Ergonomical study regarding working in standing and seating postures.
Voinescu, Mihai ; Davidescu, Arjana ; Argesanu, Veronica 等
1. INTRODUCTION
Most studies related to human ergonomics and energy consumption in
various postures are generally based on experimental determination and
less on mathematical models. The ever improving area of software related
to human ergonomics has given the possibility of further understanding
of the actual activity in the muscles and the causes of fatigue. Such
mathematical models can be used to optimize the overall processes that
involve certain human body postures for prolonged periods of time and
thus give better productivity and less fatigue or perhaps distribute
fatigue evenly and reduce the risk of certain "professional"
back and shoulder disorders.
Such mathematical model research has been done mostly in the
automotive industry for the purpose of obtaining optimal seating
conditions for the average population of certain areas, but the
importance of studying human body mechanics goes far beyond and can
extend to any field in which involves human operators.
Regarding the working environment in the field of the dental
physician, most data available on ergonomics is based on observation and
on personal experience of the physicians themselves. Most of the
work-related physical problems are, in general, only discovered after
the harm has already been done and in most cases the need for resting
the damaged muscles and tending to the affected part leads to the need
of a certain period of less to no activity for the physician. (Hokwerda
et al., 2007).
Because of these problems, a great deal of research has been made
on the mechanics that cause fatigue and pain in different muscle groups.
Doctor Oene Hokwerda's work in the field of ergonomics for dental
medicine is impressive and offers a great deal of information regarding
the correct theoretical postures and on the possible injuries that might
occur over a prolonged period of time.
2. GENERATING HUMAN BODY MODEL POSTURES AND PRELEVATION OF DATA
The major problem in such an undertaking is building a suitable
virtual environment and most of all an accurate model of the human body.
Because of this, the models used have been selected from human physiques
that Aalborg University has previously constructed, with precise muscle
definition and insertion points for applications that involve human
movement.
[FIGURE 1 OMITTED]
All these models are available in the standard demo package that
can be used in conjunction with the ANYBODY human body simulation
software. Starting from a standing human model, using pre-defined
muscles and bone attachments, and building both seated position and
standing position scenarios, eight simulated situations, five seated and
three standing have been developed.
For the seated position, the chair was added virtually through a
node that offers a stabile platform for the pelvis region. The angles
for the legs were obtained from an ideal theoretical position for the
purpose of minimising their involvement in the general muscle activity
of the body system.
The work space of the arms was obtained from measurements of
rotation and position during the physician's activity (Hokwerda et
al., 2005). All simulated positions have been designed for similar hands
and shoulders activity. The overall angle differences for the hands in
the eight simulations is minimised so that the general difference in the
total muscle activity is given by the other muscles. This allows for a
clear view of the influence of the different postures on the system.
All activities include certain tensions in the hands given by an
external load. Because of this factor, all models have forces attached
to the nodes belonging to each of the hands. This ensures that the data
output is similar to that which would be obtained from a real life model
and further adds to the accuracy of the model.
[FIGURE 2 OMITTED]
Generally, all postures considered have the same work routine and
are designed in such a way that only the patient's position changes
and the body model has to adapt to that change. As such, certain
positions required a change in posture to ensure proper balance. A few
of the models required a certain adjustment of the position of the knee;
for this purpose, a rotation of the upper part of the legs was
conducted. For this rotation, the outward rotation angles were
considered positive and those inward negative.
All movement patterns were carefully studied for muscle collision
and kinematical correctitude; after all data was considered viable, the
next phase of the study--using inverse dynamics, was conducted. The data
was then extracted from the output of the program for the various muscle
groups that were of interest (shoulders and arms, legs, general muscle
activity). The most relevant data was considered the overall muscle
fatigue per cycle investigated; in this way, a clear analysis of the
movement can be done referring to all muscles and not just individual
parts of the body model. Muscle fatigue (Activity) is defined by the
ANYBODY solver as muscle force divided by strength (anybodytech.com,
2007).
Some extreme situations were taken into consideration to allow the
prelevation of data regarding object placement in the work space
(Hokwerda, 2008); seated body rotations for various reaches left and
right. The data was taken in a fixed time interval of 10 seconds per
movement and the data can be multiplied by the assumed times a physician
has to repeat the same move, giving us an overall muscle activity per
work day.
One very important aspect of this study is the taking into account
of the feet in the positioning of the body and thus, its equilibrium.
Most previous work disregards leg muscles because of the seated
position, but, as the results show, these muscles are not entirely
inactive while seated and play a great role in equilibrium particularly
in positions involving lateral bending of the trunk. The muscle models
used are both simple and Hill-type muscles. Previous work (Dragulescu,
2005) has shown the advantages of Hill-type muscle models over the
simpler models that only take into consideration the presumed strength
of the muscle; for this reason, the leg muscles have been selected as
Hill-type to better model their involvement in the movement.
[FIGURE 3 OMITTED]
3. RESULTS AND CONCLUSIONS
[FIGURE 4 OMITTED]
After a careful examination of the data and statistical analysis, a
clear distinction between the energy consumption for seated compared to
standing position became apparent. Even the extreme positions offered
better energy consumption and less stress for the back and shoulders for
most of the movement. Clearly the seated position is better in regard to
the overall energy consumption, as was indicated by previous work.
The data resembles the results of previous studies done by
observation of real situations and is both a validation for previous
work and an improvement of the available data. Furthermore, by using the
general data from each of the eight movement paths investigated, a
maximum allowed bending and rotation of the thorax can be established.
The drawback of the method is the lack of experimental investigation
using real life subjects.
Additional studies can be conducted by replicating the motion
trajectories using test subjects and heat sensors to detect muscle
activity from the thermal variations in the body. The final result of
further study can be the improvement of the layout of dental equipment
in the dentist's workspace. The numerical model obtained can also
be used to read data from various muscle groups, determine better work
trajectories, and perhaps even ergonomically improve the way certain
tools are gripped and handled.
Another aspect of the study is the possibility of pointing out the
individual muscle strain for the various shoulder and arm muscles. The
impact of such data can lead to improvement of movement ergonomics and
development of specific work training that can relieve the tension
generated during certain stages of physical activity.
4. REFERENCES
Dragulescu, D. (2005). Modelarea in Biomecanica, Editura Didactica
si Pedagogica, ISBN 973-30-1725-6, Bucuresti
Hokwerda, O (2008). Ergonomic objections against a unit-cart on the
right or left side of the patient chair, Available from:
www.esde.org/docs/ergonomic_objectioins_against_a_unit
_or_cart_next_to_patient_cha.pdf Accessed: 2009-01-25
Hokwerda, O.; Wouters J. & de Ruijter, R. (2007). Ergonomic
requirements for dental equipment, Available from:
www.optergo.com/images/Ergonomic_req_april2007.pdf Accessed: 2009-01-15
Hokwerda, O.; de Ruijter, R. & Zijlstra-Shaw, S. (2005).
Adopting healthy sitting posture during patient treatment, Available
from: www.optergo.com/uk/images/Adopting.pdf Accessed: 2009-01-14
*** (2007) www.anybodytech.com/index.php?id=691--ANYBODY technology
tutorials, inverse dynamics analysis, Accesed on: 2009-03-20
Tab. 1. Major rotational angles
Pos. Thorax Thorax Knee
rotation bending adjustment
1 70[degrees] 0[degrees] 3[degrees]
2 70[degrees] 30[degrees] 6[degrees]
3 0[degrees] 30[degrees] 3[degrees]
4 0[degrees] 45[degrees] 10[degrees]
5 45[degrees] 25[degrees] -10[degrees]
6 20[degrees] 40[degrees] 1[degrees]
7 30[degrees] 50[degrees] 2[degrees]
8 0[degrees] 40[degrees] 1[degrees]
Pos. Max. Seated
Activity Yes/No
1 0,39 Yes
2 0,98 Yes
3 0,31 Yes
4 0,36 Yes
5 0,55 Yes
6 0,40 No
7 0,44 No
8 0,33 No
