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The influence of different space-related physiological variations on exercise capacity determined by oxygen uptake kinetics
Vol.27,pp, 65 - 69, 1992 Printed in Great Britain
0094-5765D2 $5.00+0.00 Pergamon Press Ltd
Acta Astronautica
THE INFLUENCE OF DIFFERENT SPACE-]~ R1ATED PHYSIOLOGICAL VARIATIONS ON EXERCISE CAPACITY DETERMINED BY OXYGEN UPTAKE KINETICS J. STEGEMANN Physiologisches Institut der Deutschen Sporthochschule K61n Carl-Diem-Weg6, D 5000 K61n 41, FRG
ABSTRACT Oxygen uptake kinetics, following defined variations of work load changes allow to estimate the contribution of aerob and anaerob energy supply which is the base for determining work capacity. Under the aspect of long duration missions with appfication of adequate dosed countermeasures, a reliable estimate of the astronaut's work capacity is important to adjust the necessary inflight training. Since the kinetics of oxygen uptake originate in the working muscle group itself, while measurements are performed at the mouth, ~trious influences within the oxygen transport system might disturb the determinations. There are not only detraining effects but also well-known other influences, such as blood- and fluid shifts induced by weightlessness. They might have an impact on the circulatorysystem+ Some of these factors have been simulated by immersion, blood donation, and changing of the body position. KEYWORDS Weightlessness; bed rest; oxygen uptake kinetics; exercise capacity; Pseudo Random Binary Sequence technique; INTRODUCTION The objective of this paper is to summarise the results of some preparatory investigations for our experiment planned to be performed on the German spacelab mission D-2 within the frame of the Anthrorack payload element. The aim can be summarised as determining work capacity in space by means of a new method. The ext~riment is based on the results of various authors (Cerretelli and Brambilla (1958), Ceretelli et aL (1979)) who have shown that high endurance capacity is associated with faster increase of oxygen uptake at the onset of physical exercise. However, quantifying this effect has to be shown as rather difficult despite modern breath by breath algorithms are available (Stegemann and Et~feld ,1984). Obviously there is an extreme nonlinearity in the system which does not allow to fit the single breath data to a simple model function. During the transition from rest to exercise there are various changes in the overall system, such as mobilisation of the central blood volume, rapid changes in the blood distribution and an altered equilibrium in the sympathetic- and parasympathetic tone. This was described already in detail by Liunarsson 1974). Until now obviously these and other factors did not allow to construct a suitable test for endurance fitness. The problem can be solved, however, if instead of transition from rest to exercise, two diszinct workloads are used and during the transition from one to the other the oxygen uptake is recorded breath by breath. To avoid other nonlinearities, it is recommended to use workloads under the aerob anaerob threshold. From our experience steps between 20W to 80 or 100W are well suited. Oxygen uptake kinetics under these conditions has to be proven to fulfil the requirements that are needed for determining work capacity. Someone may ask why not using a con
DETERMINING OXYGEN UPTAKE KINETICS Therefore, we have developed an oxygen uptake kinetics test in order to quantify the expected decrease of endurance capacity applying only moderate work-loads. This test could be hopefully a base for optimisation of countermeasures during long-term space flights. To reach this goal a special form for determining the oxygen uptake kinetics was adapted: the Pseudo Random Binary Sequence (PRBS)technique. 65
66
9th IAA Man in Space Symposium
The base of this technique, however, it is more difficult to understand than the base of conventional tests and needs probably some clarification: As already mentioned kinetics could be evaluated in the time domain by applying a step function to the system's input and recording the system's output. This means the workload has to be changed from a defined level to another defined level while the subject's oxygen uptake is recorded continuously. This technique suffers from the fact that human ventilation does not work like a sinus generator and the resolution is limited to the time between each breath. The resulting scatter impairs the evaluation. Multiple repetitions are not possible due to the limited crew-time during a spacelab mission. In the frequency domain kinetics experiments could be evaluated by applying sinusoidat workloads between two levels and various frequencies as input and the oxygen uptake as output. Two parameters the size of the resulting amplitude (A) and the phase angle as a function of the input frequency allow to describe the kinetics of the system. The results are usually presented as Bode plots. The abscissa contains the frequency, the ordinate the amplitude ratio (A/Ao = IFI)[A0=amplitude at frequency A 0 ] and the phase angle respectively. Unfortunately this technique would also overcharge the available crew time during spacelab missions. The trick of the PRBS - technique - originally designed for engineering purposes -- is that the input pattern (Fig.l) to the system contains all interesting frequencies and the output from the system can be mathematically processed (Fig. 2) so that the frequency domain can be evaluated. The prerequisite, however, is that the system is linear over the interesting range. Similar to the above mentioned sinusoidal stimulation technique, the results can also be presented as Bode plot with the exception that -
Power E W 3
240 1
//1 160
L
L
L
80t 0
!
0
I
i
300 600 900
|
!
1 3 5 0 1800 2250 /~30
1CS3
Fig. 1: Method for comparison of different techniques for dertermining work capacity. The first part consists of 3 workload - steps, then follows the PRBS pattern, and finally an increase of performance until exhaustion of the subject, to determine V'Oz (max). Moreover, Lactate concentration (L)is measured in the arterialised blood.
because of the harmonic analysis applied, the frequencies are limited to the harmonics of the basic frequency that can be adjusted by the adequate PRBS - pattern. In the present ease, the input function will be generated by the PRBS - modulated friction of the bicycle ergometer varying the performance of the subject, and the output is the oxygen uptake measured breath by breath. A very good relationship between the PRBS- method and V'Oz (max) results could be established (EBfeld et al. (1984)). The diagram (Fig.3) demonstrates the relation between the amplitude ratio and the different harmonics of the basic frequency (period 400 s) and the relative V'O2 (max) determined in 38 subjects. The higher the work capacity of a subject, the smaller the decrease of the amplitude ratio. This means that the kinetics of the subjects with higher V'O2 max is faster. The largest difference of the amplitude ratios can be found between the second and third harmonics. In preparation for the mission we have performed several investigations not only to meet the main objective - the quantification of decrease of work capacity - but also to find out whether concomitant effect - such as blood- and fluid shift - might influence the test results. One has to take in account that the kinetics originate in the working muscle and it would be therefore correct to measure the time course of the gas exchange between the supplying artery and the vein. Since this is not possible as an approximation we are measuring the gas exchange at the upper airways. So it might be possible that on the way from the muscle to the mouth the signal may be deformed by weightlessness induced effects pretending erroneous results ( Egfeld and Hoffmann ,1990 ;Egfeld et al. 1991 ).
9th IAA Man in Space Symposium
INPUT
SYSTEM
OUTPUT gh, I
i
.~'..
I .t
I..
I ,I
I1 _ _ I "+"'"+t\J\ ~L,I, IGI"R.I I ." :" I
ii
,'l
111 ill.,-" ] °uc:o HU""
liTi' ®
'"]
Fig. 2. The PRBS - technique and its evaluation: 1. The power changed according to the PRBS pattern and the system - response (breath by breath recorded 02 consumption) is the weight function in the time domain. 2.. Auto- and Crosscorrelation for noise reduction. 3. Transformation into the frequency domain: The shaded columns are the real, the white ones are the imaginary part. PSD =Power-spectral density, CPSD= Cross- power-spectral density. 4. Final results in form of a Bode plot.
IFI
,~
Fig.3 : The mean amplitude ratio F and the power-VO2 relation for the first 6 harmonics of the PRBS-frequency as a double logarithmic plot. The period is used instead of the usual angular frequency (~ = rad , s"1) in order to characterise the calculated sinusoidal input frequency BED~ Bedrest simulates some of the space related influences: the hypokinesia during a mission caused by the limited loading of the muscles for locomotion and keeping the body in an adequate position. Furthermore, blood is shifted in a similar manner as it happens in space. Fig. 4 depicts the results of 1 week bedrest study using the I-IDT technique (Stegemann et at 1985 ). Because of the limited time it might be enough to present only the behaviour of the amplitude ratio before and 1 day after the cessation
67
68
9th IAA Man in Space Symposium
of the experiment. Data are representing the mean and the standard error of 7 subjects. It can clearly be seen that IFI differs significantly in the range from the second harmonics upwards. The results demonstrate that the kinetics of oxygen uptake was slowed down by a bedrest period of 1 week. Not quite clear, however, whether this slowing down can be attributed to the muscle only and/or to the circulation system. 6 h H E A D - O U T IMMERSION It is a well-known fact that immersion in thermo-indifferent water leads to a diuresis since the different volume receptors in the body are stimulated by the resulting blood shift. We utilised a 6 hour head-out immersion to dehydrate 9 male subjects by 1.2 litres. From experiments already published in 1969 ( Stegemann et aL ), it was known that a 6 h immersion reduces maximal oxygen intake by 10 to 20%.
tp
[deg
J
WL -902 - R~ation : -- PRE e-.--e POSTI
0
(~i = i,0.0¼0 r~l Is
-40
-80 - 120
-
160
I
I
'"
l
!
I
I
WI
Fig. 4: Amplitude ratio determined using the PRBS-technique (Bodeplot). Wn represents the n th harmonics of the basic frequency before (pre) and after (post) 1 week of head down tilt (6*). It can clearly be observed that in the range of higher harmonics the response pre and post are different. Wewere interested whether this could be reproduced and what would be result of the PRBS- technique under these circumstances. The amplitude ratio before and after the water-immersion was practically unchanged at least up to the fourth harmonics. In the 5 th and 6th harmonics a slowing down of the kinetics could be observed although the difference was statistically not significant. The maximal oxygen intake after the immersion was decreased by 10%. BLOOD WITHDRAWAL It is well-known that during space-flight there is some substantial decrease of blood volume. Therefore we were interested to know how an isolated loss of 500ml blood would alter the V'O2 kinetics of the overall system ( Hoffmann et al. 1987). For this purpose we investigated 7 male and 7 female subjects before, 2 hours, and 2 weeks after a withdrawal of 500 ml of blood. Additionally to the PRBS-test, a V'Oz (max) test was performed. The range up to the 4th harmonics, which was significantly changed during the bedrest study is now found to be almost the same. The 6th harmonics is slightly smaller, but not significantly. Comparing the transfere function before and 2 weeks later shows that the blood withdrawal effect has disappeared. The maximal oxygen uptake of the same subjects 2 hours after the blood withdrawal is significantly impeded but almost normalised after 2 weeks.
CONCLUSION The conclusion may be drawn that the/~g induced deterioration effect of the muscles lies in another frequency range than the effects induced by circulation influences. Therefore it seems possible to determine the influence of weightlessness on the muscle quantitatively undisturbed by the circulatory changes. It would be interesting also if we could draw in future some more information from the changes in the higher frequency range. Unfortunately this range is nonlinear and therefore not easy to analyse.
ACKNOWLEDGEMENT All preparatory investigations for the D-2 mission are supported by D A R A GMBH ( FKZ: 01QV87345)
9th IAA Man in Space Symposium
REFERENCES Baum, K., U. Hoffmann, D. EBfeld, J. Stegemann (1990). Time courses of endurance performance paramters following 500ml blood withdrawal. Proceedingsof the fourth life sciences research in space Trieste ESA Sp 307, 73-76 Ccrretelli, P., I. Brambilla (1958). Cinetica della contrazione di un debito di 0 2 nell'uomo. Boll. Soc. itaL Biol. Sper. 34,679-682 Cerretelli, P., D. Pendergast, W. C. Paganelli, D.W.Rennie (1979). Effect of specific muscle training on V'O2, on response and early blood lactate. Z AppL PhysioL 47,761-769 EBfeld, D., J. Stegemann U.Hoffmann (1987). VO2 kinetics in subjects differing in aerobic capacity: investigations by analysis. Eur. J..AppL Physio156, 508-515 Stegemann, J (1976). Rcchncrgestcucrte Spiroergometrie nach dcr Methode der Einzclatcmzuganalyse. Sportarzt u. Sportmedizin 27, 1-5 Egfeld, D., U.Hoffmann (1990). Influence of various parameters on the V'O2 frequency response of the body: A theoretical study on the basis of a nonlinear modeL Proceedingsof the fourth life sciences research in space Trieste ESA Sp 307, 103-106 Egfeld, D. U.Hoffmann J.Stegemann (1991). A model for studying the distortion of muscle oxygen uptake patterns by circulation parameters. Eur.J.appLphysiol 62:83-90 Hoffmann, U., IC Baum, F.M. Baer, D. Essfeld, J.Stegemann (1987.)Influence ofa 6-h head-out water immersion on different tests of endurance performance capacity. Proceedings of the 3rd European Symposium on Life Sciences Research in Space, Graz, . (ESA SP-271), 51-54 Linnarsson. D. (1974) Dynamics of pulmonary gas exchange and heart rate at start and end of exercise Acta Physiol Scand [Suppl] 415, 1-68 Stegemann, J., HD. Framing, M. Schiefeling (1969). Der EinfluB einer 6-stiindigen Immersion in thermoindifferentem Wasser auf die Regulation des Kreislaufs und die Leistungsfahigkeit bei Trainierten und Untrainierten. PftiigersArch. ges. Physiol. 312, 129-138, Stegemann, J., D. Essfeld: Advantages of the computerized breath-by-breath method for the interpretation of spiroergometric data (1984). In L611gen.,Mellerowicz H. (eds.): Progressin Ergometry: Quality Control and Test Criteria. Fifth International Seminar on Ergometry.Springer-Verlag, Berlin - Heidelberg - New York - Tokyo 47 - 25 Stegemann, J., D. El3feld, U. Hoffmann (1985). Effects of a 7-day head-down-tilt (-6o) on the dynamics of oxygen uptake and heart rate adjustment in uptight exercise. Aviat. Space Envir 56, 410 -414
69
0094-5765D2 $5.00+0.00 Pergamon Press Ltd
Acta Astronautica
THE INFLUENCE OF DIFFERENT SPACE-]~ R1ATED PHYSIOLOGICAL VARIATIONS ON EXERCISE CAPACITY DETERMINED BY OXYGEN UPTAKE KINETICS J. STEGEMANN Physiologisches Institut der Deutschen Sporthochschule K61n Carl-Diem-Weg6, D 5000 K61n 41, FRG
ABSTRACT Oxygen uptake kinetics, following defined variations of work load changes allow to estimate the contribution of aerob and anaerob energy supply which is the base for determining work capacity. Under the aspect of long duration missions with appfication of adequate dosed countermeasures, a reliable estimate of the astronaut's work capacity is important to adjust the necessary inflight training. Since the kinetics of oxygen uptake originate in the working muscle group itself, while measurements are performed at the mouth, ~trious influences within the oxygen transport system might disturb the determinations. There are not only detraining effects but also well-known other influences, such as blood- and fluid shifts induced by weightlessness. They might have an impact on the circulatorysystem+ Some of these factors have been simulated by immersion, blood donation, and changing of the body position. KEYWORDS Weightlessness; bed rest; oxygen uptake kinetics; exercise capacity; Pseudo Random Binary Sequence technique; INTRODUCTION The objective of this paper is to summarise the results of some preparatory investigations for our experiment planned to be performed on the German spacelab mission D-2 within the frame of the Anthrorack payload element. The aim can be summarised as determining work capacity in space by means of a new method. The ext~riment is based on the results of various authors (Cerretelli and Brambilla (1958), Ceretelli et aL (1979)) who have shown that high endurance capacity is associated with faster increase of oxygen uptake at the onset of physical exercise. However, quantifying this effect has to be shown as rather difficult despite modern breath by breath algorithms are available (Stegemann and Et~feld ,1984). Obviously there is an extreme nonlinearity in the system which does not allow to fit the single breath data to a simple model function. During the transition from rest to exercise there are various changes in the overall system, such as mobilisation of the central blood volume, rapid changes in the blood distribution and an altered equilibrium in the sympathetic- and parasympathetic tone. This was described already in detail by Liunarsson 1974). Until now obviously these and other factors did not allow to construct a suitable test for endurance fitness. The problem can be solved, however, if instead of transition from rest to exercise, two diszinct workloads are used and during the transition from one to the other the oxygen uptake is recorded breath by breath. To avoid other nonlinearities, it is recommended to use workloads under the aerob anaerob threshold. From our experience steps between 20W to 80 or 100W are well suited. Oxygen uptake kinetics under these conditions has to be proven to fulfil the requirements that are needed for determining work capacity. Someone may ask why not using a con
DETERMINING OXYGEN UPTAKE KINETICS Therefore, we have developed an oxygen uptake kinetics test in order to quantify the expected decrease of endurance capacity applying only moderate work-loads. This test could be hopefully a base for optimisation of countermeasures during long-term space flights. To reach this goal a special form for determining the oxygen uptake kinetics was adapted: the Pseudo Random Binary Sequence (PRBS)technique. 65
66
9th IAA Man in Space Symposium
The base of this technique, however, it is more difficult to understand than the base of conventional tests and needs probably some clarification: As already mentioned kinetics could be evaluated in the time domain by applying a step function to the system's input and recording the system's output. This means the workload has to be changed from a defined level to another defined level while the subject's oxygen uptake is recorded continuously. This technique suffers from the fact that human ventilation does not work like a sinus generator and the resolution is limited to the time between each breath. The resulting scatter impairs the evaluation. Multiple repetitions are not possible due to the limited crew-time during a spacelab mission. In the frequency domain kinetics experiments could be evaluated by applying sinusoidat workloads between two levels and various frequencies as input and the oxygen uptake as output. Two parameters the size of the resulting amplitude (A) and the phase angle as a function of the input frequency allow to describe the kinetics of the system. The results are usually presented as Bode plots. The abscissa contains the frequency, the ordinate the amplitude ratio (A/Ao = IFI)[A0=amplitude at frequency A 0 ] and the phase angle respectively. Unfortunately this technique would also overcharge the available crew time during spacelab missions. The trick of the PRBS - technique - originally designed for engineering purposes -- is that the input pattern (Fig.l) to the system contains all interesting frequencies and the output from the system can be mathematically processed (Fig. 2) so that the frequency domain can be evaluated. The prerequisite, however, is that the system is linear over the interesting range. Similar to the above mentioned sinusoidal stimulation technique, the results can also be presented as Bode plot with the exception that -
Power E W 3
240 1
//1 160
L
L
L
80t 0
!
0
I
i
300 600 900
|
!
1 3 5 0 1800 2250 /~30
1CS3
Fig. 1: Method for comparison of different techniques for dertermining work capacity. The first part consists of 3 workload - steps, then follows the PRBS pattern, and finally an increase of performance until exhaustion of the subject, to determine V'Oz (max). Moreover, Lactate concentration (L)is measured in the arterialised blood.
because of the harmonic analysis applied, the frequencies are limited to the harmonics of the basic frequency that can be adjusted by the adequate PRBS - pattern. In the present ease, the input function will be generated by the PRBS - modulated friction of the bicycle ergometer varying the performance of the subject, and the output is the oxygen uptake measured breath by breath. A very good relationship between the PRBS- method and V'Oz (max) results could be established (EBfeld et al. (1984)). The diagram (Fig.3) demonstrates the relation between the amplitude ratio and the different harmonics of the basic frequency (period 400 s) and the relative V'O2 (max) determined in 38 subjects. The higher the work capacity of a subject, the smaller the decrease of the amplitude ratio. This means that the kinetics of the subjects with higher V'O2 max is faster. The largest difference of the amplitude ratios can be found between the second and third harmonics. In preparation for the mission we have performed several investigations not only to meet the main objective - the quantification of decrease of work capacity - but also to find out whether concomitant effect - such as blood- and fluid shift - might influence the test results. One has to take in account that the kinetics originate in the working muscle and it would be therefore correct to measure the time course of the gas exchange between the supplying artery and the vein. Since this is not possible as an approximation we are measuring the gas exchange at the upper airways. So it might be possible that on the way from the muscle to the mouth the signal may be deformed by weightlessness induced effects pretending erroneous results ( Egfeld and Hoffmann ,1990 ;Egfeld et al. 1991 ).
9th IAA Man in Space Symposium
INPUT
SYSTEM
OUTPUT gh, I
i
.~'..
I .t
I..
I ,I
I1 _ _ I "+"'"+t\J\ ~L,I, IGI"R.I I ." :" I
ii
,'l
111 ill.,-" ] °uc:o HU""
liTi' ®
'"]
Fig. 2. The PRBS - technique and its evaluation: 1. The power changed according to the PRBS pattern and the system - response (breath by breath recorded 02 consumption) is the weight function in the time domain. 2.. Auto- and Crosscorrelation for noise reduction. 3. Transformation into the frequency domain: The shaded columns are the real, the white ones are the imaginary part. PSD =Power-spectral density, CPSD= Cross- power-spectral density. 4. Final results in form of a Bode plot.
IFI
,~
Fig.3 : The mean amplitude ratio F and the power-VO2 relation for the first 6 harmonics of the PRBS-frequency as a double logarithmic plot. The period is used instead of the usual angular frequency (~ = rad , s"1) in order to characterise the calculated sinusoidal input frequency BED~ Bedrest simulates some of the space related influences: the hypokinesia during a mission caused by the limited loading of the muscles for locomotion and keeping the body in an adequate position. Furthermore, blood is shifted in a similar manner as it happens in space. Fig. 4 depicts the results of 1 week bedrest study using the I-IDT technique (Stegemann et at 1985 ). Because of the limited time it might be enough to present only the behaviour of the amplitude ratio before and 1 day after the cessation
67
68
9th IAA Man in Space Symposium
of the experiment. Data are representing the mean and the standard error of 7 subjects. It can clearly be seen that IFI differs significantly in the range from the second harmonics upwards. The results demonstrate that the kinetics of oxygen uptake was slowed down by a bedrest period of 1 week. Not quite clear, however, whether this slowing down can be attributed to the muscle only and/or to the circulation system. 6 h H E A D - O U T IMMERSION It is a well-known fact that immersion in thermo-indifferent water leads to a diuresis since the different volume receptors in the body are stimulated by the resulting blood shift. We utilised a 6 hour head-out immersion to dehydrate 9 male subjects by 1.2 litres. From experiments already published in 1969 ( Stegemann et aL ), it was known that a 6 h immersion reduces maximal oxygen intake by 10 to 20%.
tp
[deg
J
WL -902 - R~ation : -- PRE e-.--e POSTI
0
(~i = i,0.0¼0 r~l Is
-40
-80 - 120
-
160
I
I
'"
l
!
I
I
WI
Fig. 4: Amplitude ratio determined using the PRBS-technique (Bodeplot). Wn represents the n th harmonics of the basic frequency before (pre) and after (post) 1 week of head down tilt (6*). It can clearly be observed that in the range of higher harmonics the response pre and post are different. Wewere interested whether this could be reproduced and what would be result of the PRBS- technique under these circumstances. The amplitude ratio before and after the water-immersion was practically unchanged at least up to the fourth harmonics. In the 5 th and 6th harmonics a slowing down of the kinetics could be observed although the difference was statistically not significant. The maximal oxygen intake after the immersion was decreased by 10%. BLOOD WITHDRAWAL It is well-known that during space-flight there is some substantial decrease of blood volume. Therefore we were interested to know how an isolated loss of 500ml blood would alter the V'O2 kinetics of the overall system ( Hoffmann et al. 1987). For this purpose we investigated 7 male and 7 female subjects before, 2 hours, and 2 weeks after a withdrawal of 500 ml of blood. Additionally to the PRBS-test, a V'Oz (max) test was performed. The range up to the 4th harmonics, which was significantly changed during the bedrest study is now found to be almost the same. The 6th harmonics is slightly smaller, but not significantly. Comparing the transfere function before and 2 weeks later shows that the blood withdrawal effect has disappeared. The maximal oxygen uptake of the same subjects 2 hours after the blood withdrawal is significantly impeded but almost normalised after 2 weeks.
CONCLUSION The conclusion may be drawn that the/~g induced deterioration effect of the muscles lies in another frequency range than the effects induced by circulation influences. Therefore it seems possible to determine the influence of weightlessness on the muscle quantitatively undisturbed by the circulatory changes. It would be interesting also if we could draw in future some more information from the changes in the higher frequency range. Unfortunately this range is nonlinear and therefore not easy to analyse.
ACKNOWLEDGEMENT All preparatory investigations for the D-2 mission are supported by D A R A GMBH ( FKZ: 01QV87345)
9th IAA Man in Space Symposium
REFERENCES Baum, K., U. Hoffmann, D. EBfeld, J. Stegemann (1990). Time courses of endurance performance paramters following 500ml blood withdrawal. Proceedingsof the fourth life sciences research in space Trieste ESA Sp 307, 73-76 Ccrretelli, P., I. Brambilla (1958). Cinetica della contrazione di un debito di 0 2 nell'uomo. Boll. Soc. itaL Biol. Sper. 34,679-682 Cerretelli, P., D. Pendergast, W. C. Paganelli, D.W.Rennie (1979). Effect of specific muscle training on V'O2, on response and early blood lactate. Z AppL PhysioL 47,761-769 EBfeld, D., J. Stegemann U.Hoffmann (1987). VO2 kinetics in subjects differing in aerobic capacity: investigations by analysis. Eur. J..AppL Physio156, 508-515 Stegemann, J (1976). Rcchncrgestcucrte Spiroergometrie nach dcr Methode der Einzclatcmzuganalyse. Sportarzt u. Sportmedizin 27, 1-5 Egfeld, D., U.Hoffmann (1990). Influence of various parameters on the V'O2 frequency response of the body: A theoretical study on the basis of a nonlinear modeL Proceedingsof the fourth life sciences research in space Trieste ESA Sp 307, 103-106 Egfeld, D. U.Hoffmann J.Stegemann (1991). A model for studying the distortion of muscle oxygen uptake patterns by circulation parameters. Eur.J.appLphysiol 62:83-90 Hoffmann, U., IC Baum, F.M. Baer, D. Essfeld, J.Stegemann (1987.)Influence ofa 6-h head-out water immersion on different tests of endurance performance capacity. Proceedings of the 3rd European Symposium on Life Sciences Research in Space, Graz, . (ESA SP-271), 51-54 Linnarsson. D. (1974) Dynamics of pulmonary gas exchange and heart rate at start and end of exercise Acta Physiol Scand [Suppl] 415, 1-68 Stegemann, J., HD. Framing, M. Schiefeling (1969). Der EinfluB einer 6-stiindigen Immersion in thermoindifferentem Wasser auf die Regulation des Kreislaufs und die Leistungsfahigkeit bei Trainierten und Untrainierten. PftiigersArch. ges. Physiol. 312, 129-138, Stegemann, J., D. Essfeld: Advantages of the computerized breath-by-breath method for the interpretation of spiroergometric data (1984). In L611gen.,Mellerowicz H. (eds.): Progressin Ergometry: Quality Control and Test Criteria. Fifth International Seminar on Ergometry.Springer-Verlag, Berlin - Heidelberg - New York - Tokyo 47 - 25 Stegemann, J., D. El3feld, U. Hoffmann (1985). Effects of a 7-day head-down-tilt (-6o) on the dynamics of oxygen uptake and heart rate adjustment in uptight exercise. Aviat. Space Envir 56, 410 -414
69