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BALANCE ABILITIES OF WORKERS IN PHYSICALLY DEMANDING JOBS: WITH
SPECIAL REFERENCE TO FIREFIGHTERS OF DIFFERENT AGES*
*Doctoral
dissertation presented on the 5th November 2004 at the Large Lecture
Hall of the Haartman Institute, Helsinki, Finland by permission
of Faculty of Medicine of the University of Kuopio, Finland.
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Department of Physiology, University of Kuopio, Department of Physiology,
Finnish Institute of Occupational Health, Kuopio, Finland
| Published
(Online) |
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01
May 2005 |
© Journal of Sports Science
and Medicine (2005) 4, Suppl.8, 1 - 47
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This
review is based on the following original publications, which will be referred
to in the text as Studies 1-5:
1.
Punakallio, A. (2003) Balance abilities of different-aged workers
in physically demanding Jobs. Journal of Occupational Rehabilitation
13, 33-43.
2.
Punakallio, A., Lusa, S. and Luukkonen, R. (2003) Protective equipment
affects balance abilities differently in younger and older firefighters.
Aviation, Space, and Environmental Medicine 74, 1151-1156.
3.
Punakallio, A., Hirvonen, M. and Grönqvist, R. Slip and fall risk among
firefighters in relation to balance, muscular capacities and age. Safety
Science (submitted).
4.
Punakallio, A., Lusa, S. and Luukkonen, R. (2004) Functional, postural
and perceived balance for predicting the work ability of firefighters.
International Archives of Occupational and Environmental Health
77, 482-490 (in press).
5. Punakallio, A. (2004) Trial-to-trial reproducibility and test-retest
stability of two dynamic balance tests among male firefighters. International
Journal of Sports Medicine 25, 163-169.
| ABSTRACT |
|
The
objectives of the present study were to investigate the associations
between balance abilities and age, occupation and the use of fire-protective
equipment (FPE) in different visual conditions, and the associations
of slip and fall risk with balance abilities among workers in physically
demanding jobs, especially among workers in fire and rescue work.
The reliability and predictive values of balance tests in respect
to perceived work ability were also studied. The professional firefighters
aged 30 to 56-years (n = 29-135), construction workers (n = 52),
home care workers (n = 66) and nursing workers (n = 51) aged 23
to 61 years participated in this study. The data were obtained with
balance tests with the use of a force platform, functional balance
tests, slipping tests and questionnaires. In one study the balance
tests were carried out with and without FPE. The slipping tests
with FPE were carried out on a straight 8-m long path that had one
area covered by water and detergent or glycerol. Perceived work
ability at baseline and after a 3-year follow-up was determined
with the use of the work ability index (WAI). In the reliability
study, the dynamic balance tests were repeated six times in two
testing periods at an interval of 2 months. The results indicated
that the balance abilities of firefighters over 49 years of age
were significantly poorer than those of firefighters in the age
groups of <40 and 40-49 years. The decline of balance abilities
among construction, home care and nursing workers was not as consistent.
Postural balance was also more harmfully affected among the older
firefighters (43-56 years) than among the younger ones (33-38 years)
by the use of FPE without visual input. Self-contained breathing
apparatus was the most significant single piece of FPE to impair
balance in both groups. Furthermore, fast and controlled performance
in the dynamic stability test based on visual feedback was related
to smaller slip and fall risk with FPE in both age groups. The older
firefighters tended to have longer and more serious slips than the
younger ones. In addition, the construction workers were significantly
faster and made fewer errors than the firefighters in the functional
balance test. Among the firefighters, poor performance on the balance
tests significantly predicted a reduced WAI after a follow-up of
3 years. The dynamic stability and functional balance tests showed
reasonable reliability, especially when the reliability was estimated
from the best of at least three repeated trials. The present results
suggest that balance abilities should be taken into account in follow-ups
of the work ability of firefighters, as well as in the organization
of work tasks and the development of the characteristics of FPE.
It is also essential to provide ample balance training opportunities
for firefighters with and without FPE. The balance assessments of
the present study can be included when prerequisites of work ability
are evaluated and followed-up for firefighters.
KEY
WORDS: Musculoskeletal equilibrium, posture, aging, occupations,
rescue work, protective devices, risk factors, occupational exposure,
comparative study, cross-sectional studies, follow-up studies, reproducibility
of results.
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| INTRODUCTION |
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The
ability to balance is a basic element of daily activity. Due to
the high balance demands, sufficient balance abilities are especially
important in physically demanding jobs, such as firefighting. Roof
work, smoke-diving or the handling of patients and heavy tools are
examples of typical tasks carried out in fire and rescue work, in
which good individual balance ability can be critical for safe and
efficient work performance. Temporary and difficult work conditions
and the use of protective equipment further increase the challenges
placed on the balance control system. Balance abilities play an
important role also in other dynamic physical occupations that include
the handling of heavy objects and locomotion in complex environments.
High balance demands of a particular type of work may also develop
balance abilities, but only a couple of studies have compared the
balance abilities of people in different jobs. For instance, construction
workers working on high buildings have been shown to sway less than
people not engaged in physical work (Gantchev and Dunev, 1978).
The workforce is aging globally. For example, the mean age of Finnish
fire and rescue, construction, home care and nursing workers is
39, 41, 45 and 41 years, respectively (Tilastokeskus, 2003).
Most studies of age-related differences in balance have shown that
older people are less stable than younger ones. In physically demanding
jobs, balance demands are, however, equally high for workers of
different ages. Moreover, age-related problems in balance control
may increase accident risk (Gauchard et al., 2001).
The risk increases when visual or proprioceptive inputs are disturbed
due to challenging work conditions. Fall victims have been shown
to have poor balance control, particularly with their eyes closed
(Vouriot et al., 2004).
Limited data are available about the age and balance abilities of
workers in specific physically demanding jobs. Thus far, only one
study, that of Pohjonen (2001a),
has reported the age-related declines in functional balance among
home care workers.
Furthermore, accidents requiring three or more days of sick leave
as a result of lose of balance because of slips, trips and falls
on level surfaces or falls and jumps from upper to lower levels
have been reported to account for considerable proportions of work-related
accidents among certain groups of workers in Finland, for example,
for 30% and 6% among firefighters, for 22% and 12% among construction
workers and for 25% and 3% among workers in health and social services,
respectively (Tilastokeskus, 1996-2001;
Tilastokeskus, 2002).
The corresponding proportions for Finnish workers in general are
clearly lower (20% and 8%) among men and higher (33% and 4%) among
women (Tilastokeskus, 2002).
Slips, trips and falls account for 20% to 40% of disabling occupational
injuries in Sweden, the United Kingdom, and the United States (Courtney
et al., 2001).
Moreover, compared with younger workers, workers over 45 years of
age have a greater number of slip- , trip- and fall-related accidents
(Kemmlert and Lundholm 2001,
Tilastokeskus, 2002).
Good balance control in relation to a specific task may also promote
health and work ability. Pohjonen (2001a)
showed that poor balance is a strong predictor of reduced work ability
among home care workers and suggested that, in addition to tests
of muscular capacities, balance tests should be included in evaluations
of the work-related fitness of home care personnel. Several field
and laboratory methods are available for evaluating balance abilities,
but their relevance and validity to evaluate balance among active
working populations with high balance demands have not been established,
and, in most cases, their reliability has not been studied according
to current recommendations.
This study aimed at investigating associations between balance abilities
and age, aspects of work demands and workers' safety in physically
demanding jobs, and it especially focused on firefighters. The reliability
and predictive value of balance tests in respect to perceived work
ability were studied also.
|
| REVIEW
OF THE LITERATURE |
Concept
of balance control and the measurement of balance
Balance is a complex motor skill that describes the dynamics of body
posture in preventing falling. Balance control can be examined from
neurophysiological, biomechanical, and functional perspectives depending
on the goals of the study. There is, however, no solid consensus regarding
the definition of balance control or globally approved "gold
standards" for measuring it (Berg, 1989;
Ekdahl et al., 1989;
Pollock et al., 2000).
Definitions of balance vary according to the scientific background
of the research team using them, and measurements depend on what information
is needed and why. The terms "equilibrium", "postural
stability" and "postural control" are used as synonyms
for balance control (Horak, 1987;
Karlsson and Frykberg, 2000;
Shumway-Cook and Woollacott, 1995).
From a neurophysiological perspective, balance studies involve the
interaction of different levels of balance control mechanisms, whereas,
biomechanically, balance can be defined as the ability (balance ability)
to maintain or return the body's center of gravity (COG) within the
limits of stability (LOS), as determined by the base of support (BOS)
(i.e., the area of the feet) (Horak, 1987;
Nashner, 1997).
Balance is related to the inertial forces acting on the body and the
inertial characteristics of body segments. Furthermore, the LOS are
the boundaries of an area of space in which the body can maintain
its position without changing the BOS (Nashner, 1997).
The ability to maintain COG within BOS is a typically used definition
of "static balance". The term "static" is, however,
imperfect, as it ignores the minor automatic adjustments that occur
continuously when a body maintains a stable position (Berg, 1989).
Furthermore, it is the same organ system that is involved in regulating
posture in static and dynamic conditions. Mechanisms and strategies
for balance control can act differently, however, in static and dynamic
tasks. For example, during quiet standing, balance is usually controlled
by the ankle strategy, whereas ankle muscle activity alone is insufficient
to maintain balance during walking (Winter, 1995; Woollacott and Tang,
1997).
This is one reason for the low correlations between static and dynamic
balance tests (Patla et al., 1990;
Shimada et al., 2003;
Tsigilis et al., 2001).
Definitions of static and dynamic balance may, however, be useful
when the character and goal of the tool measuring individual balance
ability is described.
In static balance tests, the aim is to keep the center of pressure
(COP) of the body as immobile as possible within the BOS during standing
or sitting (Woollacott and Tang, 1997).
COP can be calculated from the forces needed when maintaining balance
applied to the surface of a force platform (Hirvonen et al., 2002).
According to Hasan et al., (1996a)
COP is the position of applied force vector that is influenced by
the shear forces produced by body segment accelerations. Its displacement
is a reaction to body dynamics representing all the vertical forces
acting on the BOS. When two feet are in contact with a surface, COP
is situated between the feet and depends on the relative weight taken
by each foot. Whereas COP itself is easily quantified and directly
measured, body COG is not directly accessible (Hasan et al., 1996a).
COG (also referred as center of mass=COM) is the point at which the
vector of total body weight passes. It depends only on the displacements
of the body segments, but it is not influenced by the dynamics. Although
COG and COP occupy different roles in the balance control system,
according to Hasan et al., (1996b),
their amplitude and frequency measures are highly correlated. This
correlation supports the use of COP-based measures in quantifying
standing balance. In the present study, static standing balance tests
using a force platform are called postural balance tests.
As opposed to the goal of static tests, that of dynamic balance tests
is to actively move the COP while standing, walking or different tasks
of daily activities are performed. In this study, the term "dynamic
stability" refers to a person's ability to move the COP in a
given direction within the LOS in force platform tests. The feet of
the testee are not allowed to move. In general, LOS are not fixed
boundaries. They change according to the task, the person's biomechanics,
and environmental aspects (Nashner, 1997;
Shumway-Cook and Woollacott, 1995).
For example, in walking, the COG is kept within the BOS only during
short double-limb support periods (Woollacott and Tang, 1997).
In all dynamic movements, COG can move outside the LOS for a moment,
but BOS has to be changed to bring COG back within BOS; otherwise
a fall results. Some researchers have defined static and dynamic balance
measurements according to whether the support surface is stable or
movable. In dynamic posturography, the support surface can move in
a horizontal plane or pitch the person either forward or backward,
but the person tries to stand in place (Monsell et al., 1997).
Functionally directed balance tests are typically dynamic tests that
measure a person's ability to maintain balance as he or she walks
or performs tasks as fast as possible or reaches as far as possible
(Hertel et al., 2000;
Podsiadlo and Richardson, 1991;
Rinne et al., 2001).
Ideally, the aim of functionally oriented balance tests is to simulate
the tasks and actions of daily activities and work, because balance
is one of the baseline requirements necessary for these tasks.
Balance is an integral component of almost all daily actions. Stability
and orientation demands of balance changes with each task, and they
are higher for activities of greater force, velocity, or magnitude
(Berg, 1989).
Postural orientation is the ability to maintain an appropriate relationship
between the body segments and between the body and the environment
during a task (Shumway-Cook and Woollacott, 1995).
In addition to individual and task-related factors, environmental
factors and their interaction affect balance control (Figure
1) (Shumway-Cook and Woollacott, 1995).
Individual
aspects of balance control
Sensory components
The ability to control balance is dependent on sensory inputs from
somatosensory, visual and vestibular systems (Table
1). Information concerning the position and movement of body segments
with reference to each other and the support surface and the distension
of the respective muscles is provided through the somatosensory system,
the proprioceptors and the mechanical sensitivity of cutaneous and
subcutaneous tissue (Nashner, 1997).
Proprioceptors are located in muscles, tendons and joints, and they
include the following receptor systems: primary endings of muscle
spindles (type I), secondary endings of muscle spindles (type II),
the Golgi tendon organ and joint receptors (McComas, 1996).
Muscle stretching activates primary (type I) and secondary (type II)
endings of muscle spindles and releases a stretch reflex that monosynaptically
facilitates the agonistic muscles and inhibits the activity of the
antagonistic muscles (Noback and Demarest, 1981).
Muscle spindle type II (secondary endings) mediates the information
on the length of the muscles to the central nervous system (CNS) as
well. Increasing of the tension of the muscles also activates Golgi
tendon organs (Noback and Demarest, 1981).
The Golgi tendon reflex effect in respective muscles is opposite of
the muscle spindle reflex, namely the activation of Golgi tendon organs
causes the facilitation of antagonist muscles, whereas agonistic muscles
are inhibited (Prochazka and Wand, 1980).
These three receptor systems work in harmony by releasing segmental
reflexes and mediating information on balance changes to the CNS (Prochazka
and Hullinger, 1983).
Information is also derived from receptors located in the cutaneous
and subcutaneous tissue (Johansson and Vallbo, 1980)
of the sole of the foot. These receptors adapt either slowly or quickly
and they can detect changes in pressure, for example, postural sway,
and can react to the acceleration and magnitude of skin stretching
on the sole of the foot during standing and walking (Johansson and
Vallbo, 1980;
Magnusson et al., 1990;
Toppila and Pyykkö, 2000).
Furthermore, the visual system provides information about the body's
position and motion in relation to the environment. Vision has an
important role in balance control, but it is not essential because
it can also be compensated by other sensory inputs (Brandt et al.,
1986).
According to Brandt et al., (1986)
visual signals, which start postural corrections, seem to react to
motion as a relative image shift on the retina when visual surroundings
are stationary. Visual input is needed not only for the continuous
evaluation of head sway, but it also seems to trigger required muscle
activation in controlling postural perturbations (Brandt et al., 1986).
In general, the central area of the visual field, when compared with
that of the peripheral retina, is more important for balance control,
and the foveal region contributes powerfully, especially to lateral
postural sway (Paulus et al., 1984).
When the direction of movement is rapidly changed, it would be impossible
to maintain a stable image on the retina without some automatic control
mechanism to stabilize the direction of the gaze of the eyes. Therefore,
the purpose of the vestibulo-ocular reflex is to stabilize vision
by producing eye movements in opposite directions during the turning
of the head (Baloh et al., 1993;
Noback and Demarest, 1981).
Through this phenomenon the vestibulo-spinal reflex stabilizes the
whole body.
Input concerning the position of the head in relation to gravity,
as well as to motion through the linear and angular acceleration of
the head, is provided by the vestibular system (Noback and Demarest,
1981).
Vertical and horizontal semicircular canals sense rotational movement
of the head in the sagittal and frontal, as well as horizontal, planes,
respectively. Canals are the most sensitive to fast movements, for
example, those occurring during sudden slips and trips, and they detect
movement with a large dynamic range (frequencies 0.5-10 Hz) (Horak
and Schubert, 1994;
Toppila and Pyykkö, 2000).
Furthermore, the otoliths sense position relative to the earth's gravitation
axis and linear and slow (range 0.2-0.5 Hz) acceleration of the head
(Toppila and Pyykkö, 2000).
Saccular otoliths sense vertical linear accelerations of the head
(e.g., gravity), for example, head translations generated during deep
knee bends, whereas utricular otoliths sense horizontal linear accelerations
like head movements generated during forward walking (Horak and Schubert,
1994).
The input of otoliths and semicircular canals converges in the vestibular
nuclei in the same neuron, which also receives visual and proprioceptive
input (Toppila and Pyykkö, 2000).
Sensory strategies
Sensory strategies, which refer to the relative weight given to a
sense by the CNS, vary as a function of such individual aspects as
age, task and environment (Table
1). It has been suggested that, under normal conditions, the nervous
system weights the importance of somatosensory information for postural
control among healthy adults (aged 20-70 years) more heavily than
vision does, but, when reliable proprioceptive information is removed,
vision becomes more important to the maintenance of balance (Colledge
et al., 1994).
It has also been suggested that there may be a systematic change in
the multisensorial process, which controls balance throughout life
(Straube et al., 1988).
Young children (2-8 years) rely more on visual inputs (Straube et
al., 1988),
whereas pressoreceptor and proprioceptor systems are important for
balance control among children aged 6 to 16 years (Hytönen et al.,
1993).
Furthermore, the importance of vision in balance control increases
again among people over 60 years of age (Hytönen et al., 1993;
Straube et al., 1988),
and persons over 85 years of age become especially dependent on vision
(Pyykkö et al., 1990).
Vision, together with vestibular input, becomes especially important
with respect to compensation for continuously applied low-frequency
balance disturbances, for example, when the support surface is unstable
(Diener et al., 1986).
Furthermore, vestibular input is critical for balance control when
somatosensory and visual inputs are unavailable, as well as under
sensory conflict conditions (Allum et al., 1989)
(Table 1). According to Colledge
et al., (1994),
the vestibular system alone can only partially compensate, however,
for proprioceptive loss. Several studies concern the importance of
vestibular inputs in different aspects of balance control; for example,
Runge et al. (1998)
suggested that vestibular information is not criticalas regards selecting
and triggering hip strategy, although it may be meaningful in controlling
hip strategy in some environments. Because the environment is constantly
changing, the CNS also has to adapt according to information for multiple
sensory modalities in order for balance to be maintained. Therefore,
the most appropriate inputs have to be selected according to the requirements
of the task and environment (Shumway-Cook and Woollacott, 1995).
Integration of motor responses according to sensory input
According to sensory systems and demands of the situation, sensory
information is organized, and motor responses are chosen in the CNS
to stabilize posture or prevent a harmful change in it (Table
2). The ability to control balance emerges from a reciprocal action
of biomechanical, musculoskeletal and sensory systems and the CNS
(Nashner, 1997).
The CNS consists of the spinal cord and the brain. Several parts of
the CNS take part in posture control, and there are three motor systems
involved in balance control. The first and fastest response to a change
in posture is triggered by the myotatic stretch reflex (spinal
cord), which regulates contractile muscle forces (Noback and Demarest,
1981).
Reflexes are activated by an external stimulus and are highly stereotyped
(Nashner, 1997).
The earliest functionally effective responses to balance perturbations
are called automatic postural responses also referred to as
the long-loop reflex or the functional stretch reflex. Their postural
latencies (mean 94 ms and 120 ms for medium and long latencies, respectively)
are much longer than the spinal stretch reflex latencies (35-40 ms),
but they are shorter than voluntary reaction times (>150
ms) (Diener and Dichgans, 1986;
Nashner, 1997).
Automatic postural responses, which are mediated in the brain stem
and subcortical area, coordinate movements across joints (Nashner,
1997).
They are stereotyped, but adaptable. Like reflexes, automatic responses
are activated by external stimuli. Their responses can be thought
of as overlearned, "long-loop" reflexes that rapidly respond
by resisting disturbances (Diener and Dichgans, 1986).
Automatic responses depend on movement strategy, which in turn is
dependent on the experience of the person and on surface conditions
(Nashner, 1997).
Automatic reactions are also adaptable to specific balance demands.
Contrary to reflexes and automatic responses, voluntary postural
movements, mediated by the brainstem and cortical area, can be
initiated in response to an external stimulus, or they can be self-initiated
and generate purposeful movements and behavior (Nashner, 1997).
Postural adjustments associated with voluntary movements are organized
on the basis of internal representation (Massion, 1994).
In order to know when and how to apply restoring forces to keep the
COG within the BOS, the CNS must have an accurate picture of where
the body is in space and whether it is stationary or in motion (Shumway-Cook
and Woollacott, 1995).
Therefore, internal representations (provided by the postural body
scheme) of body geometry, body dynamics (support conditions) and body
orientation through sensory inputs are essential for the mapping of
sensation to action (Massion, 1994)
(Table 2).
Motor components
Balance control requires an ability to produce adequate muscle contractions
according to task demands. In simplifying the control demands for
the CNS, independent, although related, muscles are combined into
muscle synergies by the nervous system (Shumway-Cook and Woollacott,
1995).
Because the muscles act around the joints when balancing the body,
the role of ankle and hip strategies and their related muscle synergies
are especially important. The ankle strategy produces shifts in the
COG by rotating the body about the ankle joints. It elicits a distal-to-proximal
muscle activation of ankle, hip and trunk musculature (Horak and Nashner,
1986).
The ankle strategy uses compensatory ankle torques that are believed
to correct for small postural perturbations on firm support surfaces
(Nashner, 1997).
Therefore efficient use of the ankle strategy depends on accurate
sensations from somatosensory inputs (Horak et al., 1990).
The hip strategy controls movement of the COG primarily by flexing
and extending the hips, and it uses early proximal hip and trunk muscle
activation (Horak and Nashner, 1986).
Hip strategy occurs when the ankle is unable to exert the appropriate
torque necessary to regain balance, for example, when the support
surface is smaller than the feet or is compliant or narrow when perturbations
are large and fast or when the body COG is near the limits of stability,
as it is during walking (Nashner, 1997;
Horak and Nashner, 1986).
The third strategy used to achieve balance is the stepping strategy,
which is used when the COG is displaced outside the BOS. The stepping
strategy uses early activation of hip abductors and ankle co-contraction
(Horak and Nashner, 1986).
The muscle strategies are not as stereotyped as reflexes are. They
can also be learned with experience in new environmental contexts
(Horak and Nashner, 1986).
When conditions are intermediate, between favoring the use of the
ankle or hip strategy, and when a person must adapt to a new surface
condition, for example, to control balance on a flat support as the
magnitude of postural perturbations increases, the use of combinations
of these strategies (mixed strategies) is common (Runge et al., 1999).
Effects of age and sex on balance control
Decrease of balance abilities with aging
An age-related decline in balance abilities has been shown in several
cross-sectional studies of postural sway and functional balance among
working-aged populations and the elderly (Colledge et al., 1994;
Du Pasquier et al., 2003;
Ekdahl et al., 1989;
Era and Heikkinen, 1985;
Gill et al., 2001;
Matheson et al., 1999;
Pohjonen 2001a;
Røgind et al., 2003;
Straube et al., 1988).
Children below 10 years of age and elderly people over 60 years of
age have the most pronounced postural sway (Hytönen et al., 1993;
Pyykkö et al., 1988;
Sihvonen et al., 1998).
Some investigations have shown that differences in postural balance
abilities in a normal standing position are minor between the ages
of 17 and 54 years (Sihvonen et al., 1998)
or that no correlation exists at all between age and postural sway
(Juntunen et al., 1987)
or that body sway is the most stable among 46-to-60-year-old people
(Hytönen et al., 1993).
When the balance demands of a test are increased (i.e., the BOS is
smaller or balance is tested on a compliant surface), when the eyes
are closed, or, especially, when the eyes are closed on a compliant
surface when two sensory cues are affected, some studies have shown
detected balance declines to be greater between younger people and
the elderly (>60 years) and between different age groups of the
working population (comparisons made for people below and over 40
years of age), and others report that, among working-aged subjects,
declines can only be detected (Colledge et al., 1994;
Era and Heikkinen, 1985;
Gill et al., 2001;
Matheson et al., 1999;
Straube et al., 1988).
It should, however, be remembered that comparisons of the age-related
results reported in different studies are complicated due to differences
in the methods, the selection of the subjects, the different age ranges
of the subjects and the study design. Moreover, most studies on the
relationship between balance and age are cross-sectional in design
(Colledge et al., 1994;
Ekdahl et al., 1989;
Era and Heikkinen, 1985;
Gill et al., 2001;
Hytönen et al., 1993;
Kollegger et al., 1992;
Matheson et al., 1999;
Pohjonen, 2001a;
Røgind et al., 2003;
Sihvonen et al., 1998;
Straube et al., 1988),
and a cross-sectional design is not ideal for obtaining an understanding
of the aging phenomenon. In longitudinal studies findings of age-related
changes have, however, confirmed the results of cross-sectional studies,
and balance deterioration in people over 75 years of age has been
shown to be even more pronounced (Baloh et al., 1998;
Du Pasquier et al., 2003;
Era et al., 2002).
Despite the considerable amount of research available on age-related
differences in balance, very little is known about age-related differences
in the balance of workers in specific physical jobs.
Age-related changes in senses, reflexes and automatic muscle responses
Changes in balance control mechanisms due aging take place at different
levels of the balance control system (Woollacott et al., 1986).
There is, however, no solid consensus concerning mechanisms contributing
to these changes (Colledge et al., 1994;
Laughton et al., 2003).
In addition, most of the studies of mechanisms contributing to age-related
balance changes have dealt with elderly subjects (mean age about 70
years) and young controls of about 20 years of age. Age-related deficits
in peripheral sensory systems are important factors, as an elderly
person's balance is more altered than that of younger adults when
visual and proprioceptive or both inputs are eliminated or reduced
(Matheson et al., 1999;
Shumway-Cook and Woollacott, 2000;
Straube et al., 1988;
Woollacott et al., 1986).
For example, the following changes take place in the senses as a result
of aging: older people have higher proprioceptive thresholds (Stelmach
and Sirica, 1987)
and anatomical changes take place in the semicircular canals, the
saccule and the utricule of the inner ear (Johnson and Hawkins, 1972).
Furthermore, visual acuity declines with age, depth perception and
contrast sensitivity are poorer among the elderly, and older people
progressively lose their peripheral vision (Cohn and Lasley, 1985;
Gittings and Fozard, 1986).
Age-related changes in the aforementioned sensory inputs could decrease
the redundancy of sensory information that is normally available and
can, therefore, make a shift in the relative weighting of inputs less
effective, depending on the environmental demands (Woollacott et al.,
1986).
Colledge et al. (1994)
studied 20-to-70-year-old healthy adults and suggested that the relative
contributions of sensory inputs to balance do not, however, alter
with advancing age.
Age-related changes in the long latency automatic postural response
systems characterize reduced muscle coordination (e.g., temporal breakdown
of distal and proximal muscle activation) and increase the absolute
latency of distal muscles within a muscle response synergy (Woollacott
et al., 1986).
The variability of the relative constancy of the contraction amplitude
of distal and proximal synergies also increases with aging (Woollacott
et al., 1986).
The latency of the monosynaptic stretch reflex, which is the lowest
level of the balance control hierarchy, increases in the achilles
tendon with age (Carel et al., 1979),
whereas in the patellar tendon no significant differences have been
found between 20- and 60-year- old men (Clarkson, 1978).
Furthermore, Woollacott et al. (1986)
found impaired integration of sensory inputs among older subjects.
Changes in higher integrative mechanisms
In their study Stelmach et al. (1989)
found that elderly subjects adapt more poorly to repeated small-slow
balance perturbations, which activate integrative mechanisms through
sensory inputs, than young controls did, although the two groups responded
similarly to large-fast rotations (elicit reflexive postural responses).
The continuous control of balance is, however, highly dependent upon
the quality of the proprioceptive information, and its integration
with visual and vestibular information, and it is not based upon reflex
mechanisms (Woollacott et al., 1986;
Stelmach et al., 1989).
Therefore, older people are at some disadvantage when balance is under
the control of slower, higher level sensory integrative mechanisms
(Stelmach et al., 1989).
More recent studies have also provided additional evidence that higher
integrative levels dominate in the decline of postural control (Colledge
et al., 1994;
Rankin et al., 2000;
Shumway-Cook and Woollacott, 2000).
For example, an enriched sensory context is not necessarily related
to more stable behavior among elderly people because the reintegration
of ankle proprioceptive input causes faster postural sway both with
the eyes open and with them closed (Teasdale and Simoneau, 2001).
Therefore, in addition to decreased peripheral acuity among older
people, their sensory reweighting process is limited by the capacity
of the central integrative mechanisms that reorganize the hierarchy
among the sensory inputs (Teasdale and Simoneau, 2001).
Colledge et al. (1994)
also suggested that the increase in body sway demonstrated with normal
aging is more likely to be due to the slowing of central integrative
processes than to altered peripheral sensibility.
Furthermore, people have shown poorer balance when they perform a
simultaneous cognitive task than when they do a balance task only
(Shumway-Cook et al., 1997;
Shumway-Cook and Woollacott, 2000).
Balance during a simultaneous cognitive task is more affected among
the elderly than among younger persons when the accuracy of visual
and somatosensory inputs is reduced (Shumway-Cook et al., 1997;
Shumway-Cook and Woollacott, 2000).
A recent experiment of Rankin et al. (2000)
demonstrated that, especially among older people, muscle activity
declines significantly in the gastrocnemius and tibialis anterior
muscles if balance is measured when a math task is performed concurrently.
These findings suggest that the elderly have a lower attentional processing
capacity for balance control during a dual-task paradigm.
Age-related changes in neuromuscular synergies and muscle function
With respect to output, the slowing of peripheral nerve conduction
velocity and the decrease in the number of motor units may also be
related to balance changes among the elderly (Leonard et al., 1997).
Furthermore, elderly people have been found to have mixed hip-ankle
activation when the BOS is narrowed during standing, whereas young
subjects adapt to increased postural demands by using the ankle strategy
only (Amiridis et al., 2003).
Okada et al. (2001)
also showed that older people rely more on hip movements to control
balance while young controls rely on ankle movements. The greater
hip muscle activation in the elderly may be caused by insufficient
torque production of the ankle muscles, which is needed to counteract
the great moment of inertia in the anteroposterior direction (Amiridis
et al., 2003;
Kuo and Zajac, 1993).
According to Kuo and Zajac (1993)
the hip strategy is the most effective in controlling the COM with
minimal muscle activation.
The greater hip muscle activation in the elderly may also be caused
by greater loss of motor units in the distal than the proximal muscles
or an insufficient proprioceptive contribution (Amiridis et al., 2003;
Stelmach and Sirica, 1987).
The elderly have also been found to have a greater amount of muscle
activity during postural sway in a quiet stance than younger persons
do (Laughton et al., 2003).
The authors concluded that it is, however, unclear if high muscle
activity of the legs precedes greater postural instability or if increased
muscle activity is a compensatory response to increases in postural
sway (Laughton et al., 2003).
Effects of sex on balance control
In some studies, men have been reported to sway more than women (Ekdahl
et al., 1989;
Era et al., 1996;
Juntunen et al., 1987;
Kollegger et al., 1992;
Maki et al., 1990;
Matheson et al., 1999).
Other studies have reported contradictory findings (Panzer et al.,
1995)
or no difference between the sexes (Colledge et al., 1994;
Hageman et al., 1995;
Røgind et al., 2003).
Moreover, no results of functional balance tests (walking time, standing
on one leg) have depended on sex (Ekdahl et al., 1989).
It has been shown, however, that standardizing the balance results
by the length of the base of support or body weight removes the difference
between the sexes (Era et al., 1996;
Maki et al., 1990).
Effects of vision on balance control
It is well known that, in the eyes-closed condition, the velocity
and amplitude of postural sway is higher than in the eyes-open condition
(Colledge et al., 1994;
Matheson et al., 1999;
Stelmach et al., 1989).
For example, postural sway was shown to increase two- to threefold
with the eyes closed in all age groups of men and women aged 15-25,
45-55 and 65-75 years and also among men aged 31-35, 51-55 and 71-75
years (Era and Heikkinen, 1985;
Gill et al., 2001).
The increase in sway in a normal standing position after the eyes
are closed is clearer in the anteroposterior direction (Era and Heikkinen,
1985).
Paulus et al. (1984)
showed that especially anteroposterior sway increased gradually as
visual acuity decreased. Slower speed and smaller amplitude of COP
movement in postural balance tests are associated with better visual
acuity among elderly men and women (Era et al., 1996).
Work
task demands and balance control
Characteristics of task-related balance control demands in physical
jobs
Physically demanding work includes several complicated tasks in which
the perturbation of balance is expected. The expected threat to balance
also causes anticipatory postural adjustment (APA) (Belen'kii et al.,
1967; Cham et al., 2002; Commissaris and Toussaint, 1997a; Cordo and Nashner, 1982; Marigold and Patla, 2002). APA activates the postural muscles and actively initiates
movements, which in advance counteract the possible disturbances of
balance associated with a voluntary task and locomotion. For example,
pushing a rigid handle while standing is associated with tibialis
anterior activation, which precedes the onset of the handle force
signal (Cordo and Nashner, 1982). This APA compensates for the body displacement induced
by the voluntarily performed handle movement.
In physical jobs APA are needed in respect to work safety, but it
is as important in respect to the fluency and efficiency of work (Belen'kii
et al., 1967).
The anticipatory activation of muscles before a voluntary task is
associated with the need to maintain balance a the minimum expenditure
of energy (Belen'kii et al., 1967).
According to Zettel et al., (2002) the CNS is able to use exteroceptive
visual input to alter balance control parameters in an anticipatory
manner, even when the characteristics of the forthcoming perturbation
cannot be predicted in advance. When the conditions in which a task
is performed change, the preparation for the movement also changes
in a way that ensures balance in the new situation (Belen'kii et al.,
1967). Balance adjustments and the sequence in which postural
muscles have been activated are also task-specific (Belen'kii et al.,
1967;
Commissaris and Toussaint, 1997a;
Toussaint et al., 1997).
The work tasks that challenge balance control the most among firefighters
are associated with work with ladders, on roofs and when smoke-diving
(Gledhill and Jamnik, 1992a;
Lusa et al., 1994).
Tasks demanding good balance abilities are involved in work on scaffoldings
and roofs among construction workers (Hsiao and Simeonov, 2001).
Furthermore, in nursing and home care work, as well as in construction,
firefighting and rescue work, manual lifting of clients or construction
material and the handling of heavy tools are essential parts of the
work. For example, the clients of municipal home care workers in Finland
are frail and elderly, and they need help with daily living activities
several times a day (Pohjonen, 2001b).
Lifting a load in front of the body creates a risk of falling forward
because adding the extra mass causes the COP to shift forward in relation
to the BOS (Commissaris and Toussaint, 1997b).
Pan et al. (2003) examined postural stability in association with
four lifting methods commonly used by drywall installers and carpenters.
They found that vertical lifting of drywall sheet placed greater demands
on their subjects' balance control (i.e., higher COM accelerations
and greater postural sway) than horizontal lifting of drywall. To
minimize the effects of balance-threatening events during lifting,
preparatory actions immediately before a load, placed in front of
the toes in lifting is grasped, are characterized by a lower forward
rotational velocity, a clear increase in the backward-directed horizontal
momentum of the body COP and a backward-directed horizontal force
vector, as well as forward displacement of the COP (Commissaris and
Toussaint, 1997b;
Toussaint et al., 1997).
During lifting the electromyographic (EMG) activity of ankle plantar
flexors increases considerably before contact with the load (Commissaris
and Toussaint, 1997b).
In one study, if the weight of the lifted load was reduced unexpectedly
(16 kg to 6 kg), balance was disturbed in most of the trials (Commissaris
and Toussaint, 1997a).
In general, the APA scales the amplitude of adjustment according to
the size or amplitude of the expected perturbation (Shumway-Cook and
Woollacott, 1995).
When a worker overestimates the weight of a load to be lifted, an
unnecessarily high linear and angular momentum of the body occurs.
This momentum can lead to disturbed balance and possibly to a fall
(Commissaris and Toussaint, 1997a;
Toussaint et al., 1997a).
Moreover, Aruin et al. (1998) suggested that higher instability during
task performance can also be expected to lead to smaller APA because
the CNS makes a logical and deliberate choice to help decrease the
probability that the APA itself will produce postural instability.
Effects
of protective equipment on balance control
The use of both bulky tools and protective equipment increases the
demand for highly developed and flexible balance skills (Hsiao and
Simeonov, 2001).
A worker's balance control can be affected by specific items of protective
equipment, such as footwear, clothing, eyeglasses and respirators
(Hsiao and Simeonov, 2001).
For example, shoes act as a sensory interface between the foot and
the BOS. The properties of the shoes affect the functional limit and
the slip resistance of the BOS and the sensitivity of the foot according
to extent, friction, firmness and incline of the surface (Hsiao and
Simeonov, 2001).
The soles of a person's shoes considerably affect the frictional properties
of the shoes (Grönqvist, 1995).
It has been shown that shoes with thin hard soles provided good walking
stability, whereas shoes with thick, soft soles reduced foot position
awareness and destabilized the walking stability of men of different
ages (Robbins et al., 1997;
Waked et al., 1997).
Protective eyeglasses, masks and other face or head protectors can
form a sensory interface to the visual system. They restrict the peripheral
visual field, and therefore protective eyewear may have harmful effects
on balance (Samo et al., 2003).
Respirators and protective clothing can also influence balance control.
Firefighters have to carry out tasks with fire-protective equipment
(FPE), consisting of specialized clothing and a self- contained breathing
apparatus (SCBA), at least a few times each year (Lusa et al., 1994).
Furthermore, firefighters need to be able to work safely with FPE,
while still maintaining sufficient physical capacity for the most
demanding tasks. However, the use of FPE also has negative effects
on performance. For example, for submaximal work in a thermoneutral
environment, the use of standard European FPE (Committee of Standardization
1995)
with SCBA weighing 15 kg increases cardiorespiratory strain by 20%
(Louhevaara et al., 1984),
and it also significantly increases thermal strain (Ilmarinen and
Mäkinen, 1992).
The strain caused by SCBA is partly due to the weight of the equipment
(Louhevaara et al., 1984),
which, together with increased heat stress, may also affect postural
stability (Kincl et al., 2002).
Kincl et al. (2002) showed that standard United States FPE impaired
postural stability in terms of sway length and sway area, especially
after physical loading (sustained squatting). These findings indicate
that wearing a heavy respirator during demanding physical work may
disturb balance (Kincl et al., 2002;
Seliga et al., 1991).
Previous studies on postural sway and FPE have, however, used subjects
in a narrow age range (24-34 years) (Seliga et al., 1991)
or failed to examine age-related effects (Kincl et al., 2002).
Although the effects of European FPE (Committee of Standardization,
1995)
on the cardiorespiratory system have been well quantified (Ilmarinen
and Mäkinen, 1992;
Louhevaara et al., 1984),
no data are available on balance control.
Differences in the balance abilities of workers in various occupations
In general, only a few studies have dealt with balance control in
different occupations. According to Kohen-Ratz et al., (1994)
fighter pilots demonstrate superior and more-mature postural control
than candidates for flight training, whereas helicopter pilots show
intermediate balance values. Although helicopter pilots are also highly
selected, they represent a group not chosen as fighter pilots (Kohen-Ratz
et al., 1994).
Diard et al. (1997) also reported that pilots on active duty had significantly
better performance in dynamic posturography than former fighter pilots,
firefighters and a control group of the general working population.
Nurses showed better performance in a Flamingo balance test than a
reference group of adults with various occupations, whereas public
servants scored better than nurses (Zinzen et al., 1996)
and construction workers had better stability than workers not engaged
in physically demanding work (Gantchev and Dunev, 1978).
Reported differences in balance abilities could be caused either by
an innate ability or by training and learning (Kohen-Ratz et al.,
1994).
Furthermore, dancers are found to be able to minimize COG displacement
towards the supporting side when raising one leg laterally to an angle
of 45 degrees in response to a light (Mouchnino et al., 1992).
Compared with naive subjects, dancers reach the new COG position faster
and require only a short adjustment period. Especially under sensory
challenged conditions professional dancers were better than controls
in maintaining their balance in a one-legged stance (Crotts et al.,
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