High content of MYHC II in vastus lateralis is accompanied by higher VO2/power output ratio during moderate intensity cycling performed both at low and at high pedalling rates.

The aim of this study was to examine the relationship between the content of various types of myosin heavy chain isoforms (MyHC) in the vastus lateralis muscle and pulmonary oxygen uptake during moderate power output incremental exercise, performed at low and at high pedalling rates. Twenty one male subjects (mean +/- SD) aged 24.1 +/- 2.8 years; body mass 72.9 +/- 7.2 kg; height 179.1 +/- 4.8 cm; BMI 22.69 +/- 1.89 kg.m(-2); VO2max 50.6 +/- 5.3 ml.kg.min(-1), participated in this study. On separate days, they performed two incremental exercise tests at 60 rev.min(-1) and at 120 rev.min(-1), until exhaustion. Gas exchange variables were measured continuously breath by breath. Blood samples were taken for measurements of plasma lactate concentration prior to the exercise test and at the end of each step of the incremental exercise. Muscle biopsies were taken from the vastus lateralis muscle, using Bergstr?m needle, and they were analysed for the content of MyHC I and MyHC II using SDS--PAGE and two groups (n=7, each) were selected: group H with the highest content of MyHC II (60.7 % +/- 10.5 %) and group L with the lowest content of MyHC II (27.6 % +/- 6.1 %). We have found that during incremental exercise at the power output between 30-120 W, performed at 60 rev.min(-1), oxygen uptake in the group H was significantly greater than in the group L (ANCOVA, p=0.003, upward shift of the intercept in VO2/power output relationship). During cycling at the same power output but at 120 rev.min(-1), the oxygen uptake was also higher in the group H, when compared to the group L (i.e. upward shift of the intercept in VO2/power output relationship, ANCOVA, p=0.002). Moreover, the increase in pedalling rate from 60 to 120 rev.min(-1) was accompanied by a significantly higher increase of oxygen cost of cycling and by a significantly higher plasma lactate concentration in subjects from group H. We concluded that the muscle mechanical efficiency, expressed by the VO2/PO ratio, during cycling in the range of power outputs 30-120 W, performed at 60 as well as 120 rev.min(-1), is significantly lower in the individuals with the highest content of MyHC II, when compared to the individuals with the lowest content of MyHC II in the vastus lateralis.     

The effect of pedaling frequency on glycogen depletion rates in type I and type II quadriceps muscle fibers during submaximal cycling exercise

This study was conducted to determine whether the pedaling frequency of cycling at a constant metabolic cost contributes to the pattern of fiber-type glycogen depletion. On 2 separate days, eight men cycled for 30 min at approximately 85% of individual aerobic capacity at pedaling frequencies of either 50 or 100 rev.min-1. Muscle biopsy samples (vastus lateralis) were taken immediately prior to and after exercise. Individual fibers were classified as type I (slow twitch), or type II (fast twitch), using a myosin adenosine triphosphatase stain, and their glycogen content immediately prior to and after exercise quantified via microphotometry of periodic acid-Schiff stain. The 30-min exercise bout resulted in a 46% decrease in the mean optical density (D) of type I fibers during the 50 rev.min-1 condition [0.52 (0.07) to 0.28 (0.04) D units; mean (SEM)] which was not different (P > 0.05) from the 35% decrease during the 100 rev.min-1 condition [0.48 (0.04) to 0.31 (0.05) D units]. In contrast, the mean D in type II fibers decreased 49% during the 50 rev.min-1 condition [0.53 (0.06) to 0.27 (0.04) units]. This decrease was greater (P < 0.05) than the 33% decrease observed in the 100 rev.min-1 condition [0.48 (0.04) to 0.32 (0.06) units). In conclusion, cycling at the same metabolic cost at 50 rather than 100 rev.min-1 results in greater type II fiber glycogen depletion. This is attributed to the increased muscle force required to meet the higher resistance per cycle at the lower pedal frequency.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of different cycling frequencies during incremental exercise on the venous plasma potassium concentration in humans

The effect of different muscle shortening velocity was studied during cycling at a pedalling rate of 60 and 120 rev.min(-1) on the [K+]v in humans. Twenty-one healthy young men aged 22.5+/-2.2 years, body mass 72.7+/-6.4 kg, VO2 max 3.720+/-0.426 l. min(-1), performed an incremental exercise test until exhaustion. The power output increased by 30 W every 3 min, using an electrically controlled ergometer Ergoline 800 S (see Zoladz et al. J. Physiol. 488: 211-217, 1995). The test was performed twice: once at a cycling frequency of 60 rev.min(-1) (test A) and a few days later at a frequency of 120 rev. min(-1) (test B). At rest and at the end of each step (i.e. the last 15 s) antecubital venous blood samples for [K+]p were taken. Gas exchange variables were measured continuously (breath-by-breath) using Oxycon Champion Jaeger. The pre-exercise [K+]v in both tests was not significantly different amounting to 4.24+/-0.36 mmol.l(-1) in test A, and 4.37+/-0.45 mmol.l(-1) in test B. However, the [K+]p during cycling at 120 rev. min(-1) was significantly higher (p<0.001, ANOVA for repeated measurements) at each power output when compared to cycling at 60 rev.min(-1). The maximal power output reached 293+/-31 W in test A which was significantly higher (p<0.001) than in test B, which amounted to 223+/-40 W. The VO2max values in both tests reached 3.720+/-0.426 l. min(-1) vs 3.777+/-0.514 l. min(-1). These values were not significantly different. When the [K+]v was measured during incremental cycling exercise, a linear increase in [K+]v was observed in both tests. However, a significant (p<0.05) upward shift in the [K+]v and a % VO2max relationship was detected during cycling at 120 rev.min(-1). The [K+]v measured at the VO2max level in tests A and B amounted to 6.00+/-0.47 mmol.l-1 vs 6.04+/-0.41 mmol.l-1, respectively. This difference was not significant. It may thus be concluded that: a) generation of the same external mechanical power output during cycling at a pedalling rate of 120 rev.min(-1) causes significantly higher [K+]v changes than when cycling at 60 rev.min(-1), b) the increase of venous plasma potassium concentration during dynamic incremental exercise is linearly related to the metabolic cost of work expressed by the percentage of VO2max (increase as reported previously by Vollestad et al. J. Physiol. 475: 359-368, 1994), c) there is a tendency towards upward up shift in the [K+]v and % VO2max relation during cycling at 120 rev.min(-1) when compared to cycling at 60 rev.min(-1).     

Determinants of endurance in well-trained cyclists

Fourteen competitive cyclists who possessed a similar maximum O2 consumption (VO2 max; range, 4.6-5.0 l/min) were compared regarding blood lactate responses, glycogen usage, and endurance during submaximal exercise. Seven subjects reached their blood lactate threshold (LT) during exercise of a relatively low intensity (group L) (i.e., 65.8 +/- 1.7% VO2 max), whereas exercise of a relatively high intensity was required to elicit LT in the other seven men (group H) (i.e., 81.5 +/- 1.8% VO2 max; P less than 0.001). Time to fatigue during exercise at 88% of VO2 max was more than twofold longer in group H compared with group L (60.8 +/- 3.1 vs. 29.1 +/- 5.0 min; P less than 0.001). Over 92% of the variance in performance was related to the % VO2 max at LT and muscle capillary density. The vastus lateralis muscle of group L was stressed more than that of group H during submaximal cycling (i.e., 79% VO2 max), as reflected by more than a twofold greater (P less than 0.001) rate of glycogen utilization and blood lactate concentration. The quality of the vastus lateralis in groups H and L was similar regarding mitochondrial enzyme activity, whereas group H possessed a greater percentage of type I muscle fibers (66.7 +/- 5.2 vs. 46.9 +/- 3.8; P less than 0.01). The differing metabolic responses to submaximal exercise observed between the two groups appeared to be specific to the leg extension phase of cycling, since the blood lactate responses of the two groups were comparable during uphill running. These data indicate that endurance can vary greatly among individuals with an equal VO2 max.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of different muscle shortening velocities during prolonged incremental cycling exercise on the plasma growth hormone, insulin, glucose, glucagon, cortisol, leptin and lactate concentrations.

BACKGROUND: Strenuous exercise was reported to involve the alteration in the release of some "stress" hormones such as growth hormone (GH), cortisol, catecholamines and appropriate adjustment of energy metabolism but the relative contribution of these hormones to metabolic response, to cycling exercise performed at different muscle shortening velocities, has not been clarified. AIMS: The purpose of this experiment was to assess the effect of applying different pedalling rates during a prolonged incremental cycling exercise test on the changes in the plasma levels of growth hormone, cortisol, insulin, glucagon and leptin in humans. Material and METHODS: Fifteen healthy non-smoking men (means +/- SD: age 22.9 +/- 2.4 years; body mass 71.9 +/- 8.2 kg; height 178 +/- 6 cm; with VO2max of 3.896 +/- 0.544 1 x min(-1), assessed in laboratory tests, were subjects in this study. The subjects performed in two different days a prolonged incremental exercise tests at two different pedalling rates, one of them at 60 and another at 120 rev x min(-1). During this tests the power output has increased by 30 W every 6 minutes. The tests were stopped when the subject reached about 70 % of the VO2max. RESULTS AND CONCLUSIONS: We have found that choosing slow or fast pedalling rates (60 or 120 rev min(-1)), while generating the same external mechanical power output, had no effect on the pattern of changes in plasma cortisol, insulin, glucagon, glucose and leptin concentrations. But, generation of the same external mechanical power output at 120 rev min(-1) causes more stepper increase (p < 0.01) in the plasma growth hormone concentration [GH]pl and plasma lactate concentrations [La]pl when compared to that observed during cycling at 60 rev x min(-1). We have also found that the onset of a significant increase in [GH]pl during cycling at 60 rev x min(-1) was not accompanied by significant increase in [La]pl. While during cycling at 120 rev x min(-1) the onset of a significant increase in [La]pl occurred without increase in [GH]pl, but with continuation of exercise when plasma [La]pl increased, there was also a parallel rise in plasma [GH]pl, as reported before. This results indicates that the increase in [GH]pl during exercise is not closely related to the increase in [La]pl.     

Muscle fiber type effects on energetically optimal cadences in cycling

Fast-twitch (FT) and slow-twitch (ST) muscle fibers vary in their mechanical and energetic properties, and it has been suggested that muscle fiber type distribution influences energy expenditure and the energetically optimal cadence during pedaling. However, it is challenging to experimentally isolate the effects of muscle fiber type on pedaling energetics. In the present study, a modeling and computer simulation approach was used to test the dependence of muscle energy expenditure on pedaling rate during submaximal cycling. Simulations were generated using a musculoskeletal model at cadences from 40 to 120 rev min(-1), and the dynamic and energetic properties of the model muscles were scaled to represent a range of muscle fiber types. Energy expenditure and the energetically optimal cadence were found to be higher in a model with more FT fibers than a model with more ST fibers, consistent with predictions from the experimental literature. At the muscle level, mechanical efficiency was lower in the model with a greater proportion of FT fibers, but peaked at a higher cadence than in the ST model. Regardless of fiber type distribution, mechanical efficiency was low at 40 rev min(-1), increased to a broad plateau between 60 and 100 rev min(-1) , and decreased substantially at 120 rev min(-1). In conclusion, muscle fiber type distribution was confirmed as an important determinant of the energetics of pedaling.     

Effect of fatigue on maximal power output at different contraction velocities in humans.

The effect of fatigue as a result of a standard submaximal dynamic exercise on maximal short-term power output generated at different contraction velocities was studied in humans. Six subjects performed 25-s maximal efforts on an isokinetic cycle ergometer at five different pedaling rates (60, 75, 90, 105, and 120 rpm). Measurements of maximal power output were made under control conditions [after 6 min of cycling at 30% maximal O2 uptake (VO2max)] and after fatiguing exercise that consisted of 6 min of cycling at 90% VO2max with a pedaling rate of 90 rpm. Compared with control values, maximal peak power measured after fatiguing exercise was significantly reduced by 23 +/- 19, 28 +/- 11, and 25 +/- 11% at pedaling rates of 90, 105, and 120 rpm, respectively. Reductions in maximum peak power of 11 +/- 8 and 14 +/- 8% at 60 and 75 rpm, respectively, were not significant. The rate of decline in peak power during the 25-s control measurement was least at 60 rpm (5.1 +/- 2.3 W/s) and greatest at 120 rpm (26.3 +/- 13.9 W/s). After fatiguing exercise, the rate of decline in peak power at pedaling rates of 105 and 120 rpm decreased significantly from 21.5 +/- 9.0 and 26.3 +/- 13.9 W/s to 10.0 +/- 7.3 and 13.3 +/- 6.9 W/s, respectively. These experiments indicate that fatigue induced by submaximal dynamic exercise results in a velocity-dependent effect on muscle power. It is suggested that the reduced maximal power at the higher velocities was due to a selective effect of fatigue on the faster fatigue-sensitive fibers of the active muscle mass.     

Skeletal muscle oxygenation during incremental exercise

The purpose of this study was to investigate the relationship between muscle oxygenation level at exhaustion and maximal oxygen uptake (VO2max) in an incremental cycling exercise. Nine male subjects took part in an incremental exhaustive cycling exercise, and then cuff occlusion was performed. Changes in oxy-(deltaHbO2) and deoxy-(deltaHb) hemoglobin concentrations in the vastus lateralis muscle were measured with a near infrared spectroscopy (NIRS). Muscle oxygenation during incremental exercise was expressed as a percentage (%Moxy) of the maximal range observed during an arterial occlusion as the lower reference point. A systematic decrease was observed in %Moxy with increasing intensity. A significant relationship was observed between %Moxy at exhaustion and VO2max (p < 0.01). We concluded that the one of the limiting factor of VO2max is the muscle oxygen diffusion capacity, and %Moxy during exercise could be one of the indexes of muscle oxygen diffusion capacity.     

Reproducibility of eight lower limb muscles activity level in the course of an incremental pedaling exercise.

Despite the wide use of surface electromyography (EMG) recorded during dynamic exercises, the reproducibility of EMG variables has not been fully established in a course of a dynamic leg exercise. The aim of this study was to investigate the reproducibility of eight lower limb muscles activity level during a pedaling exercise performed until exhaustion. Eight male were tested on two days held three days apart. Surface EMG was recorded from vastus lateralis, rectus femoris (RF), vastus medialis, semimembranosus, biceps femoris, gastrocnemius lateral, gastrocnemius medianus and tibialis anterior during incremental exercise test. The root mean square, an index of global EMG activity, was averaged every five crank revolutions (corresponding to about 3 s at 85 rpm) throughout the tests. Despite inter-subjects variations, we showed a high reproducibility of the activity level of lower limb muscles during a progressive pedaling exercise performed until exhaustion. However, RF muscle seemed to be the less reproducible of the eight muscles investigated during incremental pedaling exercise. These results suggest that each subject adopt a personal muscle activation strategy in a course of an incremental cycling exercise but fatigue phenomenon can induce some variations in the most fatigable muscles (RF).     

Alterations in human muscle fibre type distribution induced by acute exercise

Fibre type distributions in the vastus lateralis muscles of six male subjects (age 18 to 22 years) have been compared at rest and during exercise. Exercise consisted of one leg cycling at 60% VO2 max (one leg) for 120 min. The increased contractile activity was associated with a 28.8% (p less than 0.05) decrease in the distribution of Type I fibres in the exercised leg. This change in fibre type distribution was manifested early in the exercise (15 min), and was also evident in muscle samples obtained after 60 and 120 min of activity. Reductions in the Type I fibre distribution was accompanied by an increase in the Type II fibres, specifically the Type IIA distribution (p less than 0.05). Comparable alterations in the specific fibre distribution were also found in the non exercising leg. These observations indicate that alterations in the muscle cell induced either directly or indirectly by the increased contractile activity interact with normal pre-incubation conditions to effect changes in the stability of the myofibrillar ATPase reaction. Specifically, it appears that a percentage of the Type I fibre population becomes acid labile and alkali stable.     

Regulation of PDH in human arm and leg muscles at rest and during intense exercise

To test the hypothesis that pyruvate dehydrogenase (PDH) is differentially regulated in specific human muscles, regulation of PDH was examined in triceps, deltoid, and vastus lateralis at rest and during intense exercise. To elicit considerable glycogen use, subjects performed 30 min of exhaustive arm cycling on two occasions and leg cycling exercise on a third day. Muscle biopsies were obtained from deltoid or triceps on the arm exercise days and from vastus lateralis on the leg cycling day. Resting PDH protein content and phosphorylation on PDH-E1 alpha sites 1 and 2 were higher (P < or = 0.05) in vastus lateralis than in triceps and deltoid as was the activity of oxidative enzymes. Net muscle glycogen utilization was similar in vastus lateralis and triceps ( approximately 50%) but less in deltoid (likely reflecting less recruitment of deltoid), while muscle lactate accumulation was approximately 55% higher (P < or = 0.05) in triceps than vastus lateralis. Exercise induced (P < or = 0.05) dephosphorylation of both PDH-E1 alpha site 1 and site 2 in all three muscles, but it was more pronounced at PDH-E1 alpha site 1 in triceps than in vastus lateralis (P < or = 0.05). The increase in activity of the active form of PDH (PDHa) after 10 min of exercise was more marked in vastus lateralis ( approximately 246%) than in triceps ( approximately 160%), but when it was related to total PDH-E1 alpha protein content, no difference was evident. In conclusion, PDH protein content seems to be related to metabolic enzyme profile, rather than myosin heavy chain composition, and less PDH capacity in triceps is a likely contributing factor to higher lactate accumulation in triceps than in vastus lateralis.     

EMG versus oxygen uptake during cycling exercise in trained and untrained subjects.

The aim of this study was to test the hypothesis that bicycle training may improve the relationship between the global SEMG energy and VO2. We already showed close adjustment of the root mean square (RMS) of the surface electromyogram (SEMG) to the oxygen uptake (VO2) during cycling exercise in untrained subjects. Because in these circumstances an altered neuromuscular transmission which could affect SEMG measurement occurred in untrained individuals only, we searched for differences in the SEMG vs. VO2 relationship between untrained subjects and well-trained cyclists. Each subject first performed an incremental exercise to determine VO2max and the ventilatory threshold, and second a constant-load threshold cycling exercise, continued until exhaustion. SEMG from both vastus lateralis muscles was continuously recorded. RMS was computed. M-Wave was periodically recorded. During incremental exercise: (1) a significant non-linear positive correlation was found between RMS increase and VO2 increase in untrained subjects, whereas the relationship was best fitted by a straight line in trained cyclists; (2) the RMS/VO2 ratio decreased progressively throughout the incremental exercise, its decline being significantly and markedly accentuated in trained cyclists; (3) in untrained subjects, significant M-wave alterations occurred at the end of the trial. These M-wave alterations could explain the non-linear RMS increase in these individuals. During constant-load exercise: (1) after an initial increase, the VO2 ratio decreased progressively to reach a plateau after 2 min of exercise, but no significant inter-group differences were noted; (2) no M-wave changes were measured in the two groups. We concluded that the global SEMG energy recorded from the vastus lateralis muscle is a good estimate of metabolic energy expenditure during incremental cycling exercise only in well-trained cyclists.     

VO2/power output relationship and the slow component of oxygen uptake kinetics during cycling at different pedaling rates: relationship to venous lactate accumulation and blood acid-base balance.

In this experiment we studied the effect of different pedalling rates during cycling at a constant power output (PO) 132+/-31 W (mean+/-S.D.), corresponding to 50% VO2 max, on the oxygen uptake and the magnitude of the slow component of VO2 kinetics in humans. The PO corresponded to 50% of VO2 max, established during incremental cycling at a pedalling rate of 70 rev.min(-1). Six healthy men aged 22.2+/-2.0 years with VO2 max 3.89+/-0.92 l.min(-1), performed on separate days constant PO cycling exercise lasting 6 min at pedalling rates 40, 60, 80, 100 and 120 rev.min(-1), in random order. Antecubital blood samples for plasma lactate [La]pl and blood acid-base balance variables were taken at 1 min intervals. Oxygen uptake was determined breath-by-breath. The total net oxygen consumed throughout the 6 min cycling period at pedalling rates of 40, 60, 80, 100 and 120 rev.min(-1) amounted to 7.727+/-1.197, 7.705+/-1.548, 8.679+/-1.262, 9.945+/-1.435 and 13.720+/-1.862 l, respectively for each pedalling rate. The VO2 during the 6 min of cycling only rose slowly by increasing the pedalling rate in the range of 40-100 rev.min(-1). This increase, was 0.142 l per 20 rev.min(-1) on the average. Plasma lactate concentration during the sixth minute of cycling changed little within this range of pedalling rates: the values were 1.83+/-0.70, 1.80+/-0.48, 2.33+/-0.88 and 2.52+/-0.33 mmol.l(-1). The values of [La]pl reached in the 6th minute of cycling were not significantly different from the pre-exercise levels. Blood pH was also not affected by the increase of pedalling rate in the range of 40-100 rev.min(-1). However, an increase of pedalling rate from 100 to 120 rev.min(-1) caused a sudden increase in the VO2 amounting to 0.747 l per 20 rev.min(-1), accompanied by a significant increase in [La]pl from 1.21+/-0.26 mmol.l(-1) in pre-exercise conditions to 5.92+/-2.46 mmol.l(-1) reached in the 6th minute of cycling (P<0.01). This was also accompanied by a significant drop of blood pH, from 7.355+/-0.039 in the pre-exercise period to 7.296+/-0.060 in the 6th minute of cycling (P < 0.01). The mechanical efficiency calculated on the basis of the net VO2 reached between the 4th and the 6th minute of cycling amounted to 26.6+/-2.7, 26.4+/-2.0, 23.4+/-3.4, 20.3+/-2.6 and 14.7+/-2.2%, respectively for pedalling rates of 40, 60, 80, 100 and 120 rev.min(-1). No significant increase in the VO2 from the 3rd to the 6th min (representing the magnitude of the slow component of VO2 kinetics) was observed at any of the pedalling rates (-0.022+/-0.056, -0.009+/-0.029, 0.012+/-0.073, 0.030+/-0.081 and 0.122+/-0.176 l.min(-1) for pedalling rates of 40, 60, 80, 100 and 120 rev.min(-1), respectively). Thus a significant increase in [La]pl and a decrease in blood pH do not play a major role in the mechanism(s) responsible for the slow component of VO2 kinetics in humans.     

Effect of induced metabolic alkalosis on human skeletal muscle metabolism during exercise

The purpose of the study was to examine the roles of active pyruvate dehydrogenase (PDH(a)), glycogen phosphorylase (Phos), and their regulators in lactate (Lac(-)) metabolism during incremental exercise after ingestion of 0.3 g/kg of either NaHCO(3) [metabolic alkalosis (ALK)] or CaCO(3) [control (CON)]. Subjects (n = 8) were studied at rest, rest postingestion, and during constant rate cycling at three stages (15 min each): 30, 60, 75% of maximal O(2) uptake (VO(2 max)). Radial artery and femoral venous blood samples, leg blood flow, and biopsies of the vastus lateralis were obtained during each power output. ALK resulted in significantly (P < 0.05) higher intramuscular Lac(-) concentration ([Lac(-)]; ALK 72.8 vs. CON 65.2 mmol/kg dry wt), arterial whole blood [Lac(-)] (ALK 8.7 vs. CON 7.0 mmol/l), and leg Lac(-) efflux (ALK 10.0 vs. CON 4.2 mmol/min) at 75% VO(2 max). The increased intramuscular [Lac(-)] resulted from increased pyruvate production due to stimulation of glycogenolysis at the level of Phos a and phosphofructokinase due to allosteric regulation mediated by increased free ADP (ADP(f)), free AMP (AMP(f)), and free P(i) concentrations. PDH(a) increased with ALK at 60% VO(2 max) but was similar to CON at 75% VO(2 max). The increased PDH(a) may have resulted from alterations in the acetyl-CoA, ADP(f), pyruvate, NADH, and H(+) concentrations leading to a lower relative activity of PDH kinase, whereas the similar values at 75% VO(2 max) may have reflected maximal activation. The results demonstrate that imposed metabolic alkalosis in skeletal muscle results in acceleration of glycogenolysis at the level of Phos relative to maximal PDH activation, resulting in a mismatch between the rates of pyruvate production and oxidation resulting in an increase in Lac(-) production.     

Impaired muscle glycogen resynthesis after eccentric exercise

Eight men performed 10 sets of 10 eccentric contractions of the knee extensor muscles with one leg [eccentrically exercised leg (EL)]. The weight used for this exercise was 120% of the maximal extension strength. After 30 min of rest the subjects performed two-legged cycling [concentrically exercised leg (CL)] at 74% of maximal O2 uptake for 1 h. In the 3 days after this exercise four subjects consumed diets containing 4.25 g CHO/kg body wt, and the remainder were fed 8.5 g CHO/kg. All subjects experienced severe muscle soreness and edema in the quadriceps muscles of the eccentrically exercised leg. Mean (+/- SE) resting serum creatine kinase increased from a preexercise level of 57 +/- 3 to 6,988 +/- 1,913 U/l on the 3rd day of recovery. The glycogen content (mmol/kg dry wt) in the vastus lateralis of CL muscles averaged 90, 395, and 592 mmol/kg dry wt at 0, 24, and 72 h of recovery. The EL muscle, on the other hand, averaged 168, 329, and 435 mmol/kg dry wt at these same intervals. Subjects receiving 8.5 g CHO/kg stored significantly more glycogen than those who were fed 4.3 g CHO/kg. In both groups, however, significantly less glycogen was stored in the EL than in the CL.     

Metabolic adaptations to training precede changes in muscle mitochondrial capacity.

To determine whether increases in muscle mitochondrial capacity are necessary for the characteristic lower exercise glycogen loss and lactate concentration observed during exercise in the trained state, we have employed a short-term training model involving 2 h of cycling per day at 67% maximal O2 uptake (VO2max) for 5-7 consecutive days. Before and after training, biopsies were extracted from the vastus lateralis of nine male subjects during a continuous exercise challenge consisting of 30 min of work at 67% VO2max followed by 30 min at 76% VO2max. Analysis of samples at 0, 15, 20, and 60 min indicated a pronounced reduction (P less than 0.05) in glycogen utilization after training. Reductions in glycogen utilization were accompanied by reductions (P less than 0.05) in muscle lactate concentration (mmol/kg dry wt) at 15 min [37.4 +/- 9.3 (SE) vs. 20.2 +/- 5.3], 30 min (30.5 +/- 6.9 vs. 17.6 +/- 3.8), and 60 min (26.5 +/- 5.8 vs. 17.8 +/- 3.5) of exercise. Maximal aerobic power, VO2max (l/min) was unaffected by the training (3.99 +/- 0.21 vs. 4.05 +/- 0.26). Measurements of maximal activities of enzymes representative of the citric acid cycle (succinic dehydrogenase and citrate synthase) were similar before and after the training. It is concluded that, in the voluntary exercising human, altered metabolic events are an early adaptive response to training and need not be accompanied by changes in muscle mitochondrial capacity.     

Sequential muscle biopsies during a 6-h tracer infusion do not affect human mixed muscle protein synthesis and muscle phenylalanine kinetics

Stable isotope tracer experiments of human muscle amino acid and protein kinetics often involve a sequential design, with the same subject studied at baseline and during an intervention. However, prolonged fasting and sequential muscle biopsies from the same area could theoretically affect muscle protein metabolism. The purpose of this study was to determine if sequential muscle biopsies and extended fasting significantly affect parameters of muscle protein and amino acid kinetics in six human subjects. After a 12-h overnight fast, a primed continuous infusion of L-[ring-(2)H(5)]phenylalanine was started. After 120 min, we took the first of a series of five hourly muscle biopsies from the same vastus lateralis to measure mixed muscle protein fractional synthetic rate. Furthermore, between 150-180, 210-240, and 330-360 min, we measured leg phenylalanine kinetics using the two-pool and the three-pool arteriovenous balance models. Tracer enrichments were at steady state, and muscle protein FSR and phenylalanine kinetics did not change throughout the experiment (P=not significant). We conclude that a 6-h tracer infusion during extended fasting (up to 18 h) with five sequential muscle biopsies from the same muscle do not affect basal mixed muscle protein synthesis and muscle phenylalanine kinetics in human subjects. Thus, when using a sequential study design over this period of time, it is unnecessary to include a saline only control group to account for these variables.     

Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women.

This study examined the ability of well-trained eumenorrheic women to increase muscle glycogen content and endurance performance in response to a high-carbohydrate diet (HCD; approximately 78% carbohydrate) compared with a moderate-carbohydrate diet (MD; approximately 48% carbohydrate) when tested during the luteal phase of the menstrual cycle. Six women cycled to exhaustion at approximately 80% maximal oxygen uptake (VO(2 max)) after each of the randomly assigned diet and exercise-tapering regimens. A biopsy was taken from the vastus lateralis before and after exercise in each trial. Preexercise muscle glycogen content was high after the MD (625.2 +/- 50.1 mmol/kg dry muscle) and 13% greater after the HCD (709.0 +/- 44.8 mmol/kg dry muscle). Postexercise muscle glycogen was low after both trials (MD, 91.4 +/- 34.5; HCD, 80.3 +/- 19.5 mmol/kg dry muscle), and net glycogen utilization during exercise was greater after the HCD. The subjects also cycled longer at approximately 80% VO(2 max) after the HCD vs. MD (115:31 +/- 10:47 vs. 106:35 +/- 8:36 min:s, respectively). In conclusion, aerobically trained women increased muscle glycogen content in response to a high-dietary carbohydrate intake during the luteal phase of the menstrual cycle, but the magnitude was smaller than previously observed in men. The increase in muscle glycogen, and possibly liver glycogen, after the HCD was associated with increased cycling performance to volitional exhaustion at approximately 80% VO(2 max).     

Effect of aerobic capacity on the T(2) increase in exercised skeletal muscle.

The increase in nuclear magnetic resonance transverse relaxation time (T(2)) of muscle water measured by magnetic resonance imaging after exercise has been correlated with work rate in human subjects. This study compared the T(2) increase in thigh muscles of trained (cycling VO(2 max) = 54.4 +/- 2.7 ml O(2). kg(-1). min(-1), mean +/- SE, n = 8, 4 female) vs. sedentary (31.7 +/- 0.9 ml O(2). kg(-1). min(-1), n = 8, 4 female) subjects after cycling exercise for 6 min at 50 and 90% of the subjects' individually determined VO(2 max). There was no significant difference between groups in the T(2) increase measured in quadriceps muscles within 3 min after the exercises, despite the fact that the absolute work rates were 60% higher in the trained group (253 +/- 15 vs. 159 +/- 21 W for the 90% exercise). In both groups, the increase in T(2) of vastus muscles was twofold greater after the 90% exercise than after the 50% exercise. The recovery of T(2) after the 90% exercise was significantly faster in vastus muscles of the trained compared with the sedentary group (mean recovery half-time 11.9 +/- 1.2 vs. 23.3 +/- 3.7 min). The results show that the increase in muscle T(2) varies with work rate relative to muscle maximum aerobic power, not with absolute work rate.     

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content.

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O(2) uptake (Vo(2 max))] were measured in eight well-trained male subjects [Vo(2 max) = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in (13)C was ingested: plasma glucose oxidation was computed from (13)C/(12)C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% Vo(2 max), respectively) and (13)CO(2) production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% Vo(2 max) [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% Vo(2 max) [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% Vo(2 max). However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.