Load on the lumbar spine of flight attendants - North Wales Spine ...

866
2.3. Indicators of lumbar load
Computerized calculations for quantifying measures of
lumbar load were performed which were based on the
recorded data on posture and exerted hand forces, both
varying in the course of a push-or-pull action, as described
above. Individual stature and body mass, ranging between
the 5th and 95th percentiles of the sample ‘‘German flight
attendants’’, were considered (cf. Schaub et al., 2007).
Several mechanical indicators of lumbar load, i.e. the
bending and torsional moments of force or the compressive
and shear forces at the lowest intervertebral disc of
the spine, were predicted applying a previously developed
‘‘biomechanical model’’. As common in comparable
ergonomic investigations, the lumbosacral disc ‘‘L5–S1’’—
the transition between the 5th lumbar and the 1st sacral
vertebrae, the latter representing the upper part of the
sacrum—was chosen as the reference point due to its
relatively high disease prevalence and biomechanical overexertion
risk. The indirect method of biomechanical
modelling was applied, since direct determination via
invasive measurement of mechanical indicators of lumbar
load such as the intradiscal pressure (e.g. Nachemson and
Elfstro¨ m, 1970; Wilke et al., 1999) cannot be performed in
ergonomic investigations for ethical reasons.
2.3.1. Biomechanical modelling
The basic approach, the underlying procedures and
details on the modellings of the skeletal and muscular
structures, of intra-abdominal-pressure efficacy and of
inertia effects are described formerly (cf. Ja¨ ger et al., 1991,
2001); a few aspects provided in the following may give a
sketchy insight.
A spatial dynamic multi-segmental biomechanical model
(‘‘THE DORTMUNDER’’) was used for quantifying the lumbar
load during trolley handling. The human skeletal structure
in this analysis tool is represented by 30 rigid body
segments, 14 of which are located within the trunk
according to the location of the intervertebral discs
between pelvis and shoulder-joint height (T3–T4); in the
superior joint, a neck–head segment is connected. Individual
postures are replicated via angle variation in up to 26
joints, which particularly enables the imitation of realistic
spinal curvatures via sagittal and lateral flexion as well as
twisting. Foot, lower and upper legs, shoulder, upper arm
and forearm as well as hand are included in the model
bilaterally.
The muscular structure in the lower trunk region which,
in particular, is spread over the lumbar discs, is replicated
by the effect of 14 muscles or muscle cords in total: the
Longissimus Thoracis and Iliocostalis Lumborum as
relevant cords of the Erector Spinae muscle group at the
back, and the Rectus abdominis as well as the medial and
lateral parts of both the External and Internal Obliques at
the frontal side. Their functional effects correspond to nine
‘‘muscle equivalents’’ or ‘‘resultant force vectors’’ in
anatomically justified distances. The mathematical problem
ARTICLE IN PRESS
M. Jäger et al. / International Journal of Industrial Ergonomics 37 (2007) 863–876
of redundant muscle force distribution is solved via a linear
optimization technique.
The effect of trunk stabilization according to intraabdominal
pressure is considered in the model by referring
to the measurements of Morris et al. (1961) and the
consecutive evaluations of Chaffin (1969). For purposes of
validation, for example, specific electromyographical measurements
were performed, the underlying top-down
principle in THE DORTMUNDER was compared to
corresponding bottom-up application, and, where possible,
results of modelling were verified by findings from
measurements of lumbar intradiscal pressure (Nachemson,
1966; Andersson et al., 1977; Wilke et al., 1999).
The model was applied ‘‘quasi-statically’’, i.e. the action
forces during trolley handling included the inertial effects,
however, acceleration of body segments’ masses remained
unconsidered in this study. A sample of 468 out of 3,410
usable actions was analysed with regard to the prediction
of several mechanical indicators of lumbar load. Selection
was performed arbitrarily in principle, but under the
restrictions to receive data for each of the 48 different task
configurations in total and to reject trials involving
uncommon handling performance with respect to posture
(e.g. twisted), force exertion (e.g. single-handed) or, in
some cases, gliding foot.
2.3.2. Evaluation criteria
The mechanical load on the lumbar spine during trolley
handling is described on the basis of force and moment of
force acting at the lumbosacral disc. For both indicators,
classification approaches provided in the literature are
applied, as customary in ergonomics and occupational
health in the assessment of activities involving lumbar load.
Based on total values for the vectorial quantity ‘‘moment
of force’’, Tichauer (1978) provides moment categories with
relevance to the selection and training of working persons or
to the adherence of rest brakes. This classification is based on
decades of experience of load analyses in ergonomic and
biomechanical studies with a special emphasis on muscle
physiology and lumbar-load model calculations. The limit
values 40, 85 and 135 N m serve for defining the respective
classes. According to Tichauer’s specifications, a criterion of
85 N m was chosen for the evaluation of moment values
resulting for flight attendants while trolley handling. The
properties ‘‘good body structure’’ and ‘‘some training’’ were
attributed to that group of working persons, whereas
‘‘selective recruitment of labour’’ regarding musculoskeletal
or cardiopulmonary performances, ‘‘careful training’’ and
‘‘attention to rest pauses’’ were estimated as too severe
conditions. An incorrect, too insensible categorization would
be carried out, if flight attendants—according to the 3rd
category of Tichauer—were described as ‘‘untrained’’ or
‘‘irrespective of body build’’. In total, the 85-N m criterion
was applied to the sagittal–moment component, since this
direction clearly dominates the total moment in the analysed
spectrum of tasks, i.e. pushing or pulling on a straight way
(Glitsch et al., 2004).