HUMAN
FACTORS,
1997,39(1),111-118
The Retention of Balance: An Exploratory Study
into the Limits of Acceleration the Human Body
Can Withstand without Losing Equilibrium
BERND DE GRAAF,l rNO Human Factors Research Institute, Soesterberg, Netherlands, and
WILLEM VAN WEPEREN, Consumer Safety Institute, Amsterdam, Netherlands
The human limiting values for sudden accelerations we have determined can be
used to evaluate specific physical conditions that cause problems in maintaining
postural balance. A comparison between the data obtained in the laboratory and the
situations occurring during public transport by tram, bus, and metro revealed that
both the initial impetus ("jerk") and the level of acceleration found in practice were
sufficient to ensure that none of the individuals measured in the laboratory would
have been able to retain their balance without extra support. The study suggests that
limiting the initial jerk component of the acceleration would help considerably in
alleviating the problem.
INTRODUCTION
Everyone who regularly travels by public transportation has seen someone lose his or her equilibrium because of acceleration of the vehicle. In
the Netherlands, passengers have been swung
against the door and have fallen out of the bus in
a sharp turn, resulting in serious injuries or even
death. A recent poll (Consumentengids, 1995, pp.
128-131) shows that complaints regarding excessive braking and acceleration are common, especially in buses and trams. The records of the Consumer Safety Institute show that each year 2300
passengers need first aid treatment in a hospital
because of accidents occurring in the Dutch public transportation systems (Mulder, 1993). In
accidents in which people lost their balance
(claimed in 1227 cases), the direct cause is not
1 Requests for reprints should be sent to Bernd de Graaf,
TNO Human Factors Research Institute, P.O. Box 23, 3769 ZG
Soesterberg, Netherlands; [email protected].
unequivocally clear. However, it seems reasonable to look for a clue in the nature of the accelerations or decelerations to which the victims
were exposed.
Worldwide, some research has been done on
the effect of acceleration on postural balance, but
this research is of a fragmentary nature and has
frequently focused on compensatory responses to
very small disturbances (Allum, 1983; Dietz,
1986; Nashner, Woollacott, & Tuma, 1979). Guedry (1974) reported on measurements of the
(lower) perceptual threshold for accelerations,
and human reactions to extreme stresses are
known from military aviation research. However,
there is a paucity of data in the international scientific literature on experiences with accelerations in everyday life and their role in causing
people to lose their postural balance completely.
In the 1940s, however, Jongkees and Groen
(1942) investigated a series of sudden accelerations causing people to lose their balance. They
used a small vehicle to accelerate 50 standing
© 1997, Human Factors and Ergonomics Society. All rights resexved.
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1I2-March 1997
HUMAN
participants in a forward, backward, and sideward direction. They found that healthy individuals who stand upright with closed eyes and feet
together are able to endure an acceleration of up
to 76 cm/s2 in a backward direction, a forward
acceleration of up to 48 cm/s2, and a sideward
acceleration of up to 33 cm/s2. (An acceleration in
a backward direction is produced by a sudden
backward movement of the floor surface-in
their case a small vehicle-relative to the body.)
The stimulus Jongkees and Groen used in their
experiments was a sudden, constant acceleration.
It is arguable that higher limiting values might be
obtainable if the level of acceleration were increased gradually rather than in steps: This
would give the participant's
balance reflexes
some time to adapt to the disturbance.
In practice, different figures from those longkees and Groen (1942) reported are found. The
standards for acceleration and deceleration in
European road traffic define acceleration levels
of 100 to 150 cm/s2 in a longitudinal direction as
manageable and decelerations of 150 cm/s2 as
comfortable. The most "sensitive" road users (i.e.,
standing bus passengers; Westerduin, 1974) were
taken as the criterion in choosing these values. If
we evaluate these data in light of Jongkees and
Groen's findings, we can see that these bus passengers are at minimum assumed to be capable
of holding on tightly. Accelerations in a transverse or vertical direction should not exceed a
value of 50 or 25 cm/s2, respectively (Westerduin,
1974). No justification is provided for the European road traffic standards figures, and we do
not know of any fundamental research on this
topic.
The chief aim of the present study is to explore
the most extreme human limiting values for linear accelerations-that
is, the values obtained
under the most favorable sensory conditions. The
scope of the study was restricted to stationary
people (either standing still or moving steadily in
relation to the surface of the earth, such as in the
bus) exposed to a sudden (constant or gradual)
acceleration or deceleration. The data obtained
in the laboratory were compared with situations
occurring during travel on public transport.
FACTORS
STUDY 1: LABORATORY MEASUREMENTS
ON ACCELERATION
Method
A treadmill with a conveyor belt (ENRAF NONIUS, Delft, Netherlands) was used to expose 22
standing people in succession to backward,
sideward, and forward accelerations of the floor
surface. The group consisted of 11 men and 11
women in normal health, ranging in age from 26
to 63 years, who were randomly selected from the
population of the TNO Human Factors Research
Institute. Because of the exploratory character of
the study, it did not appear useful to consider a
sample more refined and extensive than 22 normal. healthy people chosen by chance from the
population of one of our institutes.
The total distance moved was 80 em, 10 cm of
which was taken up by the initial movement and
25 em by the braking distance. (In order to ensure that the increase in the speed of the treadmill had a genuinely linear character-constant
acceleration-the
treadmill was first brought to a
low constant speed to overcome the initial frictional resistance. Participants had to correct their
postural balance briefly at this point, but they
were fully stabilized again before the real acceleration occurred. The time window of this initial
period differed slightly for each stimulus exposure. This way the participant could not build up
an expectancy pattern about the moment of
stimulus acceleration.) This therefore left 45 cm
over which the participants were exposed to a
single constant acceleration. The treadmill was
controlled by a computer. The level of acceleration for each of the stimulus profiles was regularly calibrated with an accelerometer (triaxial
acceleration transducer AS-2TG, KYOWA Electronic Instruments, Tokyo, Japan). Figure 1 illustrates the type of stimulus profile used.
The acceleration was varied in steps of 0.1 mls2
over a range from 0.3 to 1.6 mls2• The sequence in
which the acceleration levels were presented was
randomly determined, but we sometimes omitted
acceleration levels that would clearly fall outside
the coping capacity of the individual participant.
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March 1997-113
THE RETENTION OF BALANCE
hold on, or even needing to be caught in order to
500
aduldiv
100
cm/s2/div
/\\
~
1
'\"".~\....•....•.•...•• \...
.•.,
'
;
'
I
3
1s
Figure 1. Exampleof a stimulus profile.The top tracing
represents the input (speed) of the treadmill (a drive
voltage expressed in analog/digital units); the middle
curve shows the total movement of the floor surface
(one revolution corresponds to 32.7 cm); and the bottom curve showsthe accelerationvaluesmeasured during that movement (here, 100 cm/s2). The small numbers on the graph correspond respectively to (1) the
initial, very short acceleration phase required to overcome the frictional resistance, (2) the acceleration that
is the subjectof this study, and (3) the deceleration.The
x axis represents time in seconds.
For instance, someone who was already having
severe difficulties with an acceleration of 0.5 rnIs2
would not be exposed to an acceleration of 1.6
rnIs2•
Participants were told that they were standing
in the bus with a bus driver who could be badtempered and were instructed to keep their eyes
open, their hands free, their shoes on, the heels of
their feet together, and their toes about 3 to 4 cm
apart. Each participant was then exposed to accelerations of the floor surface, twice in a backward direction (-Gx), then twice in a sideward
direction (+Gy), and then twice in a forward direction (+Gx). The stimulus profile used was the
same throughout, but the participant was turned
in the desired direction. This meant that the participant knew the direction of the next acceleration but did not know when it would occur.
We investigated whether the participants were
capable of coping with the acceleration without
showing a drastic loss of postural balance (large
sway movements with their body and arms), being forced to take one or more steps, having to
avoid falling. We assumed that all the participants felt sufficiently safe in the experimental
situation because of the support railing and the
proximity of the leader of the experiment (to
catch them, if necessary). The whole range of accelerations was presented quickly and in a relaxed atmosphere. The threshold values at which
the participants were just able to retain their balance over two exposures without problems were
noted.
The complete series of measurements of the
limiting values for accelerations in a forward,
sideward, and backward direction took 10 min.
The complete procedure (a repeated-measures 3
x 14 design, with acceleration levels randomly
mixed) was repeated one week later with a randomly chosen subgroup of 12 participants (of the
22) in order to identify possible learning effects.
This same subgroup was asked to get back on the
treadmill yet again a week after the one-week follow-up; on this occasion, however, they were allowed to choose their favorite pose for standing
in the bus (whatever they preferred, but without
holding on). The rest of the procedure remained
the same.
In order to explore a possible relationship
between postural behavior on the treadmill
and during normal upright stance, we briefly
examined the participants on a stabilimeter platform. The stabilimeter, on which the participant
stands, is a stable horizontal platform that records the excursions of the body. The measure of
stability used in this study was the RMS value
over a period of SO s. (For details of this method,
see Bles and De Jong, 1986.)
Results
Despite clear differences in approach between
Jongkees and Groen's (1942) experimental design
and our design (moving vehicle vs. treadmill;
closed eyes vs. open eyes; barefoot vs. wearing
shoes; threshold measurements
made during
the deceleration
phase vs. the acceleration
phase), the measurement data obtained in the
two studies are very similar (see Table 1). (Although the two threshold measurement methods
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114-March 1997
HUMAN
FACTORS
TABLE 1
Group Means (and Standard Deviations,in m/s2) for the Three Directionsof Acceleration
N=22
Jongkees & Groen
De Graaf & Van Weperen
Forward
Sideward
Backward
Sum
0.48
0.33
0.45
(0.12)
0.76
0.61
(0.16)
1.57
1.60
0.54
(0.16)
are equivalent in physical terms, they could create differences in physiological-for instance, visual-terms).
The differences between the directions of acceleration are significant for our sample: post
hoc Newman-Keuls on analysis of variance
(ANOYA)p < .01. The participants were found to
be most vulnerable to sideward accelerations and
least vulnerable to backward accelerations.
For the purpose of further comparison, we determined a persistence index (a kind of global indication of one's resistance to acceleration,
which also identifies someone's position relative
to a peer group) for each of the participants, in
accordance with the method used by Jongkees
and Groen (1942). This was the sum of the highest acceleration levels recorded for each participant in the three directions, divided by the sum of
the group's mean values, or I = (af + as + ab)/
(mean af + mean as + mean ab)'
The persistence index (I) in our experimental
group (from 0.44 to 1.5) shows a somewhat
greater spread than in Jongkees and Groen's
group (0.7 to 1.3). We were unable to detect any
differences in persistence between the male and
female participants, but there was a (negative)
correlation between persistence and age. Within
the age range of our participants (26-63), the persistence index value decreased significantly (p <
.01) as the participant's age increased (see Figure
2). (A recalculation with omission of the two individuals with the highest and lowest persistence
indexes still yields a correlation coefficient r =
-.41 and a probability level p = .07.)
A comparison of the data for the 12 (of 22)
participants who repeated the complete procedure one week later suggests a slight learning effect: ANOYA:F(1, 11) = 27.1,p < .01, 4.5%vari-
ance explained. This is shown in the first two
rows of Table 2. Because the effect applies to the
whole group (a structural effect), on average the
persistence index remains the same (given that it
is a relative measure). In other words, on average,
someone's position relative to the group remains
the same because the performance of everyone in
that group changes in the same direction and by
the same amount.
When the third test was carried out, during
which the same 12 participants were tested in
their own choice of pose for standing in the bus
(feet further apart and not completely in line), the
strategy adopted appeared to be particularly
functional for sideward accelerations. The limiting values for accelerations in a forward and
backward direction could not (on average) be improved any further (see Table 2, Test 3).
We found no correlation for our sample between the data from the posturographical examination and the persistence index. All the RMS
values recorded by the stabilimeter for our group
fell well within the norms for normal postural
balance behavior (Roos, 1991) and, indeed, differed little from one another. Consequently, postural control during normal upright stance with
the eyes open is not demanding enough of the
equilibrium system to allow prediction of postural stability on the treadmill.
Conclusions
Despite the differences in approach, our data
replicate those obtained by Jongkees and Groen
(1942). It is true that the limiting values for linear
accelerations in the three directions lie somewhat
closer, but the sum for the group corresponds
very closely to the value found in 1942 (Table 1).
Bearing in mind this correspondence, and the
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March 1997-115
THE RETENTION OF BALANCE
The limiting values can be increased further
with the benefit of some experience, but only
marginally (Table 2, Tests 1 and 2). However, if
the participants stand with their legs well apart,
the limiting value for a sideward acceleration can
be increased substantially (Table 2, Test 3).
We found no correlation between the stabilimeter data obtained from the posturographical examination and the limiting values found for applied accelerations. Such a relationship, if it
exists at aU, is probably too weak to be detected
in the limited size of our sample .
Regression of index on age
, .6
,.4
~ 1.2
'0
.!:
~-
Ql
o
•..5i
1.0
(/l
'iii
.-
•..•. •..•.
.•..•. •... ,,
Qj
a. 0.8
•..•. •..•.
0.6
•..•. •..•.
•..•. •..•.
0.4
26
36
-- --.....
,.•...
.•...
,
' •..•.
""
..... ,
46
56
" " ,.•...
STUDY 2: MEASUREMENTS TAKEN ON
PUBLIC TRANSPORT
"
66
Age
Figure 2. The straight line represents the correlation
between persistence and age in our sample (N = 22).
The correlation coefficientis -0.64 and is significantat
the 1% levelof probability.The two pairs of dotted lines
represent the 95% confidence limits (inner lines) and
the 96% prediction limits (outer lines).
fact that the experimental procedures differed
substantially, this means that the human limiting
values for linear accelerations are constant in
nature.
The persistence indexes in our group had a
somewhat greater degree of spread than did
Jongkees and Groen's (1942), but this may be attributable to the spread of ages in our sample:
Our group of participants included people who
were either younger or somewhat older than
were those in Jongkees and Groen's study. The
older people had lower limiting values than the
younger ones, and this could have had an effect
on the persistence index.
In order to gain an impression of the situation
in practice, we measured the acceleration levels
that occur during travel by public transport in
Amsterdam on a random weekday using accelerometers (g meters: AS-2TG, KYOWA Electronic
Instruments). The drivers of the vehicles were not
informed about these measurements.
Results
A randomly selected sample (see Figure 3) of
the figures recorded on the tram, express tram,
bus, and metro services reveals immediately that
the amplitude of the acceleration levels on all
these modes of transport is high enough to ensure that none of the people we tested would have
been able to maintain their postural balance
without support. The initial acceleration in a longitudinal direction regularly lies between 1 and 2
m/s2, which is certainly higher than the level at
which people can cope in an optimal situation,
but without support, without being in danger of
TABLE 2
Subgroup Means (and Standard Deviations,in m/s2) for the Three Directions of Acceleration
N= 12
Forward
Sideward
Backward
Sum
Test 1
0.56
(0.18)
0.65
(0.17)
0.60
(0.19)
0.47
(0.14)
0.53
(0.17)
0.93
(0.19)
0.64
(0.18)
0.75
(0.21)
0.68
(0.24)
1.67
Test 2
Test 3
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1.93
2.24
116-March 1997
HUMAN
FACTORS
Metro
G.
1.00
m/s2/div
Express tram
G.
1.00
..... ... .......
m/s2/div
. ..........
"
......
Bus
1.00
G.
m/s2/div
G
m/s2/div
y
1.00
--.
Time (s/div)
Examples of the acceleration profiles occurring on public transport as measured with accelerometers for the metro, express tram, and bus. The top three curves
represent the acceleration or deceleration along the longitudinal axis of the vehicle (div
= division = 1.00 m/s2), and the bottom curve represents the associated lateral acceleration values for the bus only. The time in seconds is shown along the x axis.
Figure 3.
losing their balance. This applies primarily to the
bus; not only is the acceleration measured in a
longitudinal direction the highest (2.15 mls2), but
the associated lateral acceleration levels occurring on bends and when swerving into and out of
stops are substantial (up to about 4 mls2). For
details, see de Graaf (1993).
Conclusions
The accelerations that are commonly encountered in practice appear to be impossible to endure without support. Handgrips will increase
the coping ability of standing passengers in a longitudinal
direction
to 1.50 m/s2 (Browning,
1974), but this does not eliminate all problems.
The initial impetus ("jerk") with which the acceleration begins is also relevant to postural balance. It is this jerk that can throw passengers off
balance in the first place. The issue is seldom
broached, but Vuchic (1981) recommended that
vehicles for public transport should be designed
in such a way that their acceleration rate does not
change more quickly than 0.50 to 0.60 rn/S3. At
the moment, this recommendation
does not appear to have been adopted. The initial acceleration levels found on public transport in our data
set appeared to vary from 1.5 mls3 in practice in
a longitudinal direction to 3.5 mls3 in a transverse direction. The jerk that occurred on the
treadmill varied between 1.0 and 7.0 m/S3. The
jerk on the treadmill was undoubtedly sometimes
somewhat higher but continued for only 0.2 s,
whereas the jerk measured on public transport
could continue for several seconds.
STUDY 3: MEASUREMENTS IN THE
LABORATORY ON JERK
It appeared worthwhile to carry out a second
experiment in the laboratory focusing specifically on the jerk with which acceleration begins
(the change in acceleration per unit of time). The
question we investigated in this experiment was
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March 1997-117
THE RETENTION OF BALANCE
whether there was any benefit to be gained-that
is, would the human limiting value for acceleration levels increase if the acceleration of the vehicle began somewhat less abruptly?
TABLE3
Stimulus Conditions in the Second Laboratory Experiment
Acceleration
(m/s2)
Jerk (m/fiJ)
Method
Ten new participants
(five men and five
women, haphazardly chosen from the population
of the Human Factors Research Institute) were
exposed on the treadmill to a standard forward
acceleration (1.00 mls2) that was higher than the
mean limiting value of the former participants.
This meant that, as a rule, the participant would
have to take one or more corrective steps at each
stimulus in order to avoid falling. The participant
was now repeatedly exposed to this acceleration value but with differences in the onset of
the stimulus (the jerk component; see Figure 4).
This produced the four conditions specified in
Table 3.
Each participant was presented with the four
conditions in a fixed sequence (balanced among
participants so that the sequence could vary for
each participant), and four measurements were
I
I
2'
1.0
2.0
5.0
10.0
1.0
1.0
1.0
1.0
(:t:0.2)
(:t:0.2)
(:t:0.2)
(±0.2)
taken per condition. The participant's score was
determined by the number of times that he or she
coped with a stimulus without problems (maximum score: 4 x 4 = 16). The score per condition
is thus the sum of the individual scores in that
condition.
Results
The results, summarized in Table 4, indicate
that some benefit can be gained from the human
limiting values for accelerations in cases in which
the onset of the acceleration
is sufficiently
smooth. It is clear that the recommendation formulated by Vuchic (1981) to limit the onset of
accelerations to 0.50-0.60 mls3 must be taken seriously by anyone wanting to prevent accidents
involving falls on public transport. Unfortunately, the stimulus equipment available in the
TABLE4
Scores of 10 Participants in Response to a Stimulus
with a Constant AccelerationBut Variable Onset Levels
Jerk (m/fiJ)
Figure 4. The four stimulus (speed) profiles of the second laboratory experiment. In all four conditions, participants were exposed to the same constant acceleration (1.00 mis2) but with variations in the onset (see
numbers 1-4, which represent 10 mis3, 6 mis3, 2 mis3,
and 1 mis3, respectively).Note that the slopes of the
onsets, close to the numbers, are different. The x axis is
a time axis, and the treadmill input drive voltage is
represented on the y axis.
Participant
1.0
2.0
5.0
1
2
3
4
5
6
7
8
9
10
4
2
0
0
0
4
4
4
4
4
4
2
0
0
0
4
3
0
4
2
0
0
0
0
0
0
0
0
4
1
Retention
of balance
65%
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47.5%
12.5%
10.0
0
0
0
0
0
0
0
0
0
1
2.5%
UB-March 1997
HUMAN
laboratory did not permit an acceleration rate of
1 mls2 to be paired with a jerk level of less than 1
mls3, and hence the value proposed by Vuchic
could not be tested. However, bearing in mind
that 35% of the participants were unable to continue standing without difficulty under the mildest stimulus condition, one can assume that the
optimal jerk level must be well below 1 m/s3.
GENERAL CONCLUSIONS
The combined approach, using tests of participants in the laboratory and measurements of accelerations occurring in practice on public transport, appears to provide meaningful data for the
issue of accidents involving falls.
Laboratory research reveals that the human
limiting values for applied acceleration levels are
constant (replication of data obtained in the
1940s) and that there is a negative correlation
with age. A comparison between the human limiting value for acceleration levels and the acceleration levels measured on public transport services reveals that standing passengers will never
be able to retain their balance during the journey
without support. However, restricting or changing the onset of acceleration to 0.5-0.6 m/s3
should in principle enable passengers to cope
with the acceleration levels in a longitudinal direction that now occur on the metro, tram, and
bus. (Any such restriction should never be allowed to affect the vehicle's ability to brake immediately and sharply.) In the case of the bus,
sideward accelerations must also be taken into
account.
These data, however, can be generalized only
to apply to people standing "passively" without
support. In the case of people behaving actively,
the acceptable limiting values can sometimes
tum out to be higher (anticipation), but they are
frequently also lower (e.g., one has less supporting surface when walking toward a seat). Because
the aim is to ensure that increasing numbers of
people will make use of public transport, future
research should investigate this question, together with the human limiting values for (combinations of) applied acceleration levels with the
use of support.
FACTORS
ACKNOWLEDGMENTS
Part of these data were presented at the Third International
Conference on Product Safety Research, Amsterdam, March
6-7, 1995.
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Bernd de Graaf received his Ph.D in biology at the University of
Utrecht in 1990. He is senior scientist in the Equilibrium &
Orientation Research Group at TNO Human Factors Research
Institute.
willem Van Weperen received his M.S. in physics from the
University of Groningen and is head of the Product Safety Department at the Consumer Safety Institute, Amsterdam.
Date received: March 30, 1995
Date accepted: May 15, 1996
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