Factors affecting the rate of phosphocreatine resynthesis following intense exercise

Within the skeletal muscle cell at the onset of muscular contraction, phosphocreatine (PCr) represents the most immediate reserve for the rephosphorylation of adenosine triphosphate (ATP). As a result, its concentration can be reduced to less than 30% of resting levels during intense exercise. As a fall in the level of PCr appears to adversely affect muscle contraction, and therefore power output in a subsequent bout, maximising the rate of PCr resynthesis during a brief recovery period will be of benefit to an athlete involved in activities which demand intermittent exercise. Although this resynthesis process simply involves the rephosphorylation of creatine by aerobically produced ATP (with the release of protons), it has both a fast and slow component, each proceeding at a rate that is controlled by different components of the creatine kinase equilibrium. The initial fast phase appears to proceed at a rate independent of muscle pH. Instead, its rate appears to be controlled by adenosine diphosphate (ADP) levels; either directly through its free cytosolic concentration, or indirectly, through its effect on the free energy of ATP hydrolysis. Once this fast phase of recovery is complete, there is a secondary slower phase that appears almost certainly rate-dependent on the return of the muscle cell to homeostatic intracellular pH. Given the importance of oxidative phosphorylation in this resynthesis process, those individuals with an elevated aerobic power should be able to resynthesise PCr at a more rapid rate than their sedentary counterparts. However, results from studies that have used phosphorus nuclear magnetic resonance ((31)P-NMR) spectroscopy, have been somewhat inconsistent with respect to the relationship between aerobic power and PCr recovery following intense exercise. Because of the methodological constraints that appear to have limited a number of these studies, further research in this area is warranted.     

ATP production and mechanical work in exercising skeletal muscle: A theoretical analysis applied to 31P magnetic resonance spectroscopic studies of dialyzed uremic patients

³¹P magnetic resonance spectroscopy (³¹P MRS) can yield much information about bioenergetics in skeletal muscle. During mixed aerobic/glycolytic exercise, changes in phos-phocreatine (PCr) concentration and pH may be abnormal because of reduced muscle mass or reduced efficiency (which the authors combine here as “effective muscle mass”) or because of reduced oxidative capacity. The authors show how these can be distinguished by calculating the nonoxidative and oxidative costs of mechanical work, and also of work per unit of effective muscle mass (measured using the initial rate of ATP turnover). These quantities are substantially time-independent during incremental exercise, and so can be used to compare exercise studies of differing duration. The authors illustrate this analysis by showing that in dialyzed patients with chronic renal failure, the substantial exercise abnormalities seen by ³¹P MRS are due mainly to a decrease in effective muscle mass, which outweighs the oxidative defect implied by the abnormal PCr recovery kinetics.     

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.     

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.     

The relationship between aerobic fitness and recovery from high intensity intermittent exercise

A strong relationship between aerobic fitness and the aerobic response to repeated bouts of high intensity exercise has been established, suggesting that aerobic fitness is important in determining the magnitude of the oxidative response. The elevation of exercise oxygen consumption (VO2) is at least partially responsible for the larger fast component of excess post-exercise oxygen consumption (EPOC) seen in endurance-trained athletes following intense intermittent exercise. Replenishment of phosphocreatine (PCr) has been linked to both fast EPOC and power recovery in repeated efforts. Although 31P magnetic resonance spectroscopy studies appear to support a relationship between endurance training and PCr recovery following both submaximal work and repeated bouts of moderate intensity exercise, PCr resynthesis following single bouts of high intensity effort does not always correlate well with maximal oxygen consumption (VO2max). It appears that intense exercise involving larger muscle mass displays a stronger relationship between VO2max and PCr resynthesis than does intense exercise utilising small muscle mass. A strong relationship between power recovery and endurance fitness, as measured by the percentage VO2max corresponding to a blood lactate concentration of 4 mmol/L, has been demonstrated. The results from most studies examining power recovery and VO2max seem to suggest that endurance training and/or a higher VO2max results in superior power recovery across repeated bouts of high intensity intermittent exercise. Some studies have supported an association between aerobic fitness and lactate removal following high intensity exercise, whereas others have failed to confirm an association. Unfortunately, all studies have relied on measurements of blood lactate to reflect muscle lactate clearance, and different mathematical methods have been used for assessing blood lactate clearance, which may compromise conclusions on lactate removal. In summary, the literature suggests that aerobic fitness enhances recovery from high intensity intermittent exercise through increased aerobic response, improved lactate removal and enhanced PCr regeneration.     

A "functional biopsy" of muscle properties in sprinters and distance runners

PURPOSE: Fast- and slow-twitch human muscle fibers exhibit large (two- to threefold) differences in metabolic enzyme activities and contractile economy. We asked whether comparable flux differences are evident in the muscles of athletes specializing in extremely different (i.e., sprint and long-distance) running events. METHODS: We took an in vivo "functional biopsy" of the ankle dorsiflexor muscles of 17 members of a university track team by using (31)P magnetic resonance spectroscopy. Ten sprinters (SPR) and seven distance runners (DIS) performed rapid isometric dorsiflexions against the resistance of a plastic foot holder. The contractile cost of exercise and glycolytic flux were calculated from changes in pH, [PCr], and [P(i)] during ischemic exercise, and oxidative capacity was calculated from PCr recovery kinetics after aerobic exercise. RESULTS: Contractile costs were 47% higher in SPR than in DIS, whereas oxidative capacities were 52% higher in DIS than in SPR. Surprisingly, glycolytic ATP production was similar in the two groups. CONCLUSION: The muscles of SPR and DIS exhibit clear differences in energetic properties, but these differences are smaller than the two- to three-fold variations seen in the properties of individual muscle fibers.     

Comparison of measuring energy metabolism by different 31P‐magnetic resonance spectroscopy techniques in resting,ischemic, and exercising muscle

Alternate methods to quantify mitochondrial activity or function have been extensively used for studying insulin resistance and type 2 diabetes mellitus, namely saturation transfer and phosphocreatine (PCr) recovery. As these methods are in fact determining different parameters, this study aimed to compare saturation transfer results to PCr recovery measurements within the same group. Fifteen subjects underwent saturation transfer and ischemic exercise‐recovery experiments. PCr decrease during ischemia (Q), induced by cuff inflation, served as an additional measure of resting ATP (adenosine triphosphate) production. ATP synthetic rate (fATP) measured by saturation transfer (0.234 ± 0.043 mM/s) was greater than (Q = 0.0077 ± 0.0011 mM/s), but correlated well with Q (r = 0.63 P = 0.013). Parameters of PCr recovery correlated well with fATP (Q_max,lin: r = 0.71, P = 0.003, Q_max,ADP: r = 0.66, P = 0.007) and Q (Q_max,lin: r = 0.92, P = 0.000002, Q_max,ADP: r = 0.76, P = 0.001). In conclusion, although saturation transfer yields higher ATP synthetic rates than PCr decrease during ischemia, their significant correlation indicates that fATP can be used as a marker of mitochondrial activity. The finding that both Q and fATP correlate with PCr recovery kinetics suggests that skeletal muscle with greater maximal aerobic ATP synthetic rates is also metabolically more active at rest. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

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.     

Postexercise recovery period: carbohydrate and protein metabolism

The essence of the postexercise recovery period is normalization of function and homeostatic equilibrium, and replenishment of energy resources and accomplishment of the reconstructive function. The repletion of energy stores is actualized in a certain sequence and followed by a transitory supercompensation. The main substrate for repletion of the muscle glycogen store is blood glucose derived from hepatic glucose output as well as from consumption of carbohydrates during the postexercise period. The repletion of liver glycogen is realized less repidly. It depends to a certain extent on hepatic gluconeogenesis but mainly on supply with exogenous carbohydrates. The constructive function is founded on elevated protein turnover and adaptive protein synthesis. Whereas during and shortly after endurance exercise intensive protein breakdown was found in less active fast-twitch glycolytic fibers, during the later course of the recovery period the protein degradation rate increased together with intensification of protein synthesis rate in more active fast-twitch glycolytic oxidative and slow-twitch oxidative fibers.     

Metabolic Factors in Fatigue

KEYPOINTS Sustainedproductionofmuscularforceduringexercise dependsonthegenerationofchemicalenergy(ATP)bynonoxidative(anaerobic)andoxidative(aerobic)metabolism.Fatiguedevelopswhenthecompoundsneededtopro duceATParedepletedorwhenby-productsof metabolismaccumulateinmuscle.Thesemetabolicchangescancausefatiguebyactingon nerveprocessesthatactivatemuscles.Bothcentral andperipheralnervoussystemsmaybeimpaired.ReductionsinmusclelevelsofATP,phosphocreatine,andglycogen,andlowbloodglucoseavailabilit…     

Citrulline/malate promotes aerobic energy production in human exercising muscle

BACKGROUND: Previous studies have shown an antiasthenic effect of citrulline/malate (CM) but the mechanism of action at the muscular level remains unknown. OBJECTIVE: To investigate the effects of CM supplementation on muscle energetics. METHODS: Eighteen men complaining of fatigue but with no documented disease were included in the study. A rest-exercise (finger flexions)-recovery protocol was performed twice before (D-7 and D0), three times during (D3, D8, D15), and once after (D22) 15 days of oral supplementation with 6 g/day CM. Metabolism of the flexor digitorum superficialis was analysed by (31)P magnetic resonance spectroscopy at 4.7 T. RESULTS: Metabolic variables measured twice before CM ingestion showed no differences, indicating good reproducibility of measurements and no learning effect from repeating the exercise protocol. CM ingestion resulted in a significant reduction in the sensation of fatigue, a 34% increase in the rate of oxidative ATP production during exercise, and a 20% increase in the rate of phosphocreatine recovery after exercise, indicating a larger contribution of oxidative ATP synthesis to energy production. Considering subjects individually and variables characterising aerobic function, extrema were measured after either eight or 15 days of treatment, indicating chronological heterogeneity of treatment induced changes. One way analysis of variance confirmed improved aerobic function, which may be the result of an enhanced malate supply activating ATP production from the tricarboxylic acid cycle through anaplerotic reactions. CONCLUSION: The changes in muscle metabolism produced by CM treatment indicate that CM may promote aerobic energy production.     

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.     

An enzymatic approach to lactate production in human skeletal muscle during exercise

PURPOSE: This paper examines the production of lactate in human skeletal muscle over a range of power outputs (35-250% VO2max) from an enzymatic flux point of view. The conversion of pyruvate and NADH to lactate and NAD in the cytoplasm of muscle cells is catalyzed by the near-equilibrium enzyme lactate dehydrogenase (LDH). As flux through LDH is increased by its substrates, pyruvate and NADH, the factors governing the production of these substrates will largely dictate how much lactate is produced at any exercise power output. In an attempt to understand lactate production, flux rates through the enzymes that regulate glycogenolysis/glycolysis, the transfer of cytoplasmic reducing equivalents into the mitochondria, and the various fates of pyruvate have been measured or estimated. RESULTS: At low power outputs, the rates of pyruvate and NADH production in the cytoplasm are low, and pyruvate dehydrogenase (PDH) and the shuttle system enzymes (SS) metabolize the majority of these substrates, resulting in little or no lactate production. At higher power outputs (65, 90, and 250% VO2max), the mismatch between the ATP demand and aerobic ATP provision at the onset of exercise increases as a function of intensity, resulting in increasing accumulations of the glycogenolytic/glycolytic activators (free ADP, AMP, and Pi). The resulting glycolytic flux, and NADH and pyruvate production, is progressively greater than can be handled by the SS and PDH, and lactate is produced at increasing rates. Lactate production during the onset of exercise and 10 min of sustained aerobic exercise may be a function of adjustments in the delivery of O2 to the muscles, adjustments in the activation of the aerobic ATP producing metabolic pathways and/or substantial glycogenolytic/glycolytic flux through a mass action effect.     

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.     

31P-MRS characterization of sprint and endurance trained athletes

Muscle metabolism and force production were studied in sprint trained runners, endurance trained runners and in untrained subjects, using 31P-MRS. 31P-spectra were obtained at a time resolution of 5 s during four maximal isometric contractions of 30-sec duration, interspersed by 60-sec recovery intervals. Resting CrP/ATP ratio averaged 3.3 +/- 0.3, with no difference among the three groups. The sprint trained subjects showed about 20 % larger contraction forces in contraction bouts 1 and 2 (p < 0.05). The groups differed with respect to CrP breakdown (p < 0.05), with sprinters demonstrating about 75 % breakdown in each contraction compared to about 60 % and 40 % for untrained and endurance trained subjects, respectively (p < 0.05). The endurance trained runners showed almost twice as fast CrP recovery (t 1/2 = 12.5 +/- 1.5) compared to sprint trained (t 1/2 = 22.5 +/- 2.53) and untrained subjects (t 1/2 = 26.4 +/- 2.8). From the initial rate of CrP resynthesis the rate of maximal aerobic ATP synthesis was estimated to 0.74 +/- 0.07, 0.73 +/- 0.10 and 0.33 +/- 0.07 mmol ATP x kg -1 wet muscle x sec -1 for sprint trained, endurance trained and untrained subjects, respectively. Only the sprint trained and the untrained subjects displayed a significant drop in pH and only during the first of the four contractions, about 0.2 and 0.1 pH units, respectively, indicating that only under those contractions was the glycolytic proton production larger than the proton consumption by the CK reaction. Also, in the first contraction the energy cost of contraction was higher for the sprinters compared to the two other groups. The simple 31P-MRS protocol used in the present study demonstrates marked differences in force production, aerobic as well as anaerobic muscle metabolism, clearly allowing differentiation between endurance trained, sprint trained and untrained subjects.     

Effect of exogenous creatine supplementation on muscle PCr metabolism

31P NMR was used to assess the influence of two weeks creatine supplementation (21g x d(-1)) on resting muscle PCr concentration, on the rate of PCr repletion (R(depl)), and on the half-time of PCr repletion (t 1/2). Body mass (BM) and volume of body water compartments were also estimated by impedance spectroscopy. Fourteen healthy male subjects (20.8+/-1.9 y) participated in this double-blind study. PCr was measured using a surface coil placed under the calf muscle, at rest and during two exercise bout the duration of which was 1 min. They were interspaced by a recovery of 10 min. The exercises comprised of 50 plantar flexions-extensions against weights corresponding to 40% and 70% of maximal voluntary contraction (MVC), respectively. Creatine supplementation increased resting muscle PCr content by approximately 20% (P= 0.002). R(depl) was also increased by approximately 15% (P< 0.001) and approximately 10% (P = 0.026) during 40% and 70% MVC exercises, respectively. No change was observed in R(repl) and t1/2. BM and body water compartments were not influenced. These results indicate that during a standardized exercise more ATP is synthesized by the CK reaction when the pre-exercise level in PCr is higher, giving some support to the positive effects recorded on muscle performance.     

Muscle high-energy metabolites and metabolic capacity in patients with heart failure

OKITA, K., K. YONEZAWA, H. NISHIJIMA, A. HANADA, T. NAGAI, T. MURAKAMI, and A. KITABATAKE. Muscle high-energy metabolites and metabolic capacity in patients with heart failure. Med Sci. Sports. Exerc., Vol. 33, No. 3, 2001, pp. 442-448. Background: Various abnormalities in skeletal muscle have been demonstrated by biopsy in patients with chronic heart failure (CHF). In mammalian muscles, high-energy metabolite composition at rest (HEMC) provides data on important metabolic characteristics; however, the significance of HEMC has not been clarified in patients with CHF. Therefore, we investigated HEMC in normal subjects and patients with CHF and examined its relation to muscle metabolic capacity and exercise tolerance. Methods: High-energy metabolites (phosphocreatine (PCr), inorganic phosphate (Pi), and ATP) in resting calf muscle were measured by 31P-magnetic resonance spectroscopy (31P-MRS), and ratios of Pi to PCr, Pi to ATP, and PCr to ATP were calculated in 34 patients with CHF and 13 age- and size-matched normal subjects. Muscle metabolism was evaluated during local exercise of unilateral plantar flexion by 31P-MRS. Metabolic capacity was estimated by the rate of PCr breakdown in relation to the workload. Systemic exercise capacity was evaluated by a bicycle ergometer. Results: The ratio of PCr to ATP was significantly increased in patients with CHF compared with controls (3.06 +/- 0.43 vs 2.72 +/- 0.36, P < 0.05) and was significantly correlated with metabolic capacity (r = -0.37, P < 0.01) and with peak oxygen uptake (r = -0.45, P < 0.01). There was a significant correlation between metabolic capacity and peak oxygen uptake (r = 0.53, P < 0.001). Conclusion: HEMC was altered in patients with CHF, and this change was related to metabolic capacity and exercise capacity. These findings provide new insight into the mechanism of impaired muscle metabolism in CHF.     

Recovery kinetics throughout successive bouts of various exercises in elite cyclists

PURPOSE: In the present study we investigated whether a high volume of cycling training would influence the metabolic changes associated with a succession of three exhaustive cycling exercises. METHODS: Seven professional road cyclists (VO2max: 74.3 +/- 3.7 mL.min.kg; maximal power tolerated: 475 +/- 18 W; training: 22 +/- 3 h.wk) and seven sport sciences students (VO2max: 54.2 +/- 5.3 mL.min.kg; maximal power tolerated: 341 +/- 26 W; training: 6 +/- 2 h.wk) performed three different exhaustive cycling exercise bouts (progressive, constant load, and sprint) on an electrically braked cycloergometer positioned near the magnetic resonance scanner. Less than 45 s after the completion of each exercise bout, recovery kinetics of high-energy phosphorylated compounds and pH were measured using P-MR spectroscopy. RESULTS: Resting values for phosphomonoesters (PME) and phosphodiesters (PDE) were significantly elevated in the cyclist group (PME/ATP: 0.82 +/- 0.11 vs 0.58 +/- 0.19; PDE/ATP: 0.27 +/- 0.03 vs 0.21 +/- 0.05). Phosphocreatine (PCr) consumption and inorganic phosphate (Pi) accumulation measured at end of exercise bouts 1 (PCr: 6.5 +/- 3.2 vs 10.4 +/- 1.6 mM; Pi: 1.6 +/- 0.7 vs 6.8 +/- 3.4 mM) and 3 (PCr: 5.6 +/- 2.4 vs 9.3 +/- 3.9 mM; Pi: 1.5 +/- 0.5 vs 7.7 +/- 3.3 mM) were reduced in cyclists compared with controls. During the recovery period after each exercise bout, the pH-recovery rate was larger in professional road cyclists, whereas the PCr-recovery kinetics were significantly faster for cyclists only for bout 3. DISCUSSION: Whereas the PDE and PME elevation at rest in professional cyclists may indicate fiber-type changes and an imbalance between glycogenolytic and glycolytic activity, the lower PCr consumption during exercise and the faster pH-recovery kinetic clearly suggest an improved mitochondrial function.     

Energetics of human muscle: exercise-induced ATP depletion

The energetics of human muscle have been investigated in vivo during and after fatiguing aerobic, dynamic exercise. Changes in cytoplasmic pH and concentrations of phosphocreatine, ATP and Pi were followed using 31P nuclear magnetic resonance spectroscopy. ATP was significantly depleted in 6 out of 12 experiments and in these 6 experiments decreased to 55 +/- 5% of the pre-exercise concentration. Depleted muscle had a lower phosphocreatine concentration (17 +/- 5% of resting value) and lower pH (6.12 +/- 0.04) than fatigued muscle in which ATP loss was not observed (26 +/- 5% for phosphocreatine and 6.37 +/- 0.09 for pH). The free energy of hydrolysis of ATP was not significantly different in the two groups and was also similar in exhausted and nonexhausted muscle. Loss of ATP was associated with altered recovery of the muscle: [phosphocreatine], [Pi], and pH returned more slowly to their pre-exercise values and the initial rate of oxidative phosphorylation was diminished. The restitution of [ATP] to its pre-exercise value was much slower than that of the other metabolites.