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While phosphorous magnetic resonance spectroscopy ((31)P-MRS) and near-infrared spectroscopy (NIRS) provide methods for measuring spinal muscle function non-invasively, their reliability is not established. The aim of this study was assess the reliability (ICC) and error magnitude (CV%) of measurements of muscle phosphocreatine (PCr), tissue oxygenation index (TOI) and muscle deoxyhaemoglobin (HHb) acquired during fatigue and in recovery after 24 s of exercise in the lumbar muscles. 10 healthy participants (19-25 years, 5 male, 5 female) performed exercise that involved holding the upper body unsupported in slight extension until fatigue and then, after 30 min of rest, for repeated bursts of 24 s. ICCs indicated good to excellent reliability of baseline measures (TOI: 0.75) and of amplitude changes during fatigue (PCr: 0.73, TOI: 0.69, HHb: 0.80) and recovery (HHb: 0.96), and poor to fair reliability for time constants describing rates of change during fatigue (PCr: 0.11) and recovery (PCr: 0.31, HHb: 0.47). CV% indicated varying relative measurement error across baseline measures (TOI: 5%), amplitude changes during fatigue (PCr: 7%, TOI: 38%, HHb: 31%) and recovery (HHb: 31%), and in time constants for fatigue (PCr: 39%) and recovery (PCr: 20%, HHb: 37%). The results suggested that reliability would be sufficient for future studies on spinal muscle function, but that measurement error may be too large to evaluate individuals., (© Georg Thieme Verlag KG Stuttgart · New York.)

It is not possible to attain a metabolic steady state during exercise above the so-called critical force or critical power. We tested the hypothesis that the muscle metabolic perturbations at the end of a bout of maximal isometric contractions, which yield a stable end-test force (equal to the critical force), would be similar to that at task failure following submaximal contractions performed above the critical force. Eight healthy subjects (four female) performed isometric single knee-extension exercise in the bore of a 1.5 T superconducting magnet on two occasions. Following familiarization, subjects performed the following exercises: (1) 60 maximal contractions (3 s contraction, 2 s rest); and (2) submaximal contractions (the same contraction regime performed at 54 +/- 8% maximal voluntary contraction) to task failure. Phosphocreatine (PCr), inorganic phosphate (P(i)) and diprotonated phosphate (H(2)PO(4)()) concentrations and pH were determined using (31)P magnetic resonance spectroscopy throughout both tests. During the maximal contractions, force production fell from 213 +/- 33 N to reach a plateau in the last 30 s of the test at 100 +/- 20 N. The muscle metabolic responses at the end of each test were substantial, but not different between conditions: [PCr] was reduced (to 21 +/- 12 and 17 +/- 7% of baseline for maximal and submaximal contractions, respectively; P = 0.17), [P(i)] was elevated (to 364 +/- 98 and 363 +/- 135% of baseline, respectively; P = 0.98) and pH reduced (to 6.64 +/- 0.16 and 6.69 +/- 0.17, respectively; P = 0.43). The [H(2)PO(4)()] was also elevated at the end of both tests (to 607 +/- 252 and 556 +/- 269% of baseline, respectively; P = 0.22). These data suggest that the exercise-induced metabolic perturbations contributing to force depression in all-out exercise are the same as those contributing to task failure during submaximal contractions.

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.

We hypothesized that increasing skeletal muscle total creatine (Cr) content through dietary Cr supplementation would result in slower muscle phosphocreatine concentration ([PCr]) kinetics, as assessed using (31)P magnetic resonance spectroscopy, following the onset and offset of both moderate-intensity (Mod) and heavy-intensity (Hvy) exercise. Seven healthy males (age 29 +/- 6 yr, mean +/- SD) completed a series of square-wave transitions to Mod and Hvy knee extensor exercise inside the bore of a 1.5-T superconducting magnet both before and after a 5-day period of Cr loading (4x 5 g/day of creatine monohydrate). Cr supplementation resulted in an approximately 8% increase in the resting muscle [PCr]-to-[ATP] ratio (4.66 +/- 0.27 vs. 5.04 +/- 0.22; P < 0.05), consistent with a significant increase in muscle total Cr content consequent to the intervention. The time constant for muscle [PCr] kinetics was increased following Cr loading for Mod exercise (control: 15 +/- 8 vs. Cr: 25 +/- 9 s; P < 0.05) and subsequent recovery (control: 14 +/- 8 vs. Cr: 27 +/- 8 s; P < 0.05) and for Hvy exercise (control: 54 +/- 18 vs. Cr: 72 +/- 30 s; P < 0.05), but not for subsequent recovery (control: 41 +/- 11 vs. Cr: 44 +/- 6 s). The magnitude of the increase in [PCr] following Cr loading was correlated (P < 0.05) with the extent of the slowing of the [PCr] kinetics for the moderate off-transient (r = 0.92) and the heavy on-transient (r = 0.71). These data demonstrate, for the first time in humans, that an increase in muscle [PCr] results in a slowing of [PCr] dynamics in exercise and subsequent recovery.

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.

(31)Phosphate-magnetic resonance spectroscopy and near infrared spectroscopy (NIRS) were used for the simultaneous assessment of changes in quadriceps muscle metabolism and oxygenation during consecutive bouts of high-intensity exercise. Six male subjects completed two 6 min bouts of single-legged knee-extension exercise at 80% of the peak work rate separated by 6 min of rest while positioned inside the bore of a 1.5 T superconducting magnet. The total haemoglobin and oxyhaemoglobin concentrations in the area of the quadriceps muscle interrogated with NIRS were significantly higher in the baseline period prior to the second compared with the first exercise bout, consistent with an enhanced muscle oxygenation. Intramuscular phosphorylcreatine concentration ([PCr]) dynamics were not different over the fundamental region of the response (time constant for bout 1, 51 +/- 15 s versus bout 2, 52 +/- 17 s). However, the [PCr] dynamics over the entire response were faster in the second bout (mean response time for bout 1, 72 +/- 16 s versus bout 2, 57 +/- 8 s; P < 0.05), as a consequence of a greater fall in [PCr] in the fundamental phase and a reduction in the magnitude of the 'slow component' in [PCr] beyond 3 min of exercise (bout 1, 10 +/- 6% versus bout 2, 5 +/- 3%; P < 0.05). These data suggest that the increased muscle O(2) availability afforded by the performance of a prior bout of high-intensity exercise does not significantly alter the kinetics of [PCr] hydrolysis at the onset of a subsequent bout of high-intensity exercise. The greater fall in [PCr] over the fundamental phase of the response following prior high-intensity exercise indicates that residual fatigue acutely reduces muscle efficiency.

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

The kinetics of pulmonary O(2) uptake is known to be substantially slower when exercise is initiated from a baseline of lower-intensity exercise rather than from rest. However, it is not known whether putative intracellular regulators of mitochondrial respiration (and in particular the phosphocreatine concentration, [PCr]) show similar non-linearities in their response dynamics. The purpose of this study was therefore to investigate the influence of baseline metabolic rate on muscle [PCr] kinetics (as assessed using (31)P-magnetic resonance spectroscopy) following the onset of exercise. Seven male subjects completed 'step' tests to heavy-intensity exercise (80% of peak work-rate) from a resting baseline and also from a baseline of moderate-intensity exercise (40% of peak work-rate) using a single-leg knee-extensor ergometer situated inside the bore of a 1.5 T super-conducting magnet. The time constant describing the kinetics of the initial exponential-like fall in [PCr] was significantly different between rest-to-moderate (25 +/- 14 s), rest-to-heavy (48 +/- 11 s) and moderate-to-heavy exercise (95 +/- 40 s) (P < 0.05 for all comparisons). A delayed-onset 'slow component' in the [PCr] response was observed in all subjects during rest-to-heavy exercise, but was attenuated in the moderate-to-heavy exercise condition. These data indicate that muscle [PCr] kinetics does not conform to 'linear, first-order' behaviour during dynamic exercise, and thus have implications for understanding the regulation of muscle oxidative metabolism.

We hypothesized that a period of endurance training would result in a speeding of muscle phosphocreatine concentration ([PCr]) kinetics over the fundamental phase of the response and a reduction in the amplitude of the [PCr] slow component during high-intensity exercise. Six male subjects (age 26 +/- 5 yr) completed 5 wk of single-legged knee-extension exercise training with the alternate leg serving as a control. Before and after the intervention period, the subjects completed incremental and high-intensity step exercise tests of 6-min duration with both legs separately inside the bore of a whole-body magnetic resonance spectrometer. The time-to-exhaustion during incremental exercise was not changed in the control leg [preintervention group (PRE): 19.4 +/- 2.3 min vs. postintervention group (POST): 19.4 +/- 1.9 min] but was significantly increased in the trained leg (PRE: 19.6 +/- 1.6 min vs. POST: 22.0 +/- 2.2 min; P < 0.05). During step exercise, there were no significant changes in the control leg, but end-exercise pH and [PCr] were higher after vs. before training. The time constant for the [PCr] kinetics over the fundamental exponential region of the response was not significantly altered in either the control leg (PRE: 40 +/- 13 s vs. POST: 43 +/- 10 s) or the trained leg (PRE: 38 +/- 8 s vs. POST: 40 +/- 12 s). However, the amplitude of the [PCr] slow component was significantly reduced in the trained leg (PRE: 15 +/- 7 vs. POST: 7 +/- 7% change in [PCr]; P < 0.05) with there being no change in the control leg (PRE: 13 +/- 8 vs. POST: 12 +/- 10% change in [PCr]). The attenuation of the [PCr] slow component might be mechanistically linked with enhanced exercise tolerance following endurance training.

While phosphorous magnetic resonance spectroscopy (31P-MRS) and near-infrared spectroscopy (NIRS) provide methods for measuring spinal muscle function non-invasively, their reliability is not established. The aim of this study was assess the reliability (ICC) and error magnitude (CV %) of measurements of muscle phosphocreatine (PCr), tissue oxygenation index (TOI) and muscle deoxyhaemoglobin (HHb) acquired during fatigue and in recovery after 24 s of exercise in the lumbar muscles. 10 healthy participants (19–25 years, 5 male, 5 female) performed exercise that involved holding the upper body unsupported in slight extension until fatigue and then, after 30 min of rest, for repeated bursts of 24 s. ICCs indicated good to excellent reliability of baseline measures (TOI: 0.75) and of amplitude changes during fatigue (PCr: 0.73, TOI: 0.69, HHb: 0.80) and recovery (HHb: 0.96), and poor to fair reliability for time constants describing rates of change during fatigue (PCr: 0.11) and recovery (PCr: 0.31, HHb: 0.47). CV % indicated varying relative measurement error across baseline measures (TOI: 5 %), amplitude changes during fatigue (PCr: 7 %, TOI: 38 %, HHb: 31 %) and recovery (HHb: 31 %), and in time constants for fatigue (PCr: 39 %) and recovery (PCr: 20 %, HHb: 37 %). The results suggested that reliability would be sufficient for future studies on spinal muscle function, but that measurement error may be too large to evaluate individuals. [ABSTRACT FROM AUTHOR]