Uwe Hoffmann

VO2 kinetics in subjects differing in aerobic capacity: investigation by spectral analysis

SummaryThe power-VO2 transfer functions of 38 subjects differing in aerobic capacity have been determined on the basis of breath-by-breath total oxygen uptake (VO2.1) measurements during light cycle ergometer exercise (lactic acid concentrations below 2 mmol · l−1). At constant pedalling frequency (1 Hz) pseudorandom binary sequences (PRBS) of workload were used as the testing signal. TheVO2.1 response was analysed by autocorrelating the ergometer power input and crosscorrelating the power input andVO2.1 output. From the spectra of these functions the amplitude ratios and phase relationships were computed for the first six harmonics of the PRBS fundamental (14 mrad · s−1). We found that differences in aerobic capacity are associated with significant differences in the amplitude plots of theVO2 transfer function.

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 [1a]) 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 1 · min−1 during rest, light ([la] < 2 mmol · 1−1) and moderate ([la] =2–4 mmol · 1−1) 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 (ΔW ⩾ 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 (ΔW=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.

A model for studying the distortion of muscle oxygen uptake patterns by circulation parameters

SummaryThe transmission of muscle oxygen uptake

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

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Dynamic linearity ofVO2 responses during aerobic exercise

patterns to the pulmonary site is a basically nonlinear process during unsteady state exercise. We were mainly interested in three questions concerning the dynamic relationship between power input and pulmonary

Leg blood flow during slow head-down tilt with and without leg venous congestion

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