Exercise capacity is reduced following both short and long duration exposures to microgravity. We have shown previously that supine lower body negative pressure with exercise (LBNP(sub ex) maintains upright exercise capacity in men after 5d and 15d bed rest, as a simulation of microgravity. We hypothesized that LBNP(sub ex) would protect upright exercise capacity (VO2pk) and sprint performance in eight sets of identical male twins during a 30-d bed rest. Twins within each set were randomly assigned to either a control group (CON) who performed no exercise or to an exercise group (EX) who performed a 40-min interval (40-80% pre-BR VO2pk) LBNP(sub ex) (55+/-4 mmHg) exercise protocol, plus 5 min of resting LBNP, 6 d/wk. LBNP produced footward force equivalent to 1.0- 1.2 times body weight. Pre- and post-bed rest, subjects completed an upright graded exercise test to volitional fatigue and sprint test of 30.5 m. After bed rest, VO2pk was maintained in the EX subjects (-3+/-3%), but was significantly decreased in the CON subjects (-24+/-4%). Sprint time also was increased in the CON subjects (24+/-8%), but maintained in the EX group (8+/-2%). The performance of a supine, interval exercise protocol with LBNP maintains upright exercise capacity and sprint performance during 30 d of bed rest. This exercise countermeasure protocol may help prevent microgravity-induced deconditioning during long duration space flight

Boda, Wanda L.

Hargens, Alan R.

Lee, Stuart M. C.

Macias, Brandon R.

Meyer, R. Scott

Schneider, Suzanne M.

Watenpaugh, Donald E.

NASA Technical Reports Server

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NASA Johnson Space Center 
Supine lower body negative pressure exercise maintains upright exercise capacity in male 
twins during 30 days of bed rest 
Stuart M.C. ~ e e ' ,  Suzanne M. ~chneide?, 
Wanda L. ~ o d a ~ ,  Donald E. watenpaugh4, Brandon R. ~acias ' ,  
R. Scott ~ e ~ e r ' ,  and Alan R. ~argens'  
1 Wyle Laboratories, Inc., Life Sciences Group, Houston, TX 
2~epartment of Physical Performance and Development, 
University of New Mexico, Albuquerque, NM 
3~onorna State University, Rohnert Park, CA 
4 Sleep Consultants, Inc., Fort Worth, TX 
'~e~ar tmen t  of Orthopedic Surgery, University of California-San Diego Medical Center, 
San Diego, CA 
Running Title: LBNP Exercise Maintains Post-Bed Rest Exercise Capacity 
Corresponding Author: 
Stuart M.C. Lee, MS 
Wyle Laboratories, Inc. 
Life Sciences Group 
1290 Hercules Blvd. 
Houston, TX 77058 
Telephone: (281) 483-3726 
FAX: (28 1) 483-4 18 1 
E-mail: slee1@ems.jsc.nasa.gov 
https://ntrs.nasa.gov/search.jsp?R=20080009021 2019-08-30T03:46:49+00:00Z
ABSTRACT 
Exercise capacity is reduced following both short and long duration exposures to 
microgravity. We have shown previously that supine lower body negative pressure with 
exercise (LBNP,,) maintains upright exercise capacity in men after 5d and 15d bed rest, 
as a simulation of microgravity. We hypothesized that LBNPex would protect upright 
exercise capacity (VOzpk) and sprint performance in eight sets of identical male twins 
during a 30-d bed rest. Twins within each set were randomly assigned to either a control 
group (CON) who performed no exercise or to an exercise group (EX) who performed a 
40-min interval (40-80% pre-BR V02pk) LBNP,, (55*4 mmHg) exercise protocol, plus 5 
min of resting LBNP, 6 d-wk-'. LBNP produced footward force equivalent to 1 .O- 1.2 
times body weight. Pre- and post-bed rest, subjects completed an upright graded exercise 
test to volitional fatigue and sprint test of 30.5 m. After bed rest, VOzpk was maintained 
in the EX subjects (-3&3%), but was significantly decreased in the CON subjects (- 
24&4%). Sprint time also was increased in the CON subjects (24&8%), but maintained in 
the EX group (8f2%). The performance of a supine, interval exercise protocol with 
LBNP maintains upright exercise capacity and sprint perfonnance during 30 d of bed 
rest. This exercise countermeasure protocol may help prevent microgravity-induced 
deconditioning during long duration space flight. 
Keywords: space flight, microgravity, countermeasure, LBNP, artificial gravity 
LNTRODUCTION 
Exposure to real or simulated microgravity significantly reduces aerobic exercise 
capacity [Convertino, 1996; Fortney, 19961. Self-selected exercise patterns of Space 
Shuttle crewmembers do not maintain peak [Levine, 19961 and submaximal exercise 
responses Fee, 19951 after short duration space flight (8-16 days). However, data during 
short bevine, 1996; Moore, 20011 and long duration space flight Wchel, 19771 suggest 
that in-flight exercise performance is maintained or even improved with in-flight exercise 
countermeasures, but upright exercise performance after space flight is not protected. 
Previous investigators have documented that vigorous, twice-daily supine exercise 
maintains supine exercise capacity after bed rest, as a simulation of microgravity 
[Greenleaf, 19891, but maintenance of upright exercise capacity may be more 
problematic [Convertino, 19821. After 10 days of bed rest, V02max decreases 2-23 
times more during upright exercise (- 17%) compared to supine exercise [Convertinol, 
1982b; Hung, 19831 and the oxygen uptake kinetics during steady-state exercise are 
slower during upright exercise compared to supine exercise [Convertino, 19841. Post- 
bed rest exercise in the upright posture is associated with a greater reduction in stroke 
volume and cardiac output than supine exercise [Convertino, 19961. Maintenance of 
upright exercise capacity after space flight is more important operationally than supine 
exercise capacity because crewmembers must be prepared to perform an emergency 
egress from the Space Shuttle bee,  1997; Watenpaugh, 20001. Emergency egress 
represents a significant metabolic (>2.5 lemin-') and cardiovascular (>I60 beatsemin") 
stress in normal ambulatory subjects [Bishop, 19991 and would be a much greater 
challenge after long-duration International Space Station missions that typically last six 
months or more. 
Previous investigations consistently demonstrate that plasma volume is rapidly reduced 
during exposure to space flight and bed rest. Convertino [Convertino, 19971 fomd that 
70% of the variability in V02max following bed rest can be explained by a decreased 
plasma volume. Reduced circulating plasma volume may negatively affect exercise 
stroke volume, the delivery of oxygen and nutrients to working muscle, and the removal 
of metabolic waste products. Thus, preservation of plasma volume may be an important 
factor in the maintenance of exercise capacity during bed rest. Post-bed rest relative 
hypovolernia is more problematic during upright than supine exercise because of the 
gravitational shift of blood to the lower extremities with the addition of orthostatic stress. 
The combination of a gravity-like stress during exercise training probably is necessary to 
maintain upright exercise responses after space flight and bed rest [Convertino, 19821. 
Supine exercise may maintain plasma volume [Oreenleaf, 19921, but a gravitational 
component, real or simulated, may be required to maintain venous return and stroke 
volume during post-flight exercise and work. The present study is one of a series to 
investigate the use of lower body negative pressure (LBNP) during exercise (LBNP,) as 
a countermeasure to space flight-induced deconditioning. Previous investigations from 
our group demonstrate that supine LBNP,, (Figure 1) produces exercise responses sllnular 
to upright exercise @3oda, 20001, maintains upright exercise responses after five days of 
bed rest [Lee, 19971, and protects upright exercise capacity during 15 days of bed rest 
[Watenpaugh, 20001. The purpose of this investigation is to determine whether LBNP,, 
maintains upright exercise responses following a longer bed rest study, 30 days of 6' 
head-down tilt. Specifically, we hypothesized that subjects who exercised for 40 b u t e s  
against LBNP, with five minutes of post-exercise resting LBNP, would maintain upright 
submaximal and peak exercise responses following 30 days of bed rest, while a group of 
non-exercising control subjects would not. A unique aspect of this study was that 
identical twins were used as subjects, one as control and one as countermeasure subjects, 
to reduce the overall effect of genetic variability in these comparisons. 
METHODS 
Overall Protocol 
Eight pairs ofhealthy male twins (27*5 yr, 174.6*12.9 cm; 68.0*11.0 kg, mean kSD) 
volunteered to participate in ths  30-day bed rest study. Monozygosity was co 
using DNA polymorphism analysis for STR markers, D3 S 1358, vWA, D 16 S539, D2 
S1338, D8 S1179, D21S11, Dl8 S51, Dl9 S433, Thol, and FGA. The subject's DNA 
was obtained using a cheek swab kit. Protocols were reviewed and approved by the 
Institutional Review Boards of University of Califomia-San Diego (UCSD) and NASA- 
Johnson Space Center (JSC). Subjects received verbal and written explanation of all 
procedures and signed statements of informed consent prior to participation. 
Subjects received complete physical examinations fkom qualified physicians and were 
admitted to the General Clinical Research Center (GCRC) at UCSD for the duration of 
the study. Subjects were hospitalized six days prior to bed rest for a period of 
ambulatory control during which they underwent familiarization and pre-bed rest testing 
sessions. Subjects completed a period of 30 days of strict 6" head-down tilt bed rest and 
were not allowed to be in the upright posture at any time during the bed rest. Urination, 
defecation, showering, and transport to and fiom all testing and countermeasure sessions 
were conducted in the head-down posture. 
Within each twin pair, one twin was randomly assigned to the non-exercise control 
(CON) group and his brother to the exercise countermeasure group (EX). Six days per 
week during bed rest, the EX subjects performed 40-minutes of supine treadmill exercise 
against LBNP, followed by 5 minutes of resting lower body negative pressure (without 
exercise). LBNP was adjusted during these sessions such that subjects experienced 
between 1.0 and 1.2 times body weight during the countermeasure, similar to our 
previous work bee ,  1997; Watenpaugh, 20001. 
Subjects were prescribed a diet consisting of 55% carbohydrate, 15% protein, and 30% 
fat. The dietary staff of the GCRC prepared the food, and diet records were maintained 
to insure consistency. Initial caloric consumption was prescribed for each subject using 
the Harris-Benedict equations, adjusting for self-reported activity level (correction factor: 
1.4- 1.5) prior to hospitalization warris, 19 191. Thereafter, caloric consumption was 
adjusted such that subjects maintained their body weight within *1.0 kg. Body weight 
was measured each morning at 0700 hrs, prior to breakfast, in the supine position using a 
bedside scale (Century CC894, Hill-Rom Co., Inc., Batesville, IN). Additionally, sodim 
consumption was maintained at 3,500 mglday, calcium consumption was targeted at 800- 
1200 mglday, and dietary fiber was prescribed at 25 g/day. Fluid intakes and output were 
measured daily. Fluid intake was ad libitum, but no caffeinated or alcohol beverages 
were allowed. 
Subjects completed two graded exercise tests (GXT) to volitional htigue before and one 
immediately after bed rest to determine the effectiveness of the LBNP, countermeasure 
to prevent losses in maximal and submaximal exercise responses. On the same day prior 
to the GXT, subjects also participated in an orthostatic tolerance test to presyncope, 
sprint trials, and balance testing. These tests were performed in the same order at the 
same time of day during familiarization, pre-bed rest, and post-bed rest testing sessions. 
All tests were at least two hours postprandial. Orthostatic tolerance and balance results 
will be reported elsewhere. Plasma volume was measured the day before pre- and post- 
bed rest GXT. 
Exercise Countermeasure 
The exercise device used for this study (FIGURE 1) was the same as used in our previous 
5-day [Lee, 19971 and 15-day bed rest studies watenpaugh, 20001. The device consists 
of a vacuum chamber in which subjects ran comfortably on a vertically oriented treadmill 
(PaceMaster SX-Pro, Aerobics, Little Falls, NJ). Chamber pressure was reduced using a 
high capacity vacuum cleaner. A small amount of leakage was allowed in the chamber 
such that airflow through the system minimized heat accumulation caused by the 
treadmill motor and the exercising subject. Interchangeable wooden plates were used to 
allow the adjustment of the opening size through which the subject's legs and lower torso 
projected into the chamber. The size of the elliptical opening was chosen to be 
approximately twice the subject's waist cross-sectional area. In this way, the negative 
pressure required to produce 1.0 times body weight was approximately 50 to 60 mmHg. 
INSERT FIGURE 1 HERE 
Each subject wore a neoprene waist seal, with shoulder straps attached to prevent the seal 
from sliding down the subject's body and to provide 60-65% of BW through axial 
loading [Cao, 20051. A sling supported the subject's upper body with a solid back 
support outside the chamber, and the hips were supported with a soft sling inside the 
chamber. Cuffs were placed around the knees and ankles. A bungee cord connected the 
cuffs at their respective positions through pulleys suspended &om the ceiling of the 
chamber. In this way, the legs counterbalanced each other and minimized the work 
required to move against gravity. The distance between the treadmill and the chamber 
opening was adjusted such that the iliac crest was positioned at the level of the opening 
when the subject was flat-footed against the treadmill with their legs Mly extended. 
EX subjects perfbrmed 40 minutes of exercise 6 days per week using the same protocol 
that successfully preserved upright exercise capacity during 15-day bed rest 
[Watenpaugh, 20001. Target exercise intensities for this protocol consisted of 7 minutes 
at 40%, 3 minutes at 60%, 2 minutes at 40%, 3 minutes at 70%, 2 minutes at 50%, 3 
minutes at SO%, 2 minutes at 60%, 3 minutes at SO%, 2 minutes at 50%, 3 minutes at 
70%, 2 minutes at 40%, 3 minutes at 60%, and 5 minutes at 40% pre-bed rest V02pk 
Figure 21. Target speeds to achieve these exercise intensities were prescribed based 
upon a linear relationship between treadmill speed and VO;! determined during the pre- 
bed rest GXT. Lower body negative pressure was used to produce footward force of 1.0 
times body weight at the start of bed rest and was increased up to 1.2 times body weight 
based upon subject tolerance to the exercise countermeasure. At the conclusion of the 
exercise period, LBNP was maintained for five minutes while the subjects rested -4th 
their legs outstretched to the treadmill belt. 
EX subjects participated in the complete countermeasure session each day that exeircise 
was prescribed (FIGURE 2), but two sessions were shortened in one subject (25 aiid 35 
out of 40 minutes of exercise) due to treadmill malhction. Most EX subjects completed 
their exercise at an LBNP that produced one body weight for the first two weeks. 
ThereaRer, as their tolerance to exercise increased, six of eight subjects exercised at least 
three sessions at 1.05-1.1 times body weight, and three completed some of their exercise 
sessions at 1.2 times body weight. In one subject, three of the exercise sessions were 
performed at less than one body weight (90%) as he acclimated to the exercise at the start 
of bed rest, but he later exercised at 1.2 times body weight at the end of bed rest. EX 
subjects underwent LBNP 44.8f1.6 min per session and walked and ran for an average 
distance of 4.9rt0.7 km (3.0f0.4 miles) per exercise session. Across all exercise ses,sions, 
the average LBNP was 55.4f3.4 mmHg, which corresponded to a mean loading of 
l.OfO.l times body weight. 
INSERT FIGURE 2 HERE 
Graded Exercise Test 
Subjects performed two upright GXT's prior to bed rest and one GXT on the last day of 
bed rest. The first test served as a familiarization session and consisted of one five- 
minute stage of level walking at 4.8 km-h-' (3 mph) followed by three-minute stages of 
increasing treadmill speeds (8.0,9.7, and 1 1.3 km-h-' [5,6,  and 7 mph]) at 0% grade. 


Supine Lower Body Negative Pressure Exercise Maintains Upright Exercise Capacity in Male Twins during 30 Days of Bed Rest

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