Influence of body position and pre-exercise activity on cardiac output and oxygen uptake following step changes in exercise intensity

Effects of body posture and exercise training on cardiorespiratory responses to exercise

The primary aims of the present study were to evaluate cardiorespiratory responses to incremental head down tilt exercise and to determine if the cardiorespiratory adaptations obtained from endurance training in the head down tilt posture transfer to the upright posture. 22 men (25±3 years) performed V˙O2peak cycle exercise tests in the upright and head down tilt postures. Of these, 11 men were endurance trained on a cycle ergometer in the upright posture for 8 weeks (upright training group; UTG) or in the upright posture for 4 weeks followed by 4 weeks in the head down tilt posture (head down training group; HTG). During acute exercise, V˙O2peak was decreased in the head down tilt posture compared to upright (2.01±0.51 vs. 2.32±0.61l/min respectively, P<0.05). Stroke volume (SV) at 100 W was greater during head down tilt cycling compared to the upright (77±5 vs. 71±4 ml/beat, P<0.05). Following training V˙O2peak increased in both groups during upright exercise. However, V˙O2peak during head down tilt cycling was only increased in the HTG. Sub-maximal and peak SV in the HTG increased in both upright and head down tilt postures. SV in the UTG increased only in the upright posture and was unchanged during head down tilt cycling. In conclusion, acute head down tilt exercise increases sub-maximal SV compared to upright exercise. Furthermore, training in the head down tilt posture induces cardiorespiratory adaptations in both upright and head down tilt postures, while the adaptations to upright exercise training are primarily observed when upright exercise was performed.

Effect of body incline on cardiac performance

Maximal cardiac performance is improved in man during upright compared to supine exercise. Whether cardiac performance in quadrupeds is dependent upon body position is unknown. Therefore, we undertook the present investigation to determine if peak cardiac output (Qpeak) would be influenced by body inclination in the Thoroughbred horse. To test the hypothesis, four Thoroughbred horses performed an incremental exercise protocol (speed increased by 1 m/s/min to fatigue) on both a level (L) and inclined (I: 6 degrees) treadmill. Specifically, we hypothesised that Qpeak would be increased on the incline, as this represents a progression towards upright exercise. Cardiac output was determined using the Fick relationship from continuous measurements of pulmonary VO2 and paired arterial (carotid artery or transverse facial) and mixed venous (pulmonary artery) samples. Qpeak was significantly increased on the incline (L: 279 +/- 20; I: 336 +/- 17 l/min; P<0.05), while CaO2 was not significantly different (L: 25.5 +/- 1.1; I: 25.4 +/- 1.9 ml/100 ml), and therefore, whole body O2 delivery (QO2) was significantly increased (L: 70.7 +/- 4.9; I: 84.4 +/- 3.1 l/min; P<0.05). In conclusion, within the scope of this investigation, these data suggest that cardiac performance, as judged by increased Qpeak and QO2, is enhanced in the inclined body position. Furthermore, these findings provide preliminary information that level and incline treadmill exercise tests may yield significantly different results in the Thoroughbred horse and consequently this factor should be considered when interpreting exercise testing and performance data.

Femoral artery inflow in relation to external and total work rate at different knee extensor contraction rates

Whether limb blood flow is directly regulated to match the work rate, independent of the rate of contraction, remains elusive. This study therefore investigated the relationship between femoral arterial blood flow (FABF; Doppler ultrasound) and "external" (applied load) as well as "total" [external + "internal" (potential and kinetic energy changes of the moving lower leg)] work rate, during steady-state one-legged, dynamic, knee extensor exercise (1L-KEE) in the sitting position at different contraction rates. Ten subjects performed 1L-KEE at 30, 60, and 90 contractions/min (cpm) 1) at constant resistive loads of 0.2 and 0.5 kg inducing incremental external work rates (study I) and 2) at different relative resistive loads inducing constant external work rates of 9 and 18 W (study II). Moreover, 3) six subjects performed 1L-KEE at 60 and 100 cpm at incremental total work rates of 40, 50, 60, and 70 W (study III). In study I, FABF increased (P < 0.001) with increasing contraction frequency and external work rate, for each resistive load. In study II, FABF increased (P < 0.001) with increasing contraction frequency for each constant external work rate. Of major importance in study III, however, was that FABF, although increasing linearly with the total work rate, was not different (P = not significant) between contraction rates, at the total work rates of 40, 50, 60, and 70 W, respectively. Furthermore, FABF correlated linearly and positively with both the external and total work rate for each contraction frequency. In conclusion, the findings support the concept that leg blood flow during 1L-KEE in a normal knee extensor ergometer is matched directly in relation to the total work rate and metabolic activity, irrespective of the contraction frequency. The rate of contraction seems erroneously to influence the results only when it is related to the external work rate without taking into account the internal work component.

Kinetics of oxygen uptake during supine and upright heavy exercise

It is presently unclear how the fast and slow components of pulmonary oxygen uptake (VO(2)) kinetics would be altered by body posture during heavy exercise [i.e., above the lactate threshold (LT)]. Nine subjects performed transitions from unloaded cycling to work rates representing moderate (below the estimated LT) and heavy exercise (VO(2) equal to 50% of the difference between LT and peak VO(2)) under conditions of upright and supine positions. During moderate exercise, the steady-state increase in VO(2) was similar in the two positions, but VO(2) kinetics were slower in the supine position. During heavy exercise, the rate of adjustment of VO(2) to the 6-min value was also slower in the supine position but was characterized by a significant reduction in the amplitude of the fast component of VO(2), without a significant slowing of the phase 2 time constant. However, the amplitude of the slow component was significantly increased, such that the end-exercise VO(2) was the same in the two positions. The changes in VO(2) kinetics for the supine vs. upright position were paralleled by a blunted response of heart rate at 2 min into exercise during supine compared with upright heavy exercise. Thus the supine position was associated with not only a greater amplitude of the slow component for VO(2) but also, concomitantly, with a reduced amplitude of the fast component; this latter effect may be due, at least in part, to an attenuated early rise in heart rate in the supine position.

Cardiodynamic responses during seated and supine recovery from supramaximal exercise

The cardiodynamic responses of 9 healthy men (mean age +/- SD, 22.3 +/- 2.0 yrs) were measured during seated and supine passive recovery following the Wingate Anaerobic Power Test (WAPT). Stroke index (SI) was determined noninvasively using impedance cardiographic techniques. During the initial stages of seated recovery, SI progressively increased (1 min, 60.4 +/- 6.6 ml/m2; 3 min, 64.6 +/- 5.9 ml/m2) and achieved peak levels by 5 min (70.5 +/- 6.2 ml/m2). Between Minutes 3 and 10 of seated recovery, SI was significantly (p < or = 0.05) higher than the preexercise value (46.0 +/- 4.0 ml/m2). A similar response pattern for SI was observed during supine recovery. The systemic vascular resistance index (SVRI) decreased by 101% and 114% from preexercise baseline values after 1 min of recovery in the supine and seated postures, respectively. The persistent rise in SI during the first 10 min of passive seated recovery may be explained by the sustained attenuation in SVRI coupled with the anticipated residual myocardial inotropic effects following the WAPT.

Cardiorespiratory effects of weight reduction by exercise in middle-aged women with obesity

The cardiorespiratory responses to weight reduction due to physical exercise were examined in fourteen women with obesity, aged 36 to 67 years (Body Mass Index, 32.4 +/- 1.5 kg/m2). The patients were instructed to exercise at approximately 60% of maximum oxygen uptake for 2 h every day for approximately 3 months. To evaluate physical strength, a graded cycling exercise test was performed both before and after the exercise period, monitoring gas exchange, ventilation, and heart rate. After the exercise period the body mass index and percentage fat both decreased by 11% and 18%, respectively (P < 0.001), although lean body mass did not change; maximum oxygen uptake and maximum heart rate did not change significantly, but peak ventilation equivalent, maximum metabolic equivalent and maximum load increased by 12%, 14% and 11%, respectively (P < 0.05, P < 0.001 and P < 0.001, respectively). Maximum oxygen uptake per unit body weight increased by 5% (P < 0.001). These results suggest that weight reduction as a result of exercise improves cardiorespiratory function in middle-aged women with obesity.

Prediction of individual oxygen uptake on-step transients from frequency responses

Power-oxygen uptake (VO2) frequency responses can be used to predict VO2 responses to arbitrary exercise intensity patterns. It is still an open question for which range of exercise intensities such computed VO2 response patterns yield valid predictions. In the present study, we determined the power-VO2 frequency response of nine sports students by means of pseudo-randomised switching between 20 W and 80 W during upright and supine cycle exercise. Starting from a baseline of 20 W each subject also performed sustained step increases to 40 W, 80 W, 120 W, and 160 W in both positions. The individual VO2 step responses were then compared with the expected VO2 time-courses predicted on the basis of the individual VO2 frequency responses. The comparison showed a close agreement for the 20 W-40 W and 20 W-80 W steps in both positions. With larger step amplitudes the VO2 kinetics became increasingly slower than the predicted VO2 time course in both positions. During additional ramp tests (10 W.30 s-1) whole blood lactic acid concentration [la-]b tended to be higher in the supine position at exercise intensities higher than 160 W. The mean power at 4 mmol.l-1 [la-]b amounted to 234 (SD 32) W and 253 (SD 44) W (P < 5%) in the supine and the upright position, respectively. The maximal oxygen uptake relative to body mass was not found to be significantly different [upright, mean 57 (SD 10) ml.(min.kg)-1; supine, mean 54 (SD 10) ml.(min.kg)-1].(ABSTRACT TRUNCATED AT 250 WORDS)

Postural effect on cardiac output, oxygen uptake and lactate during cycle exercise of varying intensity

Owing to changes in cardiac output, blood volume distribution and the efficacy of the muscle pump, oxygen supply may differ during upright and supine cycle exercise. In the present study we measured, in parallel, circulatory (heart rate, stroke volume, blood pressure) and metabolic parameters (oxygen uptake, lactic acid concentration [la]) during incremental-exercise tests and at constant power levels ranging from mild to severe exercise. In supine position, cardiac output exceeded the upright values by 1.0-1.5 l.min-1 during rest, light ([la] < 2 mmol.l-1) and moderate ([la] = 2-4 mmol.l-1) exercise. At higher exercise intensities the cardiac output in an upright subject approached and eventually slightly exceeded the supine values. For both rest-exercise transitions and large-amplitude steps (delta W > or = 140 W) the cardiac output kinetics was significantly faster in upright cycling. The metabolic parameters (VO2 and [la]) showed no simple relationship to the circulatory data. In light to moderate exercise they were unaffected by body position. Only in severe exercise, when cardiac output differences became minimal, could significant influences be observed: with supine body posture, [la] started to rise earlier and maximal power (delta W = 23 W) and exercise duration (64 s) were significantly reduced. However, the maximal [la] value after exercise was identical in both positions. The present findings generally show advantages of upright cycling only for severe exercise. With lower workloads the less effective muscle pump in the supine position appears to be compensated for by the improved central circulatory conditions and local vasodilatation.