Exercise over-stress and maximal muscle oxidative metabolism: a 31P magnetic resonance spectroscopy case report

Objective: ³¹P magnetic resonance spectroscopy (MRS) was used to document long lasting losses in muscle oxidative capacity after bouts of intense endurance exercise.

Methods: The subject was a 34 year old highly fit female cyclist (VO₂MAX = 53.3 ml/kg/min). Over a five month period, she participated in three separate intense bouts of acute unaccustomed exercise. ³¹P MRS measurements were performed seven weeks after the first bout and every two weeks for 14 more weeks. In all cases, ³¹P MRS measurements followed three days after each bout.

Results: The subject showed a decreased ability to generate ATP from oxidative phosphorylation and an increased reliance on anaerobic ATP production during the 70% and 100% maximal voluntary contractions after the exercise bouts. Increased rates of fatigue and increased indicators of exercise difficulty also accompanied these reductions in muscle oxidative capacity. Increased oxidative and anaerobic ATP production were needed to maintain the work level during a submaximal 45% maximal voluntary contraction exercise.

Conclusions: Acute increases in intensity accompanied by a change in exercise mode can influence the ability of muscle to generate ATP. The muscles were less economical and required more ATP to generate force during the submaximal exercises. During the maximal exercises, the muscle's mitochondria showed a reduced oxidative capacity. However, these reductions in oxidative capacity at the muscle level were not associated with changes in whole body maximal oxygen uptake. Finally, these reductions in muscular oxidative capacity were accompanied by increased rates of anaerobic ATP production, fatigue, and indicators of exercise difficulty.

     

Measurement of human skeletal muscle oxidative capacity by 31P-MR spectroscopy: a cross-validation with in vitro measurements

Purpose:

To cross‐validate skeletal muscle oxidative capacity measured by ³¹P‐MR spectroscopy with in vitro measurements of oxidative capacity in mitochondria isolated from muscle biopsies of the same muscle group in 18 healthy adults.

Materials and Methods:

Oxidative capacity in vivo was determined from PCr recovery kinetics following a 30‐s maximal isometric knee extension. State 3 respiration was measured in isolated mitochondria using high‐resolution respirometry. A second cohort of 10 individuals underwent two ³¹P‐MRS testing sessions to assess the test–retest reproducibility of the method.

Results:

Overall, the in vivo and in vitro methods were well‐correlated (r = 0.66–0.72) and showed good agreement by Bland Altman plots. Excellent reproducibility was observed for the PCr recovery rate constant (CV = 4.6%; ICC = 0.85) and calculated oxidative capacity (CV = 3.4%; ICC = 0.83).

Conclusion:

These results indicate that ³¹P‐MRS corresponds well with gold‐standard in vitro measurements and is highly reproducible. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Rapid 3D‐imaging of phosphocreatine recovery kinetics in the human lower leg muscles with compressed sensing

The rate of phosphocreatine (PCr) resynthesis following physical exercise is an accepted index of mitochondrial oxidative metabolism and has been studied extensively with unlocalized ³¹P‐MRS methods and small surface coils. Imaging experiments using volume coils that measure several muscles simultaneously can provide new insights into the variability of muscle function in healthy and diseased states. However, they are limited by long acquisition times relative to the dynamics of PCr recovery. This work focuses on the implementation of a compressed sensing technique to accelerate imaging of PCr resynthesis following physical exercise, using a modified three‐dimensional turbo‐spin‐echo sequence and principal component analysis as sparsifying transform. The compressed sensing technique was initially validated using 2‐fold retrospective undersampling of fully sampled data from four volunteers acquired on a 7T MRI system (voxel size: 1.6 mL, temporal resolution: 24 s), which led to an accurate estimation of the mono‐exponential PCr resynthesis rate constant (mean error <6.4%). Acquisitions with prospective 2‐fold acceleration (temporal resolution: 12 s) demonstrated that three‐dimensional mapping of PCr resynthesis is possible at a temporal resolution that is sufficiently high for characterizing the recovery curve of several muscles in a single measurement. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Reproducibility assessment of metabolic variables characterizing muscle energetics in Vivo: A 31P‐MRS study

The purpose of the present study was to assess the reliability of metabolic parameters measured using ³¹P magnetic resonance spectroscopy (³¹P MRS) during two standardized rest‐exercise‐recovery protocols. Twelve healthy subjects performed the standardized protocols at two different intensities; i.e., a moderate intensity (MOD) repeated over a two‐month period and heavy intensity (HEAVY) repeated over a year's time. Test‐retest reliability was analyzed using coefficient of variation (CV), limits of agreement (LOA), and intraclass correlation coefficients (ICC). During exercise and recovery periods, most of the metabolic parameters exhibited a good reliability. The CVs of individual concentration of phosphocreatine ([PCr]), concentration of adenosine diphosphate ([ADP]), and pH values recorded at end of the HEAVY exercise were lower than 15%. The CV calculated for the rate of PCr resynthesis and the maximal oxidative capacity were less than 13% during the HEAVY protocol. Inferred parameters such as oxidative and total adenosine triphosphate (ATP) production rates exhibited a good reliability (ICC ≈ 0.7; CV < 15% during the HEAVY protocol). Our results demonstrated that measurement error using ³¹P‐MRS during a standardized exercise was low and that biological variability accounted for the vast majority of the measurement variability. In addition, the corresponding metabolic measurements can reliably be used for longitudinal studies performed even over a long period of time. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Phosphorus‐31 magnetic resonance spectroscopy of skeletal muscle in maternally inherited diabetes and deafness A3243G mitochondrial mutation carriers

Purpose

To investigate high‐energy phosphate metabolism in striated skeletal muscle of patients with Maternally Inherited Diabetes and Deafness (MIDD) syndrome.

Materials and Methods

In 11 patients with the MIDD mutation (six with diabetes mellitus [DM] and five non‐DM) and eight healthy subjects, phosphocreatine (PCr) and inorganic phosphate (Pi) in the vastus medialis muscle was measured immediately after exercise using ³¹P‐magnetic resonance spectroscopy (MRS). The half‐time of recovery (t1/2) of monoexponentially fitted (PCr+Pi)/PCr was calculated from spectra obtained every 4 seconds after cessation of exercise. A multiple linear regression model was used for statistical analysis.

Results

Patients with the MIDD mutation showed a significantly prolonged t1/2 (PCr+Pi)/PCr after exercise as compared to controls (13.6±3.0 vs. 8.7±1.3 sec, P = 0.01). No association between the presence of DM and t1/2 (PCr + Pi)/PCr was found (P = 0.382).

Conclusion

MIDD patients showed impaired mitochondrial oxidative phosphorylation in skeletal muscle shortly after exercise, irrespective of the presence of DM. J. Magn. Reson. Imaging 2009;29:127–131. © 2008 Wiley‐Liss, Inc.     

Comparisons of ATP turnover in human muscle during ischemic and aerobic exercise using 31P magnetic resonance spectroscopy

To investigate human muscle bioenergetics quantitatively in vivo, we used ³¹P magnetic resonance spectroscopy to study the flexor digitorum superficialis of four adult males during dynamic ischemic and aerobic exercise at 0.50–1.00 W and during recovery from aerobic exercise. During exercise, changes in pH and [PCr] were larger at higher power, but in aerobic exercise neither end-exercise [ADP] nor the initial postexercise PCr resynthesis rate altered with power. In ischemic exercise we estimated total ATP synthesis from the rates of PCr depletion and glycogenolysis (inferred using an analysis of proton buffering); this was linear with power output. In aerobic exercise, again we estimated ATP synthesis rates due to phosphocreatine hydrolysis and glycogenolysis (incorporating a correction for proton efflux) and also estimated oxidative ATP synthesis by difference, using the total ATP turnover rate established during ischemic exercise. We conclude that in early exercise oxidative ATP synthesis was small, increasing by the end of exercise to a value close (as predicted) to the initial postexercise rate of PCr resynthesis. Furthermore, a plausible estimate of proton efflux during aerobic exercise can be inferred from the pH-dependence of proton efflux in recovery.     

Evaluation of muscle diseases using artificial neural network analysis of 31P MR spectroscopy data

Dermatomyositis is an autoimmune disease characterized by an erythematous rash and severe muscle weakness. ³¹P Magnetic resonance spectroscopy (MRS) provides quantitative data for longitudinal monitoring of disease status and responses to immunosuppressive therapy. A disease variant, amyopathic dermatomyositis, presents with a typical rash but no clinical muscle weakness. However, metabolic abnormalities in the oxidative capacity of muscles of amyopathic patients during exercise were detected with ³¹P MRS. Because MRS provided the best quantitative data for evaluating dermatomyositis, the ³¹P metabolic parameters derived from the MR spectra were further processed using an artificial neural network (XERION). The neural network analyses provided additional clinical information from the weighted correlations of multiple ³¹P parameters, namely, inorganic phosphate, phos-phocreatine, ATP, phosphodiesters, and selected ratios. This investigation analyzes the relative importance of the various metabolic parameters for accurate patient characterization and provides insights into the pathogenesis of the disease.     

Contributions of dynamic phosphorus-31 magnetic resonance spectroscopy to the analysis of muscle fiber distribution

RATIONALE AND OBJECTIVES: In high-performance athletes, conclusions regarding the muscle fiber distribution were to be drawn from dynamic 31phosphorus magnetic resonance spectroscopy (31P MRS). METHODS: Eleven volleyball players (V), eight bodybuilders (B), and 22 nonathletic volunteers (N) were examined by dynamic 31P MRS. During rest, exhaustive exercise, and recovery, respectively, up to 60 consecutive phosphorus spectra of the quadriceps muscle were acquired by "time series" in 36 s each. Two main spectroscopic approaches to the spectroscopic analysis of muscle fiber distribution were applied: evaluation of the ratio Pi/PCr at rest and the computer-assisted analysis of the Pi-peak at its exercise-induced line width maximum. RESULTS: At rest, the bodybuilders showed a significant lower Pi/PCr (0.07 +/- 0.03), in comparison with the volleyball players (0.11 +/- 0.03) and the nonathletic volunteers (0.11 +/- 0.02). The computer-assisted analysis of the Pi-peak at its line width maximum revealed a significantly lower pH of both of the subpeaks in the bodybuilders [6.30 versus 6.37 (V) and 6.38 (N); 6.89 versus 6.92 (V, N)], whereas the volleyball players provided the largest proportion of oxidative muscle fibers (68%), compared to bodybuilders (64%) and nonathletic volunteers (59%). A correlation between the ratio Pi/PCr and the area of the subpeak with the high pH (representing oxidative fibers) could not be demonstrated. CONCLUSIONS: Spectroscopic results during rest and exercise may be influenced by the muscle fiber distribution of the respective volunteer. The applied spectroscopic approaches to the analysis of muscle fiber composition are not compatible with each other; depending on the applied method, the classification of a muscle fiber as type I or type II fiber may change. The influence of physiologic factors like muscle fiber distribution on spectroscopic results has to be considered in the interpretation of pathological conditions.     

Experimental design of 31P MRS assessment of human forearm muscle function: Restrictions imposed by functional anatomy

The restrictions imposed by the functional anatomy of the finger flexor muscles on the experimental design of ³¹P MRS assessment of human forearm muscle function employing surface coil localization and voluntary exercise were investigated. It was found that ³¹P MRS metabolic data of finger flexor muscle should be correlated with mechanical data of combined flexion of only the ring and little fingers, rather than all four fingers as has been commonly the case in previously reported studies.     

Phosphocreatine resynthesis during recovery in different muscles of the exercising leg by 31P‐MRS

To investigate the high‐energy phosphate metabolism by ³¹P‐nuclear magnetic resonance spectroscopy during off‐transition of exercise in different muscle groups, such as calf muscles and biceps femoris muscles, seven male long‐distance runners (LDR) and nine untrained males (UT) performed both submaximal constant and incremental exercises. The relative exercise intensity was set at 60% of the maximal work rate (60%Wmax) during both knee flexion and plantar flexion submaximal constant load exercises. The relative areas under the inorganic phosphate (P_i) and phosphocreatine (PCr) peaks were determined. During the 5‐min recovery following the 60%Wmax, the time constant for the PCr off‐kinetics was significantly faster in the plantar flexion (LDR: 17.3 ± 3.6 s, UT: 26.7 ± 6.7 s) than in the knee flexion (LDR: 29.7 ± 4.7 s, UT: 42.7 ± 2.8 s, P < 0.05). In addition, a significantly faster PCr off‐kinetics was observed in LDR than in UT for both exercises. The ratio of P_i to PCr (P_i/PCr) during exercise was significantly lower during the plantar flexion than during the knee flexion (P < 0.01). These findings indicated that the calf muscles had relatively higher potential for oxidative capacity than that of biceps femoris muscles with an association of training status.     

Exertional heatstroke and muscle metabolism: an in vivo 31P-MRS study.

An impairment of muscle energy metabolism has been suggested as a predisposing factor for, as well as a consequence of exertional heatstroke (EHS). Thirteen young men were investigated 6 months after a well-documented EHS using 31Phosphorus Magnetic Resonance Spectroscopy (31P-MRS). The relative concentrations of ATP, phosphocreatine (PCr), inorganic phosphate (Pi), phosphomonoesters (PME), and the intracellular pH (pHi) were determined at rest, during a graded standardized exercise protocol (360 active plantar flexions) and during recovery. Also the leg tissue blood flow was determined by venous occlusion plethysmography during the MRS procedure. Sixteen age-matched healthy male volunteers served as control group. In resting muscle, there were no significant differences between the groups as regards pHi, Pi/PCr, and ATP/PCr+Pi+PME ratios. During steady state exercise conditions, effective power outputs were similar for both groups at each level of exercise: 20, 35, and 50% of maximal voluntary contraction (MVC) of the calf muscle. No significant differences were shown between the two groups in Pi/PCr, pHi, or changes of leg blood flow at each level of exercise. At 50% MVC, Pi/PCr was 0.48 +/- 0.08 vs 0.47 +/- 0.05 (P = 0.96), pHi was 6.94 +/- 0.03 vs 6.99 +/- 0.02, respectively (P = 0.13). Finally, the rate of PCr resynthesis during recovery was not significantly different between the two groups: t1/2 PCr = 0.58 +/- 0.07 vs 0.50 +/- 0.05 min, respectively (P = 0.35). Therefore, no evidence of an impairment of muscle energy metabolism was shown in the EHS group during a standardized submaximal exercise using 31P-MRS performed 6 months after an EHS.     

The use of nuclear magnetic resonance to evaluate muscle injury.

Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are new and powerful tools to study tissue biochemistry, and to provide precise anatomical visualization of soft tissue structures. This review focuses on the use of these techniques to study exercise-induced muscle injury. MRS measurements show an increase in the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) 1-7 d after eccentric exercise. This increase in Pi/PCr could be due to either increases in extracellular Pi or small increases in resting muscle metabolism. Increased Pi/PCr is also seen during training programs and may indicate persistent muscle injury. Increased resting Pi/PCr with injury was not associated with altered metabolism during exercise. Elevations in resting Pi/PCr have been used to show increased susceptibility of dystrophic muscle to exercise-induced injury. Progressive clinical deterioration in dystrophic dogs is marked by impaired muscle metabolism, and the presence of low oxidative muscle fibers not seen in normal dogs. MRI shows increased proton T2 relaxation times following eccentric exercise that last up to 80 d after injury, and can reflect muscle edema as well as longer lasting changes in the characteristics of cell water. MRI demonstrate precise localization of the injured area, with large differences in both location and degree of injury in different subjects following the same exercise protocol. Thus, MRS can provide information on the metabolic response to injury, while MRI provides information regarding the site and extent of the injury. These tools have promise in helping to understand exercise-induced muscle injury.     

Creatine supplementation--part II: in vivo magnetic resonance spectroscopy

PURPOSE: Our purpose was to study effects of creatine (Cr) supplementation on muscle metabolites noninvasively by means of magnetic resonance spectroscopy (MRS) before and after supplementation with Cr or placebo. METHODS: 1H-MRS was used in a comprehensive, double-blind, cross-over study in 10 volunteers to measure Cr in m. tibialis anterior and m. rectus femoris at rest. PCr/ATP was observed in m. quadriceps femoris by 31P-MRS at rest and after exercise. RESULTS: A significant increase in total Cr was observed with Cr intake in m. tibialis anterior (+9.6 +/- 1.7%, P = 0.001) and in m. rectus femoris (+18.0 +/- 1.8%, P < 0.001). PCr/ATP showed a significant increase (+23.9 +/- 2.3%, P < 0.001) in m. quadriceps femoris at rest with Cr supplementation. Post-Cr supplementation recovery rates from exercise were significantly lower (k = 0.029 s(-1), P < 0.01) compared with postplacebo consumption (k = 0.034 s(-1)) and presupplementation (k = 0.037 s(-1)). However, higher levels of PCr/ATP at rest compensate for this reduction of the recovery rate after Cr supplementation. The increase of PCr/ATP determined by 31P-MRS correlates with the increase of Cr observed by 1H-MRS (r = 0.824, P < 0.001). CONCLUSION: Noninvasive observation of Cr and PCr after Cr supplementation shows an increase in a muscle specific manner. Higher preexercise levels of PCr/ATP at rest compensate for significantly slower recovery rates of PCr/ATP after Cr supplementation.     

Direct noninvasive quantification of lactate and high energy phosphates simultaneously in exercising human skeletal muscle by localized magnetic resonance spectroscopy.

A novel method based on interleaved localized 31P- and 1H MRS is presented, by which lactate accumulation and the accompanying changes in high energy phosphates in human skeletal muscle can be monitored simultaneously during exercise and recovery. Lactate is quantified using a localized double quantum filter suppressing the abundant lipid signals while taking into account orientation dependent signal modulations. Lactate concentration after ischemic exercise directly quantified by DQF 1H spectroscopy was 24 +/- 3 mmol/L cell water, while 22 +/- 3 mmol/L was expected on the basis of 31P MRS acquired simultaneously. Lactate concentration in a sample of porcine meat was estimated to be 40 +/- 7 mmol/L by means of DQF quantitation, versus 39 +/- 5 mmol/L by biochemical methods. Excellent agreement is shown between lactate concentrations measured noninvasively by 1H MRS, measured biochemically ex vivo, and inferred indirectly in vivo from changes in pH, P(i), and PCr as obtained from 31P MRS data.     

Metabolic response to exercise in malignant hyperthermia-sensitive patients measured by 31P magnetic resonance spectroscopy

The currently favored theory of pathogenesis of malignant hyperthermia (MH) implicates an abnormality in skeletal muscle calcium ion transport. During a MH crisis a profound lactic acidosis occurs and in MH-sensitive individuals a delayed recovery of venous lactate has been previously noted postexercise. We have used ³¹P magnetic resonance spectroscopy to follow noninvasively in vivo changes in muscle of intracellular pH and high-energy phosphate metabolites during rest, exercise, and recovery of MH-sensitive subjects. Eleven biopsy-positive MH-sensitive patients have been studied and compared to 26 normal subjects. The MH-sensitive subjects as a group prematurely dropped their intracellular pH during mild aerobic exercise and they demonstrated a marked delay before the recovery of pH after maximal exercise. PCr/(PCr + P_i) ratios also dropped early during exercise but recovered normally. The observed changes in pH and PCr/(PCr + P_i) are consistent with a myopathy in MH-susceptible individuals. © 1990 Academic Press, Inc.     

Impact of exercise capacity on myocardial high-energy phosphate metabolism

31-Phosphorous magnetic resonance spectroscopy (31P MRS) is a unique tool to investigate IN VIVO high-energy phosphates (HEP) in the human heart. We hypothesized that physical capacity may be associated with myocardial HEP status. Healthy, male volunteers (n = 105, mean age 51 +/- 7 years) underwent bicycle ergometry with a stepwise increasing workload to determine maximal working capacity (MWC). Heart rate (HR) and blood pressure (BP) were measured continuously during exercise and 4 minutes of recovery. Further 31-Phosphorous 2-dimensional chemical shift imaging (31P 2D CSI) MRS was performed to assess myocardial HEP metabolism by determining phosphocreatinine to beta-ATP ratios (PCr/b-ATP) using a 1.5 tesla scanner. Volunteers with MWC > 230 Watt had significantly higher PCr/b-ATP ratios than those with MWC < 200 Watt (1.93 +/- 0.36 vs. 1.59 +/- 0.35; p < 0.001). Additionally, those with a recovery systolic (S)BP < 195 mmHg had significantly higher ratios than those with a recovery SBP > 195 mmHg (1.74 +/- 0.3 vs. 1.51 +/- 0.2; p < 0.05). We observed a linear correlation between the PCr/b-ATP ratio and MWC (r = 0.411; p < 0.001) and recovery SBP (r = - 0.290; p < 0.01). After statistical correction for age, these correlations remained significant. In this study, we observed a correlation of parameters of physical fitness determined by bicycle exercise testing and cardiac PCr/b-ATP ratios.     

Isokinetic eccentric exercise as a model to induce and reproduce pathophysiological alterations related to delayed onset muscle soreness

Physiological alterations following unaccustomed eccentric exercise in an isokinetic dynamometer of the right m. quadriceps until exhaustion were studied, in order to create a model in which the physiological responses to physiotherapy could be measured. In experiment I (exp. I), seven selected parameters were measured bilaterally in 7 healthy subjects at day 0 as a control value. Then after a standardized bout of eccentric exercise the same parameters were measured daily for the following 7 d (test values). The measured parameters were: the ratio of phosphocreatine to inorganic phosphate (PCr/P_i), the ratio of inorganic phosphate to adenosintriphosphate (Pi/ATP), the ratio of phosphocreatine to adenosintriphosphate (PCr/ATP) (all three ratios measured with ³¹P-nuclear magnetic resonance spectroscopy), dynamic muscle strength, plasma creatine kinase (CK), degree of pain and “muscle” blood flow rate (¹³³Xenon washout technique). This was repeated in experiment II (exp. II) 6–12 months later in order to study reproducbility. In experiment III (exp. III), the normal fluctuations over 8 d of the seven parameters were measured, without intervention with eccentric exercise in 6 other subjects. All subjects experienced pain, reaching a maximum 48 h after eccentric exercise in both exp. I and II. A systematic effect over time for CK (increasing 278% resp. 308%), muscle strength (decreasing more than 10%), PCr/Pi (decreasing 31% resp. 43%) and Pi/ATP (increasing 55% resp. 99%) was found in both exp. I and II (P<0.05), but not in exp. III. No significant difference was observed between exp. I and II for CK, blood-flow rate, concentric muscle strength, PCr/Pi, Pi/ATP and PCr/ATP. It is concluded that pathophysiological alterations in m. quadriceps following eccentric exercise can be induced and can be reproduced after an interval of 6 months. Thus, this model can be used to study the effects of physiotherapy.     

Fiber recruitment affects oxidative recovery measurements of human muscle in vivo

PURPOSE: Fast-twitch and slow-twitch muscle fibers are known to have distinct metabolic properties. However, it has not been clearly established whether such heterogeneity within mixed-fiber muscles can influence measurements of energy metabolism in vivo. We therefore tested the hypothesis that differences in muscle fiber recruitment can cause differences in whole-muscle oxidative recovery from exercise. METHODS: We used (31)P magnetic resonance spectroscopy to measure oxidative ATP synthesis in the ankle dorsiflexor muscles of eight healthy volunteers under a variety of recruitment conditions. Oxidative ATP synthesis after isometric exercise was quantified as the rate constant k(PCr), the reciprocal of the time constant of PCr recovery. RESULTS: k(PCr) was 37% higher after low-force ramp contractions (which primarily recruit slow-twitch fibers) than after ballistic contractions to the same peak force (which recruit both fast- and slow-twitch fibers). k(PCr) was also 24% higher after low-force ramp contractions than after high-force ramp contractions, presumably reflecting the recruitment of fast-twitch fibers at high forces. CONCLUSION: Our results indicate that the muscle fibers recruited first in voluntary contractions have a higher oxidative capacity than those recruited last. Such metabolic differences among fibers can confound whole-muscle measurements and thus need to be taken into account when studying voluntary exercise.     

In vivo 31P magnetic resonance spectroscopy using a needle microcoil

Batch fabrication methods have been used to produce low‐cost microcoils for magnetic resonance spectroscopy (MRS) that could be discarded after applications such as insertion into tissue during interventional surgery. Needle‐shaped microcoils were constructed using electroplated conductors buried in shafts formed with different combinations of silicon and plastic and used to acquire in vivo ³¹P spectra of rat thigh muscle at 81 MHz. The designs in this study achieved a maximum signal‐to‐noise ratio (SNR) for phosphocreatine (PCr) of 10.4 in a 10‐min acquisition, with the three adenosine triphosphate (ATP) multiplets also clearly visible. An average 20% reduction in PCr occurred over a 60‐min period, and intracellular pH was estimated to be 6.6, which are both evidence of ischemia. A needle microcoil design could have applications in real‐time MRS of tumors or in evaluating pathology in general during surgical investigations. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Peak oxygen consumption and skeletal muscle bioenergetics in African-American and Caucasian men

OBJECTIVE: To compare peak oxygen consumption (VO2peak) and skeletal muscle oxidative metabolism between nine African-American and nine Caucasian men. METHODS: Subjects performed arm ergometry to exhaustion. On a separate occasion 31phosphorous-nuclear magnetic resonance spectroscopy (31P-NMRS) was used to determine the concentrations of phosphorous (Pi), phosphocreatine (PCr), and the intracellular pH of the flexor carpi radialis before and during 4 min of steady-state, wrist flexion exercise performed at 28% (15 W) of each subject's peak voluntary contraction. RESULTS: The Pi/PCr ratio was used as an indirect measure of skeletal muscle oxidative metabolism. VO2peak was lower in the African-Americans compared with the Caucasians (means +/- SD, 19.4 +/- 3.4 vs 23.3 +/- 4.0 mL x kg(-1) x min(-1)) (P < 0.05). No significant between group difference was noted in the Pi/PCr ratio at rest (0.10 +/- 0.02 both groups). However, resting pH was lower in the African-Americans (6.99 +/- 0.04 vs 7.03 +/- 0.05) (P < 0.05). Exercise caused an increase in the Pi/PCr ratio in the African-Americans (1.06 +/- 0.11), which was higher than the increase observed in the Caucasians (0.50 +/- 0.14) (P < 0.05). pH levels decreased to a lower level during exercise in the African-Americans (6.89 +/- 0.04) than in the Caucasians (6.98 +/- 0.05) (P < 0.05). CONCLUSIONS: This select group of African-American men achieved a lower VO2peak than the Caucasian men. Variations in skeletal muscle oxidative metabolic components may explain this difference.