Time to exhaustion and time spent at a high percentage of VO2max in severe intensity domain in children and adults

The aim of the study was to compare time spent at a high percentage of VO2max (>90% of VO2max) (ts90%), time to achieve 90% of VO2max (ta90%), and time to exhaustion (TTE) for exercise in the severe intensity domain in children and adults. Fifteen prepubertal boys (10.3 ± 0.9 years) and 15 men (23.5 ± 3.6 years) performed a maximal graded exercise to determine VO2max, maximal aerobic power (MAP) and power at ventilatory threshold (PVTh). Then, they performed 4 constant load exercises in a random order at PVTh plus 50 and 75% of the difference between MAP and PVTh (PΔ50 and PΔ75) and 100 and 110% of MAP (P100 and P110). VO2max was continuously monitored. The P110 test was used to determine maximal accumulated oxygen deficit (MAOD). No significant difference was found in ta90% between children and adults. ts90% and TTE were not significantly different between children and adults for the exercises at PΔ50 and PΔ75. However, ts90% and TTE during P100 (p < 0.05 and p < 0.01, respectively) and P110 (p < 0.001) exercises were significantly shorter in children. Children had a significantly lower MAOD than adults (34.3 ± 9.4 ml · kg vs. 53.6 ± 11.1 ml · kg). A positive relationship (p < 0.05) was obtained between MAOD and TTE values during the P100 test in children. This study showed that only for intensities at, or higher than MAP, lower ts90% in children was linked to a reduced TTE, compared to adults. Shorter TTE in children can partly be explained by a lower anaerobic capacity (MAOD). These results give precious information about exercise intensity ranges that could be used in children's training sessions. Moreover, they highlight the implication of both aerobic and anaerobic processes in endurance performances in both populations.     

The influence of exercise duration at VO2 max on the off-transient pulmonary oxygen uptake phase during high intensity running activity

The purpose of this study was to examine the influence of time run at maximal oxygen uptake (VO2 max) on the off-transient pulmonary oxygen uptake phase after supra-lactate threshold runs. We hypothesised: 1) that among the velocities eliciting VO2 max there is a velocity threshold from which there is a slow component in the VO2-off transient, and 2) that at this velocity the longer the duration of this time at VO2 max (associated with an accumulated oxygen kinetics since VO2 can not overlap VO2 max), the longer is the off-transient phase of oxygen uptake kinetics. Nine long-distance runners performed five maximal tests on a synthetic track (400 m) while breathing through the COSMED K4b2 portable, telemetric metabolic analyser: i) an incremental test which determined VO2 max, the minimal velocity associated with VO2 max (vVO2 max) and the velocity at the lactate threshold (vLT), ii) and in a random order, four supra-lactate threshold runs performed until exhaustion at vLT + 25, 50, 75 and 100% of the difference between vLT and vVO2 max (vdelta25, vdelta50, vdelta75, vdelta100). At vdelta25, vdelta50 (= 91.0 +/- 0.9% vVO2 max) and vdelta75, an asymmetry was found between the VO2 on (double exponential) and off-transient (mono exponential) phases. Only at vdelta75 there was at positive relationship between the time run at VO2 max (%tlimtot) and the VO2 recovery time constant (Z = 1.8, P = 0.05). In conclusion, this study showed that among the velocities eliciting VO2 max, vdelta75 is the velocity at which the longer the duration of the time at VO2 max, the longer is the off-transient phase of oxygen uptake kinetics. It may be possible that at vdelta50 there is not an accumulated oxygen deficit during the plateau of VO2 at VO2 max and that the duration of the time at VO2 max during the exhaustive runs at vdelta100, could be too short to induce an accumulating oxygen deficit affecting the oxygen recovery.     

NADH content in type I and type II human muscle fibres after dynamic exercise.

The effect of dynamic exercise on the NADH content of human type I (slow-twitch) and II (fast-twitch) muscle fibres was investigated. Muscle biopsy samples were obtained from the quadriceps femoris of seven healthy subjects at rest and after bicycle exercise at 40, 75 and 100% of the maximal oxygen uptake [VO2(max.)]. At rest and after exercise at 100% VO2(max.), muscle NADH content was significantly higher (P less than 0.05) in type I than in type II fibres. After exercise at 40% VO2(max.), muscle NADH decreased in type I fibres (P less than 0.01), but was not significantly changed in type II fibres. After exercise at 75 and 100% VO2(max.), muscle NADH increased above the value at rest in both type I and II fibres (P less than 0.05). Muscle lactate was unchanged at 40% VO2(max.), but increased 20- and 60-fold after exercise at 75 and 100% VO2(max.) respectively. The finding that NADH decreased only in type I fibres at 40% VO2(max.) supports the idea that type I is the fibre type predominantly recruited during low-intensity exercise. The increase of NADH in both fibre types after exercise at 75% and 100% VO2(max.) suggests that the availability of oxygen relative to the demand is decreased in both fibre types at high exercise intensities.     

Gastric emptying during walking and running: effects of varied exercise intensity

Gastric emptying is increased during running (50%-70% maximal aerobic uptake, VO2max) as compared to rest. Whether this increase varies as a function of mode (i.e. walking vs running) and intensity of treadmill exercise is unknown. To examine the gastric emptying characteristics of water during treadmill exercise performed over a wide range of intensities relative to resting conditions, 10 men ingested 400 ml of water prior to each of six 15 min exercise bouts or 15 min of seated rest. Three bouts of walking exercise (1.57 m.s-1) were performed at increasing grades eliciting approximately 28%, 41% or 56% of VO2max. On a separate day, three bouts of running (2.68 ms-1) exercise were performed at grades eliciting approximately 57%, 65% or 75% of VO2max. Gastric emptying was increased during treadmill exercise at all intensities excluding 75% VO2max as compared to rest. Gastric emptying was similar for all intensities during walking and at 57% and 65% VO2max during running. However, running at 74% VO2max decreased the volume of original drink emptied as compared to all lower exercise intensities. Stomach secretions were markedly less during running as compared to walking and rest. These data demonstrate that gastric emptying is similarly increased during both moderate intensity (approximately 28%-65% VO2max) walking or running exercise as compared to resting conditions. However, gastric emptying decreases during high intensity exercise. Increases in gastric emptying during moderate intensity treadmill exercise may be related to increases in intragastric pressure brought about by contractile activity of the abdominal muscles.     

Anaerobic threshold and maximal steady-state blood lactate in prepubertal boys

To elucidate further the special nature of anaerobic threshold in children, 11 boys, mean age 12.1 years (range 11.4-12.5 years), were investigated during treadmill running. Oxygen uptake, including maximal oxygen uptake (VO2max), ventilation and the "ventilatory anaerobic threshold" were determined during incremental exercise, with determination of maximal blood lactate following exercise. Within 2 weeks following this test four runs of 16-min duration were performed at a constant speed, starting with a speed corresponding to about 75% of VO2max and increasing it during the next run by 0.5 or 1.0 km.h-1 according to the blood lactate concentrations in the previous run, in order to determine maximal steady-state blood lactate concentration. Blood lactate was determined at the end of every 4-min period. "Anaerobic threshold" was calculated from the increase in concentration of blood lactate obtained at the end of the runs at constant speed. The mean maximal steady-state blood lactate concentration was 5.0 mmol.l-1 corresponding to 88% of the aerobic power, whereas the mean value of the conventional "anaerobic threshold" was only 2.6 mmol.l-1, which corresponded to 78% of the VO2max. The correlations between the parameters of "anaerobic threshold", "ventilatory anaerobic threshold" and maximal steady-state blood lactate were only poor. Our results demonstrated that, in the children tested, the point at which a steeper increase in lactate concentrations during progressive work occurred did not correspond to the true anaerobic threshold, i.e. the exercise intensity above which a continuous increase in lactate concentration occurs at a constant exercise intensity.     

Exercise and nasal patency

Nasal airflow resistances were studied in 20 healthy subjects at rest, with exercise, and during recovery from exercise. Resistances were first measured under resting conditions. As a basis for comparison 0.1% xylometazoline was applied by insufflation; it reduced nasal resistance by an average of 49%. On a subsequent occasion, the degree and time course of changes in resistance were measured 1) during 5-min exercise bouts at rest 25, 50, and 75% of predicted maximum O2 intake (VO2max), 2) during 5-, 10-, and 15-min exercise bouts at 50% of VO2max, and 3) during recovery from exercise. Resistance decreased with intensity but not duration of exercise; an initial sudden decrease was followed by a more gradual but progressive decrease, which continued for several minutes following vigorous short duration exercise. Thus following 5 min of effort at 75% of VO2max, resistance reached a nadir (46% fall) 5 min after cessation of exercise. Recovery of preexercise values required 5 min after 5 min of exercise at 25% of VO2max and 10 min after 5 min of exercise at 50% of VO2max. Some decrease persisted 15 min after 5 min of exercise at 75% of VO2max.     

Exercise training prevents decline in stroke volume during exercise in young healthy subjects.

Stroke volume (SV) increases above the resting level during exercise and then declines at higher intensities of exercise in sedentary subjects. The purpose of this study was to determine whether an attenuation of the decline in SV at higher exercise intensities contributes to the increase in maximal cardiac output (Qmax) that occurs in response to endurance training. We studied six men and six women, 25 +/- 1 (SE) yr old, before and after 12 wk of endurance training (3 days/wk running for 40 min, 3 days/wk interval training). Cardiac output was measured at rest and during exercise at 50 and 100% of maximal O2 uptake (Vo2max) by the C2H2-rebreathing method. VO2max was increased by 19% (from 2.7 +/- 0.2 to 3.2 +/- 0.3 l/min, P less than 0.001) in response to the training program. Qmax was increased by 12% (from 18.1 +/- 1 to 20.2 +/- 1 l/min, P less than 0.01), SV at maximal exercise was increased by 16% (from 97 +/- 6 to 113 +/- 8 ml/beat, P less than 0.001) and maximal heart rate was decreased by 3% (from 185 +/- 2 to 180 +/- 2 beats/min, P less than 0.01) after training. The calculated arteriovenous O2 content difference at maximal exercise was increased by 7% (14.4 +/- 0.4 to 15.4 +/- 0.4 ml O2/100 ml blood) after training. Before training, SV at VO2max was 9% lower than during exercise at 50% VO2max (P less than 0.05). In contrast, after training, the decline in SV between 50 and 100% VO2max was only 2% (P = NS). Furthermore, SV was significantly higher (P less than 0.01) at 50% VO2max after training than it was before. Left ventricular hypertrophy was evident, as determined by two-dimensional echocardiography at the completion of training. The results indicate that in young healthy subjects the training-induced increase in Qmax is due in part to attenuation of the decrease in SV as exercise intensity is increased.     

Neural blockade during exercise augments central command's contribution to carotid baroreflex resetting

This investigation was designed to determine central command's role on carotid baroreflex (CBR) resetting during exercise. Nine volunteer subjects performed static and rhythmic handgrip exercise at 30 and 40% maximal voluntary contraction (MVC), respectively, before and after partial axillary neural blockade. Stimulus-response curves were developed using the neck pressure-neck suction technique and a rapid pulse train protocol (+40 to -80 Torr). Regional anesthesia resulted in a significant reduction in MVC. Heart rate (HR) and ratings of perceived exertion (RPE) were used as indexes of central command and were elevated during exercise at control force intensity after induced muscle weakness. The CBR function curves were reset vertically with a minimal lateral shift during control exercise and exhibited a further parallel resetting during exercise with neural blockade. The operating point was progressively reset to coincide with the centering point of the CBR curve. These data suggest that central command was a primary mechanism in the resetting of the CBR during exercise. However, it appeared that central command modulated the carotid-cardiac reflex proportionately more than the carotid-vasomotor reflex.     

Oxygen uptake kinetics in treadmill running and cycle ergometry: a comparison.

The purpose of the present study was to comprehensively examine oxygen consumption (VO(2)) kinetics during running and cycling through mathematical modeling of the breath-by-breath gas exchange responses to moderate and heavy exercise. After determination of the lactate threshold (LT) and maximal oxygen consumption (VO(2 max)) in both cycling and running exercise, seven subjects (age 26.6 +/- 5.1 yr) completed a series of "square-wave" rest-to-exercise transitions at running speeds and cycling power outputs that corresponded to 80% LT and 25, 50, and 75%Delta (Delta being the difference between LT and VO(2 max)). VO(2) responses were fit with either a two- (LT) exponential model. The parameters of the VO(2) kinetic response were similar between exercise modes, except for the VO(2) slow component, which was significantly (P < 0.05) greater for cycling than for running at 50 and 75%Delta (334 +/- 183 and 430 +/- 159 ml/min vs. 205 +/- 84 and 302 +/- 154 ml/min, respectively). We speculate that the differences between the modes are related to the higher intramuscular tension development in heavy cycle exercise and the higher eccentric exercise component in running. This may cause a relatively greater recruitment of the less efficient type II muscle fibers in cycling.     

Oxygen cost and oxygen uptake dynamics and recovery with 1 min of exercise in children and adults.

To test the hypothesis that O2 uptake (VO2) dynamics are different in adults and children, we examined the response to and recovery from short bursts of exercise in 10 children (7-11 yr) and 13 adults (26-42 yr). Each subject performed 1 min of cycle ergometer exercise at 50% of the anaerobic threshold (AT), 80% AT, and 50% of the difference between the AT and the maximal O2 uptake (VO2max) and 100 and 125% VO2max. Gas exchange was measured breath by breath. The cumulative O2 cost [the integral of VO2 (over baseline) through exercise and 10 min of recovery (ml O2/J)] was independent of work intensity in both children and adults. In above-AT exercise, O2 cost was significantly higher in children [0.25 +/- 0.05 (SD) ml/J] than in adults (0.18 +/- 0.02 ml/J, P less than 0.01). Recovery dynamics of VO2 in above-AT exercise [measured as the time constant (tau VO2) of the best-fit single exponential] were independent of work intensity in children and adults. Recovery tau VO2 was the same in both groups except at 125% VO2max, where tau VO2 was significantly smaller in children (35.5 +/- 5.9 s) than in adults (46.3 +/- 4 s, P less than 0.001). VO2 responses (i.e., time course, kinetics) to short bursts of exercise are, surprisingly, largely independent of work rate (power output) in both adults and children. In children, certain features of the VO2 response to high-intensity exercise are, to a small but significant degree, different from those in adults, indicating an underlying process of physiological maturation.     

Cardiovascular responses to exercise as functions of absolute and relative work load

The roles of absolute and relative oxygen uptake (VO2 and percent of muscle group specific VO2 max) as determinants of the cardiovascular and ventilatory responses to exercise over a wide range of active muscle mass have not previously been defined. Six healthy men performed four types of dynamic exercise--one-arm curl, one-arm cranking, and one- and two-leg cycling at four different relative work loads--25, 50, 75, and 100% of VO2 max for the corresponding muscle group. VO2 during maximal one-arm curl, one-arm cranking, and one-leg cycling averaged 20, 50, and 75%, respectively, of that for maximal two-leg cycling. Cardiac output was linearly related to VO2 with a similar slope and intercept for each type of exercise. Heart rate at a given %VO2 max was higher with larger active muscle mass. In relation to %VO2 max, systemic resistance was lower and plasma catecholamine levels were higher with larger active muscle mass. The cardiovascular responses to exercise are determined to a large extent by the active muscle mass and the absolute oxygen uptake, with the principal feature appearing to be the tight linkage between systemic oxygen transport and utilization.     

Are indices of free radical damage related to exercise intensity

The possibility that plasma levels of malonaldehyde (MDA) are altered by exercise has been examined. The presence of MDA has been recognized to reflect peroxidation of lipids resulting from reactions with free radicals. Maximal exercise, eliciting 100% of maximal oxygen consumption (VO2max) resulted in a 26% increase in plasma MDA (P less than 0.005). Short periods of intermittent exercise, the intensity of which was varied, indicated a correlation between lactate and MDA (r2 = 0.51) (p less than 0.001). Blood lactate concentrations increased throughout this exercise regimen. A significant decrease (10.3%) in plasma MDA occurred at 40% VO2max. At 70% VO2max plasma MDA was still below resting values, however the trend to an increase in MDA with exercise intensity was evident. At exhaustion, plasma MDA and lactate were significantly greater than at rest. These results suggest, that exhaustive maximal exercise induces free radical generation while short periods of submaximal exercise (i.e. less than 70% VO2max) may inhibit it and lipid peroxidation.     

Influence of respiratory muscle work on VO(2) and leg blood flow during submaximal exercise.

The work of breathing (W(b)) normally incurred during maximal exercise not only requires substantial cardiac output and O(2) consumption (VO(2)) but also causes vasoconstriction in locomotor muscles and compromises leg blood flow (Q(leg)). We wondered whether the W(b) normally incurred during submaximal exercise would also reduce Q(leg). Therefore, we investigated the effects of changing the W(b) on Q(leg) via thermodilution in 10 healthy trained male cyclists [maximal VO(2) (VO(2 max)) = 59 +/- 9 ml. kg(-1). min(-1)] during repeated bouts of cycle exercise at work rates corresponding to 50 and 75% of VO(2 max). Inspiratory muscle work was 1) reduced 40 +/- 6% via a proportional-assist ventilator, 2) not manipulated (control), or 3) increased 61 +/- 8% by addition of inspiratory resistive loads. Increasing the W(b) during submaximal exercise caused VO(2) to increase; decreasing the W(b) was associated with lower VO(2) (DeltaVO(2) = 0.12 and 0.21 l/min at 50 and 75% of VO(2 max), respectively, for approximately 100% change in W(b)). There were no significant changes in leg vascular resistance (LVR), norepinephrine spillover, arterial pressure, or Q(leg) when W(b) was reduced or increased. Why are LVR, norepinephrine spillover, and Q(leg) influenced by the W(b) at maximal but not submaximal exercise? We postulate that at submaximal work rates and ventilation rates the normal W(b) required makes insufficient demands for VO(2) and cardiac output to require any cardiovascular adjustment and is too small to activate sympathetic vasoconstrictor efferent output. Furthermore, even a 50-70% increase in W(b) during submaximal exercise, as might be encountered in conditions where ventilation rates and/or inspiratory flow resistive forces are higher than normal, also does not elicit changes in LVR or Q(leg).     

The effect of exercise intensity and duration on the oxygen deficit and excess post-exercise oxygen consumption

Nine males with mean maximal oxygen consumption (VO2max) = 63.0 ml.kg-1.min-1, SD 5.7 and mean body fat = 10.6%, SD 3.1 each completed nine counterbalanced treatments comprising 20, 50 and 80 min of treadmill exercise at 30, 50 and 70% VO2max. The O2 deficit, 8 h excess post-exercise oxygen consumption (EPOC) and EPOC:O2 deficit ratio were calculated for all subjects relative to mean values obtained from 2 control days each lasting 9.3 h. The O2 deficit, which was essentially independent of exercise duration, increased significantly (P less than 0.05) with intensity such that the overall mean values for the three 30%, 50% and 70% VO2max workloads were 0.83, 1.89 and 3.09 l, respectively. While there were no significant differences (P greater than 0.05) between the three EPOCs after walking at 30% VO2max for 20 (1.01 l), 50 (1.43 l) and 80 min (1.04 l), respectively, the EPOC thereafter increased (P less than 0.05) with both intensity and duration such that the increments were much greater for the three 70% VO2max workloads (EPOC: 20 min = 5.68 l; 50 min = 10.04 l; 80 min = 14.59 l) than for the three 50% VO2max workloads (EPOC: 20 min = 3.14 l; 50 min = 5.19 l; 80 min = 6.10 l). An analysis of variance indicated that exercise intensity was the major determinant of the EPOC since it explained five times more of the EPOC variance than either exercise duration or the intensity times duration interaction. The mean EPOC:O2 deficit ratio ranged from 0.8 to 4.5 and generally increased with both exercise intensity and duration.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Dynamics of changes in the cardiovascular response to submaximal exercise during low-intensity endurance training with particular reference to the systolic time intervals

Eighteen male volunteers (aged 20-23 years), not involved in any sporting activities, were submitted to 13 weeks of training consisting of 30 min exercise [at 50%-75% maximal oxygen intake (VO2max)] on a cycle ergometer, performed 3 times a week. Every 4 weeks cardiac function was evaluated by measuring the systolic time intervals at rest and during submaximal cycle exercise. Stroke volume (SV), heart rate (HR) and blood pressure (BP) responses to submaximal exercise, VO2max and anaerobic threshold (AT) were also determined. Significant increases in VO2max, increases in AT and SV at the submaximal exercise intensities, as well as decreases in HR and BP were found after 4 weeks of training. Resting systolic time intervals were not affected by training, but during the submaximal cycle exercise the values of the pre-ejection period (PEP) and isovolumic contraction time (ICT) corresponding to HR of 100 beats.min-1 were significantly lowered after 13 weeks of training, whereas PEP, ICT and total electromechanical systole corresponding to HR of 130 beats.min-1 were significantly shortened by the 4th week. The ratios of PEP:LVET (left ventricular ejection time) and ICT:LVET during submaximal exercise were significantly lowered by training starting from the 8th week. These changes might be interpreted as evidence of the training-induced enhancement of the "contractility reserve", i.e. the ability to increase heart muscle contractility with increasing exercise intensity.     

Exercise intensity: effect on postexercise O2 uptake in trained and untrained women

Despite many reports of long-lasting elevation of metabolism after exercise, little is known regarding the effects of exercise intensity and duration on this phenomenon. This study examined the effect of a constant duration (30 min) of cycle ergometer exercise at varied intensity levels [50 and 70% of maximal O2 consumption (VO2max)] on 3-h recovery of oxygen uptake (VO2). VO2 and respiratory exchange ratios were measured by open-circuit spirometry in five trained female cyclists (age 25 +/- 1.7 yr) and five untrained females (age 27 +/- 0.8 yr). Postexercise VO2 measured at intervals for 3 h after exercise was greater (P less than 0.01) after exercise at 50% VO2max in trained (0.40 +/- 0.01 l/min) and untrained subjects (0.39 +/- 0.01 l/min) than after 70% VO2max in (0.31 +/- 0.02 l/min) and untrained subjects (0.29 +/- 0.02 l/min). The lower respiratory exchange ratio values (P less than 0.01) after 50% VO2max in trained (0.78 +/- 0.01) and untrained subjects (0.80 +/- 0.01) compared with 70% VO2max in trained (0.81 +/- 0.01) and untrained subjects (0.83 +/- 0.01) suggest that an increase in fat metabolism may be implicated in the long-term elevation of metabolism after exercise. This was supported by the greater estimated fatty acid oxidation (P less than 0.05) after 50% VO2max in trained (147 +/- 4 mg/min) and untrained subjects (133 +/- 9 mg/min) compared with 70% VO2max in trained (101 +/- 6 mg/min) and untrained subjects (85 +/- 7 mg/min).     

Carbohydrate metabolism in human skeletal muscle during exercise is not regulated by G-1,6-P2

Glucose 1,6-bisphosphate (G-1,6-P2) is a potent activator of phosphofructokinase (PFK) and an inhibitor of hexokinase in vitro. It has been suggested that increases in G-1,6-P2 are a main means by which PFK can achieve significant catalytic function in vivo despite falling pH and that increases in G-1,6-P2 will inhibit hexokinase in vivo. The purpose of the present study was to determine whether contraction-induced changes in flux through PFK and hexokinase are associated with changes in G-1,6-P2 in skeletal muscle. Ten men performed bicycle exercise for 10 min at 40 and 75% of maximal O2 uptake (VO2max) and to fatigue [4.8 +/- 0.6 (SE) min] at 100% VO2max. Biopsies were obtained from the quadriceps femoris muscle at rest and after each work load and analyzed for G-1,6-P2. G-1,6-P2 averaged 111 +/- 13 mumol/kg dry wt at rest and 121 +/- 16, 123 +/- 15, and 123 +/- 11 mumol/kg dry wt after the low-, moderate-, and high-intensity exercise bouts, respectively (P less than 0.05 for all means vs. rest). Flux through PFK was estimated to increase exponentially as the exercise intensity increased and muscle pH decreased at the higher work loads, whereas flux through hexokinase was estimated to increase during exercise at 40 and 75% VO2max but decrease sharply at 100% VO2max. These data demonstrate that flux through neither PFK nor hexokinase is mediated by changes in G-1,6-P2 in human skeletal muscle during short-term dynamic exercise.     

The effects of one- and two-legged exercise on the lactate and ventilatory threshold

The purpose of this investigation was to compare differences between one- and two-legged exercise on the lactate (LT) and ventilation (VT) threshold. On four separate occasions, eight male volunteer subjects (1-leg VO2max = 3.36 l X min-1; 2-leg VO2max = 4.27 l X min-1) performed 1- and 2-legged submaximal and maximal exercise. Submaximal threshold tests for 1- and 2-legs, began with a warm-up at 50 W and then increased every 3 minutes by 16 W and 50 W, respectively. Similar increments occurred every minute for the maximal tests. Venous blood samples were collected during the last 30 s of each work load, whereas noninvasive gas measures were calculated every 30 s. No differences in VO2 (l X min-1) were found between 1- and 2-legs at LT or VT, but significant differences (p less than 0.05) were recorded at a given power output. Lactate concentration ([LA]) was different (p less than 0.05) between 1- and 2-legs (2.52 vs. 1.97 mmol X l-1) at LT. This suggests it is VO2 rather than muscle mass which affects LT and VT. VO2max for 1-leg exercise was 79% of the 2-leg value. This implies the central circulation rather than the peripheral muscle is limiting to VO2max.