Reliability of 31P-magnetic resonance spectroscopy during an exhaustive incremental exercise test in children

High-field (11.75T) multimodal MR imaging of exercising hindlimb mouse muscles using a non-invasive combined stimulation and force measurement device

We have designed and constructed an experimental set-up allowing electrical stimulation of hindlimb mouse muscles and the corresponding force measurements at high-field (11.75T). We performed high-resolution multimodal MRI (including T2 -weighted imaging, angiography and diffusion) and analysed the corresponding MRI changes in response to a stimulation protocol. Mice were tested twice over a 1-week period to investigate the reliability of mechanical measurements and T2 changes associated with the stimulation protocol. Additionally, angiographic images were obtained before and immediately after the stimulation protocol. Finally, multislice diffusion imaging was performed before, during and immediately after the stimulation session. Apparent diffusion coefficient (ADC) maps were calculated on the basis of diffusion weighted images (DWI). Both force production and T2 values were highly reproducible as illustrated by the low coefficient of variation (<8%) and high intraclass correlation coefficient (≥0.75) values. Maximum intensity projection angiographic images clearly showed a strong vascular effect resulting from the stimulation protocol. Although a motion sensitive imaging sequence was used (echo planar imaging) and in spite of the strong muscle contractions, motion artifacts were minimal for DWI recorded under exercising conditions, thereby underlining the robustness of the measurements. Mean ADC values increased under exercising conditions and were higher during the recovery period as compared with the corresponding control values. The proposed experimental approach demonstrates accurate high-field multimodal MRI muscle investigations at a preclinical level which is of interest for monitoring the severity and/or the progression of neuromuscular diseases but also for assessing the efficacy of potential therapeutic interventions.

Changes in phosphocreatine concentration of skeletal muscle during high-intensity intermittent exercise in children and adults

PURPOSE

The aim of the present study was to test the hypotheses that a greater oxidative capacity in children results in a lower phosphocreatine (PCr) depletion, a faster PCr resynthesis and a lower muscle acidification during high-intensity intermittent exercise compared to adults.

METHODS

Sixteen children (9.4 ± 0.5 years) and 16 adults (26.1 ± 0.3 years) completed a protocol consisting of a dynamic plantar flexion (10 bouts of 30-s exercise at 25 % of one repetition maximum separated by 20-s recovery), followed by 10 min of passive recovery. Changes of PCr, ATP, inorganic phosphate, and phosphomonoesters were measured by means of (31)Phosphorous-magnetic resonance spectroscopy during and post-exercise.

RESULTS

Average PCr (percentage of [PCr] at initial rest (%[PCr]i)) at the end of the exercise (adults 17 ± 12 %[PCr]i, children 38 ± 17 %[PCr]i, P < 0.01) and recovery periods (adults 37 ± 14 %[PCr]i, children 57 ± 17 %[PCr]i, P < 0.01) was significantly lower in adults compared to children, induced by a stronger PCr decrease during the first exercise interval (adults -73 ± 10 %[PCr]i, children -55 ± 15 %[PCr]i, P < 0.01). End-exercise pH was significantly higher in children compared to adults (children 6.90 + 0.20, -0.14; adults 6.67 + 0.23, -0.15, P < 0.05).

CONCLUSIONS

From our results we suggest relatively higher rates of oxidative ATP formation in children's muscle for covering the ATP demand of high-intensity intermittent exercise compared to adults, enabling children to begin each exercise interval with significantly higher PCr concentrations and leading to an overall lower muscle acidification.

Influence of intermittent hypoxic training on muscle energetics and exercise tolerance

Intermittent hypoxic training (IHT) is sometimes used by athletes to enhance nonhematological physiological adaptations to simulated altitude. We investigated whether IHT would result in greater improvements in muscle energetics and exercise tolerance compared with work-matched intermittent normoxic training (INT). Nine physically active men completed 3 wk of intensive, single-leg knee-extensor exercise training. Each training session consisted of 25 min of IHT (FiO2 14.5 ± 0.1%) with the experimental leg and 25 min of INT with the alternate leg, which served as a control. Before and after the training intervention, subjects completed a test protocol consisting of a bout of submaximal constant-work-rate exercise, a 24-s high-intensity exercise bout to quantify the phosphocreatine recovery time constant ([PCr]-τ), and an incremental test to the limit of tolerance. The tests were completed in normoxia and hypoxia in both INT and IHT legs. Muscle metabolism was assessed noninvasively using 31P-magnetic resonance spectroscopy. Improvements in the time-to-exhaustion during incremental exercise were not significantly different between training conditions either in normoxia (INT, 28 ± 20% vs. IHT, 25 ± 9%; P = 0.86) or hypoxia (INT, 21 ± 10% vs. IHT, 15 ± 11%; P = 0.29). In hypoxia, [PCr]-τ was speeded slightly but significantly more post-IHT compared with post-INT (−7.3 ± 2.9 s vs. −3.7 ± 1.7 s; P < 0.01), but changes in muscle metabolite concentrations during exercise were essentially not different between IHT and INT. Under the conditions of this investigation, IHT does not appreciably alter muscle metabolic responses or incremental exercise performance compared with INT.

The reproducibility of 31-phosphorus MRS measures of muscle energetics at 3 Tesla in trained men

Objective

Magnetic resonance spectroscopy (MRS) provides an exceptional opportunity for the study of in vivo metabolism. MRS is widely used to measure phosphorus metabolites in trained muscle, although there are no published data regarding its reproducibility in this specialized cohort. Thus, the aim of this study was to assess the reproducibility of ³¹P-MRS in trained skeletal muscle.

Methods

We recruited fifteen trained men (VO₂peak = 4.7±0.8 L min⁻¹/58±8 mL kg⁻¹ min⁻¹) and performed duplicate MR experiments during plantar flexion exercise, three weeks apart.

Results

Measures of resting phosphorus metabolites were reproducible, with 1.7 mM the smallest detectable difference in phosphocreatine (PCr). Measures of metabolites during exercise were less reliable: exercising PCr had a coefficient of variation (CV) of 27% during exercise, compared with 8% at rest. Estimates of mitochondrial function were variable, but experimentally useful. The CV of PCr_1/2t was 40%, yet much of this variance was inter-subject such that differences of <20% were detectable with n = 15, given a significance threshold of p<0.05.

Conclusions

31-phosphorus MRS provides reproducible and experimentally useful measures of phosphorus metabolites and mitochondrial function in trained human skeletal muscle.

Age- and sex-related differences in muscle phosphocreatine and oxygenation kinetics during high-intensity exercise in adolescents and adults

The aim of this investigation was to examine the adaptation of the muscle phosphates (e.g. phosphocreatine (PCr) and ADP) implicated in regulating oxidative phosphorylation, and oxygenation at the onset of high intensity exercise in children and adults. The hypotheses were threefold: primary PCr kinetics would be faster in children than adults; the amplitude of the PCr slow component would be attenuated in children; and the amplitude of the deoxyhaemoglobin/myoglobin (HHb) slow component would be reduced in children. Eleven children (5 girls, 6 boys, 13 +/- 1 years) and 11 adults (5 women, 6 men, 24 +/- 4 years) completed two to four constant work rate exercise tests within a 1.5 T MR scanner. Quadriceps muscle energetics during high intensity exercise were monitored using (31)P-MRS. Muscle oxygenation was monitored using near-infrared spectroscopy. The time constant for the PCr response was not significantly different in boys (31 +/- 10 s), girls (31 +/- 10 s), men (44 +/- 20 s) or women (29 +/- 14 s, main effects: age, p = 0.37, sex, p = 0.25). The amplitude of the PCr slow component relative to end-exercise PCr was not significantly different between children (23 +/- 23%) and adults (17 +/- 13%, p = 0.47). End-exercise [PCr] was significantly lower, and [ADP] higher, in females (18 +/- 4 mM and 53 +/- 16 microM) than males (23 +/- 4 mM, p = 0.02 and 37 +/- 11 microM, p = 0.02), but did not differ with age ([PCr]: p = 0.96, [ADP]: p = 0.72). The mean response time for muscle tissue deoxygenation was significantly faster in children (22 +/- 4 s) than adults (27 +/- 7 s, p = 0.01). The results of this study show that the control of oxidative metabolism at the onset of high intensity exercise is adult-like in 13-year-old children, but that matching of oxygen delivery to extraction is more precise in adults.

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 (31)P magnetic resonance spectroscopy ((31)P 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 approximately 0.7; CV < 15% during the HEAVY protocol). Our results demonstrated that measurement error using (31)P-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.

Comparative analysis of skeletal muscle oxidative capacity in children and adults: a 31P-MRS study

The aim of the present study was to compare the oxidative capacity of the forearm flexor muscles in vivo between children and adults using 31-phosphorus magnetic resonance spectroscopy. Seven boys (11.7 +/- 0.6 y) and 10 men (35.6 +/- 7.8 year) volunteered to perform a 3 min dynamic finger flexions exercise against a standardized weight (15% of the maximal voluntary contraction). Muscle oxidative capacity was quantified on the basis of phosphocreatine (PCr) post-exercise recovery kinetics analysis. End-of-exercise pH was not significantly different between children and adults (6.6 +/- 0.2 vs. 6.5 +/- 0.2), indicating that indices of PCr recovery kinetics can be reliably compared. The rate constant of PCr recovery (kPCr) and the maximum rate of aerobic ATP production were about 2-fold higher in young boys than in men (kPCr: 1.7 +/- 1.2 vs. 0.7 +/- 0.2 min(-1); Vmax: 49.7 +/- 24.6 vs. 29.4 +/- 7.9 mmol.L(-1).min(-1), p < 0.05). Our results clearly illustrate a greater mitochondrial oxidative capacity in the forearm flexor muscles of young children. This larger ATP regeneration capacity through aerobic mechanisms in children could be one of the factors accounting for their greater resistance to fatigue during high-intensity intermittent exercise.

Muscle phosphocreatine kinetics in children and adults at the onset and offset of moderate-intensity exercise

The splitting of muscle phosphocreatine (PCr) plays an integral role in the regulation of muscle O2 utilization during a "step" change in metabolic rate. This study tested the hypothesis that the kinetics of muscle PCr would be faster in children compared with adults both at the onset and offset of moderate-intensity exercise, in concert with the previous demonstration of faster phase II pulmonary O2 uptake kinetics in children. Eighteen peri-pubertal children (8 boys, 10 girls) and 16 adults (8 men, 8 women) completed repeated constant work-rate exercise transitions corresponding to 80% of the Pi/PCr intracellular threshold. The changes in quadriceps [PCr], [Pi], [ADP], and pH were determined every 6 s using 31P-magnetic resonance spectroscopy. No significant (P>0.05) age- or sex-related differences were found in the PCr kinetic time constant at the onset (boys, 21+/-4 s; girls, 24+/-5 s; men, 26+/-9 s; women, 24+/-7 s) or offset (boys, 26+/-5 s; girls, 29+/-7 s; men, 23+/-9 s; women 29+/-7 s) of exercise. Likewise, the estimated theoretical maximal rate of oxidative phosphorylation (Qmax) was independent of age and sex (boys, 1.39+/-0.20 mM/s; girls, 1.32+/-0.32 mM/s; men, 2.36+/-1.18 mM/s; women, 1.51+/-0.53 mM/s). These results are consistent with the notion that the putative phosphate-linked regulation of muscle O2 utilization is fully mature in peri-pubertal children, which may be attributable to a comparable capacity for mitochondrial oxidative phosphorylation in child and adult muscle.

Accurate work-rate measurements during in vivo MRS studies of exercising human quadriceps

INTRODUCTION

Given that we have reached a point in the field of muscle energetics where absolute measurements are warranted to take the area forward, we designed an ergometer, including two force and two displacement transducers, allowing dynamic and isometric knee extension within a MR system and accurate measurements of power output.

METHODS

On the basis of repeated measurements, the force and displacement transducers accuracy was 1% for values ranging from 0 to 394 N and 4% for values ranging from 0 to 20 cm. In addition, measurements were not affected by magnetic field. MRS experiments in exercising muscle were conducted in eight subjects. They performed two standardized dynamic alternate leg extension exercises (25 and 35% of MVC) while the corresponding metabolic changes were measured using (31)P-MRS.

RESULTS

The mean power output produced during both exercises were 63 +/- 16 and 81 +/- 15 W while the eccentric work was reduced i.e. 12 +/- 14 and 21 +/- 6 W for the moderate and heavy exercise respectively. The corresponding metabolic changes were significant with a 20-40% PCr depletion and an end of exercise pH ranging from 0.02 to 0.70 pH units.

CONCLUSION

Overall, the present ergometer allows quadriceps exercise in a MR system and should be useful for future metabolic studies for which reliable and absolute quantification of power output is warranted.

Non-invasive methods in paediatric exercise physiology

Oded Bar-Or’s hypothesis that children may be “metabolic non-specialists”, even when engaging in specialized sports, has stimulated the study of paediatric exercise metabolism since the publication of his classic text Pediatric sports medicine for the practitioner in 1983. Evidence drawn from several methodologies indicates an interplay of anaerobic and aerobic exercise metabolism in which children have a relatively higher metabolic contribution from oxidative energy pathways than adolescents or adults, whereas there is a progressive increase in glycolytic support of exercise with age, at least into adolescence and possibly into young adulthood. The picture is generally consistent but incomplete, as research with young people has been limited by both ethical and methodological constraints. The recent rigorous introduction of non-invasive techniques such as breath-by-breath respiratory gas analysis and magnetic resonance spectroscopy into paediatric exercise physiology promises to open up new avenues of research and generate unique insights into the metabolism of the exercising muscle during growth and maturation. It therefore appears that we might have available the tools necessary to answer some of the elegant questions raised by Professor Bar-Or over 25 years ago.

Muscle phosphocreatine and pulmonary oxygen uptake kinetics in children at the onset and offset of moderate intensity exercise

To further understand the mechanism(s) explaining the faster pulmonary oxygen uptake (p(VO)(2)) kinetics found in children compared to adults, this study examined whether the phase II p(VO)(2) kinetics in children are mechanistically linked to the dynamics of intramuscular PCr, which is known to play a principal role in controlling mitochondrial oxidative phosphorylation during metabolic transitions. On separate days, 18 children completed repeated bouts of moderate intensity constant work-rate exercise for determination of (1) PCr changes every 6 s during prone quadriceps exercise using (31)P-magnetic resonance spectroscopy, and (2) breath by breath changes in p(VO)(2) during upright cycle ergometry. Only subjects (n = 12) with 95% confidence intervals <or=+/-7 s for all estimated time constants were considered for analysis. No differences were found between the PCr and phase II p(VO)(2) time constants at the onset (PCr 23 +/- 5 vs. p(VO)(2) 23 +/- 4 s, P = 1.000) or offset (PCr 28 +/- 5 vs. p(VO)(2) 29 +/- 5 s, P = 1.000) of exercise. The average difference between the PCr and phase II p(VO)(2) time constants was 4 +/- 4 s for the onset and offset responses. Pooling of the exercise onset and offset responses revealed a significant correlation between the PCr and p(VO)(2) time constants (r = 0.459, P = 0.024). The close kinetic coupling between the p(VO)(2) and PCr responses at the onset and offset of exercise in children is consistent with our current understanding of metabolic control and suggests that an age-related modulation of the putative phosphate linked controller(s) of mitochondrial oxidative phosphorylation may explain the faster p(VO)(2) kinetics found in children compared to adults.