Near-infrared assessments of skeletal muscle oxidative capacity in persons with spinal cord injury

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.

Association of skeletal muscle oxidative capacity with muscle function, sarcopenia-related exercise performance, and intramuscular adipose tissue in older adults

Muscle function and exercise performance measures, such as muscle endurance capacity, maximal strength, chair stand score, gait speed, and Timed Up and Go score, are evaluated to diagnose sarcopenia and frailty in older individuals. Furthermore, intramuscular adipose tissue (IntraMAT) content increases with age. Skeletal muscle oxidative capacity determines muscle metabolism and maintains muscle performance. This study aimed to investigate the association of skeletal muscle oxidative capacity with muscle function, exercise performance, and IntraMAT content in older individuals. Thirteen older men and women participated in this study. Skeletal muscle oxidative capacity was assessed by the recovery speed of muscle oxygen saturation after exercise using near-infrared spectroscopy from the medial gastrocnemius. We assessed two muscle functions, peak torque and time to task failure, and four sarcopenia-related exercise performances: handgrip strength, gait speed, 30-s chair stand, and Timed Up and Go. The IntraMAT content was measured using axial magnetic resonance imaging. The results showed a relationship between skeletal muscle oxidative capacity and gait speed but not with muscle functions and other exercise performance measures. Skeletal muscle oxidative capacity was not related to IntraMAT content. Skeletal muscle oxidative capacity, which may be indicative of the capacity of muscle energy production in the mitochondria, is related to locomotive functions but not to other functional parameters or skeletal fat infiltration.

The Clinical Relevance of Autonomic Dysfunction, Cerebral Hemodynamics, and Sleep Interactions in Individuals Living With SCI

A myriad of physiological impairments is seen in individuals after a spinal cord injury (SCI). These include altered autonomic function, cerebral hemodynamics, and sleep. These physiological systems are interconnected and likely insidiously interact leading to secondary complications. These impairments negatively influence quality of life. A comprehensive review of these systems, and their interplay, may improve clinical treatment and the rehabilitation plan of individuals living with SCI. Thus, these physiological measures should receive more clinical consideration. This special communication introduces the under investigated autonomic dysfunction, cerebral hemodynamics, and sleep disorders in people with SCI to stakeholders involved in SCI rehabilitation. We also discuss the linkage between autonomic dysfunction, cerebral hemodynamics, and sleep disorders and some secondary outcomes are discussed. Recent evidence is synthesized to make clinical recommendations on the assessment and potential management of important autonomic, cerebral hemodynamics, and sleep-related dysfunction in people with SCI. Finally, a few recommendations for clinicians and researchers are provided.

Assessing the efficacy of duration and intensity prescription for physical activity in mitigating cardiometabolic risk after spinal cord injury

PURPOSE OF REVIEW

Spinal cord injury (SCI) heightens susceptibility to cardiometabolic risk (CMR), predisposing individuals to cardiovascular disease. This monograph aims to assess the optimal duration and intensity of physical activity (PA) for managing CMR factors, particularly obesity, after SCI and provide modality-specific PA durations for optimal energy expenditure.

RECENT FINDINGS

PA guidelines recommend at least 150 min/week of moderate-intensity activity. However, non-SCI literature supports the effectiveness of engaging in vigorous-intensity PA (≥6 METs) and dedicating 250-300 min/week (≈2000 kcal/week) to reduce CMR factors. Engaging in this volume of PA has shown a dose-response relationship, wherein increased activity results in decreased obesity and other CMR factors in persons without SCI.

SUMMARY

To optimize cardiometabolic health, individuals with SCI require a longer duration and higher intensity of PA to achieve energy expenditures comparable to individuals without SCI. Therefore, individuals with SCI who can engage in or approach vigorous-intensity PA should prioritize doing so for at least 150 min/wk. At the same time, those unable to reach such intensities should engage in at least 250-300 min/week of PA at a challenging yet comfortable intensity, aiming to achieve an optimal intensity level based on their abilities. Given the potential to decrease CMR after SCI, increasing PA duration and intensity merits careful consideration in future SCI PA directives.

Muscle Oxidative Capacity in Vivo Is Associated With Physiological Parameters in Trained Rowers

Purpose: The muscle oxygen uptake (m V ˙ O 2 ) kinetics following exercise, measured by near-infrared spectroscopy, has been used as a functional evaluation of muscle oxidative metabolism. This study aimed to determine the m V ˙ O 2 off-kinetics and verify the relationship of the recovery rate of m V ˙ O 2 (k) with time-trial performance and different aerobic parameters in trained rowers. Methods: Eleven male rowers (age: 20 ± 3 years; V ˙ O 2 m a x : 4.28 ± 0.35 L·min-1) used a rowing ergometer to perform (I) an incremental test to determine the maximal oxygen uptake (V ˙ O 2 m a x ) and peak power output (Ppeak); (II) several visits to determine maximal lactate steady state (MLSS); and (III) a 2000-m rowing ergometer performance test. Also, one test to determine m V ˙ O 2 off-kinetics of the vastus lateralis muscle using a repeated arterial occlusions protocol. Results: The m V ˙ O 2 generated a good monoexponential fit (R2 = 0.960 ± 0.030; SEE = 0.041 ± 0.018%.s-1). The k of m V ˙ O 2 (2.06 ± 0.58 min-1) was associated with relative V ˙ O 2 m a x (r = 0.79), power output at MLSS (r = 0.76), and Ppeak (r = 0.83); however, it was not related with 2000-m rowing performance (r = -0.38 to 0.52; p > .152). Conclusion: These findings suggest that although not associated with rowing performance, the m V ˙ O 2 off-kinetics determined after a submaximal isometric knee extension may be a practical and less-exhaustive approach than invasive responses and incremental tests to assess the muscle oxidative metabolism during a training program.

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.

SS-31 does not prevent or reduce muscle atrophy 7 days after a 65 kdyne contusion spinal cord injury in young male mice

Spinal cord injury (SCI) leads to major reductions in function, independent living, and quality of life. Disuse and paralysis from SCI leads to rapid muscle atrophy, with chronic muscle loss likely playing a role in the development of the secondary metabolic disorders often seen in those with SCI. Muscle disuse is associated with mitochondrial dysfunction. Previous evidence has suggested targeting the mitochondria with the tetrapeptide SS-31 is beneficial for muscle health in preclinical models that lead to mitochondrial dysfunction, such as cast immobilization or burn injury. We gave young male mice a sham (n = 8) or 65 kdyne thoracic contusion SCI with (n = 9) or without (n = 9) daily administration of 5.0 mg/kg SS-31. Hindlimb muscle mass and muscle bundle respiration were measured at 7 days post-SCI and molecular targets were investigated using immunoblotting, RT-qPCR, and metabolomics. SS-31 did not preserve body mass or hindlimb muscle mass 7 days post-SCI. SS-31 had no effect on soleus or plantaris muscle bundle respiration. SCI was associated with elevated levels of protein carbonylation, led to reduced protein expression of activated DRP1 and reductions in markers of mitochondrial fusion. SS-31 administration did result in reduced total DRP1 expression, as well as greater expression of inhibited DRP1. Gene expression of proinflammatory cytokines and their receptors were largely stable across groups, although SS-31 treatment led to greater mRNA expression of IL1B, TNF, and TNFRSF12A. In summation, SS-31 was not an efficacious treatment acutely after a moderate thoracic contusion SCI in young male mice.

Assessment of mitochondrial respiratory capacity using minimally invasive and noninvasive techniques in persons with spinal cord injury

Purpose

Muscle biopsies are the gold standard to assess mitochondrial respiration; however, biopsies are not always a feasible approach in persons with spinal cord injury (SCI). Peripheral blood mononuclear cells (PBMCs) and near-infrared spectroscopy (NIRS) may alternatively be predictive of mitochondrial respiration. The purpose of the study was to evaluate whether mitochondrial respiration of PBMCs and NIRS are predictive of respiration of permeabilized muscle fibers after SCI.

Methods

Twenty-two individuals with chronic complete and incomplete motor SCI between 18–65 years old were recruited to participate in the current trial. Using high-resolution respirometry, mitochondrial respiratory capacity was measured for PBMCs and muscle fibers of the vastus lateralis oxidizing complex I, II, and IV substrates. NIRS was used to assess mitochondrial capacity of the vastus lateralis with serial cuff occlusions and electrical stimulation.

Results

Positive relationships were observed between PBMC and permeabilized muscle fibers for mitochondrial complex IV (r = 0.86, P < 0.0001). Bland-Altman displayed agreement for complex IV (MD = 0.18, LOA = -0.86 to 1.21), between PBMCs and permeabilized muscles fibers. No significant relationships were observed between NIRS mitochondrial capacity and respiration in permeabilized muscle fibers.

Conclusions

This is the first study to explore and support the agreement of less invasive clinical techniques for assessing mitochondrial respiratory capacity in individuals with SCI. The findings will assist in the application of PBMCs as a viable alternative for assessing mitochondrial health in persons with SCI in future clinical studies.

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.

Neuromuscular electrical stimulation resistance training enhances oxygen uptake and ventilatory efficiency independent of mitochondrial complexes after spinal cord injury: a randomized clinical trial

The purpose of the study was to determine whether neuromuscular electrical stimulation resistance training (NMES-RT)-evoked muscle hypertrophy is accompanied by increased V̇o₂ peak, ventilatory efficiency, and mitochondrial respiration in individuals with chronic spinal cord injury (SCI). Thirty-three men and women with chronic, predominantly traumatic SCI were randomized to either NMES-RT (n = 20) or passive movement training (PMT; n = 13). Functional electrical stimulation-lower extremity cycling (FES-LEC) was used to test the leg V̇o₂ peak, V̇E/V̇co₂ ratio, and substrate utilization pre- and postintervention. Magnetic resonance imaging was used to measure muscle cross-sectional area (CSA). Finally, muscle biopsy was performed to measure mitochondrial complexes and respiration. The NMES-RT group showed a significant increase in postintervention V̇o₂ peak compared with baseline (ΔV̇o₂ = 14%, P < 0.01) with no changes in the PMT group (ΔV̇o₂ = 1.6%, P = 0.47). Similarly, thigh (ΔCSA_thigh = 19%) and knee extensor (ΔCSA_knee = 30.4%, P < 0.01) CSAs increased following NMES-RT but not after PMT. The changes in thigh and knee extensor muscle CSAs were positively related with the change in V̇o₂ peak. Neither NMES-RT nor PMT changed mitochondrial complex tissue levels; however, changes in peak V̇o₂ were related to complex I. In conclusion, in persons with SCI, NMES-RT-induced skeletal muscle hypertrophy was accompanied by increased peak V̇o₂ consumption which may partially be explained by enhanced activity of mitochondrial complex I.NEW & NOTEWORTHY Leg oxygen uptake (V̇o₂) and ventilatory efficiency (V̇E/V̇co₂ ratio) were measured during functional electrical stimulation cycling testing following 12-16 wk of either electrically evoked resistance training or passive movement training, and the respiration of mitochondrial complexes. Resistance training increased thigh muscle area and leg V̇o₂ peak but decreased V̇E/V̇co₂ ratio without changes in mitochondrial complex levels. Leg V̇o₂ peak was associated with muscle hypertrophy and mitochondrial respiration of complex I following training.

Bilateral NIRS measurements of muscle mitochondrial capacity: Feasibility and repeatability

Background

Non‐invasive determination of mitochondrial capacity via near infrared spectroscopy (NIRS) typically involves voluntary exercise of a single muscle group followed by as many as 26 brief ischemic cuff occlusions to determine a single recovery rate constant (k).

Purpose

To determine the within‐ and between‐visit repeatability of a shortened bilateral NIRS protocol, and to establish the feasibility of hamstring k measurements.

Methods

Sixteen young (eight women, eight men; 22 ± 3 years) active adults underwent a bilateral electrical stimulation protocol in which multiple (n = 4) measurements of k for the vastus lateralis (VL) and medial hamstring (MH) muscles were determined on two visits. Repeatability (CV% and intraclass correlations, ICC) and equivalency across visits were assessed for both muscles.

Results

Mean k values in the VL were consistent with published values and within‐visit ICCs were moderately high for both muscles in both sexes. In men, average k values on visit 2 were within 1% (VL muscle) and 5% (MH muscle) of the values on visit 1 (all p > 0.78). In women, average k values were 10%–15% lower on visit 2 (p = 0.01 and p = 0.15 for MH and VL) with the largest between‐visit differences in a subset of participants with the most days between visits.

Conclusions

This bilateral NIRS protocol is time efficient and provides valid estimates of k in both sexes and muscle groups with acceptable within‐visit repeatability. Lower than expected between‐visit repeatability in some participants reinforces the need for further investigation of this newly developed protocol to identify and control for experimental and behavioral sources of variation.

Role of exercise on visceral adiposity after spinal cord injury: a cardiometabolic risk factor

PURPOSE

Visceral adipose tissue (VAT) is associated with cardiometabolic disease risk in able-bodied (AB) populations. However, the underlying mechanisms of VAT-induced disease risk are unknown in persons with spinal cord injury (SCI). Potential mechanisms of VAT-induced cardiometabolic dysfunction in persons with SCI include systemic inflammation, liver adiposity, mitochondrial dysfunction, and anabolic deficiency. Moreover, how exercise interventions impact these mechanisms associated with VAT-induced cardiometabolic dysfunction are still being explored.

METHODS

A search for relevant scientific literature about the effects of exercise on VAT and cardiometabolic health was conducted on the PubMed database. Literature from reference lists was also included when appropriate.

RESULTS

Both aerobic and resistance exercise training beneficially impact health and VAT mass via improving mitochondrial function, glucose effectiveness, and inflammatory signaling in SCI and AB populations. Specifically, aerobic exercise appears to also modulate cellular senescence in AB populations and animal models, while resistance exercise seems to augment anabolic signaling in persons with SCI.

CONCLUSIONS

The current evidence supports regular engagement in exercise to reduce VAT mass and the adverse effects on cardiometabolic health in persons with SCI. Future research is needed to further elucidate the precise mechanisms by which VAT negatively impacts health following SCI. This will likely facilitate the development of rehabilitation protocols that target VAT reduction in persons with SCI.

Methodological considerations for near-infrared spectroscopy to assess mitochondrial capacity after spinal cord injury

Background: Skeletal muscle mitochondrial activity is reduced by ∼ 50-60% after SCI, resulting in impaired energy expenditure, glucose utilization and insulin sensitivity. Near infra-red spectroscopy (NIRS) is a non-invasive tool that can be used to assess mitochondrial capacity. Objectives: (1) Highlight methodological limitations impacting data acquisition and analysis such as subcutaneous adipose tissue (SAT) thickness, movement artifacts, inadequate muscle stimulation, light interference, and ischemic discomfort. (2) Provide technical considerations to improve data acquisition and analysis. This may serve as guidance to other researchers and clinicians using NIRS. Study Design: cross-sectional observational design. Settings: Clinical research medical center. Participants: Sixteen men with 1 > year post motor complete SCI. Methods: NIRS signals were obtained from right vastus lateralis muscle utilizing a portable system. Signals were fit to a mono-exponential curve. Outcome Measures: Rate constant and r² values for the fit curve, indirectly measures mitochondrial capacity. Results: Only four participants produced data with accepted rate constants of 0.002-0.013 s^-1 and r² of 0.71-0.87. Applications of studentized residuals ≥2.5 resulted in sparing data from another four participants with rate constants of 0.010-0.018 s^-1and r² values ranging from 0.86-0.99. Conclusions: Several limitations may challenge the use of NIRS to assess mitochondrial capacity after SCI. Acknowledging these limitations and applying additional data processing techniques may overcome the discussed limitations and facilitate data sparing.

Tissue oxygenation in peripheral muscles and functional capacity in cystic fibrosis: a cross-sectional study

NEW FINDINGS

What is the central question of this study? How do peripheral muscle tissue oxygenation and physical conditioning levels of children and adolescents with cystic fibrosis compare to demographically matched controls? What is the main finding and its importance? Children and adolescents with cystic fibrosis consumed more oxygen, more quickly and exhibited slower recovery, demonstrating that there may have been deficiencies in oxygen supply related to both oxygen uptake and oxygen transport.

ABSTRACT

Cystic fibrosis affects skeletal muscle performance and functional capacity. However, it is currently unclear how peripheral muscle behaviour is affected, especially in children and adolescents. To examine this, we compared tissue oxygenation of children and adolescents with cystic fibrosis against healthy volunteers. We also evaluated the functional capacity of participants via the modified shuttle test (MST) and assessed for associations between performance and near-infrared spectroscopy. A total of 124 participants enrolled. Participants were divided into either the cystic fibrosis group (CFG) or the healthy group (HG). Statistical comparisons between groups were evaluated with the Mann-Whitney U test and associations with functional capacity were evaluated using Spearman's correlation coefficient. CFG volunteers scored lower on the MST compared to the HG. They walked shorter distances (P = 0.001) with less efficiency because they performed the tests with a less efficient walking economy (P = 0.001) and a greater deoxyhaemoglobin concentration (P = 0.001). Further, they experienced reduced tissue oxygen saturation (P = 0.037) faster than the HG. As a result, they presented lower respiratory (P = 0.001) and lower heart (P = 0.001) rate values at the end of the MST, with a longer post-test heart rate recovery time (P = 0.005). There was a significant association between deoxygenation time and functional capacity. The CFG consumed more oxygen, more quickly, with a slower recovery, reflecting impairments in the dynamics of muscle oxygen extraction. The results suggest differences in functional capacity and haemodynamic recovery in children and adolescents with cystic fibrosis.

Association between muscle aerobic capacity and whole-body peak oxygen uptake

PURPOSE

Decline in skeletal muscle mitochondrial oxidative capacity (MOC) is associated with reduced aerobic capacity and increased risk of cardiovascular and metabolic disease. Measuring skeletal muscle MOC may be an alternative method to assess aerobic capacity, especially for individuals unable to perform a whole-body maximum oxygen uptake protocol. In this study, linear regression analysis in two leg muscles was performed to determine whether MOC values could be used to predict whole-body peak oxygen uptake.

METHODS

MOC was measured with near infrared spectroscopy (NIRS) in the medial gastrocnemius (MG) and vastus lateralis (VL) muscles of 26 participants (age, 27.1 ± 5.8 years old). Whole-body peak oxygen uptake (VO₂ peak) was determined by indirect calorimetry during a continuous ramp protocol on a cycle ergometer.

RESULTS

VO₂ peak values were significantly correlated with the muscle recovery rate constant (k) of the MG (kMG, r = 0.59; p < 0.01) and VL (kVL, r = 0.63; p < 0.01) muscles. Summing recovery rate constants of both muscles together (kMG + kVL) improved the strength of the correlation with VO₂ peak (r = 0.78; p < 0.0001) and could explain a majority of the variance (R² = 0.61) between the two measurements.

CONCLUSION

Data suggest that NIRS can provide reliable MOC measurements on two leg muscles that correlate well with whole-body peak oxygen uptake.

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.

Sixteen weeks of testosterone with or without evoked resistance training on protein expression, fiber hypertrophy and mitochondrial health after spinal cord injury

We investigated the effects of testosterone replacement therapy (TRT) with and without evoked resistance training (RT) on protein expression of key metabolic and hypertrophy regulators, muscle fiber cross-sectional area (CSA), and markers of mitochondrial health after spinal cord injury (SCI). Twenty-two men with chronic motor complete SCI were randomly assigned to either TRT + RT (n = 11) or TRT (n = 11) for 16 wk. TRT + RT men underwent twice weekly progressive RT using electrical stimulation with ankle weights. TRT was administered via testosterone patches (2-6 mg/day). Muscle biopsies were obtained before and after 16 wk from the right vastus lateralis. Expression of proteins associated with oxidative muscles and mechanical loading (PGC-1α and FAK), muscle hypertrophy (total and phosphorylated Akt, total and phosphorylated mTOR), and cellular metabolism (total and phosphorylated AMPK and GLUT4) were evaluated. Immunohistochemistry analysis was performed to measure fiber CSA and succinate dehydrogenase (SDH) activity as well as mitochondrial citrate synthase (CS) activity and complex III (CIII) activities. TRT + RT demonstrated a robust 27.5% increase in average fiber CSA compared with a -9% decrease following TRT only (P = 0.01). GLUT4 protein expression was elevated in the TRT + RT group compared with TRT only (P = 0.005). Total Akt (P = 0.06) and phosphorylated Akt Ser³⁸⁹ (P = 0.049) were also elevated in the TRT + RT group. Mitochondrial activity of SDH (P = 0.03) and CS (P = 0.006) increased in the TRT + RT group, with no changes in the TRT-only group. Sixteen weeks of TRT with RT resulted in fiber hypertrophy and beneficial changes in markers of skeletal muscle health and function.NEW & NOTEWORTHY Fiber cross-sectional area (CSA), protein expression, mitochondrial citrate synthase (CS), and succinate dehydrogenase (SDH) were measured following 16 wk of low-dose testosterone replacement therapy (TRT) with and without electrically evoked resistance training (RT) in men with spinal cord injury (SCI). Fiber CSA and protein expression of total GLUT4, total Akt, and phosphorylated Akt increased following TRT + RT but not in the TRT-only group. Mitochondrial CS and SDH increased after TRT + RT but not in TRT-only group.

EMG median frequency shifts without change in muscle oxygenation following novel locomotor training in individuals with incomplete spinal cord injury

Objectives: To examine the effect of muscle fiber recruitment patterns on muscle oxygen utilization during treadmill walking in a group of individuals who have incomplete spinal cord injury.Methods: 5 participants with motor incomplete spinal cord injury (Age; 42.2 ± 18.8 years, Male; n = 4) completed an over ground locomotor training program. Muscle utilization/oxygenation and activation of the medial gastrocnemius were measured by near infrared spectroscopy and surface electromyography pre- and post-over ground locomotor training during two separate treadmill walking bouts at self-selected speeds. Outcomes were changes in deoxygenation hemoglobin/myoglobin concentrations, and the change in median power of the power spectrum of the electromyography after training.Results: A significant increase in median power of the power spectrum of the electromyography signal was observed during both bouts of treadmill walking, 6-minute walking bout and longer fatiguing bout (49% p = 0.047 and 48% p = 0.035, respectively) post-over ground locomotor training. There was no significant change in muscle utilization/oxygenation post-over ground locomotor training. There was no significant effect of median power of the power spectrum on deoxygenation hemoglobin/myoglobin during either of the walking bouts.Conclusions: The main finding of the current study was that median power of the power spectrum significantly increased following 12 weeks of over ground locomotor training, with no significant change in deoxygenation hemoglobin/myoglobin. The recruitment of more and/or larger motor units was seen in conjunction with no changes in muscle oxygen utilization for the same walking task.Implications for RehabilitationThe reduction of skeletal muscle innervation in Spinal Cord Injury may adversely affect the orderly recruitment of motor units, which could in turn blunt the oxidative metabolic response during physical activity.Over-ground locomotor could be a useful tool in the rehabilitative process following an incomplete spinal cord injury.

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.

Skeletal muscle V̇o2 kinetics by the NIRS repeated occlusions method during the recovery from cycle ergometer exercise

Near-infrared spectroscopy (NIRS) has been utilized as a noninvasive method to evaluate skeletal muscle mitochondrial function in humans, by calculating muscle V̇o2 (V̇o2 m) recovery (off-) kinetics following short light-intensity plantar flexion exercise. The aim of the present study was to determine V̇o2 m off- kinetics following standard cycle ergometer exercise of different intensities. Fifteen young physically active healthy men performed an incremental exercise (INCR) up to exhaustion and two repetitions of constant work-rate (CWR) exercises at 80% of gas exchange threshold (GET; MODERATE) and at 40% of the difference between GET and peak pulmonary V̇o2 (V̇o2 p; HEAVY). V̇o2 p and vastus lateralis muscle fractional O2 extraction by NIRS (Δ[deoxy(Hb+Mb)]) were recorded continuously. Transient arterial occlusions were carried out at rest and during the recovery for V̇o2 m calculation. All subjects tolerated the repeated occlusions protocol without problems. The quality of the monoexponential fitting for V̇o2 m off-kinetics analysis was excellent (0.93≤ r2≤0.99). According to interclass correlation coefficient, the test-retest reliability was moderate to good. V̇o2 m values at the onset of recovery were ~27, ~38, and ~35 times higher (in MODERATE, HEAVY, and INCR, respectively) than at rest. The time constants (τ) of V̇o2 m off-kinetics were lower ( P < 0.001) following MODERATE (29.1 ± 6.8 s) vs. HEAVY (40.8 ± 10.9) or INCR (42.9 ± 10.9), suggesting an exercise intensity dependency of V̇o2 m off-kinetics. Only following MODERATE the V̇o2 m off-kinetics were faster than the V̇o2 p off-kinetics. V̇o2 m off-kinetics, determined noninvasively by the NIRS repeated occlusions technique, can be utilized as a functional evaluation tool of skeletal muscle oxidative metabolism also following conventional cycle ergometer exercise.

NEW & NOTEWORTHY This is the first study in which muscle V̇o2 recovery kinetics, determined noninvasively by near-infrared spectroscopy (NIRS) by utilizing the repeated occlusions method, was applied following standard cycle ergometer exercise of different intensities. The results demonstrate that muscle V̇o2 recovery kinetics, determined noninvasively by the NIRS repeated occlusions technique, can be utilized as a functional evaluation tool of skeletal muscle oxidative metabolism also following conventional cycle ergometer exercise, overcoming significant limitations associated with the traditionally proposed protocol.

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.

MUSCLE MITOCHONDRIAL CAPACITY AND ENDURANCE IN ADULTS WITH TYPE 1 DIABETES

The impact of type 1 diabetes (T1D) on muscle endurance and oxidative capacity is currently unknown.

Purpose

Measure muscle endurance and oxidative capacity of adults with T1D compared to controls.

Methods

A cross-sectional study design with a control group was used. Subjects (19-37 years old) with T1D (n=17) and controls (n=17) were assessed with hemoglobin A1c (HbA1c) and casual glucose. Muscle endurance was measured with an accelerometer at stimulation frequencies of 2, 4, and 6 Hz for a total of nine minutes. Mitochondrial capacity was measured using near-infrared spectroscopy after exercise as the rate constant of the rate of recovery of oxygen consumption.

Results

T1D and control groups were similar in age, sex, height, and race. The T1D group had slightly higher BMI values and adipose tissue thickness over the forearm muscles. Casual glucose was 150±70 mg/dL for T1D and 98±16 mg/dL for controls (P=0.006). HbA1c of T1D subjects was 7.1±0.9% and 5.0±0.4% for controls (P<0.01). Endurance indexes at 2, 4, and 6 Hz were 94.5±5.2%, 81.8±8.4%, and 68.6±13.5% for T1D and 94.6±4.1%, 85.9±6.3%, and 68.7±15.4% for controls (p = 0.97, 0.12, 0.99, respectively). There were no differences between groups in mitochondrial capacity (T1D= 1.9±0.5 min^-1 and control=1.8±0.4 min^-1, P=0.29) or reperfusion rate (T1D= 8.8±2.8s and control=10.3±3.0s, P=0.88). There were no significant correlations between HbA1c and either muscle endurance, mitochondrial capacity or reperfusion rate.

Conclusions

Adults with T1D did not have reduced oxidative capacity, muscle endurance or muscle reperfusion rates compared to controls. HbA1c also did not correlate with muscle endurance, mitochondrial capacity or reperfusion rates. Future studies should extend these measurements to older people or people with poorly-controlled T1D.

Ischemic preconditioning has no effect on maximal arm cycling exercise in women

PURPOSE

We investigated the effect of ischemic preconditioning (IPC) on performance of a 3 min maximal effort arm ergometer test in young women.

METHODS

Twenty healthy women (23.1 (SD 3.3) years) performed a 3 min maximal effort arm cycling exercise, preceded by IPC on both arms or SHAM in a counterbalanced randomized crossover design. Both blood flow (via high resolution ultrasound; n = 17) and muscle oxygenation/deoxygenation (via near infrared spectroscopy; n = 5) were measured throughout the IPC/SHAM. Performance and perceptual/physiological (i.e., heart rate, blood lactate, rating of perceived exertion, and triceps brachialis oxygenation) parameters were recorded during the exercise test.

RESULTS

Occlusion during IPC completely blocked brachial artery blood flow, decreased oxygenated hemoglobin/myoglobin (Δ[oxy(Hb + Mb)]), and increased deoxygenated Hb/Mb (Δ[deoxy(Hb + Mb)]). There were no differences (P > 0.797) in performance (peak, mean, and end power output) or in any perceptual/physiological variables during the 3 min all-out test between IPC/SHAM. During exercise, Δ[oxy(Hb + Mb)] initially decreased with no differences (P ≥ 0.296) between conditions and returned towards baseline by the completion of the test while Δ[deoxy(Hb + Mb)] increased with no differences between conditions and remained elevated until completion of the test (P ≥ 0.755).

CONCLUSIONS

We verified the successful application of IPC via blood flow and NIRS measures but found no effects on performance of a 3 min maximal effort arm cranking test in young women.

Electrically evoked wrist extensor muscle fatigue throughout repetitive motion as measured by mechanomyography and near-infrared spectroscopy

Repetitive electrically-evoked muscle contraction leads to accelerated muscle fatigue. This study assessed electrically-evoked fatiguing muscle with changes to mechanomyography root mean square percentage (%RMS-MMG) and tissue saturation index (%TSI) in extensor carpi radialis. Forty healthy volunteers (n=40) performed repetitive electrical-evoked wrist extension to fatigue and results were analyzed pre- and post-fatigue, i.e. 50% power output (%PO) drop. Responses of %PO, %TSI and %RMS-MMG were correlated while the relationships between %RMS-MMG and %TSI were investigated using linear regression. The %TSI for both groups were negatively correlated with declining %PO as the ability of the muscle to take up oxygen became limited due to fatigued muscle. The %RMS-MMG behaved in two different patterns post-fatigue against declining %PO whereby; (i) group A showed positive correlation (%RMS-MMG decreased) throughout the session and (ii) group B demonstrated negative correlation (%RMS-MMG increased) with declining %PO until the end of the session. Regression analysis showed %TSI was inversely proportional to %RMS-MMG during post-fatigue in group A. Small gradients in both groups suggested that %TSI was not sensitive to the changes in %RMS-MMG and they were mutually exclusive. Most correlation and regression changed significantly post-fatigue indicating that after fatigue, the condition of muscle had changed mechanically and physiologically.

Key Glycolytic Metabolites in Paralyzed Skeletal Muscle Are Altered Seven Days after Spinal Cord Injury in Mice

Spinal cord injury (SCI) results in rapid muscle atrophy and an oxidative-to-glycolytic fiber-type shift. Those with chronic SCI are more at risk for developing insulin resistance and reductions in glucose clearance than able-bodied individuals, but how glucose metabolism is affected after SCI is not well known. An untargeted metabolomics approach was utilized to investigate changes in whole-muscle metabolites at an acute (7-day) and subacute (28-day) time frame after a complete T9 spinal cord transection in 20-week-old female C57BL/6 mice. Two hundred one metabolites were detected in all samples, and 83 had BinBase IDs. A principal components analysis showed the 7-day group as a unique cluster. Further, 36 metabolites were altered after 7- and/or 28-day post-SCI (p values <0.05), with 12 passing further false discovery rate exclusion criteria; of those 12 metabolites, three important glycolytic molecules-glucose and downstream metabolites pyruvic acid and lactic acid-were reduced at 7 days compared to those values in sham and/or 28-day animals. These changes were associated with altered expression of proteins associated with glycolysis, as well as monocarboxylate transporter 4 gene expression. Taken together, our data suggest an acute disruption of skeletal muscle glucose uptake at 7 days post-SCI, which leads to reduced pyruvate and lactate levels. These levels recover by 28 days post-SCI, but a reduction in pyruvate dehydrogenase protein expression at 28 days post-SCI implies disruption in downstream oxidation of glucose.

Muscle oxygenation during hybrid arm and functional electrical stimulation-evoked leg cycling after spinal cord injury

This study compared muscle oxygenation (StO2) during arm cranking (ACE), functional electrical stimulation-evoked leg cycling (FES-LCE), and hybrid (ACE+FES-LCE) exercise in spinal cord injury individuals. Eight subjects with C7-T12 lesions performed exercises at 3 submaximal intensities. StO2 was measured during rest and exercise at 40%, 60%, and 80% of subjects' oxygen uptake (VO2) peak using near-infrared spectroscopy. StO2 of ACE showed a decrease whereas in ACE+FES-LCE, the arm muscles demonstrated increasing StO2 from rest in all of VO2) peak respectively. StO2 of FES-LCE displayed a decrease at 40% VO2 peak and steady increase for 60% and 80%, whereas ACE+FES-LCE revealed a steady increase from rest at all VO2 peak. ACE+FES-LCE elicited greater StO2 in both limbs which suggested that during this exercise, upper- and lower-limb muscles have higher blood flow and improved oxygenation compared to ACE or FES-LCE performed alone.

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.

Enhanced Local Skeletal Muscle Oxidative Capacity and Microvascular Blood Flow Following 7-Day Ischemic Preconditioning in Healthy Humans

Ischemic preconditioning (IPC), which involves intermittent periods of ischemia followed by reperfusion, is an effective clinical intervention that reduces the risk of myocardial injury and confers ischemic tolerance to skeletal muscle. Repeated bouts of IPC have been shown to stimulate long-term changes vascular function, however, it is unclear what metabolic adaptations may occur locally in the muscle. Therefore, we investigated 7 days of bilateral lower limb IPC (4 × 5 min) above limb occlusion pressure (220 mmHg; n = 10), or sham (20 mmHg; n = 10), on local muscle oxidative capacity and microvascular blood flow. Oxidative capacity was measured using near-infrared spectroscopy (NIRS) during repeated short duration arterial occlusions (300 mmHg). Microvascular blood flow was assessed during the recovery from submaximal isometric plantar flexion exercises at 40 and 60% of maximal voluntary contraction (MVC). Following the intervention period, beyond the late phase of protection (72 h), muscle oxidative recovery kinetics were speeded by 13% (rate constant pre 2.89 ± 0.47 min^-1 vs. post 3.32 ± 0.69 min^-1; P < 0.05) and resting muscle oxygen consumption (mO₂) was reduced by 16.4% (pre 0.39 ± 0.16%.s^-1 vs. post 0.33 ± 0.14%.s^-1; P < 0.05). During exercise, changes in deoxygenated hemoglobin (HHb) from rest to steady state were reduced at 40 and 60% MVC (16 and 12%, respectively, P < 0.05) despite similar measures of total hemoglobin (tHb). At the cessation of exercise, the time constant for recovery in oxygenated hemoglobin (O₂Hb) was accelerated at 40 and 60% MVC (by 33 and 43%, respectively) suggesting enhanced reoxygenation in the muscle. No changes were reported for systemic measures of resting heart rate or blood pressure. In conclusion, repeated bouts of IPC over 7 consecutive days increased skeletal muscle oxidative capacity and microvascular muscle blood flow. These findings are consistent with enhanced mitochondrial and vascular function following repeated IPC and may be of clinical or sporting interest to enhance or offset reductions in muscle oxidative capacity.

Near-infrared spectroscopy detects age-related differences in skeletal muscle oxidative function: promising implications for geroscience

Age is the greatest risk factor for chronic disease and is associated with a marked decline in functional capacity and quality of life. A key factor contributing to loss of function in older adults is the decline in skeletal muscle function. While the exact mechanism(s) remains incompletely understood, age-related mitochondrial dysfunction is thought to play a major role. To explore this question further, we studied 15 independently living seniors (age: 72 ± 5 years; m/f: 4/11; BMI: 27.6 ± 5.9) and 17 young volunteers (age: 25 ± 4 years; m/f: 8/9; BMI: 24.0 ± 3.3). Skeletal muscle oxidative function was measured in forearm muscle from the recovery kinetics of muscle oxygen consumption using near-infrared spectroscopy (NIRS). Muscle oxygen consumption was calculated as the slope of change in hemoglobin saturation during a series of rapid, supra-systolic arterial cuff occlusions following a brief bout of exercise. Aging was associated with a significant prolongation of the time constant of oxidative recovery following exercise (51.8 ± 5.4 sec vs. 37.1 ± 2.1 sec, P = 0.04, old vs. young, respectively). This finding suggests an overall reduction in mitochondrial function with age in nonlocomotor skeletal muscle. That these data were obtained using NIRS holds great promise in gerontology for quantitative assessment of skeletal muscle oxidative function at the bed side or clinic.

Metabolic inflexibility in individuals with obesity assessed by near-infrared spectroscopy

OBJECTIVE

To non-invasively evaluate differences in oxidative metabolism in individuals with obesity compared to normal weight using the near-infrared spectroscopy and vascular occlusion technique during hyperglycaemia.

METHODS

In all, 16 normal-weight individuals (body mass index: 21.3 ± 1.7 kg/m²) and 13 individuals with obesity (body mass index: 34.4 ± 2.0 kg/m²) had five vascular occlusion tests (pre, 30, 60, 90 and 120 min after glucose ingestion). Oxygen utilization was estimated from the area under the curve of the deoxyhemoglobin [HHb] signal during occlusion. Muscle reperfusion was derived from the area above the curve after cuff release.

RESULTS

The deoxyhemoglobin area under the curve during occlusion of the normal-weight individuals increased from 15,732 ± 2344 (% . s) at pre to 18,930 ± 3226 (% . s) ( p < 0.05) at 90 min after glucose ingestion. The deoxyhemoglobin area under the curve during occlusion decreased significantly from 14,695 ± 3341 (% . s) at pre to 11,273 ± 1825 (% . s) ( p < 0.05) and 11,360 ± 1750 (% . s) ( p < 0.05) at 30 and 60 min, respectively, after glucose ingestion. The area above the curve of deoxyhemoglobin during reperfusion decreased significantly from 6450 ± 765 (% . s) at pre to 4830 ± 963 (% . s) ( p < 0.05) at 60 min and to 4210 ± 595 (% . s) ( p < 0.01) at 90 min in normal-weight individuals after glucose ingestion, with no changes observed in individuals with obesity.

CONCLUSION

This study confirmed in vivo and non-invasively the metabolic inflexibility of skeletal muscle in individuals with obesity during hyperglycaemia.

In Vivo Assessment of Mitochondrial Dysfunction in Clinical Populations Using Near-Infrared Spectroscopy

The ability to sustain submaximal exercise is largely dependent on the oxidative capacity of mitochondria within skeletal muscle, and impairments in oxidative metabolism have been implicated in many neurologic and cardiovascular pathologies. Here we review studies which have demonstrated the utility of Near-infrared spectroscopy (NIRS) as a method of evaluating of skeletal muscle mitochondrial dysfunction in clinical human populations. NIRS has been previously used to noninvasively measure tissue oxygen saturation, but recent studies have demonstrated the utility of NIRS as a method of evaluating skeletal muscle oxidative capacity using post-exercise recovery kinetics of oxygen metabolism. In comparison to historical methods of measuring muscle metabolic dysfunction in vivo, NIRS provides a more versatile and economical method of evaluating mitochondrial oxidative capacity in humans. These advantages generate great potential for the clinical applicability of NIRS as a means of evaluating muscle dysfunction in clinical populations.

Fatigability, oxygen uptake kinetics and muscle deoxygenation in incomplete spinal cord injury during treadmill walking

PURPOSE

The purpose of the present study was to characterize hypothesized relationships among fatigability and cardiorespiratory fitness in individuals with chronic motor-incomplete SCI (iSCI) during treadmill walking. The theoretical framework was that exacerbated fatigability would occur concomitantly with diminished cardiorespiratory fitness in people with iSCI.

METHODS

Subjects with iSCI (n = 8) and an able-bodied reference group (REF) (n = 8) completed a 6-min walking bout followed by a walking bout of 30-min or until volitional exhaustion, both at a self-selected walking speed. Fatigability was assessed using both perceived fatigability and performance fatigability measures. Pulmonary oxygen uptake kinetics (VO₂ on-kinetics) was measured breath-by-breath and changes in deoxygenated hemoglobin/myoglobin concentration (∆[HHb]) of the lateral gastrocnemius was measured by near-infrared spectroscopy. Adjustment of VO₂ and ∆[HHb] on-kinetics were modeled using a mono-exponential equation.

RESULTS

Perceived fatigability and performance fatigability were 52% and 44% greater in the iSCI group compared to the REF group (p = 0.003 and p = 0.004). Phase II time constant (τp) of VO₂ on-kinetics and ∆[HHb] ½ time during resting arterial occlusion were 55.4% and 16.3% slower in iSCI vs REF (p < 0.01 and p = 0.047, respectively).

CONCLUSIONS

The results of the present study may suggest that compromised O₂ delivery and/or utilization may have contributed to the severity of fatigability in these individuals with iSCI. The understanding of the extent to which fatigability and VO₂ and Δ[HHb] on-kinetics impacts locomotion after iSCI will assist in the future development of targeted interventions to enhance function.

Effects of Testosterone and Evoked Resistance Exercise after Spinal Cord Injury (TEREX-SCI): study protocol for a randomised controlled trial

INTRODUCTION

Individuals with spinal cord injury (SCI) are at a lifelong risk of obesity and chronic metabolic disorders including insulin resistance and dyslipidemia. Within a few weeks of injury, there is a significant decline in whole body fat-free mass, particularly lower extremity skeletal muscle mass, and subsequent increase in fat mass (FM). This is accompanied by a decrease in anabolic hormones including testosterone. Testosterone replacement therapy (TRT) has been shown to increase skeletal muscle mass and improve metabolic profile. Additionally, resistance training (RT) has been shown to increase lean mass and reduce metabolic disturbances in SCI and other clinical populations.

METHODS AND ANALYSIS

26 individuals with chronic, motor complete SCI between 18 and 50 years old were randomly assigned to a RT+TRT group (n=13) or a TRT group (n=13). 22 participants completed the initial 16-week training phase of the study and 4 participants withdrew. 12 participants of the 22 completed 16 weeks of detraining. The TRT was provided via transdermal testosterone patches (4-6 mg/day). The RT+TRT group had 16 weeks of supervised unilateral progressive RT using surface neuromuscular electrical stimulation with ankle weights. This study will investigate the effects of evoked RT+TRT or TRT alone on body composition (muscle cross-sectional area, visceral adipose tissue, %FM) and metabolic profile (glucose and lipid metabolism) in individuals with motor complete SCI. Findings from this study may help in designing exercise therapies to alleviate the deterioration in body composition after SCI and decrease the incidence of metabolic disorders in this clinical population.

ETHICS AND DISSEMINATION

The study is currently approved by the McGuire VA Medical Center and Virginia Commonwealth University. All participants read and signed approved consent forms. Results will be submitted to peer-reviewed journals and presented at national and international conferences.

TRIAL REGISTRATION NUMBER

Pre-result, NCT01652040.

Mitochondrial mass and activity as a function of body composition in individuals with spinal cord injury

Spinal cord injury (SCI) is accompanied by deterioration in body composition and severe muscle atrophy. These changes put individuals at risk for insulin resistance, type II diabetes, and cardiovascular disease. To determine the relationships between skeletal muscle mitochondrial mass, activity, and body composition, 22 men with motor complete SCI were studied. Body composition assessment was performed using dual-energy X-ray absorptiometry and magnetic resonance imaging. Skeletal muscle biopsies were obtained from the vastus lateralis muscle to measure citrate synthase (CS) and complex III (CIII) activity. CS activity was inversely related to %body fat (r = -0.57, P = 0.013), %leg fat (r = -0.52, P = 0.027), %trunk fat (r = -0.54, P = 0.020), and %android fat (r = -0.54, P = 0.017). CIII activity was negatively related to %body fat (r = -0.58, P = 0.022) and %leg fat (r = -0.54, P = 0.037). Increased visceral adipose tissue was associated with decreased CS and CIII activity (r = -0.66, P = 0.004; r = -0.60, P = 0.022). Thigh intramuscular fat was also inversely related to both CS and CIII activity (r = -0.56, P = 0.026; r = -0.60, P = 0.024). Conversely, lean mass (r = 0.75, P = 0.0003; r = 0.65, P = 0.008) and thigh muscle cross-sectional area (CSA; r = 0.82, P = 0.0001; r = 0.84; P = 0.0001) were positively related to mitochondrial parameters. When normalized to thigh muscle CSA, many body composition measurements remained related to CS and CIII activity, suggesting that %fat and lean mass may predict mitochondrial mass and activity independent of muscle size. Finally, individuals with SCI over age 40 had decreased CS and CIII activity (P = 0.009; P = 0.004), suggesting a decrease in mitochondrial health with advanced age. Collectively, these findings suggest that an increase in adipose tissue and decrease in lean mass results in decreased skeletal muscle mitochondrial activity in individuals with chronic SCI.

Endurance neuromuscular electrical stimulation training improves skeletal muscle oxidative capacity in individuals with motor-complete spinal cord injury

INTRODUCTION

Spinal cord injury (SCI) results in skeletal muscle atrophy, increases in intramuscular fat, and reductions in skeletal muscle oxidative capacity. Endurance training elicited with neuromuscular electrical stimulation (NMES) may reverse these changes and lead to improvement in muscle metabolic health.

METHODS

Fourteen participants with complete SCI performed 16 weeks of home-based endurance NMES training of knee extensor muscles. Skeletal muscle oxidative capacity, muscle composition, and blood metabolic and lipid profiles were assessed pre- and post-training.

RESULTS

There was an increase in number of contractions performed throughout the duration of training. The average improvement in skeletal muscle oxidative capacity was 119%, ranging from -14% to 387% (P = 0.019). There were no changes in muscle composition or blood metabolic and lipid profiles.

CONCLUSION

Endurance training improved skeletal muscle oxidative capacity, but endurance NMES of knee extensor muscles did not change blood metabolic and lipid profiles. Muscle Nerve 55: 669-675, 2017.

Skeletal muscle mitochondrial capacity in people with multiple sclerosis

Background

People with multiple sclerosis (MS) have functional disability and may have reduced muscle mitochondrial capacity.

Objective

The objective of this paper is to measure muscle mitochondrial capacity of leg muscles using near-infrared spectroscopy (NIRS) and compare to functional status.

Materials and methods

People with MS (n = 16) and a control (CON) group (n = 9) were evaluated for 25-ft walk time. Mitochondrial capacity of both gastrocnemius muscles were measured with NIRS as the rate of recovery of oxygen consumption in after exercise.

Results

Mitochondrial capacity was lower in the MS group compared to the CON group (rate constants: 1.13 ± 0.29 vs. 1.68 ± 0.37 min⁻¹, p < 0.05). There was a tendency for people with MS who used assistive devices to have lower mitochondrial capacity in the weaker leg (p = 0.07).

Conclusion

NIRS measurements of mitochondrial capacity suggest a 40% deficit in people with MS compared to CONs and this may contribute to walking disability.

Skeletal muscle mitochondrial health and spinal cord injury

Mitochondria are the main source of cellular energy production and are dynamic organelles that undergo biogenesis, remodeling, and degradation. Mitochondrial dysfunction is observed in a number of disease states including acute and chronic central or peripheral nervous system injury by traumatic brain injury, spinal cord injury (SCI), and neurodegenerative disease as well as in metabolic disturbances such as insulin resistance, type II diabetes and obesity. Mitochondrial dysfunction is most commonly observed in high energy requiring tissues like the brain and skeletal muscle. In persons with chronic SCI, changes to skeletal muscle may include remarkable atrophy and conversion of muscle fiber type from oxidative to fast glycolytic, combined with increased infiltration of intramuscular adipose tissue. These changes contribute to a proinflammatory environment, glucose intolerance and insulin resistance. The loss of metabolically active muscle combined with inactivity predisposes individuals with SCI to type II diabetes and obesity. The contribution of skeletal muscle mitochondrial density and electron transport chain activity to the development of the aforementioned comorbidities following SCI is unclear. A better understanding of the mechanisms involved in skeletal muscle mitochondrial dynamics is imperative to designing and testing effective treatments for this growing population. The current editorial will review ways to study mitochondrial function and the importance of improving skeletal muscle mitochondrial health in clinical populations with a special focus on chronic SCI.

Reproducibility of NIRS assessment of muscle oxidative capacity in smokers with and without COPD

Low muscle oxidative capacity contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). Near-infrared spectroscopy (NIRS) allows non-invasive determination of the muscle oxygen consumption (mV̇O₂) recovery rate constant (k), which is proportional to oxidative capacity assuming two conditions are met: 1) exercise intensity is sufficient to fully-activate mitochondrial oxidative enzymes; 2) sufficient O₂ availability. We aimed to determine reproducibility (coefficient of variation, CV; intraclass correlation coefficient, ICC) of NIRS k assessment in the gastrocnemius of 64 participants with (FEV₁ 64±23%predicted) or without COPD (FEV₁ 98±14%predicted). 10-15s dynamic contractions preceded 6min of intermittent arterial occlusions (5-10s each, ∼250mmHg) for k measurement. k was lower (P<0.05) in COPD (1.43±0.4min^-1; CV=9.8±5.9%, ICC=0.88) than controls (1.74±0.69min^-1; CV=9.9±8.4%; ICC=0.93). Poor k reproducibility was more common when post-contraction mV̇O₂ and deoxygenation were low, suggesting insufficient exercise intensity for mitochondrial activation and/or the NIRS signal contained little light reflected from active muscle. The NIRS assessment was well tolerated and reproducible for muscle dysfunction evaluation in COPD.

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

It is now clear that mitochondria are involved as either a cause or consequence of many chronic diseases. This central role of the mitochondria is due to their position in the cell as important integrators of cellular energetics and signaling. Mitochondrial function affects many aspects of the cellular environment such as redox homeostasis and calcium signaling, which then also exert control over mitochondrial function. This complex dynamic between mitochondrial function and the cellular environment highlights the value of examining mitochondria in vivo in the intact physiological environment. This review discusses NMR and optical approaches used to measure mitochondria ATP and oxygen fluxes that provide in vivo measures of mitochondrial capacity and quality in animal and human models. Combining these in vivo measurements with more traditional ex vivo analyses can lead to new insights into the importance of the cellular environment in controlling mitochondrial function under pathological conditions. Interpretation and underlying assumptions for each technique are discussed with the goal of providing an overview of some of the most common approaches used to measure in vivo mitochondrial function encountered in the literature.

Skeletal muscle oxidative capacity in patients with cystic fibrosis

NEW FINDINGS

What is the central question of this study? Do patients with cystic fibrosis have reduced skeletal muscle oxidative capacity, measured with near-infrared spectroscopy, compared with demographically matched control subjects? What is the main finding and is its importance? Patients with cystic fibrosis have impairments in skeletal muscle oxidative capacity. This reduced skeletal muscle oxidative capacity not only appears to be accelerated by age, but it may also contribute to exercise intolerance in patients with cystic fibrosis. Exercise intolerance predicts mortality in patients with cystic fibrosis (CF); however, the mechanisms have yet to be elucidated fully. Using near-infrared spectroscopy, in this study we compared skeletal muscle oxidative capacity in patients with CF versus healthy control subjects. Thirteen patients and 16 demographically matched control subjects participated in this study. Near-infrared spectroscopy was used to measure the recovery rate of oxygen consumption ( mus V̇O2max) of the vastus lateralis muscle after 15 s of electrical stimulation (4 Hz) and subsequent repeated transient arterial occlusions. The mus V̇O2max was reduced in patients with CF (1.82 ± 0.4 min(-1) ) compared with control subjects (2.13 ± 0.5 min(-1) , P = 0.04). A significant inverse relationship between age and mus V̇O2max was observed in patients with CF (r = -0.676, P = 0.011) but not in control subjects (r = -0.291, P = 0.274). Patients with CF exhibit a reduction in skeletal muscle oxidative capacity compared with control subjects. It appears that the reduced skeletal muscle oxidative capacity is accelerated by age and could probably contribute to exercise intolerance in patients with CF.

Pilot study: evaluation of the effect of functional electrical stimulation cycling on muscle metabolism in nonambulatory people with multiple sclerosis

OBJECTIVE

To investigate the changes in muscle oxygen consumption (mV˙O2) using near-infrared spectroscopy (NIRS) after 4 weeks of training with functional electrical stimulation (FES) cycling in nonambulatory people with multiple sclerosis (MS).

DESIGN

Four-week before-after trial to assess changes in mV˙O2 after an FES cycling intervention.

SETTING

Rehabilitation hospital.

PARTICIPANTS

People (N=8; 7 men, 1 women) from a volunteer/referred sample with moderate to severe MS (Expanded Disability Status Scale score>6.0).

INTERVENTION

Participants cycled 30 minutes per session, 3d/wk for 4 weeks or a total of 12 sessions.

MAIN OUTCOME MEASURES

mV˙O2 of the right vastus lateralis muscle was measured with NIRS before and within 1 week after the intervention. Six bouts of 15-second electrical stimulation increasing from 2 to 7Hz were used to activate the muscle. mV˙O2 was assessed by analyzing the slope of the NIRS oxygen signal during a 10-second arterial occlusion after each electrical stimulation bout.

RESULTS

Significant FES training by electrical stimulation frequency level interaction was observed (P=.031), with an average increase in mV˙O2 of 47% across frequencies with a main effect of training (P=.047).

CONCLUSIONS

FES cycling for 4 weeks improved mV˙O2, suggesting that FES cycling is a potential therapy for improving muscle health in people with MS who are nonambulatory.

Assessment of in vivo skeletal muscle mitochondrial respiratory capacity in humans by near-infrared spectroscopy: a comparison with in situ measurements

The present study aimed to compare in vivo measurements of skeletal muscle mitochondrial respiratory capacity made using near-infrared spectroscopy (NIRS) with the current gold standard, namely in situ measurements of high-resolution respirometry performed in permeabilized muscle fibres prepared from muscle biopsies. Mitochondrial respiratory capacity was determined in 21 healthy adults in vivo using NIRS to measure the recovery kinetics of muscle oxygen consumption following a ∼15 s isometric contraction of the vastus lateralis muscle. Maximal ADP-stimulated (State 3) respiration was measured in permeabilized muscle fibres using high-resolution respirometry with sequential titrations of saturating concentrations of metabolic substrates. Overall, the in vivo and in situ measurements were strongly correlated (Pearson's r = 0.61-0.74, all P < 0.01). Bland-Altman plots also showed good agreement with no indication of bias. The results indicate that in vivo NIRS corresponds well with the current gold standard, in situ high-resolution respirometry, for assessing mitochondrial respiratory capacity.

In vivo (31)P NMR spectroscopy assessment of skeletal muscle bioenergetics after spinal cord contusion in rats

PURPOSE

Muscle paralysis after spinal cord injury leads to muscle atrophy, enhanced muscle fatigue, and increased energy demands for functional activities. Phosphorus magnetic resonance spectroscopy ((31)P-MRS) offers a unique non-invasive alternative of measuring energy metabolism in skeletal muscle and is especially suitable for longitudinal investigations. We determined the impact of spinal cord contusion on in vivo muscle bioenergetics of the rat hind limb muscle using (31)P-MRS.

METHODS

A moderate spinal cord contusion injury (cSCI) was induced at the T8-T10 thoracic spinal segments. (31)P-MRS measurements were performed weekly in the rat hind limb muscles for 3 weeks. Spectra were acquired in a Bruker 11 T/470 MHz spectrometer using a 31P surface coil. The sciatic nerve was electrically stimulated by subcutaneous needle electrodes. Spectra were acquired at rest (5 min), during stimulation (6 min), and recovery (20 min). Phosphocreatine (PCr) depletion rates and the pseudo first-order rate constant for PCr recovery (k PCr) were determined. The maximal rate of PCr resynthesis, the in vivo maximum oxidative capacity (V max) and oxidative adenosine triphosphate (ATP) synthesis rate (Q max) were subsequently calculated.

RESULTS

One week after cSCI, there was a decline in the resting total creatine of the paralyzed muscle. There was a significant reduction (~24 %) in k PCr measures of the paralyzed muscle, maximum in vivo mitochondrial capacity (V max) and the maximum oxidative ATP synthesis rate (Q max) at 1 week post-cSCI. During exercise, the PCr depletion rates in the paralyzed muscle one week after injury were rapid and to a greater extent than in a healthy muscle.

CONCLUSIONS

Using in vivo MRS assessments, we reveal an acute oxidative metabolic defect in the paralyzed hind limb muscle. These altered muscle bioenergetics might contribute to the host of motor dysfunctions seen after cSCI.

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.

Case report: endurance electrical stimulation training improves skeletal muscle oxidative capacity in chronic spinal cord injury

OBJECTIVE

To describe the use of a novel neuromuscular electrical stimulation (NMES) endurance exercise protocol and its effects on skeletal muscle oxidative capacity.

DESIGN

Case report, pre/post intervention.

SETTING

University-based trial.

PARTICIPANT

A 39-year-old man who suffered a motor complete spinal cord injury (C5-6, ASIA Impairment Scale grade A).

INTERVENTION

Twenty-four weeks of endurance NMES that consisted of progressive increases in the twitch frequency, duration of sessions, and sessions per week.

MAIN OUTCOME MEASURE

Mitochondrial capacity was measured, in vivo, as the rate of recovery of muscle oxygen consumption using near-infrared spectroscopy.

RESULTS

The rate of recovery of muscle oxygen consumption increased approximately 3-fold from 0.52 to 1.43, 1.46, and 1.40/min measured on 3 separate occasions during week 12 of training, and 1.57/min after 24 weeks of NMES endurance training.

CONCLUSION

The findings of this study suggest that NMES endurance training using twitches can increase mitochondrial capacity to comparable levels measured in nonparalyzed muscles of sedentary able-bodied controls.