Multivariate analysis of muscular fatigue during bicycle ergometer exercise

The purpose of this study is to estimate the endurance threshold in terms of muscular fatigue during bicycle ergometer exercise. The problems to be solved are induced by dynamic movement and the physiological variation of muscle activity: that is, the progression and impairment of muscle activity occur simultaneously. First of all, we used multichannel recordings of myoelectric (ME) signals to reduce the effect by the movement of a bipolar surface electrode relative to the innervation zones. Second, since even the different types of ME parameters contain redundant information on muscular fatigue, we used the principal component analysis (PCA) to represent the meaningful information by small dimensions. Moreover, we proposed a total evaluation pattern to discriminate muscular fatigue from progression of muscle force at a glance. The total evaluation pattern shows the proportion of first principal component, the components of the first eigenvector, and the correlation coefficients as a function of the work load. The assessment using the total evaluation pattern divided eight subjects into three groups, whereas these subjects were not identified by a specific ME parameter.     

Validity of oxygen uptake measurements during exercise under moderate hyperoxia

PURPOSE: The validity of oxygen uptake in hyperoxia (FIO2 = 30%) measured by an automated system (MedGraphics, CPX/D system) was assessed during the simulation of gas exchanges during exercise with a mechanical system and during submaximal exercise by human subjects. METHODS: The simulation system reproduced a stable and accurate VO2 for 30 min (sim-test). This trial was repeated nine times in normoxia and nine times in hyperoxia. Ten subjects also performed two submaximal exercises (55% of normoxic VO2max) on a cycle ergometer at the same absolute power in normoxia and in hyperoxia (ex-test). RESULTS: There was a significant downward drift of the oxygen fraction measurement in hyperoxia (< or = 0.10% for FIO2 and FEO2) during sim-test, but VO2 measurement remained stable in the two conditions. There was also a downward drift of the oxygen fraction measurement in the two conditions (< or = 0.07% for FIO2) during ex-test. VO2 was significantly higher in hyperoxia (+4.6%), and this result was confirmed using a modified Douglas bag method. CONCLUSIONS: These findings show that the CPX/D system is stable and valid for assessing VO2 in moderate hyperoxia.     

First-pass radionuclide angiography during bicycle and treadmill exercise

BACKGROUND: Treadmill testing is usually preferred over cycle ergometry because of the greater sensitivity in diagnosing coronary artery disease. Treadmill testing has only recently been used with radionuclide angiography (RNA) because patient motion makes RNA imaging difficult. In this study we evaluate the comparability of treadmill and cycle exercise RNA with a dual isotope motion correction technique. METHODS AND RESULTS: Volunteer patients (n = 27) performed first-pass RNA during maximal exercise using both cycle ergometer and treadmill. Exercise capacity was greater during treadmill exercise (8.1 +/- 2.4 vs 7.5 +/- 2.2 METs). Twenty-three of 27 treadmill and all cycle ergometer exercise studies were technically adequate. Maximal heart rate was greater during treadmill exercise (150 +/- 24 vs 143 +/- 25 beats * min-l), however, systolic blood pressure was greater during cycle ergometry (174 +/- 23 vs 188 +/- 25 mmHg), resulting in no difference in heart rate times systolic blood pressure (25.7 +/- 7.2 vs 26.9 +/- 6.0). There were no differences between treadmill and cycle ergometer for peak exercise left ventricular ejection fraction (56% +/- 13% vs 57% +/- 14%) (r = 0.89). Calculated left ventricular end-diastolic volume was not different at rest (183 +/- 42 ml vs 176 +/- 44 ml) but differed significantly at peak exercise (282 +/- 75 ml vs 231 +/- 60 ml). The clinical impression, based on wall motion and left ventricular ejection fraction was very similar between treadmill and cycle ergometer. CONCLUSION: Treadmill exercise RNA is feasible, with about 85% of studies likely to be technically adequate. The overall clinical results are very similar to cycle exercise RNA, although the ordinarily expected advantages of treadmill exercise were largely absent.     

Maximal oxygen uptake during exercise with various combinations of arm and leg work

Oxygen uptake (VO2) was determined in 10 males during the following types of maximal exercise (work time: about 5 min): uphill running, bicycling, arm work (cranking), and combined arm work and bicycling (A + L). The A + L exercise was performed in four different ways, the arms doing 10%, 20%, 30%, or 40% of the same total rate of work; and also with the maximal bicycle work load plus either maximal or submaximal arm work. VO2 was the same in running as in all types of A + L exercise, except when the arm work load was 10% and 40% of the total rate of work, where VO2 was 2.5% (P less than 0.05) and 9.4% (P less than 0.001) lower, respectively. Bicycle VO2 was lower than VO2 in running but equal to A + L VO2 when arm work intensity was 40% of the total rate of work. It is concluded that VO2 during maximal exercise a) to a certain extent depends on the exercising muscle mass, b) is lower than the oxygen-consuming potential of the muscles involved in A + L exercise, and c) in A + L exercise is influenced by the ratio of arm work to total rate of work and the subject's fitness for arm work and bicycling.     

Changes in maximum oxygen uptake during prolonged training, overtraining, and detraining in horses

Thirteen standardbred horses were trained as follows: phase 1 (endurance training, 7 wk), phase 2 (high-intensity training, 9 wk), phase 3 (overload training, 18 wk), and phase 4 (detraining, 12 wk). In phase 3, the horses were divided into two groups: overload training (OLT) and control (C). The OLT group exercised at greater intensities, frequencies, and durations than group C. Overtraining occurred after 31 wk of training and was defined as a significant decrease in treadmill run time in response to a standardized exercise test. In the OLT group, there was a significant decrease in body weight (P < 0.05). From pretraining values of 117 +/- 2 (SE) ml.kg-1.min-1, maximal O2 uptake (VO2max) increased by 15% at the end of phase 1, and when signs of overtraining were first seen in the OLT group, VO2max was 29% higher (151 +/- 2 ml.kg-1.min-1 in both C and OLT groups) than pretraining values. There was no significant reduction in VO2max until after 6 wk detraining when VO2max was 137 +/- 2 ml.kg-1.min-1. By 12 wk detraining, mean VO2max was 134 +/- 2 ml.kg-1.min-1, still 15% above pretraining values. When overtraining developed, VO2max was not different between C and OLT groups, but maximal values for CO2 production (147 vs. 159 ml.kg-1.min-1) and respiratory exchange ratio (1.04 vs. 1.11) were lower in the OLT group. Overtraining was not associated with a decrease in VO2max and, after prolonged training, decreases in VO2max occurred slowly during detraining.     

Determinants of oxygen uptake. Implications for exercise testing

For exercise modalities such as cycling which recruit a substantial muscle mass, muscle oxygen uptake (VO2) is the primary determinant of pulmonary VO2. Indeed, the kinetic complexities of pulmonary VO2 associated with exercise onset and the non-steady state of heavy (> lactate threshold) and severe [> asymptote of power-time relationship for high intensity exercise (W)] exercise reproduce with close temporal and quantitative fidelity those occurring across the exercising muscles. For moderate (< lactate threshold) exercise and also rapidly incremental work tests, pulmonary (and muscle) VO2 increases as a linear function of work rate (approximately equal to 9 to 11 ml O2/W/min) in accordance with theoretical determinations of muscle efficiency (approximately equal to 30%). In contrast, for constant load exercise performed in the heavy and severe domains, a slow component of the VO2 response is manifest and pulmonary and muscle VO2 increase as a function of time as well as work rate beyond the initial transient associated with exercise onset. In these instances, muscle efficiency is reduced as the VO2 cost per unit of work becomes elevated, and in the severe domain, this VO2 slow component drives VO2 to its maximum and fatigue ensues rapidly. At pulmonary maximum oxygen uptake (VO2max) during cycling, the maximal cardiac output places a low limiting ceiling on peak muscle blood flow, O2 delivery and thus muscle VO2. However, when the exercise is designed to recruit a smaller muscle mass (e.g. leg extensors, 2 to 3kg), mass-specific muscle blood flow and VO2 at maximal exercise are 2 to 3 times higher than during conventional cycling. consequently, for any exercise which recruits more than approximately equal to 5 to 6kg of muscle at pulmonary VO2max, there exists a mitochondrial or VO2 reserve capacity within the exercising muscles which cannot be accessed due to oxygen delivery limitations. The implications of these latter findings relate to the design of exercise tests. Specifically, if the purpose of exercise testing is to evaluate the oxidative capacity of a small muscle mass (< 5 to 6kg), the testing procedure should be designed to restrict the exercise to those muscles so that a central (cardiac output, muscle O2 delivery) limitation is not invoked. It must be appreciated that exercise which recruits a greater muscle mass will not stress the maximum mass-specific muscle blood flow and VO2 but rather the integration of central (cardiorespiratory) and peripheral (muscle O2 diffusing capacity) limitations.     

Prediction of maximal aerobic power from a submaximal exercise test performed by paraplegics on a wheelchair ergometer

The aim of the present study was to verify the basic principles underlying the prediction of VO2 peak from a submaximal exercise test performed by paraplegics on a wheelchair ergometer and thus to propose regression equations of VO2 peak prediction. Forty-six paraplegic subjects (mean age = 33.2 +/- 8.7 years) with a traumatic lesion (T1-L3) performed a graded exercise test on a wheelchair ergometer until exhaustion. The test started with an initial workload of 0 watts, with an increment of 6 watts per 2 minutes. Measurements included power output (W), heart rate (HR) and oxygen consumption (VO2) throughout the test. Linear relationships were observed between VO2 and W (VO2 = 0.79 + 0.02 W, r = 0.80, SEE = 0.22 l min-1) as well as between %VO2 max and % maximal heart rate (% VO2 max = 8.7 + 0.83 %HR, r = 0.83 SEE = 10.5%). Combination of the two equations for estimating VO2 peak led to a linear relationship between the estimated and measured VO2 peak. Nonetheless, the strength and accuracy of the prediction were low (r = 0.49, SEE = 0.29 l min-1). Participation in aerobic exercise, body mass and lean body mass, introduced as correction factors in the regression equation, significantly improved the strength and the accuracy of the prediction (r = 0.85, SEE = 0.29 l min-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle function during brief maximal exercise: accurate measurements on a friction-loaded cycle ergometer

A friction loaded cycle ergometer was instrumented with a strain gauge and an incremental encoder to obtain accurate measurement of human mechanical work output during the acceleration phase of a cycling sprint. This device was used to characterise muscle function in a group of 15 well-trained male subjects, asked to perform six short maximal sprints on the cycle against a constant friction load. Friction loads were successively set at 0.25, 0.35, 0.45, 0.55, 0.65 and 0.75 N.kg-1 body mass. Since the sprints were performed from a standing start, and since the acceleration was not restricted, the greatest attention was paid to the measurement of the acceleration balancing load due to flywheel inertia. Instantaneous pedalling velocity (v) and power output (P) were calculated each 5 ms and then averaged over each downstroke period so that each pedal downstroke provided a combination of v, force and P. Since an 8-s acceleration phase was composed of about 21 to 34 pedal downstrokes, this many v-P combinations were obtained amounting to 137-180 v-P combinations for all six friction loads in one individual, over the widest functional range of pedalling velocities (17-214 rpm). Thus, the individual's muscle function was characterised by the v-P relationships obtained during the six acceleration phases of the six sprints. An important finding of the present study was a strong linear relationship between individual optimal velocity (vopt) and individual maximal power output (Pmax) (n = 15, r = 0.95, P < 0.001) which has never been observed before. Since vopt has been demonstrated to be related to human fibre type composition both vopt, Pmax and their inter-relationship could represent a major feature in characterising muscle function in maximal unrestricted exercise. It is suggested that the present method is well suited to such analyses.     

The effect of sedation on oxygen uptake during spontaneous breathing

Sedation may be used in intensive care and emergency medicine to improve the oxygen demand/delivery ratio. The influence of sedation has most frequently been investigated in a dose-related manner. The aim of the present study was to determine the effect-related influence of different sedatives on oxygen uptake (VO2) in relation to defined resting conditions. METHODS. Forty ASA I patients who had to undergo a minor surgical procedure were investigated 1.5 h before surgery at basal energy-expenditure measurement conditions. One of the following substances was given with a preset bolus rate in a double-blind, randomised order until a defined level of sleep or side effects was encountered: propofol (n = 8), midazolam (n = 8), thiopentone (n = 8), sodium chloride (n = 8), and fentanyl (n = 8). The sleep level was defined as sluggish response to a loud voice or tapping on the forearm. The variables VO2, carbon dioxide elimination (VCO2), end tidal CO2 (p(et)CO2), oxygen saturation (SaO2), heart rate, systemic blood pressure, skin temperature, and skin resistance on the sole of the foot were documented on-line on a computer. All variables were compared using differences of averages from 10-min periods before and after sedation during which the VO2 was minimal. RESULTS. The mean VO2 before sedation was 264 +/- 60 ml/min, and the measured energy expenditure did differ by -0.2% (+/- 14%) from mean predicted values using the Harris-Benedict equation. The VO2 was reduced by 15 +/- 2% with propofol, by 12 +/- 8% with midazolam, and by 10 +/- 5% with thiopentone. This was statistically significant compared to placebo treatment, as was the difference between propofol and thiopentone effects. All patients in these groups reached the defined sleep level, which was not achieved by the placebo and fentanyl groups. Placebo treatment changed the VO2 by 0.1% (+/- 2%). Fentanyl increased the VO2 by 5% (+/- 8%), which did not reach significance. In the fentanyl group the bolus application had to be stopped at a p(et)CO2 of 50 mm Hg in all patients. In the propofol, midazolam, and thiopentone groups the phasic changes of skin resistance were reduced to zero and the skin temperature increased from 27 +/- 2 degrees C to 32 +/- 2 degrees C. The fentanyl group showed an increase in changes of skin resistance without changes in temperature. CONCLUSIONS. Sleep induced by propofol, midazolam, or thiopentone to a clinically maximal desirable level in spontaneously breathing patients reduced VO2 by 10% to 15%. This level of sedation did not induce a relevant change in P(et)CO2 or SaO2. The effect of propofol appeared to be the most pronounced and least variable. This may be attributable to a more pronounced reduction in single-organ VO2 or to an undetected difference in level of sedation. Fentanyl did, in contrast to most publications on opioid effects, seem to increase VO2. Underlying mechanisms may be sought in an increased rate-pressure product and sympathetic activity on the basis of hypercapnia and changes in muscle tension.     

A cross-sectional analysis on relationships between maximum oxygen uptake and risk factors for cardiovascular diseases

We examined the relationships between maximum oxygen uptake (Vo2max) and cardiovascular risk factors including age (year), systolic blood pressure (mmHg), diastolic blood pressure (mmHg), serum total cholesterol level (mg/dl), serum high-density lipoprotein level (mg/dl), serum triglyceride level (mg/dl), blood glucose level (mg/dl), serum uric acid level (mg/dl), body fat (%bw), Body Mass Index (BMI), alcohol (points/day), cigarettes (/day), and physical activity (METs.exercise time/30 days). The alcohol point was defined as follows: beer 633ml = a glass of whiskey and water - sake 180ml = 1 point, and totaled at 30 days. The subjects of our study were 162 males (aged 40.6 +/- 13.1) and 133 females (aged 41.3 +/- 11.1) who underwent medical and physical examinations at the Fukui Industrial Health Center from April, 1991 to June, 1992. As a result of simple correlation analysis in males, Vo2max had significantly negative correlations with age (r = -0.223, p < 0.01), systolic blood pressure (r = -0.228, p < 0.01), diastolic blood pressure (r = -0.239, p < 0.01), or serum triglyceride level (r = -0.258, p < 0.001), serum uric acid level (p < 0.05), body fat (r = -0.230, p < 0.01), and BMI (r = -0.312, p < 0.001), and was positively correlated with physical activity (r = -0.249, p < 0.01). On the other hand, in females, age (r = -0.224, p < 0.01), systolic blood pressure (r = -0.222, p < 0.01), diastolic blood pressure (r = -0.267, p < 0.01), serum triglyceride level (r = -0.261, p < 0.001), body fat (r = -0.280, p < 0.01), and BMI (r = -302, p < 0.001), had significantly negative correlations with VO2max. However, partial correlations were tested after controlling body fat, BMI, cigarette, alcohol, physical activity, and age, none of the factors correlated with VO2max significantly. These findings suggest that the risk factors for cardiovascular diseases are related to VO2max, and the life style has an influence on these correlations. Thus, VO2max may be a comprehensive indicator for health promotion among the working population. Furthermore a longitudinal study is required to determine whether the increase in VO2max is related to the improvement in the risk of cardiovascular diseases.     

Effect of voluntary exercise on maximal oxygen uptake in young female Fischer 344 rats

The effect of voluntary exercise on maximal oxygen uptake (VO2 max) was studied in young female Fischer 344 rats. After 10 weeks of wheel-running training, the absolute VO2 max and VO2 max relative to body mass increased without a decline in body mass. The running speed eliciting VO2 max, heart and soleus muscle mass, and succinate dehydrogenase (SDH) activity in the soleus muscle also increased. These results suggest that voluntary exercise is an effective means of increasing the aerobic exercise capacity of young female Fischer 344 rats.     

Correlates of maximal oxygen consumption during treadmill exercise

According to the Balke treadmill protocol, 39 healthy male USAF volunteers were subjected to maximal exercise. The subjects as a group passed the anaerobic threshold by the end of exercise since average venous lactate concentrations increased from 11.2 +/- 1.6 mg% (95% confidence limits) to 93.0 +/- 8.5 mg% (95% confidence limits), and the average gas exchange ratio (R) at the end of the exercise was greater than unity (p less than 0.0005). Tests for correlations showed weak but statistically significant (p less than 0.05) relationships between change in venous lactic acid concentrations and R (r = 0.44) and maximal heart rate (r = 0.34). Maximal oxygen consumption was correlated with time of exercise (r = 0.70) and subject weight (r = 0.33). Subject age and initial plasma lactate concentrations were not significantly correlated with any other variables. Multiple linear regression yielded an equation for prediction of maximal oxygen consumption which included terms for time of exercise and subject weight. Although the multiple correlation coefficent (r = 0.75) was statistically significant (p less than 0.05), it was considered insufficient for accurate prediction of maximal oxygen consumption.     

Influence of chronic undernutrition on oxygen consumption of rats during exercise

The current study was undertaken to examine the effects of dithiothreitol (DDT), a sulfhydryl-reducing agent and heavy metal chelator, on the course of heavy metal-induced acute renal failure in the rat. Groups of rats in metabolic cages received uranyl nitrate (UN) alone, UN plus DTT, mercuric chloride (HgCl2) alone, and HgCl2 plus DTT. UN injected alone produced azotemia, decreased creatinine clearance, and rising fractional sodium excretion over the 48 hr of study. These effects of UN on renal function were not observed when DTT was administered 30 min after UN injection. Qualitatively similar results were obtained with HgCl2-induced acute renal failure. Groups of rats were killed at 6 hr after UN plus DTT, HgCl2 alone, or HgCl2 plus DTT; and determinations of plasma renin activity (PRA) and renin activities of the superficial and deep juxtaglomerular apparatus (JGA) were performed. PRA's and JGA renins were increased in animals receiving either UN or HgCl2 alone, but not in the rats receiving both DTT and UN or HgCl2. The effect of DTT on distribution of 203Hg was also examined. Treatment with DTT did not alter the renal accumulation of 203Hg, suggesting that this agent does not act by limiting renal exposure to the heavy metals. Thus, DTT ameliorates the course of heavy metal-induced ARF, and this effect is associated with prevention of heavy metal-induced alterations in sodium excretion and renin-angiotensin system activity.     

Continuous ambulatory radionuclide monitoring of left ventricular function: effect of body position during ergometer exercise

We assessed the reliability of a continuous ambulatory radionuclide monitoring system (the VEST system, Capintec, Inc., Ramsey, NJ) for measurement of left ventricular performance during exercise in the upright and supine positions. METHODS: Sixteen healthy male volunteers (aged 32-46 yr; mean age 37 +/- 4 yr) were studied. All volunteers underwent ergometer exercise testing in both the upright and supine positions, and left ventricular performance was determined with the VEST system. RESULTS: The resting heart rate, systolic blood pressure, pressure rate product, relative end-diastolic volume, relative end-systolic volume and left ventricular ejection fraction (LVEF) all showed no differences between the upright and supine positions. At peak exercise, the heart rate, systolic blood pressure and pressure rate product showed no differences between the upright and supine positions. In the upright position at peak exercise the relative end-diastolic volume was increased (83% +/- 9% to 91% +/- 11%, p < 0.001); the relative end-systolic volume remained unchanged (34% +/- 3% to 33% +/- 15%), and LVEF was significantly increased from 58% +/- 6% to 66% +/- 11% (p < 0.01). In the supine position at peak exercise, the relative end-diastolic volume remained unchanged (85% +/- 5 to 83% +/- 7%), the relative end-systolic volume was increased (35% +/- 5% to 43% +/- 13%, p < 0.01), and LVEF was decreased from 58% +/- 5% to 48% +/- 17% (p < 0.01). These results indicated inferior data collection by the VEST system in the supine position. CONCLUSION: Since the detector of the VEST system may be too small, the data collection is impaired during exercise in the supine position by shifting the heart with deep respiration. The VEST system is very useful for determining left ventricular performance when applied in the sitting or upright position. However, in the supine position during exercise, the use of the VEST system should be avoided because it might indicate an artifactual deterioration of left ventricular performance.     

Kinetics of oxygen uptake at the onset of exercise in boys and men

The objective of this study was to compare the O2 uptake (VO2) kinetics at the onset of heavy exercise in boys and men. Nine boys, aged 9-12 yr, and 8 men, aged 19-27 yr, performed a continuous incremental cycling task to determine peak VO2 (VO2 peak). On 2 other days, subjects performed each day four cycling tasks at 80 rpm, each consisting of 2 min of unloaded cycling followed twice by cycling at 50% VO2 peak for 3.5 min, once by cycling at 100% VO2 peak for 2 min, and once by cycling at 130% VO2 peak for 75 s. O2 deficit was not significantly different between boys and men (respectively, 50% VO2 peak task: 6.6 +/- 11.1 vs. 5.5 +/- 7.3 ml . min-1 . kg-1; 100% VO2 peak task: 28.5 +/- 8.1 vs. 31.8 +/- 6.3 ml . min-1 . kg-1; and 130% VO2 peak task: 30.1 +/- 5.7 vs. 35.8 +/- 5.3 ml . min-1 . kg-1). To assess the kinetics, phase I was excluded from analysis. Phase II VO2 kinetics could be described in all cases by a monoexponential function. ANOVA revealed no differences in time constants between boys and men (respectively, 50% VO2 peak task: 22. 8 +/- 5.1 vs. 26.4 +/- 4.1 s; 100% VO2 peak task: 28.0 +/- 6.0 vs. 28.1 +/- 4.4 s; and 130% VO2 peak task: 19.8 +/- 4.1 vs. 20.7 +/- 5. 7 s). In conclusion, O2 deficit and fast-component VO2 on-transients are similar in boys and men, even at high exercise intensities, which is in contrast to the findings of other studies employing simpler methods of analysis. The previous interpretation that children rely less on nonoxidative energy pathways at the onset of heavy exercise is not supported by our findings.     

Prediction of maximal oxygen uptake by squat test in men and women

To examine the reproducibility of simple tests including step, squat and double quick tests, the respective tests were performed twice in 242 college women. The step test for 1, 2 or 3 min and the squat test for 1 or 1.5 min were adopted as simple endurance tests with superior reproducibility. Then 30 men (18-26 yr) and 32 women (18-34 yr) participated in a maximal O2 uptake (VO2max) test and the simple endurance tests. The scores (the sum of heart beats for 30-60, 90-120, 150-180 seconds in a sitting posture following the exercise) in the respective simple endurance tests were significantly correlated (P < 0.001) with VO2max per kilogram body weight in the men and women. For example, the equation relating VO2max to the score (X) of the squat test for 1.5 min in men was: VO2max (ml. kg-1. min-1) = -0.261X + 85.19 (r = -0.820, P < 0.001). The discrepancies between VO2max predicted by using the respective estimation equations and that determined by the direct method were 6.3% (by the step test for 3 min) approximately 8.1% (by the squat test for 1 min) in men and 4.7% (by the step test for 2 min) approximately 6.1% (by the squat test for 1 min) in women. Significant correlations were observed between the respective scores in the simple endurance tests (P < 0.001) and between % body fat and the scores (P < 0.01) in both men and women, but not for height vs. the scores. These results suggest that VO2max can be estimated not only by the step test for 1, 2 or 3 min but also the squat test for 1 or 1.5 min.     

Improved oxygen utilization during mild exercise in heart failure

In heart failure with low cardiac output, exercise tolerance is reduced despite modulated regional blood distribution and oxygen extraction. However, low cardiac output does not necessarily lead to reduced exercise tolerance especially during mild exercise. In the present study, in order to understand the mechanisms regulating exercise tolerance in heart failure, we measured oxygen consumption (VO2) and cardiac output (CO) during both mild and intense exercise. Patients with heart failure were divided into 2 groups; group L (n = 8) consists of patients with low anaerobic threshold (AT) < 13 ml/min per kg and group H (n = 7) consisting of patients with AT > 13 ml/min per kg. At rest, VO2 was similar between groups L and H, whereas CO was lower in group L than in group H (3.5 + 0.3 vs 4.8 + 1.4 ml/min, p < 0.01). Increase in VO2 during warm-up exercise was not significant between the 2 groups (7.4 +/- 0.5 (group L) vs 6.2 +/- 0.3 ml/min per kg (group H), ns), but increase in CO was lower in group L than in group H (2.5 +/- 0.6 vs 3.4 +/- 0.4 ml/min, p < 0.01). After warm-up to the AT point, however, the increase in not only VO2 but also CO was markedly reduced in group L than in group H (VO2: 0.5 +/- 0.4 vs 3.7 +/- 0.8 ml/min per kg, p < 0.01, CO: 0.2 +/- 0.3 vs 1.1 +/- 0.3 L/min, p < 0.01). Based on these measurements, we calculated the arteriovenous oxygen difference (c(A-V)O2 difference) during exercise in individual patients using Fick's equation. The c(A-V)O2 difference was markedly increased in severe heart failure during the warm-up stage, but between the end of warm-up and the AT point, it remained at the same level as that of group H. These results suggest the presence of a unique mechanism regulating the c(A-V)O2 difference in severe heart failure patients, activation of which may, at least during mild exercise, contribute to efficient oxygen delivery to the peripheral tissues thus compensating for the jeopardized exercise tolerance in those patients.     

Unaltered oxygen uptake kinetics at exercise onset with lower-body positive pressure in humans

The purpose of this study was to determine the influence of a reduced skeletal muscle blood flow on oxygen uptake (VO2) kinetics at the onset of cycle ergometer exercise. Seven healthy subjects performed rest-to-exercise transitions with a lower-body positive pressure (LBPP) of 45 Torr. Two work rates were selected for each subject: a moderate intensity (VO2, approximately 1.9 l min-1; delta[lactate], approximately 1 mequiv l-1) below the estimated lactate threshold and a heavy intensity (VO2, approximately 2.6 l min-1; delta[lactate], approximately 3 mequiv l-1) above this threshold. Pulmonary gas exchange variables and ventilatory (VE) responses were computed breath-by-breath from mass spectrometer and turbine volume meter signals, respectively, and mean response times (MRT) calculated. Samples of 'arterialized' venous blood were used for the determination of [lactate], pH and [K+]. While the application of 45 Torr LBPP had no effects on VO2 kinetics during moderate exercise (MRT: 33.5 +/- 1.2 s at 45 Torr vs. 32.8 +/- 1.3 s at 0 Torr; P > 0.05) or on [lactate], pH or [K+], breathing frequency (f) was increased (P < 0.05) and tidal volume (VT) reduced (P < 0.05). The addition of LBPP during heavy exercise did not alter VO2 kinetics (MRT: 35.2 +/- 1.5 s at 45 Torr vs. 34.8 +/- 1.5 s at 0 Torr; P > 0.05), or [lactate], pH or [K+]. Although both the VE (via an increased f) and CO2 output (VCO2) were significantly greater with LBPP by approximately 30 l min-1 and approximately 500 ml min-1, respectively, end-tidal CO2 partial pressure was decreasing, suggesting an additional ventilatory stimulus. These data can be interpreted to suggest that oxygen delivery is not critically dependent upon blood flow to the working muscle at exercise onset, while LBPP-induced increases in VE during suprathreshold exercise may be related to an accumulation of metabolites at the working muscle or the effects of pressure per se.     

Responses to ergometer exercise in a healthy biracial population of children

To determine normal values for the exercise variables heart rate, blood pressure, maximal workload, physical working capacity index, J point displacement, and ST segment slope, we stress tested 405 healthy children. We analyzed the data for four body surface area-determined groups, to discover whether there were any racial differences between healthy white children and black children. There were numerous racial differences in blood pressure, maximal workload, and physical working capacity index; there were no differences in the heart rate values. The incidence of false-positive J point displacement was less than or equal to 3% when the PR isoelectric line method was used. The ST segment slope in healthy children was always greater than zero at maximal exercise. Thus, norms for exercise variables must be expressed in relation to both sex and race. The nomograms presented in this report provide an easy-to-use set of normative data for cycle ergometer stress testing in children.