GROUND REACTION FORCES DURING TREADMILL EXERCISE ON THE INTERNATIONAL
SPACE STATION
1
John K. De Witt, 1Renita S. Fincke, 1Mark E. Guilliams, and 2Lori L. Ploutz-Snyder
1
Wyle Science, Technology and Engineering Group , Houston, TX, USA; 2Universities Space Research Association, Houston, TX,
USA. email: [email protected]
Astronauts perform treadmill exercise during long
duration spaceflight to maintain their physical
health. There are limited data available that quantify
the ground reaction forces (GRF) developed during
exercise in microgravity [1,2,3]. The treadmill
currently used on the International Space Station
(ISS) is equipped with load sensors that measure
GRF during normal exercise. The purpose of this
investigation was to quantify the GRF during
typical exercise on the ISS. The study is ongoing
and the data presented are from a subset of subjects
who have completed the evaluation.
METHODS
Six astronauts completed preflight ground testing
and inflight testing throughout their ISS mission.
The single preflight testing session occurred in the
Exercise
Physiology
and
Countermeasures
Laboratory at NASA Johnson Space Center (1G).
Each astronaut walked or ran at speeds ranging
from 1.5 to 9.5 mph in 0.5 mph increments on a
Kistler Gaitway Instrumented Treadmill (Kistler
Instrument Corp., Amherst, NY). Vertical GRF
were recorded at 1000 Hz for 30 s at each speed.
Ground data were used as baseline for comparison
with inflight data.
Three-dimensional GRF data were collected at 250
Hz. Subjects maintained contact with the treadmill
by using a harness attached to elastomer bungees.
The ISS treadmill is a Force treadmill (Woodway
USA Inc., Waukesha, WI) customized for onboard
use. The treadmill is mounted on a dampening
system to reduce the transmission of vibration to the
ISS.
GRF data were downlinked and post-processed in
the laboratory using custom MATLAB scripts (The
MathWorks, Natick, MA). Algorithms were
developed to identify the onset of heel strike and
toe-off for each footfall. Peak force and impulse
were computed for each footfall, as well as the
respective means over each trial.
RESULTS AND DISCUSSION
Vertical GRF trajectories in 0G were very similar to
those typically obtained in 1G (Fig. 1). Footfalls
demonstrate clear impact and propulsive force
peaks.
Vertical GRF, speed = 7.3 mph, n=133
1400
1200
1000
GRF, N
INTRODUCTION
800
600
400
Once aboard ISS, each subject completed data
collection sessions in microgravity (0G) as part of
their normal exercise session for the day. The actual
protocol performed varied from subject-to-subject
and between sessions because it could not be
standardized due to logistical reasons. However,
sessions were designed to include multiple 30-s
trials at varying speeds. The 3 to 6 data collection
sessions were scheduled to span the entire mission
of each crewmember and were separated by
approximately 25 to 30 days.
200
0
0
10
20
30
40
50
60
Sample (freq = 250 hz)
70
80
90
100
Figure 1: Typical GRF trajectories during running
at 7.3 mph in 0G.
Peak GRFs were less at each speed in 0G than in
1G, which was expected given that subjects
typically exercised in 0G with external loads (EL)
that were less than their 1G body weight (BW).
However, when GRFs for each subject were
normalized (by BW for 1G and EL for each 0G
session), subjects tended to generate similar peak
forces between gravitational conditions at all
running speeds (Fig 2).
to optimize flight time by reducing propulsive GRF
proportionally to EL.
Figure 2: Mean peak GRF for two subjects in 1G
and 0G normalized by EL across exercise speeds.
Note that in 1G, bodyweight (BW) is the EL. Each
plot represents a 0G session at the given EL in
addition to the 1G plot.
Figure 3. Mean impulse per footfall for two
subjects in 1G and 0G.
The ‘dose’ of force has been hypothesized to be an
important factor for bone health [4]. Impulses,
which are analogous to the ‘force dose’ were similar
across EL and did not decrease with increasing
speed (Fig 3). However, in 1G impulse decreased as
speed increased. This suggests that at higher speeds,
impulse in 0G may be similar to that occurring in
1G.
CONCLUSIONS
GRF data from treadmill exercise on ISS suggest
that subjects generate similar footfall patterns in 0G
as in 1G. Peak forces are less in 0G, but data
suggest that subjects adjust the propulsive force
based on the EL. It is possible that subjects attempt
In addition, while peak GRF are less in 0G than 1G,
impulse generated at faster speeds between Gconditions may be similar, suggesting that
exercising at higher speeds in 0G may be more
similar to 1G than previously thought. The data
from this investigation will provide greater insight
regarding the quantification of exercise in 0G.
REFERENCES
1. Cavanagh PR, et al. J Biomech 43, 21822188, 2010.
2. De Witt JK, et al., Aviat Space Environ Med
81, 1092-1099, 2010.
3. Genc KO, et al. J Biomech 43, 3020-3027,
2010.
4. Genc KO, et al. Aviat Space Environ Med
80, 919-926, 2009.