytoarchitecture and microviscosity in these cells and the identification of intracellular impediments to the diffusion of proteins inside cells. The recent experimental data then help to refine our understanding of Mb function and establish a basis for future investigation.

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

Absolute quantification of phosphorus metabolite concentrations in human muscle in vivo by 31P MRS: a quantitative review

31P MRS offers a unique view of muscle metabolism in vivo, but correct quantification is important. Inter-study correlation of estimates of [Pi] and [phosphocreatine (PCr)] in a number of published studies suggest that the main technical problem in calibrated 31P MRS studies is the measurement of PCr and Pi signal intensities, rather than absolute quantification of [ATP]. For comparison, we discuss the few published biopsy studies of calf muscle and a selection of the many studies of quadriceps muscle. The ATP concentration is close to the value that we obtained in calf muscle in our own study, presented here, on four healthy subjects, by localised 31P MRS using a surface coil incorporating an internal reference and calibrated using an external phantom. However, the freeze-clamp biopsy PCr concentration is approximately 20% lower than the value obtained by 31P MRS, consistent with PCr breakdown by creatine kinase during freezing. Finally, we illustrate some consequences of uncertainty in resting [PCr] for analysis of mitochondrial function from PCr kinetics using a published 31P MRS study of exercise and recovery: the lower the assumed resting [PCr], the lower the absolute rate of oxidative ATP synthesis estimated from the PCr resynthesis rate; in addition, the lower the assumed resting [PCr], or the higher the assumed [total creatine], the higher the apparent resting [ADP], and therefore the more sigmoid the relationship between the rate of oxidative ATP synthesis and [ADP]. Correct quantification of resting metabolite concentrations is crucially important for this sort of analysis. Our own results ([PCr] = 33 +/- 2 mM, [Pi] = 4.5 +/- 0.2 mM, and [ATP] = 8.2 +/- 0.4 mM; mean +/- SEM) are close to the overall mean values of the 10 published studies on calf muscle by 'calibrated' 31P MRS (as in the present work), and of [PCr] and [Pi] in a representative selection of 'uncalibrated' 31P MRS studies (i.e. from measured PCr/ATP and Pi/ATP ratios, assuming a literature value for [ATP]).

A brief review of the use of near infrared spectroscopy with particular interest in resistance exercise

There is growing interest in resistance training, but many aspects related to this type of exercise are still not fully understood. Performance varies substantially depending on how resistance training variables are manipulated. Fatigue is a complex phenomenon usually attributed to central (neuronal) and/or peripheral (muscular) origin. Cerebral oxygenation may be associated with the decision to stop exercise, and muscle oxygenation may be related to resistance training responses. Near infrared spectroscopy (NIRS) is a non-invasive optical technique used to monitor cerebral and muscle oxygenation levels. The purpose of this review is to briefly describe the NIRS technique, validation and reliability, and its application in resistance exercise. NIRS-measured oxygenation in cerebral tissue has been validated against magnetic resonance imaging during motor tasks. In muscle tissue, NIRS-measured oxygenation was shown to be highly related to venous oxygen saturation and muscle oxidative rate was closely related to phosphocreatine resynthesis, measured by (31)P-magnetic resonance spectroscopy after exercise. The test-retest reliability of cerebral and muscle NIRS measurements have been established under a variety of experimental conditions, including static and dynamic exercise. Although NIRS has been used extensively to evaluate muscle oxygenation levels during aerobic exercise, only four studies have used this technique to examine these changes during typical resistance training exercises. Muscle oxygenation was influenced by different resistance exercise protocols depending on the load or duration of exercise, the number of sets and the muscle being monitored. NIRS is a promising, non-invasive technique that can be used to evaluate cerebral and muscle oxygenation levels simultaneously during exercise, thereby improving our understanding of the mechanisms influencing performance and fatigue.

Regional Differences of Hemodynamics and Oxygenation in the Human Calf Muscle Detected with Near-Infrared Spectrophotometry

PURPOSE

Measurements in muscle tissue are often performed at a selected single location over the muscle of interest. The hypothesis is that the values obtained reflect the status within the entire muscle or muscle group. This, however, may not be the case. The study was performed to investigate whether this hypothesis is true for hemodynamics and oxygenation in the healthy human calf muscle at rest.

MATERIALS AND METHODS

Hemoglobin flow, blood flow, oxygen consumption, and venous hemoglobin oxygen saturation were mapped at 22 locations in 30 legs of 15 healthy subjects (nine women, six men aged 26-37 years) simultaneously by using frequency-domain near-infrared spectrophotometry with a specially designed probe during venous occlusion.

RESULTS

For all parameters, spatial heterogeneity was found between subjects and within individual legs. All parameters were highly significantly different when comparing proximal and distal regions. Differences were also found between medial and lateral regions. The global mean values (+/-standard deviation) over all measurements were as follows: hemoglobin flow, 1.27 micromol per 100 mL/min +/- 0.88; blood flow, 0.56 mL per 100 g/min +/- 0.38; oxygen consumption, 0.016 mL per 100 g/min +/- 0.011; and venous oxygen saturation, 77.6% +/- 5.9. The thickness of the overlying adipose tissue had an influence on the measurements and must be considered.

CONCLUSION

Highly significant spatial heterogeneity of hemodynamics and oxygenation was found in the healthy human calf muscle.

Effects of Passive Leg Movement on the Oxygenation Level of Lower Limb Muscle in Chronic Stroke Patients

Objective. To evaluate the effects of passive leg movements on the muscle oxygenation level and electromyographic (EMG) activity in the lower limbs in chronic stroke patients. Methods. With a gait training apparatus, passive movements were imposed on the lower limbs of 15 chronic stroke patients at a frequency of 0.8 Hz for 10 minutes. During the passive leg movements, muscle oxygenation level and muscular EMG activity of the paretic and nonparetic calf muscles were assessed. Results. The passive leg movements caused increases in the EMG activity and muscle oxygenation level in both paretic and nonparetic lower limbs. Although a significant difference was found in the concentration changes of the oxygenated hemoglobin (Oxy-Hb), both paretic and nonparetic sides of the muscle showed enhancement of the tissue oxygenation level (TOI). The degree of the changes of the Oxy-Hb depended on the level of motor recovery after stroke; subjects with good motor recovery showed less difference in the Oxy-Hb level between the paretic and nonparetic sides of the muscle. Conclusion. Passive leg movements have the capacity to induce muscular activity and enhance oxygen metabolism, even in the paretic lower limb muscle of chronic stroke patients. This type of exercise might be a useful and efficient method for the prevention of metabolic deterioration in the lower limb paretic muscles of chronic stroke patients.

Muscle oxygenation and EMG activity during isometric exercise in children

The aim of this study was to examine the time-course of and the relationships between muscle oxygenation, blood volume and myoelectrical manifestations during isometric exercise in children. Twelve healthy children aged 12.5 +/- 1.2 years (mean +/- s) performed an isometric knee extension at 50% of their maximal voluntary contraction (MVC) until exhaustion to assess endurance time (limit time, T(lim)). Changes in muscle oxygenation and blood volume were assessed by near infrared spectroscopy (NIRS). The root mean square (RMS) amplitude and the mean power frequency (MPF) from electromyogram (EMG) signals were obtained, as NIRS parameters, from the vastus lateralis. Mean T(lim) was 117 +/- 34 s. The muscle oxygenation and blood volume curves decreased immediately at the beginning of exercise. Maximal deoxygenation occurred at 50% T(lim), and fell by 76.9% from the resting value. Similarly, minimal blood volume was observed at 50% T(lim); it reached a plateau that lasted until the end of exercise. The kinetics of the MPF and RMS curves were inversely related to time. At the last set of exercises (after 75% T(lim)), a steeper RMS curve and an abrupt decrease in the MPF curve were observed. Significant correlations (r) between muscle oxygenation, blood volume, root mean square amplitude and mean power frequency were observed, which ranged from 0.72 to 0.99. These findings suggest that the fatigue resulting from sustained isometric exercise is related to a decrease in oxygenation and blood volume.

Skeletal muscle energetics with PNMR: personal views and historic perspectives

This article reviews historical and current NMR approaches to describing in vivo bioenergetics of skeletal muscles in normal and diseased populations. It draws upon the first author's more than 70 years of personal experience in enzyme kinetics and the last author's physiological approaches. The development of in vivo PNMR jointly with researchers around the world is described. It is explained how non-invasive PNMR has advanced human exercise biochemistry, physiology and pathology. Further, after a brief explanation of bioenergetics with PNMR on creatine kinase, anerobic glycolysis and mitochondrial oxidative phosphorylation, some basic and controversial subjects are focused upon, and the authors' view of the subjects are offered, with questions and answers. Some of the research has been introduced in exercise physiology. Future directions of NMR on bioenergetics, as a part of system biological approaches, are indicated.

A novel method to measure regional muscle blood flow continuously using NIRS kinetics information

BACKGROUND

This article introduces a novel method to continuously monitor regional muscle blood flow by using Near Infrared Spectroscopy (NIRS). We demonstrate the feasibility of the new method in two ways: (1) by applying this new method of determining blood flow to experimental NIRS data during exercise and ischemia; and, (2) by simulating muscle oxygenation and blood flow values using these newly developed equations during recovery from exercise and ischemia.

METHODS

Deoxy (Hb) and oxyhemoglobin (HbO2), located in the blood of the skeletal muscle, carry two internal relationships between blood flow and oxygen consumption. One is a mass transfer principle and the other describes a relationship between oxygen consumption and Hb kinetics in a two-compartment model. To monitor blood flow continuously, we transfer these two relationships into two equations and calculate the blood flow with the differential information of HbO2 and Hb. In addition, these equations are used to simulate the relationship between blood flow and reoxygenation kinetics after cuff ischemia and a light exercise. Nine healthy subjects volunteered for the cuff ischemia, light arm exercise and arm exercise with cuff ischemia for the experimental study.

RESULTS

Analysis of experimental data of both cuff ischemia and light exercise using the new equations show greater blood flow (four to six times more than resting values) during recovery, agreeing with previous findings. Further, the simulation and experimental studies of cuff ischemia and light exercise agree with each other.

CONCLUSION

We demonstrate the accuracy of this new method by showing that the blood flow obtained from the method agrees with previous data as well as with simulated data. We conclude that this novel continuous blood flow monitoring method can provide blood flow information non-invasively with NIRS.

High-energy phosphate metabolism in the calf muscle during moderate isotonic exercise under different degrees of cuff compression: A phosphorus 31 magnetic resonance spectroscopy study

BACKGROUND

The purpose of this study was to investigate phosphocreatine (PCr) and inorganic phosphate levels as well as pH changes in exercising muscle at a workload of 4.5 W under progressive cuff stenoses, whereby the flow reduction due to cuff compression was quantified by flow-sensitive magnetic resonance imaging.

METHODS

By using a whole-body 1.5-T magnetic resonance scanner and an exercise bench, serial phosphorus 31 (31P) magnetic resonance spectroscopy with a time resolution of 30 seconds was performed in 10 healthy men. Percentage changes in PCr, inorganic phosphate (Pi), and pH were statistically evaluated in comparison with baseline. The exercise protocol was characterized by a constant workload level of 4.5 W. Ischemic conditions were achieved by a cuff that was placed at the upper leg. Consecutively, increments of 0, 60, 90, 120, and 150 mm Hg were applied. Each increment lasted for 3 minutes. The following rest period was 10 minutes.

RESULTS

Blood flow increased significantly immediately after the onset of muscle exercise. No significant changes in blood flow were detected as long as the air pressure of the pneumatic cuff was 60 to 90 mm Hg. Significant reductions in blood flow were observed immediately after inflation of the cuff to 120 and 150 mm Hg. PCr passed into a steady state during the first increment with 0 mm Hg and showed no substantial changes during the increment with 60, 90, and 120 mm Hg. PCr hydrolysis seemed progressive during the 150-mm Hg increment. Pi passed into a plateau level at the onset of exercise and increased significantly at the increment of 150 mm Hg. The pH turned into a steady state with no significant changes during the increments up to 120 mm Hg. At 150 mm Hg, pH decreased progressively. PCr levels at the end of the 150-mm Hg increment correlated significantly and moderately with the reduction in blood flow.

CONCLUSIONS

Our study shows that the ischemic condition during constant muscle exercise is clearly characterized by PCr and Pi kinetics, as well as by pH changes. The correlation between the degree of blood flow reduction and PCr levels in the exercising muscle groups, which are supplied by the stenosed arteries, is the first essential of using 31P magnetic resonance spectroscopy in the assessment of the effect of arterial stenoses on muscle function in claudicants.

Muscle Oxygenation of the Paralyzed Lower Limb in Spinal Cord???Injured Persons

PURPOSE

Even in the paralyzed lower limb muscle, EMG activity can be induced by imposing passive leg movement in standing posture in persons with spinal cord injury (SCI). The purpose of the present study was to ascertain whether the oxygenation level of the paralyzed lower limb muscle covaried with the muscle EMG activity during imposed passive leg movement.

METHODS

Six motor-complete SCI subjects and four neurologically normal controls were placed on a gait-training apparatus that enabled the SCI subjects to stand and move their legs passively. After a 1-min resting stage, consecutive passive alternate leg movements were performed at different frequencies (0.8, 1, 1.2, and 1 Hz, for 3 min at each stage). To obtain postexercise data, subjects were kept in a standing posture for 5 min after passive movement ceased. The EMG activity and concentration changes in the oxygenated (oxy-) and deoxygenated hemoglobin (Hb) (deoxy-Hb) were continuously measured using near-infrared spectroscopy (NIRS) from the gastrocnemius muscle.

RESULTS

In all SCI subjects, muscle EMG activity was observed during passive leg movement. The oxy-Hb level gradually increased, whereas the deoxy-Hb decreased, and these changes were independent of the total Hb changes. In the recovery stage, the total Hb level was found to exceed the preexercise level. In contrast to the SCI patients, the normal subjects showed neither EMG activity nor changes in oxy- or deoxy-Hb.

CONCLUSION

The present results demonstrate that passive leg movement can induce not only muscular activity but also alteration of muscle oxygenation level in the paralyzed lower leg. Particularly, induced muscular activity seems to correlate with increased perfusion of the muscle.

Muscle oxygenation trends after tapering in trained cyclists

BACKGROUND: This study examined muscle deoxygenation trends before and after a 7-day taper using non-invasive near infrared spectroscopy (NIRS). METHODS: Eleven cyclists performed an incremental cycle ergometer test to determine maximal oxygen consumption (VO2max = 4.68 +/- 0.57 L.min-1) prior to the study, and then completed two or three high intensity (85-90% VO2max) taper protocols after being randomly assigned to a taper group: T30 (n = 5), T50 (n = 5), or T80 (n = 5) [30%, 50%, 80% reduction in training volume, respectively]. Physiological measurements were recorded during a simulated 20 km time trials (20TT) performed on a set of wind-loaded rollers. RESULTS AND DISCUSSION: The results showed that the physiological variables of oxygen consumption (VO2), carbon dioxide (VCO2) and heart rate (HR) were not significantly different after tapering, except for a decreased ventilatory equivalent for oxygen (VE/VO2) in T50 (p Collapse

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Affiliation(s)

J Patrick Neary Faculty of Kinesiology, University of New Brunswick, Fredericton, New Brunswick, Canada
Donald C McKenzie Faculty of Human Kinetics, Allan McGavin Sports Medicine Centre, University of British Columbia, Vancouver, British Columbia, Canada
Yagesh N Bhambhani Faculty of Rehabilitation Medicine, Department of Occupational Therapy, University of Alberta, Edmonton, Alberta, Canada

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