Prolonged Submaximal Exercise Research Articles

Vastus lateralis oxygenation during prolonged cycling in healthy males

This study examined the relationship between acute cardiorespiratory and muscle oxygenation and blood volume changes during prolonged exercise. Eight healthy male volunteers (mean maximum oxygen uptake VO(2max) = 41.6 +/- 2.4 mL/kg/min) performed 60 min submaximal cycling at 50% VO(2max). Oxygen uptake VO(2) was measured by indirect spirometry, cardiac output (CO) was estimated using a Portapre, and right vastus lateralis oxyhemoglobin/ myoglobin (oxyHb/Mb), deoxyhemoglobin/myoglobin (deoxyHb/Mb), and total hemoglobin/myoglobin (total Hb/Mb) were recorded using near-infrared spectroscopy (NIRS). After 40 min of exercise, there was a significant increase in VO(2) due to a significantly higher arteriovenous oxygen difference ((a - v)O(2diff). After 30 min of exercise CO remained unchanged, but there was a significant decrease in stroke volume and a proportionate increase in heart rate, thus indicating the occurrence of cardiovascular drift. During the first few minutes of exercise, there was a decline in oxyHb/Mb and total Hb/Mb, whereas deoxyHb/Mb remained unchanged. Thereafter, oxyHb/Mb and total Hb/Mb increased systematically until the termination of exercise while deoxyHb/Mb declined. After 40 min of exercise, these changes were significantly different from the baseline values. There were no significant correlations between the changes in the NIRS variables and systemic VO(2) or mixed (a - v)O(2)diff during exercise. These results suggest that factors other than localized changes in muscle oxygenation and blood volume account for the increased VO(2) during prolonged submaximal exercise.

Cardiac vagal modulation of heart rate during prolonged submaximal exercise in animals with healed myocardial infarctions: effects of training

The present study investigated the effects of long-duration exercise on heart rate variability [as a marker of cardiac vagal tone (VT)]. Heart rate variability (time series analysis) was measured in mongrel dogs (n = 24) with healed myocardial infarctions during 1 h of submaximal exercise (treadmill running at 6.4 km/h at 10% grade). Long-duration exercise provoked a significant (ANOVA, all P < 0.01, means +/- SD) increase in heart rate (1st min, 165.3 +/- 15.6 vs. last min, 197.5 +/- 21.5 beats/min) and significant reductions in high frequency (0.24 to 1.04 Hz) power (VT: 1st min, 3.7 +/- 1.5 vs. last min, 1.0 +/- 0.9 ln ms(2)), R-R interval range (1st min, 107.9 +/- 38.3 vs. last min, 28.8 +/- 13.2 ms), and R-R interval SD (1st min, 24.3 +/- 7.7 vs. last min 6.3 +/- 1.7 ms). Because endurance exercise training can increase cardiac vagal regulation, the studies were repeated after either a 10-wk exercise training (n = 9) or a 10-wk sedentary period (n = 7). After training was completed, long-duration exercise elicited smaller increases in heart rate (pretraining: 1st min, 156.0 +/- 13.8 vs. last min, 189.6 +/- 21.9 beats/min; and posttraining: 1st min, 149.8 +/- 14.6 vs. last min, 172.7 +/- 8.8 beats/min) and smaller reductions in heart rate variability (e.g., VT, pretraining: 1st min, 4.2 +/- 1.7 vs. last min, 0.9 +/- 1.1 ln ms(2); and posttraining: 1st min, 4.8 +/- 1.1 vs. last min, 2.0 +/- 0.6 ln ms(2)). The response to long-duration exercise did not change in the sedentary animals. Thus the heart rate increase that accompanies long-duration exercise results, at least in part, from reductions in cardiac vagal regulation. Furthermore, exercise training attenuated these exercise-induced reductions in heart rate variability, suggesting maintenance of a higher cardiac vagal activity during exercise in the trained state.

Decreased energy expenditure during prolonged sub-maximal exercise in a warm environment

The purpose of this study was to investigate the influence of a warm environment on thermoregulation and energy expenditure during sub-maximal prolonged exercise in humans. Six healthy male subjects cycled for 120 min at an intensity of 60% maximal oxygen uptake (Vo2max) at three environmental temperatures (10°C, CT; 20°C, MT; and 30°C, WT). Although oxygen uptake at WT showed a significantly lower value compared to those at MT and CT, no significant differences of respiratory exchange ratio were observed among the three environmental trials. A remarkable decrease in total energy expenditure during the 120-min exercise at WT was observed in comparison with those at MT and CT (p<0.05). Changes in rectal temperature, mean skin temperature, and mean body temperature at WT were significantly higher than those at both MT and CT. Although increases in mean body temperature from rest every five minutes during exercise were not different among three environmental temperatures, mean energy expenditures every five minutes at WT were lower compared with those at MT and CT (p<0.05). These results suggest that the increase in energy expenditure for physical exertion is substantially reduced during prolonged sub-maximal exercise in a warm environment. This acute alteration in the energy metabolism may contribute to inhibition of excess heat production and enable prolonged exercise in a warm environment.

Relationship between ventilatory response and body temperature during prolonged submaximal exercise

We examined whether an increase in skin temperature or the rate of increase in core body temperature influences the relationship between minute ventilation (Ve) and core temperature during prolonged exercise in the heat. Thirteen subjects exercised for 60 min on a cycle ergometer at 50% of peak oxygen uptake while wearing a suit perfused with water at 10 degrees C (T10), 35 degrees C (T35), or 45 degrees C (T45). During the exercise, esophageal temperature (Tes), skin temperature, heart rate (HR), Ve, tidal volume, respiratory frequency (f), respiratory gases, blood pressure (BP), and blood lactate were all measured. We found that oxygen uptake, carbon dioxide output, BP, and blood lactate did not differ among the sessions. Tes, HR, Ve, and f remained nearly constant from minute 10 onward in the T10 session, but all of these parameters progressively increased in the T35 and T45 sessions, and significantly higher levels were seen in the T45 than the T35 session. For all but two subjects in the T35 and T45 sessions, plotting Ve as a function of Tes revealed no threshold for hyperventilation; instead, increases in Ve were linearly related to Tes, and there were no significant differences in the slopes or intercepts between the T35 and T45 sessions. Thus, during prolonged submaximal exercise in the heat, Ve increases with core temperature, and the influences of skin temperature and the rate of increase in Tes on the relationship between Ve and Tes are apparently small.

Prolonged submaximal exercise induces isoform-specific Na+-K+-ATPase mRNA and protein responses in human skeletal muscle

This study investigated effects of prolonged submaximal exercise on Na+-K+-ATPase mRNA and protein expression, maximal activity, and content in human skeletal muscle. We also investigated the effects on mRNA expression of the transcription initiator gene, RNA polymerase II (RNAP II), and key genes involved in protein translation, eukaryotic initiation factor-4E (eIF-4E) and 4E-binding protein 1 (4E-BP1). Eleven subjects (6 men, 5 women) cycled at 75.5% (SD 4.8%) peak O2 uptake and continued until fatigue. A vastus lateralis muscle biopsy was taken at rest, fatigue, and 3 and 24 h postexercise. We analyzed muscle for Na+-K+-ATPase alpha1, alpha2, alpha3, beta1, beta2, and beta3, as well for RNAP II, eIF-4E, and 4E-BP1 mRNA expression by real-time RT-PCR and Na+-K+-ATPase isoform protein abundance using immunoblotting. Muscle homogenate maximal Na+-K+-ATPase activity was determined by 3-O-methylfluorescein phosphatase activity and Na+-K+-ATPase content by [3H]ouabain binding. Cycling to fatigue [54.5 (SD 20.6) min] immediately increased alpha3 (P = 0.044) and beta2 mRNA (P = 0.042) by 2.2- and 1.9-fold, respectively, whereas alpha1 mRNA was elevated by 2.0-fold at 24 h postexercise (P = 0.036). A significant time main effect was found for alpha3 protein abundance (P = 0.046). Exercise transiently depressed maximal Na+-K+-ATPase activity (P = 0.004), but Na+-K+-ATPase content was unaltered throughout recovery. Exercise immediately increased RNAP II mRNA by 2.6-fold (P = 0.011) but had no effect on eIF-4E and 4E-BP1 mRNA. Thus a single bout of prolonged submaximal exercise induced isoform-specific Na+-K+-ATPase responses, increasing alpha1, alpha3, and beta2 mRNA but only alpha3 protein expression. Exercise also increased mRNA expression of RNAP II, a gene initiating transcription, but not of eIF-4E and 4E-BP1, key genes initiating protein translation.

Effect Of Gender On Lipid Mobilization And Lipolytic Hormones During And After Prolonged Submaximal Exercise

During prolonged, moderate-intensity exercise, lipid oxidation is a major source of ATP. If present, gender differences in lipolytic hormone concentrations may significantly alter this response. PURPOSE To examine potential gender differences in lipolytic markers and hormones before, during, and after a prolonged, moderate-intensity exercise bout. S276 Vol. 37 No. 5 Supplement METHODS Seven healthy males (age = 25.9 ± 4.9 y, height = 174.4 ± 3.0 cm, mass = 69.9 ± 6.3 kg, body fat = 10.0 ± 2.7%, VO2max = 69.1 ± 6.7 ml·kg-1·min-1) and seven healthy, eumenorrheic females (age = 29.0 ± 4.2 y, height = 166.0 ± 4.2 cm, mass = 54.6 ± 6.5 kg, body fat = 19.3 ± 3.9%, VO2max = 53.1 ± 4.5 ml·kg-1·min-1) participated in this study. All subjects were trained runners familiar with treadmill running. Subjects ran on a motor-driven treadmill for 60 min at 70% VO2max. Blood samples were obtained immediately prior to exercise, 20 and 40 minutes during exercise, and four, 30, and 60 minutes post-exercise; samples were analyzed for lactate, non-esterified fatty acids, glycerol, cortisol, prolactin, growth hormone, epinephrine, norepinephrine, and dopamine. RESULTS The majority of analytes were stimulated or unchanged during the exercise bout; except for non-esterified fatty acids in males, all measurements returned to baseline values within 60 minutes of exercise completion. Prior to exercise, females displayed significantly greater prolactin and dopamine. Four minutes post-exercise, females showed significantly lower epinephrine concentrations than males. No other physiologically or statistically significant gender differences existed. CONCLUSIONS Despite significantly different resting endocrine profiles, males and females appeared to mobilize and utilize lipid substrates similarly during prolonged, moderate-intensity exercise.

Effect Of Gender On Lipid Mobilization And Lipolytic Hormones During And After Prolonged Submaximal Exercise

Effect Of Gender On Lipid Mobilization And Lipolytic Hormones During And After Prolonged Submaximal Exercise

Exercise in the fasted state facilitates fibre type‐specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans

The effects were compared of exercise in the fasted state and exercise with a high rate of carbohydrate intake on intramyocellular triglyceride (IMTG) and glycogen content of human muscle. Using a randomized crossover study design, nine young healthy volunteers participated in two experimental sessions with an interval of 3 weeks. In each session subjects performed 2 h of constant-load bicycle exercise ( approximately 75% ), followed by 4 h of controlled recovery. On one occasion they exercised after an overnight fast (F), and on the other (CHO) they received carbohydrates before ( approximately 150 g) and during (1 g (kg bw)(-1) h(-1)) exercise. In both conditions, subjects ingested 5 g carbohydrates per kg body weight during recovery. Fibre type-specific relative IMTG content was determined by Oil red O staining in needle biopsies from m. vastus lateralis before, immediately after and 4 h after exercise. During F but not during CHO, the exercise bout decreased IMTG content in type I fibres from 18 +/- 2% to 6 +/- 2% (P = 0.007) area lipid staining. Conversely, during recovery, IMTG in type I fibres decreased from 15 +/- 2% to 10 +/- 2% in CHO, but did not change in F. Neither exercise nor recovery changed IMTG in type IIa fibres in any experimental condition. Exercise-induced net glycogen breakdown was similar in F and CHO. However, compared with CHO (11.0 +/- 7.8 mmol kg(-1) h(-1)), mean rate of postexercise muscle glycogen resynthesis was 3-fold greater in F (32.9 +/- 2.7 mmol kg(-1) h(-1), P = 0.01). Furthermore, oral glucose loading during recovery increased plasma insulin markedly more in F (+46.80 microU ml(-1)) than in CHO (+14.63 microU ml(-1), P = 0.02). We conclude that IMTG breakdown during prolonged submaximal exercise in the fasted state takes place predominantly in type I fibres and that this breakdown is prevented in the CHO-fed state. Furthermore, facilitated glucose-induced insulin secretion may contribute to enhanced muscle glycogen resynthesis following exercise in the fasted state.

Aggravated hypoxia during breath-holds after prolonged exercise

Hyperventilation prior to breath-hold diving increases the risk of syncope as a result of hypoxia. Recently, a number of cases of near-drownings in which the swimmers did not hyperventilate before breath-hold diving have come to our attention. These individuals had engaged in prolonged exercise prior to breath-hold diving and it is known that such exercise enhances lipid metabolism relative to carbohydrate metabolism, resulting in a lower production of CO(2) per amount of O(2 )consumed. Therefore, our hypothesis was that an exercise-induced increase in lipid metabolism and the associated reduction in the amount of CO(2) produced would cause the urge to breathe to develop at a lower P O(2), thereby increasing the risk of syncope due to hypoxia. Eight experienced breath-hold divers performed 5 or 6 breath-holds at rest in the supine position and then 5 or 6 additional breath-holds during intermittent light ergometer exercise with simultaneous apnoea (dynamic apnoea, DA) on two different days: control (C) and post prolonged sub-maximal exercise (PPE), when the breath-holds were performed 30 min after 2 h of sub-maximal exercise. After C and before the prolonged submaximal exercise subjects were put on a carbohydrate-free diet for 18 h to start the depletion of glycogen. The respiratory exchange ratio ( RER) and end-tidal P CO(2), P O(2), and SaO(2) values were determined and the data were presented as means (SD). The RER prior to breath-holding under control conditions was 0.83 (0.09), whereas the corresponding value after exercise was 0.70 (0.05) ( P <0.01). When the three apnoeas of the longest duration for each subject were analysed, the average duration of the dynamic apnoeas was 96 (14) s under control conditions and 96 (17) s following exercise. Both P O(2) and P CO(2) were higher during the control dynamic apnoeas than after PPE [PO(2) 6.9 (1.0) kPa vs 6.2 (1.2) kPa, P <0.01; P CO(2) 7.8 (0.5) kPa vs 6.7 (0.4) kPa, P <0.001; ANOVA testing]. A similar pattern was observed after breath-holding under resting conditions, i.e., a lower end-tidal P O(2) and P CO(2) after exercise (PPE) compared to control conditions. Our findings demonstrate that under the conditions of a relatively low RER following prolonged exercise, breath-holding is terminated at a lower P O(2) and a lower P CO(2) than under normal conditions. This suggests that elevated lipid metabolism may constitute a risk factor in connection with breath-holding during swimming and diving.

Effects of Microhydrin® Supplementation on Endurance Performance and Metabolism in Well-Trained Cyclists

This study investigated whether the supplement Microhydrin (MH) contains silica hydride bonds (Si-H) and if Microhydrin supplementation increased performance or altered metabolism compared to placebo (PL) during prolonged endurance cycling. Seven endurance-trained male cyclists consumed 9.6 g of MH or PL over 48 h in a randomized, double-blind, crossover design. Subjects cycled at approximately 70% of their VO2peak, coupled with five 2-min bursts at 85% VO2peak to simulate hill climbs over 2 h. Subjects then completed a time trial, which required them to complete 7 kJ/kg body mass as quickly as possible. Infrared spectrometry analysis showed a complete absence of Si-H bonds in MH. There was no difference in time trial performance between the 2 trials (PL: 2257+/-120 s vs. MH: 2345+/-152 s). Measured oxygen uptake, respiratory exchange ratio, carbohydrate (MH: 2.99+/-0.13 g/min; PL: 2.83+/-0.17 g/min avg. over 2 h) and fat (MH: 0.341+/-0.06 g/min; PL: 0.361+/-0.07 g/min) oxidation rates and all blood parameters (lactate, glucose, and free fatty acids) were all unaffected by MH supplementation. The volume of expired CO2 and ventilation were significantly greater with MH supplementation (P < or = 0.05). The results indicate that oral Microhydrin supplementation does not enhance cycling time trial performance or alter metabolism during prolonged submaximal exercise in endurance-trained cyclists.

Sleep Deprivation, Energy Expenditure and Cardiorespiratory Function

Participants in the sport of adventure racing often choose to go without sleep for a period of greater than 24 h while partaking in prolonged submaximal exercise. This study examined the effect of 30 h of sleep deprivation and intermittent physical exercise, on the cardiorespiratory markers of submaximal exercise in six subjects. Six subjects with the following physical characteristics participated in the study (mean +/- SD): age 22 +/- 0.3 years, height 180 +/- 5 cm, body mass: 77 +/- 5 kg, VO2peak 44 +/- 5 ml. kg (-1). min (-1). Three subjects engaged in normal sedentary activities while three others cycled on a cycle ergometer at 50 % VO2peak for 20 min out of every two hours during thirty hours of sleep deprivation. One week later sleep deprivation was repeated with a cross over of subjects. Every four hours, subjects completed assessments of cardiorespiratory function during 50 % VO2peak cycling. A 3 x 8 repeated measures ANOVA revealed a significantly lower heart rate with sleep deprivation (p < 0.05), but no other significant effects (p > 0.05) on respiratory gas exchange variables. Neither sleep deprivation, nor a combination of sleep deprivation and five hours of moderate intensity cycling, appear to be limiting factors to the physiological capacity to perform submaximal exercise.

Time course and magnitude of changes in total body water, extracellular fluid volume, intracellular fluid volume and plasma volume during submaximal exercise and recovery in horses

AbstractThe purpose of the present study was to determine the time course and magnitude of changes in extracellular and intracellular fluid volumes in relation to changes in total body water during prolonged submaximal exercise and recovery in horses. Seven horses were physically conditioned over a 2-month period and trained to trot on a treadmill. Total body water (TBW), extracellular fluid volume (ECFV) and plasma volume (PV) were measured at rest using indicator dilution techniques (D2O, thiocyanate and Evans Blue, respectively). Changes in TBW were assessed from measures of body mass, and changes in PV and ECFV were calculated from changes in plasma protein concentration. Horses exercised by trotting on a treadmill for 75–120 min incurred a 4.2% decrease in TBW. During exercise, the entire decrease in TBW (mean±standard error: 12.8±2.0 l at end of exercise) could be attributed to the decrease in ECFV (12.0±2.4 l at end of exercise), such that there was no change in intracellular fluid volume (ICFV; 0.9±2.4 l at end of exercise). PV decreased from 22.0±0.5 l at rest to 19.8±0.3 l at end of exercise and remained depressed (18–19 l) during the first 2 h of recovery. Recovery of fluid volumes after exercise was slow, and characterized by a further transient loss of ECFV (first 30 min of recovery) and a sustained increase in ICFV (between 0.5 and 3.5 h of recovery). Recovery of fluid volumes was complete by 13 h post exercise. It is concluded that prolonged submaximal exercise in horses favours net loss of fluid from the extracellular fluid compartment.

Intramyocellular lipid content is increased after exercise in nonexercising human skeletal muscle.

Intramyocellular lipid (IMCL) content has been reported to decrease after prolonged submaximal exercise in active muscle and, therefore, seems to form an important local substrate source. Because exercise leads to a substantial increase in plasma free fatty acid (FFA) availability with a concomitant increase in FFA uptake by muscle tissue, we aimed to investigate potential differences in the net changes in IMCL content between contracting and noncontracting skeletal muscle after prolonged endurance exercise. IMCL content was quantified by magnetic resonance spectroscopy in eight trained cyclists before and after a 3-h cycling protocol (55% maximal energy output) in the exercising vastus lateralis and the nonexercising biceps brachii muscle. Blood samples were taken before and after exercise to determine plasma FFA, glycerol, and triglyceride concentrations, and substrate oxidation was measured with indirect calorimetry. Prolonged endurance exercise resulted in a 20.4 +/- 2.8% (P < 0.001) decrease in IMCL content in the vastus lateralis muscle. In contrast, we observed a substantial (37.9 +/- 9.7%; P < 0.01) increase in IMCL content in the less active biceps brachii muscle. Plasma FFA and glycerol concentrations were substantially increased after exercise (from 85 +/- 6 to 1450 +/- 55 and 57 +/- 11 to 474 +/- 54 microM, respectively; P < 0.001), whereas plasma triglyceride concentrations were decreased (from 1498 +/- 39 to 703 +/- 7 microM; P < 0.001). IMCL is an important substrate source during prolonged moderate-intensity exercise and is substantially decreased in the active vastus lateralis muscle. However, prolonged endurance exercise with its concomitant increase in plasma FFA concentration results in a net increase in IMCL content in less active muscle.

Plasma thyroid hormone concentrations in dogs competing in a long-distance sled dog race.

Plasma thyroxine (T4), 3,5,3'-triiodothyronine (T3), total protein, and albumin concentrations were measured in 15 dogs both before and after completion, and in an additional 16 dogs before and 24 dogs after completion, of a long-distance sled dog race. The plasma T4 concentration (mean +/- SD) decreased significantly from 18.2 +/- 5.4 nmol/L before to 14.3 +/- 3.5 nmol/L after the race in dogs evaluated at both times and decreased significantly from 21.8 +/- 10.5 nmol/L before to 15.8 +/- 4.9 nmol/L after the race in dogs sampled only before or only after the race. The mean plasma T3 concentrations in dogs measured twice decreased significantly from 1.20 +/- 0.48 nmol/L before to 0.74 +/- 0.42 nmol/L after the race, as well as in dogs measured either before (1.28 +/- 0.36 nmol/L) or after (0.69 +/- 0.28 nmol/L) the race, respectively. Plasma total protein and albumin concentrations decreased significantly after completion of the race. No significant change was noted in 4 control dogs that did not compete in the race and were tested during a similar time period. The plasma concentrations of T4 and T3 were lower than the normal reference range established for this laboratory in 23 and 39%, respectively, of Alaskan sled dogs tested before the race. Plasma thyroid hormone concentrations frequently are below normal in conditioned Alaskan sled dogs and are further reduced after prolonged submaximal exercise.

Clinical Physiology of Exercise in Pregnancy: A Literature Review

Objectives: To review the existing literature on the physiology of exercise in pregnancy as a basis for clinical practice guidelines for prenatal exercise prescription.Methods: MEDLINE search for English language abstracts and articles published between 1966 and 2003 related to physiological adaptations to pregnancy, effects of pregnancy on responses to acute exercise and aerobic conditioning, effects of acute maternal exercise on indexes of fetal well-being, impact of physical conditioning on birth weight and other pregnancy outcomes, and use of exercise to prevent or treat gestational diabetes mellitus and preeclampsia.Results: Maximal aerobic power (V̇O2max, L/min) is well-preserved in pregnant women who remain physically active, but anaerobic working capacity may be reduced in late gestation. The increase in resting heart rate, reduction in maximal heart rate, and resulting smaller heart rate reserve render heart rate a less precise way of estimating exercise intensity. As rating of perceived exertion (RPE) is not altered by pregnancy, the use of revised pulse rate target zones along with Borg’s RPE scale is recommended to prescribe exercise intensity during pregnancy. Responses to prolonged submaximal exercise (>30 min) in late gestation include a moderate reduction in maternal blood glucose concentration, which may transiently reduce fetal glucose availability. The normal response to sustained submaximal exercise is an increase in fetal heart rate (FHR) baseline. Transient reductions in FHR reactivity, fetal breathing movements, and FHR variability may also occur in association with more strenuous exercise. Controlled prospective studies have demonstrated that moderate prenatal exercise during the second and third trimesters is useful to improve aerobic fitness and maternal-fetal physiological reserve without affecting fetal growth.Conclusions:The Physical Activity Readiness Medical Examination for Pregnancy is recommended for use by physicians and mid-wives to provide medical clearance for prenatal exercise. Evidence-based prenatal exercise guidelines are needed to counsel healthy and fit pregnant women on the safety of involvement in more strenuous physical conditioning. Future study is also recommended to determine the usefulness of prenatal exercise in the prevention and treatment of gestational diabetes mellitus and preeclampsia.

The effect of prolonged submaximal exercise on gas exchange kinetics and ventilation during heavy exercise in humans.

This study compared ventilation, gas exchange (oxygen uptake, VO(2)) and the surface electromyogram (EMG) activity of four major lower limb muscles during heavy exercise before (Pre-Ex) and after (Post-Ex) a sustained 90-min cycling exercise at 60% VO(2peak). The 90-min exercise was incorporated under the hypothesis that sustained exercise would alter substrate availability in the second exercise bout causing differences in fibre recruitment patterns, gas exchange and ventilation. Nine trained male subjects [VO(2peak)=60.2 (1.7) ml.kg(-1).min(-1)] completed two identical 6-min bouts of cycling performed at high intensity [approximately 90% VO(2peak); 307 (6) W, mean (SE)]. Ventilation and gas exchange were measured breath-by-breath and the EMG was recorded during the last 12 s of each minute of the two 6-min bouts. EMG signals were analysed to determine integrated EMG (iEMG) and mean power frequency (MPF). VO(2) at min 3 and min 6 in Post-Ex were significantly higher (i.e., +201 and 141 ml.min(-1), respectively, P<0.05) than in Pre-Ex but there was a approximately 25% decrease of the slow component, taken as the difference between min 6 and min 3 [187 (27) vs 249 (35) ml.min(-1), respectively, P<0.05]. The greater whole-body VO(2) after 3 min of exercise in Post-Ex was not accompanied by clear alterations in the iEMG and MPF of the examined leg muscles. Ventilation and heart rate were elevated (approximately 12-16 l.min(-1) and approximately 10 beats.min(-1), respectively, P<0.05) as were the ratios V(E)/O(2) and V(E)/VCO(2) in the Post-Ex tests. It was concluded that the VO(2) and ventilation responses to high-intensity exercise can be altered following prolonged moderate intensity exercise in terms of increased amplitude without associated major changes in either iEMG or MPF values among conditions.

HUMAN SKELETAL MUSCLE PDH KINASE ACTIVITY INCREASES DURING PROLONGED EXERCISE

During prolonged sub-maximal exercise, there is a shift in skeletal muscle substrate utilization from carbohydrate to fat oxidation. This decrease in carbohydrate oxidation is due at least in part to decreased activity of the active form of pyruvate dehydrogenase (PDHa). However, the exact mechanism of the observed decrease in PDHa activity is unknown. PDHa is down-regulated by an intrinsic PDH kinase (PDK) which can either be up-regulated acutely by allosteric effectors, or in an chronic manner which is independent of changes in mitochondrial effectors. PURPOSE To test the hypothesis that the observed decrease in PDHa activity during a single 4 h exercise bout is due to a chronic increase in PDK activity. METHODS Healthy male volunteers (n = 7; age, 24 ± 4 yr; mean maximal oxygen uptake or VO2max, 53.1 ± 1.6 ml/kg/min) were asked to cycle at 55% of their VO2max for 4 hours. At 0 min, 10 min, and 4 h two biopsies were taken. One biopsy was frozen immediately in liquid nitrogen for analysis of PDHa activity. A second biopsy was extracted fresh for intact mitochondria and subsequent PDK activity (calculated as the first-order rate constant of PDHa inactivation, min−1). Respiratory exchange ratio (RER) was used to calculate energy contributions from carbohydrate and fat oxidation. RESULTS RER decreased during exercise from 0.94 ± 0.01 to 0.79 ± 0.01, carbohydrate oxidation decreased from 35 ± 3 to 12.0± 1.5 kJ/min, and fat oxidation increased from 8.3 ± 0.8 to 30.2 ± 1.2 g/min. PDHa activity increased from 0 to 10 min, but was decreased at 4 h (0.79 ± 0.08, 2.86 ± 0.26, and 2.23 ± 0.24 respectively). PDK activity was increased ∼2 fold by 4 h (0 min, 0.077 ± 0.016; 10 min, 0.119 ± 0.016 and 4 h, 0.173 ± 0.019 min−1). CONCLUSION Decreased PDHa activity while cycling at 55% VO2max for 4 h is associated with a chronic increase in PDK activity. Supported by NSERC and CIHR, Canada

LIQUID CARBOHYDRATE FEEDING IMPROVES SELF-SELECTED EXERCISE PERFORMANCE DURING A 24K HIKE

During prolonged exercise, when muscle glycogen stores are depleted, blood glucose can assume 100% of the total carbohydrate (CHO) oxidation. The effectiveness of exogenous CHO during endurance exercise has been widely documented. However, CHO ingestion during extended exercise at lower intensities has received little attention. PURPOSE To determine the effects of carbohydrate feeding on blood glucose and self-selected hiking speed during a prolonged submaximal exercise bout. METHODS Subjects (n=12) completed two extended hikes (24K) in a single-blind double-crossover design 7 days apart. Blood glucose concentration was measured pre, post, and hourly during the hike using an automated glucometer. Concurrently, subjects consumed 200mL of a CHO (40g.hour-1, 20% maltodextrin) or placebo (PLA) drink hourly. Rating of perceived exertion (RPE) was also recorded hourly using the Borg 6–20 scale. Data were analyzed using apriori planned comparisons across trials. RESULTS Blood glucose was similar prior to exercise and for the first 8K of the hike. Thereafter blood glucose remained significantly higher (p < 0.05) during the CHO trial (12K: CHO=6.1 ± 0.9, PLA=5.2 ± 0.6; 16K: CHO=6.3 ± 1.0, PLA=5.4 ± 1.1; 20K: CHO=7.8 ± 1.0, PLA=5.8 ± 1.2; 24K: CHO=7.4 ± 0.9, PLA=5.7 ± 1.0 mM). Subjects self-selected a faster hiking speed during the CHO trial as demonstrated by the significant difference in return time (124 ± 15 vs. 136 ± 12 minutes for the CHO and PLA trials, respectively, p < 0.05) despite no differences in RPE values (11.92 ± 3.00 vs. 11.75 ± 2.30, p < 0.05). CONCLUSION These data demonstrate that supplemental CHO feedings can improve self-selected exercise performance during low to moderate extended hiking regardless of the subject's rating of perceived exertion.

The increase in intramyocellular lipid content is a very early response to training.

The present study investigated the influences of a 2-wk training program on intramyocellular lipid (IMCL) content, IMCL decrease during exercise, fat oxidation, and insulin sensitivity. Nine untrained men (age, 23.3 +/- 3.2 yr; body mass index, 22.6 +/- 2.6 kg/m(2); maximal power output, 3.8 +/- 0.6 W/kg body weight) trained for 2 wk. Before and after training, subjects cycled for 3 h while substrate oxidation was measured. IMCL content in the vastus lateralis muscle was determined before and after cycling by proton magnetic resonance spectroscopy. Before and after training, insulin sensitivity was assessed by an insulin tolerance test. The training period resulted in a significant increase in IMCL content by 42 +/- 14%. IMCL content decreased significantly during cycling. However, 2 wk of training were not sufficient to achieve increases in fat oxidation and/or use of IMCL during exercise. All markers used to test insulin sensitivity point toward improved insulin sensitivity, albeit not significant. We conclude that the increase in IMCL content is a very early response to training, preceding significant changes in insulin sensitivity. The results suggest that the presence of triglycerides alone does not necessarily have detrimental effects on insulin sensitivity. We confirm earlier reports that IMCL contributes to the energy used during prolonged submaximal exercise.

The effects of pre-warming on the metabolic and thermo-regulatory responses to prolonged submaximal exercise in moderate ambient temperature. Gregson, W. A., Drust, B., Batterham, A., and Cable, N. T. Eur. J. Appl. Physiol., 86(6): 26-533, 2002.(Abstracts of foreign literature)

The effects of pre-warming on the metabolic and thermo-regulatory responses to prolonged submaximal exercise in moderate ambient temperature. Gregson, W. A., Drust, B., Batterham, A., and Cable, N. T. Eur. J. Appl. Physiol., 86(6): 26-533, 2002.(Abstracts of foreign literature)