Human body posture evaluation in working activities.
Baritz, Mihaela ; Cristea, Luciana ; Cotoros, Diana 等
1. INTRODUCTION
Human body stability is one of the most complex mechanisms for the
interaction of the human body with the environment and tools or devices
in working activities. Control of standing position and the stability
behaviour of the whole body is one of the most outstanding problems in
human working activities. Normal working process causes many changes to
neuromuscular system of a human being restricting his stability
capabilities. It is very important to study the changes in the working
activities of subjects especially when these activities are daily
automatic movements. Because these changes sometimes result in an
increase the number of back pain and neuromuscular disorders or falls
during daily walking it is very important to analyze the influence of
the hands cycling movements to the hall human body stability (static and
dynamic). Many researchers studied stability of human body and it was
quoted that human body stability decreases with the increase of working
time and with the age increasing the risk of falls in worker people.
Some research is being done on the portable system to calculate the
kinematics parameters in the environment, but it requires many sensors
attached to the body, which should be linked to a laptop computer. To
use the human body in stability analyzes it is necessary to study, in
the first step of analyze, the anthropometrical dimensions of the
locomotor system and make measurements on the body segments. Also, for
stability analyzes it is necessary to locate the centre of mass of
segment, and then the weight of each segment can easily be calculated.
Averaged density of the whole body is: d=0,69+0,9 (h/[w.sup.1/3]).
Segment mass expressed by the percentage of the total mass is:
M = [[summation].sup.n.sub.i=1] [m.sub.i] (1)
Using the definition of centre of mass (COM) like the point where
the entire weight of the body is concentrated, or the point in a body
about which all the parts exactly balance each other, it can be possible
to distinguish the centre of mass from the centre of gravity (COG) or
from the centre of pressure (COP). To establish the position of the COM
there are some different techniques, one of them is the suspension
technique--in this case a body segment is suspended in a frame from only
one point and then the point where the gravity effect is equalled
represents the location of the centre of mass.
[FIGURE 1 OMITTED]
Precise location of COM is depending on: individual's
anatomical structure; habitual standing posture; current position and
external support. The most important is that the location of COM remains
fixed as long as the body does NOT change its shape; location in human
body generally accepted that it is located at ~57% of standing height in
males and ~ 55% of standing height in females COM varies with body
build, posture, age, and gender also the weight for infant > child
> adult (in % of body height from the floor).
(http://www.pt.ntu.edu.tw)
2. THEORETICAL ASPECTS
Because the human body is not a homogeneous structure, in activity
of establishing the evolution of stability in automatic movements it is
very important to calculate the centroid. The centroid is the point that
defines the geometric centre of a human body. If the material composing
a body is homogeneous, the weight can be neglected, i.e. centroid
coincides with point COM. But the human body is not homogeneous; there
is a centre of gravity (COG). To understand and to analyze the stability
of the entire human body it is important also to make a classification
of equilibrium in: stable equilibrium occurs when an object is placed in
such a position that any disturbance effort would raise its COM and
tends to fall back in its original position; unstable equilibrium occurs
when an object is placed in such a position that any disturbance effort
would lower its COM and tends to fall into a more stable position; and
neutral equilibrium occurs when an object is placed in such a position
that any disturbance effort would not change the level of its COM and
tends to fall into a more stable position. (Farrell, 2005) To maintain
the equilibrium and stability in bipedal position it is necessary to
study the factors that can affect stability like: size and shape of base
of support (BOS); wide-base stance; tandem stance: standing with one
foot ahead the other; stance with crutches; height of COM; relationship
of COG to BOS; mass of body; friction; segmental alignment; sensory
input: visual, vestibular system, pro-prioception. (Stirling &
Zakynthinaki, 2004)
3. EXPERIMENTAL SETUP
The experimental methodology structure proposed by this paper needs
to start the investigations by knowing the initial human parameters and
the human behavior of different physiological parameter are recorded and
analyzed. It is important to have the physiological parameters recorded
and the human model to obtain the answer of the interaction between
human body and environment, during the working activities with hands
cycling movements. During the first step of investigation we're
recording the physiological information about weight, height, blood
pressure and pulse, oxygen quantity in the blood, lactic acid and
quantity of glucose and temperature. The persons participated to this
investigation were monitoring three times daily (morning, afternoon and
evening) to have all kind of information's about the variations of
these parameters in the day time or about the variations of values for
human body weight.
[FIGURE 2 OMITTED]
To record the forces and moments developed in stability of the
human body we use a Kistler force plate and record the ground forces
variations when the human subject achieves an cycling movement with his
hands from position 1 to position 2 (fig.2.). In the same time we use a
high speed and thermo-vision video cameras to record the small movements
of the human body when its stability is recorded and also the
temperature gradient into the body. For that a very good and performing
computer is necessary to be used because there are many and large
recordings of the response signals from the acquisition system and also
images (movies) from high-speed video-cam and information from
electro-medical apparatus. In the next step of research it was necessary
to establish the simple theoretical models for the static, kinematical
and dynamical study of the human stability. The main and the most
important differences between both models concern to the role played by
lower limbs. The kinematical model neglects the mass of lower limbs and
all the attention are concentrated around oscillations around the axis
of sagital plane. In the dynamic model, the leg mass is translated in a
coupling between two dynamic organizers. In this way, it is possible to
incorporate inertial phenomena, which contribute to reinforce the
marginal stability around support surface. Inertial effects are crucial
for a balanced analysis between anticipatory and compensatory movements.
(Baritz, 2002)
The geometrical aspects are determined by the possibility to
restrict the analysis only in the sagital plane Oxz. Also the human body
stability and the simplest legs motions are generated at rotational
joints located at hip, knee and ankle and the trunk axis is supposed at
the upright position, at starting with the maximal length for leg. Other
aspects establish that the ranges for rotations at each rotational joint
are quite different between them. The dynamic aspects put in evidence
that the contact with the ground of the legs is inelastic, constant and
it induces discontinuities for parameters controlling the kinematics and
dynamics. The important aspect for stability is the friction effect at
the foot with the ground and the stability at different action moment is
guaranteed by the Zero-Moment Point (ZMP) to base support. (Vaughan et
al., 1999)
4. RESULTS AND CONCLUSIONS
In the following graphs it is presented the recordings made with
the same subject, in the same conditions (position of hands, day time,
the same physical and environmental conditions etc.) follow up the
initial situations presented.
He was standing for 30 sec on the force plate doing a set of
automatic movements with his hands from position 1 to position 2, making
almost 30-45 simple movements. The stability of the human body of this
person was recorded when he stand with the feet on big and small base of
support and the top of the feet in line with the trunk.
[FIGURE 3 OMITTED]
From these recordings and in according with the initial conditions
and the demands of the researches we can conclude: that he most
important force values are the components from direction Oz because they
can establish the amplitude of the balance (moments) in other two
directions Ox and Oy. Also the changes in foot position have been found
to affect measurements of standing balance, force and stability surface
and in normal conditions the size of the support is a primary determiner
of stability.
Other influences were the light stimulus on the visual system
because they are the most important stimulus induced the instability
that will be bigger in the open and fixed oriented eyes position than
free gaze even the optical stimulus was the same. This situation is due
of the unknown visual external stimulus reactions and concentration on
the automatic activities.
The results from these researches activity will be continued to
establish the model of investigations for people with simple, normal
posture or with disabilities because it is necessary to estimate the
influence of its medical treatments in the rehabilitation process or to
establish the working comfort for ergonomic places.
Also we will develop a system to evaluate the people in sitting
position by measuring the stability behavior with light stimulus in
known or unknown environment conditions and also to measure the thermal
gradient develop into human body during working activities correlated
with ergonomic aspects of working places.
5. ACKNOWLEDGEMENTS
Researches are part of Grants IDEI 722/2009 and IDEI 744/2009 with
CNCSIS, Romania.
6. REFERENCES
Baritz, M. (2002), Study of the complex shape by complementary
methods, Infomarket Publisher, Romania, ISBN 973-8204-26-7
Farrell, K. (2005). Kinematic human modeling and simulation using
optimization-based posture prediction, PhD Thesis, University of Iowa
Stirling, J. & Zakynthinaki, M., (2004) Stability and the
maintenance of balance following a perturbation from quiet stance.
CHAOS, Vol. 14, No. 1 March
Vaughan, C. L & al .(1999) Dynamic of human gait, Kiboho
Publishers, Cape Town, South Africa ISBN 0-620-23560-8
*** http://www.pt.ntu.edu.tw, accessed 2008-06-12
