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J Appl Physiol 100: 951-957, 2006. First published November 23, 2005; doi:10.1152/japplphysiol.01083.2005
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Cardiorespiratory responses to physical work during and following 17 days of bed rest and spaceflight

Todd Trappe,1 Scott Trappe,1 Gary Lee,1 Jeffrey Widrick,2 Robert Fitts,2 and David Costill1

1Human Performance Laboratory, Ball State University, Muncie, Indiana; and 2Department of Biology, Marquette University, Milwaukee, Wisconsin

Submitted 6 September 2005 ; accepted in final form 19 November 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
To determine the influence of a 17-day exposure to real and simulated spaceflight (SF) on cardiorespiratory function during exercise, four male crewmembers of the STS-78 space shuttle flight and eight male volunteers were studied before, during, and after the 17-day mission and 17 days of –6° head-down-tilt bed rest (BR), respectively. Measurements of oxygen uptake, pulmonary ventilation, and heart rate were made during submaximal cycling 60, 30, and 15 days before the SF liftoff and 12 and 7 days before BR; on SF days 2, 8, and 13 and on BR days 2, 8, and 13; and on days 1, 4, 5, and 8 after return to Earth and on days 3 and 7 after BR. During 15 days before liftoff, day 4 after return, and day 8 after return and all BR testing, each subject completed a continuous exercise test to volitional exhaustion on a semirecumbent (SF) or supine (BR) cycle ergometer to determine the submaximal and maximal cardiorespiratory responses to exercise. The remaining days of the SF testing were limited to a workload corresponding to 85% of the peak pre-SF peak oxygen uptake (VO2 peak) workload. Exposure to and recovery from SF and BR induced similar responses to submaximal exercise at 150 W. VO2 peak decreased by 10.4% from pre-SF (15 days before liftoff) to day 4 after return and 6.6% from pre-BR to day 3 after return, which was partially (SF: –5.2%) or fully (BR) restored within 1 wk of recovery. Workload corresponding to 85% of the peak pre-SF VO2 peak showed a rapid and continued decline throughout the flight (SF day 2, –6.2%; SF day 8, –9.0%), reaching a nadir of –11.3% during testing on SF day 13. During BR, VO2 peak also showed a decline from pre-BR (BR day 2, –7.3%; BR day 8, –7.1%; BR day 13, –9.0%). These results suggest that the onset of and recovery from real and simulated microgravity-induced cardiorespiratory deconditioning is relatively rapid, and head-down-tilt BR appears to be an appropriate model of this effect, both during and after SF.

space shuttle; head-down tilt bed rest; cycle ergometry; peak oxygen consumption; weightlessness

PHYSIOLOGICAL ADAPTATIONS to microgravity have been evident since the earliest human spaceflights (SFs) and continue to be documented following stays aboard the International Space Station (24). Prolonged periods of reduced gravitational demands on the cardiovascular and musculoskeletal systems have been shown to lessen human tolerance for physical exertion (5, 26, 28, 33, 38, 39). Because of the limited number of studies that can be conducted in the microgravity of space, several Earth-based simulations of microgravity have been developed, the most accepted analog being head-down-tilt bed rest (BR). By using the BR model, the cardiorespiratory responses to exercise have been extensively documented (4–14, 18, 21, 35, 41). However, comparison of the ground-based and SF data are confounded by the discrepancies among the protocols of the laboratory studies and the activities completed by the astronauts while in space. That is, rarely do ground simulations of microgravity track actual SF activities.

The data reported here are the results of two investigations examining the submaximal and maximal cardiorespiratory responses to cycling exercise before, during, and after 17 days (16 days/22 h/48 m) of real [STS-78; Life and Microgravity Sciences (LMS) Spacelab] and simulated (BR) SF. The first investigation served as a ground-based simulation of the actual SF, using the –6° head-down-tilt BR model and employing the same timeline and testing protocols planned for the SF. The second investigation studied four male crewmembers of the LMS mission and, with only a few variations, used the same protocol as the BR study. We hypothesized that alterations in the maximal and submaximal aerobic exercise responses would be similar during and following SF compared with –6° head-down-tilt BR. Because of flight restrictions, countermeasures activity, and the integrative nature of the scientific objectives of the space shuttle missions, it should be realized that the data presented here do not describe the effects of weightlessness alone (real or simulated) on physical working capacity, rather the effects of living in space on cardiorespiratory adaptations to submaximal and maximal effort.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects

The subjects of this investigation included four male crewmembers of the LMS mission who flew aboard the 17-day space shuttle flight of STS-78, and eight male volunteers who participated in the 17-day BR phase of this investigation. Subject characteristics are presented in Table 1. Subjects were judged healthy following a physical exam completed by the National Aeronautics and Space Administration (NASA) medical staff. All of the procedures, risks, and benefits associated with the experimental testing were explained to the subjects before they signed a consent form approved by the Institutional Review Boards of the participating institutions.


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  		Table 1. Subject characteristics from the spaceflight and bed rest

 
Experimental Protocol

SF.   The submaximal and maximal exercise testing completed for this investigation was one of 14 human experiments designed to examine the physiological and metabolic responses to SF. Consequently, the experiments were integrated to fit the timeline and work schedule of the astronauts before, during, and after the SF. Before the testing sequence listed below, each crewmember was familiarized with all of the testing hardware and protocols. Ninety days before the flight [launch (L) – 90 days (L-90)] each crewmember performed the cycling test protocol (described below) as an introduction to the testing regimen. After this, testing was completed on L-60, L-30, L-15; SF days 2, 8, and 13 (SF+2, SF+8, SF+13, respectively); and on days 1, 4, 5, and 8 after return to Earth (i.e., R+1, R+4, R+5, R+8, respectively). Because of time limitations on the astronauts’ schedules while on orbit, one crewmember had to complete the testing on the day following the other three (i.e., SF+3, SF+9, SF+14). Body weight was measured immediately before each pre- and postflight testing session and on launch and landing day.

BR.   The testing protocol used during the BR study was designed to mimic the timing and testing sequences that would be followed during the STS-78 space shuttle flight. Consequently, the same additional experimental protocols scheduled to be completed during the SF were integrated into the subjects’ daily schedules.

A 2-wk orientation period was completed to familiarize the subjects with the testing procedures and equipment before any experimental data were collected. Following this orientation period, the subjects were admitted to the Human Research Facility at the NASA Ames Research Center for a continuous 39-day residency period, consisting of a 14-day control period, 17 days of –6° head-down-tilt BR, and 8 days of ambulatory recovery. The control period provided time for the subjects to equilibrate to the standardized diet and for baseline data collection on each of the subjects. All meals and snacks were prepared for the subjects, and caloric intake was ad libitum from a set of 12 daily menus randomly distributed during the study. Total fluid intake and urine outputs were recorded each day. Additionally, body weight was measured in the morning after voiding (during BR in the –6° head-down-tilt position). Body weight was not measured every day during BR because of time limitations in the daily schedule. During BR, the subjects remained in the –6° head-down position or horizontal for all activities, including eating, bathing, excretory functions, and physiological testing.

Testing was completed 12 and 7 days before BR (BR-12 and BR-7, respectively), on BR days 2, 8, and 13 (BR+2, BR+8, BR+13, respectively), and on days 3 and 7 after BR (R+3 and R+7, respectively). Similar to the SF, because of time constraints in the schedule, testing was limited to one-half of the subjects on a given testing day; therefore, the other half of the subjects completed the testing on the following day (i.e., BR-11, BR+3, R+4, etc.). One of the eight subjects was not able to complete the cardiorespiratory measurements due to a cardiac abnormality apparent only at high levels of exercise. Another subject did not complete cardiorespiratory testing during the final BR testing point (BR+13) due to not feeling well. He remained in head-down-tilt BR and was well enough to participate in all recovery testing.

Cardiorespiratory Measurements

SF.   Oxygen consumption (VO2), minute ventilation (VE), and heart rate (HR) responses were measured during submaximal and maximal exercise before, during, and after flight on a semirecumbent electronically resisted cycle ergometer (Innovision, Odense, Denmark). This ergometer utilized competitive cycling pedals that secured the subject’s foot to the pedal. Leg extension and hip angle were adjusted by altering the location of the seat pad and lower back support and were maintained for each crewmember throughout the ground-based and SF testing. In addition, the subjects were restrained with only a strap around the seat pad and pelvis.

Two separate incremental exercise protocols were used to study the crewmembers. The first was a continuous exercise test to volitional exhaustion [peak VO2 (VO2 peak)], which consisted of four 3-min stages at 50, 100, 150, and 175 W followed by 25-W increments every 2 min until exhaustion. The second protocol, a "submaximal" exercise test, was implemented due to a NASA medical staff limitation to maximal exercise while on orbit and followed the same incremental sequence, but was terminated when the subjects completed a workload that was ~85% of the peak workload achieved during the L-90 VO2 peak testing (VO2-85W). The VO2 peak tests were performed on L-15 and on R+4 and R+8. The VO2-85W protocol, on the other hand, was conducted on L-60, L-30, SF+2, SF+8, and SF+13, and on R+1 and R+5.

During ground-based testing (L-90, L-60, L-30, L-15, R+1, R+4, R+5, and R+8), respiratory exchange was monitored throughout the test using a computer-based system that incorporated a low-resistance gas meter (Rayfield RAM-9200), a 3.5-liter mixing chamber, 3.49-cm inner diameter (ID) hoses, a Hans Rudolph breathing valve (2700 series), and electronic O2 and CO2 analyzers (Applied Electrochemistry-Ametek, S3A1 and CD-3A, respectively). HRs were measured throughout the tests using radiotelemetry (Polar Vantage XL, Polar Electronics, Port Washington, NY) and were confirmed via electrocardiography.

In-flight testing (SF+2, SF+8, and SF+13) utilized an on-board computer (LSLE Micro), mass spectrometer (GASMAP), a Hans Rudolph breathing valve (2700 series), Fleisch pneumotach no. 3, 3.5-liter mixing chamber, and 3.49-cm ID hoses. Three-lead electrocardiographic recordings were used to monitor HRs throughout the exercise trials during the flight.

BR.   Submaximal and maximal VO2, HR, and VE responses were measured during supine cycle ergometry (Quinton Instruments, 846T) before, during, and after the BR period. Similar to the VO2 peak protocol used for the astronauts, the BR protocol was a continuous incremental exercise test to volitional exhaustion, consisting of four 3-min stages at 50, 100, 150, and 200 W followed by 25-W increments every 2 min until exhaustion.

During the BR testing, expired gases were measured using an automated open-circuit system that incorporated a pneumotachometer (KL Engineering, K520), a 3-liter mixing chamber, 3.49-cm ID hoses, a Hans Rudolph breathing valve (2700 series), and electronic O2 and CO2 analyzers (Applied Electrochemistry-Ametek, S3A1 and CD-3A, respectively), interfaced with an IBM computer. HR was measured with radiotelemetry (Polar Vantage XL, Polar Electro, Port Washington, NY) and confirmed with a three-lead electrocardiogram (Physio-Control, Redmond, WA) throughout exercise.

Calibrations and calculations.   The gas analyzers for both the SF and BR studies were calibrated before each test with O2 and CO2 of known concentration. During the in-flight testing, Spacelab air was analyzed for O2 and CO2 before and immediately after each session. Barometric pressure was measured aboard the shuttle and at each ground-based testing site before each test. The Haldane transformation was used to calculate respiratory exchange using the O2 and CO2 gas concentrations (42). The semirecumbent and supine cycle ergometers were calibrated before each study by the NASA engineering staff.

O2 pulse (ml/beat) was calculated from the VO2 and HR data obtained during submaximal and maximal cycling (2, 3), as was ventilatory equivalent (VE/VO2) (43).

For SF (average of L-60, L-30, and L-15) and BR (average of BR-12 and BR-7), the results from 150 W were chosen to represent the submaximal responses. VO2 peak was defined by a plateau in VO2 with an increase in work rate and a respiratory exchange ratio >1.10.

Statistical Analysis

Changes in the measured variables during the SF and BR studies were evaluated with a one-way ANOVA with repeated measures. Comparison between the SF and BR responses (percent change from pre-SF or pre-BR) were made with a two-way (condition and time) ANOVA with repeated measures on the time factor. When appropriate, post hoc comparisons were made with Tukey’s test (for unequal sample sizes when comparing the SF and BR responses), and significance was accepted at P < 0.05. Means are presented ± SE.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Cardiorespiratory Measurements

Submaximal responses.   The submaximal cardiorespiratory responses to cycling at 150 W before, during, and after SF and BR are presented in Tables 2 and 3. The time course of changes in the cardiorespiratory responses was consistent between SF and BR. VO2 (l/min), VE (l/min), and ventilatory equivalent (VE/VO2) did not change from prelevels (P > 0.05) during BR or recovery, and this same trend was observed during SF and recovery. A small decrease (P < 0.05) in VO2 was apparent at SF+13 and again at the end of the first week of recovery (R+8). HR was elevated during SF+8 (11 beats/min) and SF+13 (10 beats/min), and, although not statistically significant (P > 0.05), these changes are certainly physiologically relevant. Responses to BR followed a similar trend, as HR was elevated during day 8 (10 beats/min) (P < 0.05) and day 13 (14 beats/min) (P < 0.05). Following SF, HR remained substantially elevated during R+1 (13 beats/min) (although not statistically significant, P > 0.05) and was similar to pre-SF on R+4, R+5, and R+8. Following BR, HR was still elevated at R+3 (11 beats/min) (P < 0.05), but was no different (P > 0.05) than pre-BR by R+7. O2 pulse (ml O2/beat) followed this same trend and was decreased during SF+8 and SF+13 and BR+8 and BR+13 and returned to prelevels by R+8.


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  		Table 2. Submaximal responses to recumbent cycling at 150 W before, during, and after 17 days of spaceflight

 

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  		Table 3. Submaximal responses to supine cycling at 150 W before, during, and after 17 days of bed rest

 
Maximal responses: before and after SF and BR.   The maximal cardiorespiratory responses to SF and BR during cycling are presented in Tables 4–6. VO2 peak was decreased from pre-SF by 10.4% (R+4) and pre-BR by 6.6% (R+3) (P < 0.05). Following 1 wk of recovery, VO2 peak was 5.2% below pre-SF (P < 0.05) and was unchanged from pre-BR (–3.3%, P > 0.05). Maximal HR, VE, and VE/VO2 were not altered post-SF or post-BR (P > 0.05). Similar to VO2 peak, O2 pulse was decreased from pre-SF by 10.0% (R+4) and pre-BR by 7.6% (R+3) (P < 0.05) and recovered to 6.1 and 3.7% below pre-SF (P < 0.05) and pre-BR (P > 0.05), respectively.


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  		Table 4. Maximal responses to recumbent cycling before and after 17 days of spaceflight

 

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  		Table 6. Maximal responses to supine cycling before, during, and after 17 days of bed rest

 
Maximal responses: during SF and BR.   Even though the criteria for VO2 peak (see above) were not achieved in all tests during SF, several conclusions can be drawn from the in-flight testing in regards to limitations to maximal exercise while on orbit. Of the 12 tests performed on orbit (4 astronauts x 3 in-flight sessions), three of the four astronauts achieved a maximal HR on all three tests (9 of 12 tests). In addition, given the decrease in maximal workload that was seen in the BR study, three of the four astronauts were cycling at a workload that would be expected to elicit VO2 peak. In-flight decreases in VO2-85W were similar to those observed during maximal cycling during BR (SF: SF+2, –6.2%; SF+8, –9.0%; SF+13, –11.3%; BR: BR+2, –7.3%; BR+8, –7.1%; BR+13, –9.0%). VE and VE/VO2 remained unchanged during SF and BR (P > 0.05). O2 pulse was decreased during SF (–9.9 to –17.8%) and BR (–7.4 to –11.6%) (P < 0.05).

SF vs. BR responses.   The percent change in VO2 and O2 pulse while cycling during (at 85% of pre-SF peak workload) and after (maximal) SF and while maximal cycling during and after BR are presented in Fig. 1. There was no difference between the SF and BR responses in either of these variables at any of the time points during or after SF or BR (P > 0.05). The percent change in HR and O2 pulse during submaximal cycling at 150 W during and after SF and BR are presented in Fig. 2. Similar to the maximal responses, there was no difference between the SF and BR responses in either of these variables at any of the time points during or after SF or BR (P > 0.05).


Figure 1
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  		Fig. 1. Similarities of the cardiorespiratory changes during maximal exercise during and following both spaceflight (SF) and bed rest (BR). Change is shown in oxygen consumption (VO2) and O2 pulse while cycling during (at 85% of pre-SF peak workload) and following (maximal) SF, and while maximal cycling during and after BR. BR+2, BR+8, and BR+13: BR days 2, 8, and 13, respectively; R+3/4 and R+7/8: recovery days 3 or 4 and 7 or 8, respectively.

 

Figure 2
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  		Fig. 2. Similarities of the cardiorespiratory changes during submaximal exercise during and following both SF and BR. Change is shown in heart rate and O2 pulse while cycling at 150 W during and following SF and BR.

 
Body Weight and Fluid Balance

SF induced a decrease in body weight in all four astronauts from launch to landing day, R+0 (–3.1 ± 1.1 kg) (Fig. 3), and recovered to within 0.5 kg of pre-SF values by R+9. During BR, ~70% of the total body weight loss had occurred by BR+4 (–0.8 kg). Body weight decreased to a low (–1.2 kg) on BR+12, where it remained until the end of BR. Similar to SF, body weight following BR recovered to within 0.5 kg within the first week of recovery (Fig. 3).


Figure 3
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  		Fig. 3. Change in body weight from pre-SF (launch day) and pre-BR (average of 14 days) on landing day (R+0) and during recovery.

 
Daily fluid balance (fluid intake – urine output) during BR averaged 269 ± 44 ml/day during the pre-BR period. During the first day of BR, subjects experienced a large increase in urine output, which resulted in a fluid balance of –337 ± 95 ml. Therefore, within the first 24 h of BR, there was approximately a –600-ml shift in fluid balance. Fluid balance averaged 62 ± 34 ml/day during BR and increased to pre-BR levels during recovery (315 ± 59 ml/day). A significant relationship was found between fluid balance on the first day of BR (+126 to –722 ml) and the change in VO2 peak (–0.4 to –12.6%) from pre-BR on BR+2 (r = 0.92, P < 0.05).


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
There are relatively few observations of the cardiorespiratory responses to exercise after SF and even fewer observations of astronauts while on orbit. One of the main findings from this study is that maximal exercise capacity is compromised during and following SF exposure. Another main finding from this study highlights the adequacy of BR as an analog for SF. BR has been used as a simulation of microgravity for >40 yr (18) and has been assumed to also be an adequate simulation of SF. Because of the complexity in synchronizing the protocols completed and schedule followed by the astronauts with subjects completing BR, it is really not known how well BR mimics SF in terms of physiological responses. The two studies presented here were able to do this synchronization, and the findings support the contention that –6° head-down-tilt BR is an appropriate simulation of in-flight and postflight physiological responses to exercise. This is evidenced by the fact that the direction, magnitude, and time course of the changes in the cardiorespiratory responses to exercise were similar between BR and SF (see Figs. 1 and 2).

Outside of the four men tested for the STS-78 mission, during the previous 44 yr of human SF, there are only nine astronauts (7 men and 2 women) on whom data have been reported from maximal exercise testing completed in microgravity (26, 28, 34). The three male crewmembers aboard the 84-day Skylab 4 mission were observed to increase VO2 peak during cycling (3.11 l/min preflight to 3.37 l/min on days 79–83 in-flight) (28, 34). We feel that this is most likely a training response, as opposed to a true adaptation to microgravity, due to the large amount of countermeasure exercise completed in-flight, which far exceeded their exercise regimen before the mission. Previous 9- and 14-day space shuttle missions aboard SLS-1 and SLS-2 examining maximal responses to in-flight cycling between the 5th (SF+5) and 8th (SF+8) days in-flight reported no change in VO2 peak (26). Specific reasons for the discrepancy between our in-flight findings and those of Levine et al. (26) are not clear. However, the relatively large difference in preflight fitness levels (3.59 l/min in the present study vs. 2.76 l/min) may have contributed. The length of microgravity exposure may have also played a role, as the present study showed the largest changes at SF+13. Other factors, such as the in-flight testing protocol (discussed below), potential gender differences (4 men in the present study vs. 4 men and 2 women) (23), posture during testing on Earth (semirecumbent in the present study vs. upright), nonspecified in-flight countermeasures activity, and individual responses to orbital insertion and SF, should also be considered.

A few months before the launch, we were limited to "85% of maximal exercise" by the NASA medical staff. As a result, we chose to terminate the test after completion of the workload that corresponded to ~85% of the peak workload obtained during the pre-SF testing. Thus this final workload corresponds to 90–100% of VO2 peak, owing to the fact that exercising at a workload corresponding to 85% of peak workload and 85% of VO2 peak does not elicit the same VO2. Indeed, Levine et al. (26) reported a mean decrease of ~10% in peak workload after 5–8 days in-flight. During the STS-78 flight, there was a progressive decline in VO2 of –6.2% (SF+2), –9.0% (SF+8), and –11.3% (SF+13), while cycling at the same near-peak workload compared with pre-SF. In addition, three of the four astronauts did achieve preflight maximal HR during the in-flight testing (see RESULTS). Therefore, cycling at the same near-peak workload, at maximal HR, and near VO2 peak resulted in a reduced VO2 (i.e., –11%) while in-flight. It seems physiologically implausible that in-flight VO2 (i.e., VO2-85W) could be decreased at the high level of exercise in the present study and VO2 peak not be decreased from pre-SF levels. Thus the actual decrease in VO2 peak is likely equal to, or larger than, the decrease in VO2-85W measured while on orbit in the present study. Furthermore, the responses of the in-flight testing compare favorably with the maximal testing results obtained during the BR study, in which VO2 peak was obtained.

Studies from the SLS-1 and SLS-2 flights reported a significant decrease (–17%) in plasma volume 22 h after launch (1, 25), which remained below preflight levels throughout the missions and returned to preflight levels by 6 days postflight. Decrements in plasma volume of 10–15% (300–500 ml) following 2–3 wk of BR have also been observed, with the majority of the loss occurring during the first few days (17, 19, 22, 35, 37). Given the close relationship between plasma volume and maximal VO2 (15, 16, 29, 36), it is not surprising that VO2-85W and VO2 peak during SF and BR, respectively, declined rapidly and to a large degree. The changes in body weight in the SF and BR studies also support the concept of relatively rapid fluid shifts occurring upon insertion into orbit and return to Earth, contributing to alterations in exercise capacity. While both SF and BR subjects lost weight during the "flight" period, the astronauts lost 1.5–2.0 kg more than the BR subjects. Stein et al. (40) followed the same subjects from both the SF and BR studies and reported a decrease from pre-SF in SF energy intake, whereas the BR subjects maintained energy balance throughout the BR period.

The relatively constant submaximal VO2 during exercise at an absolute workload (150 W) observed throughout SF, BR, and recovery in the present study has been shown previously during upright cycling at 150 W before and after the Apollo and Skylab missions (6–84 days of SF) (27, 28, 30–34), and during supine or upright cycling or running following several microgravity simulations (7–10, 12, 13, 35). However, mean VO2 was decreased during the in-flight measurements made on the Skylab flights (27, 28, 31, 34) and on the Russian Space Station Mir in two cosmonauts (20). The data from the four astronauts in the present SF study were relatively variable and do not absolutely support or refute the notion of a decrease in the oxygen delivery to the active musculature, or more likely an increase in the mechanical efficiency of pedaling the cycle ergometer in microgravity. Certainly the restraint system during testing on Earth and while on orbit plays a major role in the mechanical efficiency of cycling. Indeed, alterations in the restraint system during the in-flight Skylab testing may have provided the astronauts additional leverage to pedal the bicycle and may have caused the apparent increase in mechanical efficiency (28). Regardless, it is most likely that, in the present study, there was an increase in HR to counteract the decrease in stroke volume (O2 pulse) (as a result of a decreased circulating blood volume, as discussed above), which was sufficient to maintain cardiac output and oxygen delivery to the active musculature during exercise at a submaximal level.

In summary, the data from the LMS mission and the ground based-simulation strongly suggest that maximal exercise capacity is compromised in the microgravity environment of space. Furthermore, –6° head-down-tilt BR appears to be a very adequate analog of weightlessness with respect to the cardiorespiratory responses to physical work.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This research was supported by NASA Grant NAS9-18768 to R. Fitts.


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  		Table 5. Physiological responses to recumbent cycling during exercise at a workload corresponding to 85% of pre-spaceflight peak workload before, during, and after 17 days of spaceflight

 

   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
Special thanks to all of the crewmembers of STS-78 and the bed-rest volunteers for their commitment throughout the studies. Thanks to the staff of the Human Research Facility at the National Aeronautics and Space Administration (NASA) Ames Research Center, especially Dee O’Hara, Dr. Sara Arnaud, Dr. Ralph Pelligra who served as the medical monitor for all of the exercise sessions, and Dr. John Greenleaf for the use of his equipment. Also, thanks to Sam Krenek, George Lutz, Stuart Johnston, Ladonna Miller, and Mel Buderer for assistance throughout the LMS mission.


   FOOTNOTES
 

Address for reprint requests and other correspondence: T. Trappe, Human Performance Laboratory, Ball State Univ., Muncie, IN 47306 (e-mail: ttrappe{at}bsu.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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