Measurements of cardiac output in maximum exercise. Application of an acetylene rebreathing method to arm and leg exercise

Summary An acetylene rebreathing technique suitable for the measurement of cardiac output in maximum exercise is described; results have a coefficient of variation of 2.9–3.6%, and show good agreement with observations made by other methods during arm and leg work. Gas samples are analysed by gas chromatograph, using silica gel and molecular sieve/glass columns. Recirculation of the foreign gas is avoided by collection of the final gas sample after 7 sec of rebreathing. Allowance is also made for other systematic errors, including the dead space of the sampling needles and solution of gas in the lung parenchyma.Arm work has a similar oxygen cost to leg work if extraneous movements are avoided; however, the maximal attainable oxygen intake is less in arm exercise (28% in 5 young men). The heart rate in sub-maximum effort is 36–39 beats/min greater for arm work; however, the maximum heart rate is unchanged. The cardiac stroke volume is 25% smaller for both sub-maximum and maximum arm work, while the arterio-venous oxygen difference is slightly greater than in leg work.Both stroke volume and arterio-venous difference increase somewhat between 60 and 100% of aerobic power, but the main burden of increasing oxygen transport is accepted by the increase in heart rate.     

Effects of physical conditioning upon the central and peripheral circulatory responses to arm work

Summary 10 sedentary young men completed a programme of 4 weeks endurance training involving bi-weekly 30 min sessions of exercise on an arm ergometer at 80% of maximum aerobic power. Maximum oxygen intake increased by 8%, and there was also a 4% increase in the mechanical efficiency of effort. The cardiac output (measured by acetylene rebreathing) increased in both sub-maximum and maximum effort. There was an 8% increase in maximum stroke volume, but no change of maximum heart rate. Strain gauge measurements showed a diversion of blood flow from skin to muscle with training. This adaptation is of value to the athlete only after alternative methods of heat dissipation have developed. The possible application of the arm ergometer to the training of patients with leg injuries is briefly discussed.     

Oxygen deficit during incremental exercise

Summary The oxygen deficit and debt have conventionally been determined during exercise at constant work rates. During this study these were calculated during and after exercise at progressively incremented work rates. Five men performed two successive incremental exercise tests to exhaustion on an electronically braked cycle ergometer. The two tests were separated by a 5 min rest period. The oxygen deficit was defined as the sum of the minute differences between the measured oxygen uptake and the oxygen uptake occurring during steady state work at that same rate. The oxygen deficit was quantified for the work periods before and after the anaerobic threshold (AT) as determined from respiratory gas analysis (ATR). The measured deficit for the period before the ATR was smaller than the deficit measured in the same subjects during steady state work at low intensity (below the ATR) and was also less than the rapid component of the oxygen repayment as determined after the second incremental test. It was concluded that this test could be used for the determination of anaerobic capacity as represented by the total oxygen deficit (within motivational limits), but that the lactacid and alactacid components of the deficit could not be differentiated. A considerable portion of the alactacid component of the deficit was incurred after the onset of the ATR.     

Relationship between O2 consumption,high energy phosphates and the kinetics of the O2 debt in exercise

Summary The oxygen consumption together with lactic acid production and concentration of ATP, ADP, and creatinephosphate was measured during exercise and recovery on an isolated dog gastrocnemius.Oxygen debt contraction and payment follow an exponential path with a half reaction time of about 20 sec. The concentration of ATP and ADP at steady state seem to be unaffected by the intensity of the exercise when this is submaximal and no appreciable production of lactic acid takes place. The concentration of creatinephosphate in muscle at steady state decreases with the intensity of the exercise. The ratio of the oxygen consumption at steady state to the alactic oxygen debt is identified with the speed constant of the resynthesis of phosphagen in muscle; the half reaction time of this process is 17–20 sec. The total alactic oxygen debt amounts to about 50 ml/kg of muscle. These figures are in good agreement with earlier data found in man.We are greatly indebted to Dr.J. Piiper for his useful criticism and help during the preparation of this paper.     

Influence of training status on maximal accumulated oxygen deficit during all-out cycle exercise

The influence of training status on the maximal accumulated oxygen deficit (MAOD) was used to assess the validity of the MAOD method during supramaximal all-out cycle exercise. Sprint trained (ST; n = 6), endurance trained (ET; n = 8), and active untrained controls (UT; n = 8) completed a 90 s all-out variable resistance test on a modified Monark cycle ergometer. Pretests included the determination of peak oxygen uptake ( O_2peak) and a series (5–8) of 5-min discontinuous rides at submaximal exercise intensities. The regression of steady-state oxygen uptake on power output to establish individual efficiency relationships was extrapolated to determine the theoretical oxygen cost of the supramaximal power output achieved in the 90 s all-out test. Total work output in 90 s was significantly greater in the trained groups (P<0.05), although no differences existed between ET and ST. Anaerobic capacity, as assessed by MAOD, was larger in ST compared to ET and UT. While the relative contributions of the aerobic and anaerobic energy systems were not significantly different among the groups, ET were able to achieve significantly more aerobic work than the other two groups, while ST were able to achieve significantly more anaerobic work. Peak power and peak pedalling rate were significantly higher in ST. The results suggested that MAOD determined during all-out exercise was sensitive to training status and provided a useful assessment of anaerobic capacity. In our study sprint training, compared with endurance training, appeared to enhance significantly power output and high intensity performance over brief periods (up to 60 s), yet few overall differences in performance (i.e. total work) existed during 90 s of all-out exercise.     

Oxygen debt: Involvement of the Cori cycle

Summary Tryptophan and quinolinic acid, inhibitors of gluconeogenesis, were used to block the removal of lactate by the liver in order to investigate the involvement of the Cori cycle in oxygen debt. Five male, mongrel dogs were run on a treadmill at 4 mph with a 20 percent grade for 19 min. The mean exercise was 80.67±3.11 ml/kg/min for the control tests while peak arterial lactate values ranged from 3.83 to 4.98 mM/l. When removal of lactate by the liver was blocked, oxygen debt showed a mean reduction of 44 percent. Moreover, oxygen consumption during the last minute of exercise was reduced by 11 percent.Fasting (72 h) was used in 1 dog to prevent the accumulation of lactate during exercise. This procedure reduced oxygen debt to the same level as when the removal of lactate by the liver was blocked with tryptophan and quinolinic acid.The data show that the lactacid as well as the alactacid component is involved in oxygen debt when lactate is being removed by the liver during the recovery period following exercise.     

Energetics of paraplegic cycling: a new theoretical framework and efficiency characterisation for untrained subjects

Complete lower-limb paralysis resulting from spinal cord injury precludes volitional leg exercise, leading to muscle atrophy and physiological de-conditioning. Cycling can be achieved using phased stimulation of the leg muscles. With training there are positive physiological adaptations and health improvement. Prior to training, however, power output may not be sufficient to overcome losses involved in rotating the legs and little is known about the energetics of untrained paralysed muscles. Here we propose efficiency measures appropriate to subjects with severe physical impairment performing cycle ergometry. These account for useful internal work (i.e. muscular work done in moving leg mass) and are applicable even for very low work rates. Experimentally, we estimated total work efficiency of ten untrained subjects with paraplegia to be 7.6 +/- 2.1% (mean +/- SD). This is close to values previously reported for anaesthetised able-bodied individuals performing stimulated cycling exercise, but is less than 1/3 of that of able-bodied subjects cycling volitionally. Correspondingly, oxygen cost of the work (38.8 +/- 13.9 ml min(-1) W(-1)) was found to be approximately 3.5 times higher. This indicates the need, for increased power output from paralysed subjects, to maximise muscle strength through training, and to improve efficiency by determining better methods of stimulating the individual muscles involved in the exercise.     

Low intensity training, inactivity and resumed training in sedentary men

The effects of a low intensity training regimen, consisting of two 7-week periods with an interspersed 8-week inactivity period were investigated in 16 sedentary men. A follow-up was made on 7 subjects after 38 additional weeks' training. Systemic as well as local effects were studied using exercise tests and leg muscle biopsies. The two 7-week training periods both resulted in a 6% increase in Vo2 max and a lowered heart rate during submaximal work. No persisting training effects were detected by exercise tests after inactivity. In skeletal muscle, however, striking differences in enzyme activity pattern and ultrastructure were observed between the two periods, indicating that some training effect of importance for muscle metabolic adaptation might have persisted during inactivity. It is suggested that such an effect might be associated with the local oxygen supply. During the 38-week training period there was a large increase in muscle metabolic capacity, but no change in maximal oxygen uptake. This separation of systemic and local training effects indicates a lack of a direct causal relationship between muscle metabolic potential and max imal oxygen uptake. It is suggested that the elevated muscle oxidative capacity is of importance for an increased endurance capacity.     

The effects of various warming up intensities and durations upon some physiological variables during an exercise corresponding to the WC170

Summary Empirically, many procedures are used by sportsmen to warm up, such as massage, hot showers, or muscular exercise, although their objective effect on performance is still not evident. The present study investigates different warming up durations and intensities with or without a resting period between warming up and the criterion exercise, both performed on a bicycle ergometer. Heart rate, oxygen consumption, blood lactic acid level, and oxygen debt are measured. When the criterion exercise immediately follows warming up, heart rate, and oxygen consumption generally attain higher values during the criterion exercise but lactic acid level shows no important variation. When introducing a resting period between warming up and the criterion exercise, the heart rate and oxygen consumption values do not differ from those attained without warming up. The results do not conclusively show a positive effect of warming up before exercise.     

A comparison between the bicycle ergometer and the step-test for determining maximum oxygen intake on Kalahari Bushmen

Summary In March 1967 a group of scientists visited the Kalahari desert in Botswana and determined the maximum oxygen capacities of a number of Bushmen subjects by making use of the bicycle ergometer and the step-test.The mean maximum oxygen intake values obtained with the step-test were close to those determined from sub-maximum bicycle ergometer tests, 40.5 and 39.8 ml/(min · kg) respectively. Primitive people can be trained to give good submaximal results on a bicycle ergometer, but it was impossible to obtain reliable maximum work rates for these people on a bicycle ergometer.     

The effect of training and various work loads on the lactacid-alactacid oxygen debt response of exercising dogs

Summary The involvement of the lactacid and alactacid mechanisms in oxygen debt was examined in 2 dogs prior to and after a 6-week training program by using tryptophan and quinolinic acid to block the removal of lactate by the liver. The results show that the lactacid mechanism is involved at work loads resulting in sufficient elevation of blood lactate during the recovery period. It was further shown that training produced a significant decrease in both oxygen debt and blood lactate. Mechanisms responsible for the findings are discussed.     

Oxygen deficit is not affected by the rate of transition from rest to submaximal exercise

Five subjects cycled on an ergometer at power outputs corresponding to 20, 40, 60 and 80% of their maximal oxygen uptake (VO2 max). On one occasion the transition from rest to work was direct (D), while on the other occasion the power output was increased slowly (S) in a stepwise manner for 6-15 min prior to exercise at the predetermined intensity. Oxygen uptake (VO2) was measured, and O2 deficit and O2 debt were calculated. Oxygen deficit increased with the exercise intensities, the peak values being 2.1 +/- 0.2 and 1.9 +/- 0.1 litres (mean +/- SEM) at 80% of VO2 max after D and S respectively. No significant difference was observed in O2 deficit or O2 debt between D and S at any exercise intensity (P less than 0.05). The O2 debt was similar to the O2 deficit at 20, 40 and 60% of VO2 max but lower than the O2 deficit (P less than 0.05) at 80% of VO2 max. Femoral venous blood lactate remained unchanged at 20% of VO2 max but increased at the higher exercise intensities, reaching peak values of 7.6 +/- 0.6 and 7.4 +/- 1.1 mmol l-1 at 80% of VO2 max after D and S respectively. Blood lactate was not significantly different between D and S at any exercise intensity (P greater than 0.05). It is concluded that O2 deficit, O2 debt and blood lactate are not affected by the rate of transition from rest to submaximal exercise. The data contradict the hypothesis that O2 deficit is caused by an inadequate O2 transport at the onset of exercise.     

Utilization of blood-borne and intramuscular substrates during continuous and intermittent exercise in man.

1. Substrate utilization in the legs during bicycle exercise was studied in five subjects when performing intermittent intense exercise (15 sec work--15 sec rest) as well as continuous exercise during 60 min, with an almost identical average power output and oxygen uptake in both situations. 2. Muscle biopsies were obtained from vastus lateralis at rest, during, and after exercise in order to determine intramuscular lipid and carbohydrate utilization. The contribution from blood-borne substrates to total oxidative metabolism was determined by arterial-femoral venous (a-fv) differences for oxygen, FFA, glucose, and lactate and leg blood flow. 3. Intermittent and continuous exercise revealed a similar glycogen depletion and the intramuscular lactate accumulation was rather small. A similar uptake of blood-borne substrate (FFA, glucose) was found in both situations whereas a release of lactate only was observed in intermittent exercise. 4. ATP and CP levels oscillated between work and rest periods in intermittent exercise but were not resynthesized to resting levels at the end of the rest periods. The mainly aerobic energy release during each work period in intermittent exercise is partly caused by myoglobin functioning as an oxygen store; this factor was calculated to be more important than ATP and CP or lactate level oscillations. 5. The metabolic response to intermittent exercise was found to be similar to that found in continuous exercise with approximately the same average power output and oxygen uptake. This indicates that some factor in the intermediary metabolism, for instance citrate, functions as a regulator retarding glycolysis and favouring lipid utilization and an aerobic energy release in intermittent exercise.     

The influence of training on oxygen isotope fractionation in healthy subjects

We determined the oxygen isotope fractionation in expired alveolar gas relative to inspired air (delta(A-I)) in eight young, healthy subjects at rest and at five levels of exercise up to maximal workload both before and after a training period of about 4 weeks which increased maximum oxygen uptake by about 10%. The data for delta(A-I) were used to compute the relative difference (deltaU) between the resistances of 16O18O and 16O2 for oxygen transport from the alveolar space and utilization in the mitochondria. Prior to training, deltaU decreased from 15 per thousand at rest to 5 per thousand at the highest level of exercise and after training from 12 to 5 per thousand. The difference between the results for deltaU before and after training was significant for rest (P < or = 5) but not for exercise conditions. Accordingly, we conclude that for exercise conditions the non-fractionating oxygen transport by blood flow to and the fractionating oxygen transport by diffusion in the muscles have improved by training to more or less the same degree. The decrease in deltaU in rest after training suggests that oxygen transport by diffusion in other tissues also benefits from the effects of training.     

Analysis of recovery from anaerobic work

Summary Recovery processes have been studied in severe work of uniform intensity continuing from a minimum of 5 seconds to a maximum of 106 seconds. The work was largely anaerobic, requiring the accumulation of an oxygen debt, the magnitude of which was approximately a linear function of time.The rate of removal of lactic acid is a logarithmic function of time; the rate varies somewhat from time to time and from one person to another. In the type of activity studied the lactic acid accumulated is proportional to the duration of work. The total oxygen debt as well as the lactacid debt being proportional to duration of work, it follows that the so-called alactacid debt is also proportional to the duration of work. In other words, the lactacid debt and the alactacid debt in such activity are contracted concurrently.The oxygen intake in recovery may be analyzed into a rapid process nearly complete in 5 minutes which pays the alactacid debt, a process about 1/20th as rapid which pays the lactacid debt, and finally an increase in resting metabolism which decreases in a linear fashion until the pre-exercise level is reached. The excess R. Q for the entire process of work and recovery probably lies within the range 0,9 to 1,0.     

Atrial natriuretic peptide in plasma after prolonged physical strain,energy deficiency and sleep deprivation

Summary Plasma concentrations of atrial natriuretic peptide (ANP) were investigated daily in 16 male cadets during a 6-day military training course with continuous heavy physical activities, sleep and energy deficiency (course 1). At the end of another similar course (course 11) 15 cadets were studied during 30-min cycle exercise at 50% maximal oxygen uptake with and without glucose infusion. A small, but not significant increase was found in the plasma concentrations of ANP during course I from 9.6 (SEM 1.1) pmol·l^–1 in the control experiment to 11.1 (SEM 0.5) pmol·l^–1 on day 5. During course II a small but significant increase was found from 7.8 (SEM 0.5) pmol·l^–1 in the control experiment to 9.1 (SEM 0.5) pmol·l^–1 at the end of the course. Plasma osmolality and chloride concentration decreased during the course. During the exercise test a significant increase was seen in ANP concentration from 8.2 (SEM 0.8) to 13.1 (SEM 2.0) pmol·l^–1 in the control experiment and from 9.4 (SEM 0.7) to 13.5 (SEM 1.2) pmol·l^–1 during the course. This response was attenuated by glucose infusion, an effect which may have been due to an exercise induced increase in plasma chloride concentration being abolished. In contrast, the potassium concentration response to exercise was increased during the course but unaffected by glucose infusion. In conclusion, the large increases in endogenous plasma catecholamine concentration shown to take place during previous courses were not reflected in the plasma concentrations of ANP, indicating only a moderate cardiac stress or no cardiac work overload during such courses.     

Plasma vasopressin,renin activity,and aldosterone: Effect of exercise and training

Summary The influence of endurance-training on hematocrit, plasma vasopressin, renin activity, and aldosterone changes at rest and at the end of an exercise performed until exhaustion at a given and constant relative work-load (87% of maximal oxygen uptake) has been studied in four untrained subjects submitted to a 5-month training. At the end of this period, maximal oxygen uptake increased of 15.2% (p<0.01). Hematocrit at rest slightly rose after training, and if exercise constantly induced increases in hematocrit before (p<0.001) and after training (p<0.005), the per cent increase after training was lower than before (p<0.05). Comparison between the importance of weight loss and hematocrit variation showed that when untrained subjects become trained the variation of hematocrit after exercise becomes smaller while weight loss is more important (p<0.01).Plasma renin activity (PRA), aldosterone (Aldo) and vasopressin (AVP) levels, compared to control values, displayed a significant increase after exercise before as well as after training. Control values remained unchanged after training for aldosterone and AVP, but were significantly lower (p<0.05) for PRA. This latter observation could be explained by the change in blood volume induced by exercise.This work was supported by DGRST (grant nos. 77.7.0823 to Cl. Gharib and 77.7.0824 to J. R. Lacour) and Fondation pour la Recherche Médicale FranÇaise (1979)     

VE response to VCO2 during exercise is unaffected by exercise training and different exercise limbs

We designed two experiments to investigate the relationship between ventilation (VE) and CO2 output (VCO2) during exercise under the conditions of exercising different limbs, the arms as opposed to the legs (experiment 1), and of different physical training states after undergoing standard exercise training for 90 d (experiment 2). Six healthy young subjects underwent submaximal ramp exercise at an incremental work rate of 15 W/min for the arm and leg, and 11 healthy middle-aged subjects underwent an incremental exercise test at the rate of 30 W/3 min before and after exercise training. We measured pulmonary breath-by-breath VE, VCO2, oxygen uptake (VO2), tidal volume (VT), breathing frequency (bf), and end-tidal O2 and CO2 pressures (PETO2, PETCO2) via a computerized metabolic cart. In experiment 1, arm exercise produced significantly greater VE than did leg exercise at the same work rates, as well as significantly higher VO2, VCO2, and bf. The slopes of the regression lines in the VE-VCO2 relationship were not significantly different: the values were 27.8 +/- 2.1 (SD) during the arm exercise, and 25.3 +/- 3.9 during the leg exercise, with no differences in their intercepts. In experiment 2, the VO2, VCO2, and VE responses at the same work rates were similar in both before and after the 90-d exercise training, whereas the heart rate (HR) and mean blood pressure (MBP) were significantly reduced after training. Exercise training did not alter the VE-VCO2 relationship, the slope of which was 31.9 +/- 4.9 before exercise training and 34.2 +/- 4.4 after exercise training. We concluded that the VE-VCO2 relationship during exercise is unaltered, independent of not only working muscle regions but also exercise training states.     

Can human cardiovascular regulation during exercise be learnt from feedback from arterial baroreceptors?

During dynamic exercise, a large fall in systemic vascular resistance occurs. Arterial pressure (AP) is, however, maintained through a combination of central command and neural activity from muscle afferents that adjust the autonomic outflow to the circulation. How these signals are calibrated to provide accurate regulation of AP remains unclear. This study tests the hypothesis that the calibration can be 'learnt' through feedback from the arterial baroreceptors arising over multiple trials of exercise. Eight healthy subjects undertook three different protocols in random order. The test protocol consisted of 7 days' training, when subjects were exposed on 70 occasions to 4 min of exercise (50% of maximal oxygen uptake capacity) paired with neck suction (-40 mmHg) to mimic an excessive rise in AP at the carotid baroreceptors with exercise. Two control protocols involved training with either exercise or neck suction alone. No significant changes in mean AP, diastolic AP or heart rate during normal exercise were detected following training with any protocol. However, the rise in systolic AP with exercise was attenuated by an average of 7.3 +/- 2.0 mmHg (mean +/- s.e.m., P < 0.01) on the first and second days following training with the test protocol, but not with either control protocol (P < 0.05 for difference between protocols, ANOVA). In conclusion, this study failed to show that mean AP during normal exercise could be reduced through prior conditioning by overstimulation of the baroreceptors during exercise. However, a reduction in systolic AP was observed that suggests the presence of some plasticity within the autonomic response, consistent with our hypothesis.     

The Nature of the Training Response; Peripheral and Central Adaptations to One-Legged Exercise

13 male subjects were studied and placed in 3 groups. Each group exercised one leg with sprint (S), or endurance (E) training and the other leg oppositely or not at all (NT). Oxygen uptake (Vo₂), heart rate and blood lactate were measured for each leg separately and for both legs together during submaximal and maximal bicycle work before and after 4 weeks of training with 4–5 sessions per week. Muscle samples were obtained from the quadriceps muscle and assayed for succinate dehydrogenase (SDH) activity, and stained for myofibrillar AT Pase. In addition eight of the subjects performed after the training two-legged exercise at 70% Vo₂ max for one hour. The measurements included muscle glycogen and lactate concentrations of the two legs as well as the blood flow and the a-v difference for O₂, glucose and lactate. The improvement in Vo₂ max, the lowered heart rate and blood lactate response at submaximal work levels were only found when exercising with a trained leg (E or S). Part of the variables studied were markedly more changed with E as compared with S-training. Although muscle fibre composition did not change a pronounced muscle adaptation took place with the training with enhancement of the SDH activity of the S and E legs while the NT-leg did not change. Blood flow and oxygen uptake were similar in NT and S–E legs while femoral vein oxygen content was slightly lower in the trained as compared to the NT-leg. Glycogen utilization was lowest in the trained leg with similar glucose uptake in all legs regardless of training status. Moreover, lactate was only continuously released from the NT-leg. It is concluded that training induces marked local adaptations which not only affects the metabolic response to exercise but also are of importance eliciting an improved cardiovascular function.