A COMPARISON OF TORSO STABILITY BETWEEN BED REST SUBJECTS AND
ASTRONAUTS DURING TANDEM WALK: PRELIMINARY FINDINGS
1
Chris Miller, 1Brian Peters, 1Igor Kofman, 1Rachel Brady, 1Tiffany Phillips, 2Crystal Batson,
3
Ajitkumar Mulavara, 4Alan Feiveson, 4Millard Reschke and 4Jacob Bloomberg
1
Wyle Science Technology & Engineering Group, Houston, TX, USA
2
MEI Technologies Inc., Houston, TX, USA
3
Universities Space Research Association, Houston, TX, USA
4
NASA Lyndon B. Johnson Space Center, Houston, TX, USA
Email: [email protected]
INTRODUCTION
During spaceflight, crewmembers experience a
microgravity environment that results in a central
reinterpretation of both vestibular and body axialloading information by the sensorimotor system.
Bed rest studies have been developed as a groundbased analog to spaceflight. Subjects lie at 6° headdown in strict bed rest to simulate the fluid shift and
gravity-unloading of the microgravity environment.
However, bed rest subjects still sense gravity in the
vestibular organs. Therefore, bed rest isolates the
axial-unloading component, thus allowing for the
direct study of its effects.
The Tandem Walk (that is, walk on floor eyes
closed) is a standard sensorimotor test of dynamic
postural stability [1]. Normally, the performance
metric is the number of steps the subject can
complete without sidestepping. In this analysis, we
used linear acceleration of the torso as the
performance metric as it provides additional
information about the subject’s stability during the
test. The purpose of this study was to compare torso
stability during the Tandem Walk test between bed
rest control subjects, short-duration (Space Shuttle)
crewmembers, and long-duration International
Space Station (ISS) crewmembers during their
recovery from bed rest or spaceflight.
METHODS
All subjects provided written informed consent
before participating in this study that was approved
in advance by the NASA Lyndon B. Johnson Space
Center Institutional Review Board.
This study is part of a larger protocol that uses a
suite of physiologic tests and functional tasks to
relate physiologic changes to changes in functional
performance of mission-critical tasks immediately
postflight or after bed rest. Subjects who have
completed the study to date include: (a) 5 bed rest
controls (4M/1F; 36.8 ± 7.9 yr; 66 ± 2.2 in.; 163.2 ±
17.9 lb), all of whom completed 70 days of 6degrees head-down strict bed rest with no exercise
countermeasure; (b) 6 short-duration crewmembers
(4M/2F; 43.0 ± 5.6 yr; 70 ± 1.5 in.; 168.2 ± 17.6 lb)
who completed Space Shuttle missions (13.2 ± 1.5
d), and (c) 7 long-duration crewmembers (6M/1F;
47.8 ± 2.3 yr; 70 ± 2.8 in.; 188.1 ± 32.8 lb) who
completed missions aboard the ISS (152.6 ± 18.2 d).
Data collection sessions occurred twice within a 2month or 2-week time period before flight or bed
rest, respectively (PRE), and four times after
flight/bed rest: on the day of landing or end of bed
rest (Post+0d), 1 day post (Post+1d), 6 days post
(Post+6d) and a final session 10 days (bed rest) or
30 days (flight) post (Post+10/30). Due to logistical
limitations, data could not be collected from longduration crewmembers on landing day.
In the Tandem Walk test, the subject walked in a
heel-to-toe fashion at a self-selected speed for 10 to
12 steps with their arms crossed on their chest and
their eyes closed. A spotter walked next to the
subject to ensure safety and to monitor the step
count. Three Tandem Walk trials were performed
per session.
Torso and head linear accelerations (relative to their
respective local coordinate systems) were recorded
RESULTS AND DISCUSSION
All groups exhibited increased torso instability (that
is, higher RMS values) during Tandem Walk
immediately postflight/bed rest (Figure 1). This
suggests that axial-unloading alone – as is the case
in bed rest – could result in decrements in Tandem
Walk performance.
RMS of Torso Acceleration during Tandem Walk
1.2
RMS of Torso Acc (m/sec 2)
1
0.8
encompassed preponderantly positive values when
comparing the long-duration crewmembers and the
bed rest controls for the Post+1d and Post+6d
sessions (Figure 2), and the short- and long-duration
crewmembers for the Post+6d session. This seemed
to indicate that, in long-duration crewmembers, the
vestibular component further impaired torso
stability postflight (that is, increased RMS), beyond
that attributable to the axial-unloading component
alone.
Bed Rest Control vs. Long-Duration Flight
0.4
Difference (m/sec^2)
using triaxial inertial measurement units (Xsens,
North America Inc., Culver City, CA, USA)
sampled at 50 Hz. During the analysis, the start and
end of the trial were manually selected by
inspection of the plot of the vertical acceleration of
the head. Data were then rotated into the global
coordinate
system,
and
the
gravitational
acceleration was removed from the Z-component.
The resultant linear acceleration of the torso was
calculated at each time point of the trial. The rootmean-square (RMS) of the torso accelerations
within a set window centered on the mid-point of
the trial was then computed. A separate analysis
revealed no significant difference between the two
PRE sessions, so all PRE data were pooled. Finally,
a preliminary mixed-model analysis of the
differences in RMS values for each postflight/bed
rest session relative to PRE (with three outliers
removed) was used to compare group  session
interactions; that is, changes from preflight/bed rest
to postflight/bed rest values between groups.
0.2
0
-0.2
-0.4
Post+0
Post+1
Post+6
Post+10/30
Figure 2: Mean estimate with 95% confidence
intervals of the difference of the (post-PRE) session
differences for the RMS of the torso acceleration
between bed rest controls and long-duration fliers.
CONCLUSIONS
Body unloading alone, as in strict bed rest, resulted
in decreased Tandem Walk performance (as
measured by the RMS of torso acceleration)
immediately post-bed rest. Central reinterpretation
of vestibular information by the sensorimotor
system during extended exposure to microgravity
may further compromise torso stability during
Tandem Walking postflight.
0.6
REFERENCES
0.4
1. Fregly,
AR, et al. Aerospace Medicine 43, 395399, 1972.
0.2
0
PRE
Bed Rest Control
Post+0
Post+1
Short-Duration Flight
Post+6
Post+10/30
Long-Duration Flight
Figure 1: RMS estimated means (± SE) of the torso
resultant acceleration across sessions and groups.
The preliminary mixed-model analysis showed
confidence intervals for interaction effects that
ACKNOWLEDGEMENTS
This study is funded by the National Aeronautics
and Space Administration.