Evaluation of the human stability in bipedal position for person with neuromotor disabilities.

Baritz, Mihaela ; Cristea, Luciana

Abstract: This paper presents some considerations about human body stability in bipedal positions. Our studies are made with people, with neuromotor disabilities, in different conditions and we used an experimental configuration with a Kistler force plate, high-speed video-cam and some medical apparatus to establish and record pre- and post-medical treatment of this person. In the final part of the paper are presented the results and analyzes of these information's.

Key words: human stability, force plate, biomechanics.

1. INTRODUCTION

Stability is defined mechanically as resistance to both linear and angular acceleration, or resistance to disruption of equilibrium. Different mechanical factors affect a body's stability. (Kent Van der Graaff, 1998)

According to Newton's second law of motion (F=ma), the more massive an object is, the greater is the force required to produce a given acceleration. The greater the amount of friction is between an object and the surface or surface it contacts, the greater is the force requirement for initiating or maintaining motion.

Another factor affecting stability is the size of the base of support. This consists of the area enclosed by the outermost edges of the body in contact with the supporting surface or surfaces. When the line of action of a body's weight (directed from the center of gravity (CG)) moves outside the base of support, a torque is created that tends to cause angular motion of the body, thereby disrupting stability, with the CG falling toward the ground. The horizontal location of the CG relative to the base of support can also influence stability. The closer the horizontal location of the CG is to the boundary of the base of support, the smaller is the force required to push it outside the base of support, thereby disrupting equilibrium.

Alternatively, if a horizontal force must be sustained, stability is enhanced if the CG is positioned closer to the oncoming force, since the CG can be displaced farther before being moved outside the base of support. The height of the CG relative to the base of support can also affect stability. The higher the positioning of the CG, the greater the potentially disruptive torque created if the body undergoes an angular displacement.

Although these principles of stability are generally true, their applications to the human body are made only with the recognition that neuromuscular factors are also influential

2. EXPERIMENTAL ASPECTS

The experimental setup proposed by this paper it is presented in the fig.2. (Manual for force plate Kistler, 2006)

To record the stability of the human body we use a Kistler force plate with 4 (four) piezoelectric sensors to measure the forces and the moments from 3 (three) directions.

Also a very good and performing computer it is necessary to be used because there are many and bigger recordings of the response signals from acquisition system and also from high-speed video-cam.

[FIGURE 1 OMITTED]

In these activities we establish a compact methodology to record the data in different conditions and for different persons. In the first time we're recording the physiological information's about weight, high, blood pressure, temperature, oxygen quantity in the blood, lactic acid, quantity of glucose and pulse.

Each person participated to this investigation was recording in three daily time (morning, afternoon and evening) to have all kind of information's about the variations of these parameters in the day time or about the values for different human body weight.

In these two initial conditions (with big and with small contact base) the recording are made in two stimulating situations: with eyes open and with eyes close and also we want to know how the stability of human body will change if the arms are near body or they are in lateral or in the front of human body. All these recordings are made with the setup configuration presented in the fig.1 and using BioWare software.

[FIGURE 2 OMITTED]

The mechanical behavior of a body subject to force or forces is greatly influenced by the location of its center of gravity CG: the point around which the body's weight is equally balanced in all directions. A body's mechanical stability is its resistance to both linear and angular acceleration. We can assume and observe that a number of factors influence a body's stability, including mass, friction, center of gravity location, and base of support.

When a body is motionless, it is in static equilibrium. The three conditions of static equilibrium are: [SIGMA][F.sub.v] = 0; [SIGMA][F.sub.h] = 0 and [SIGMA]T = 0. A body in motion is in dynamic equilibrium when inertial factors are considered. Mechanically, muscles and bones function as levers. Most joints function as third-class lever systems, well structured for maximizing range of motion and movement speed, but requiring muscles force of greater magnitude than that of the resistance to be overcome. The angle at which a muscle pulls on a bone also affects its mechanical effectiveness, because only the rotary component of muscle force produce joint torque. (Baritz M, 2002)

3. RESULTS

In the following graphs we present the recordings made with the same subject with neuromotor disabilities (before and after a rehabilitation treatment) but in the same conditions (position with hands, day time, the same physical conditions etc.)

Test no.1--before rehabilitation treatment, big support, open eyes, hands near body:

[FIGURE 3 OMITTED]

Test no.2--before rehabilitation treatment, big support, close eyes, hands near body:

[FIGURE 4 OMITTED]

Test no.3--after rehabilitation treatment, big support, open eyes, hands near body:

[FIGURE 5 OMITTED]

Test no.3--after rehabilitation treatment, big support, close eyes, hands near body:

[FIGURE 6 OMITTED]

4. CONCLUSIONS

From these recordings and in according with the initial conditions and the demands of the researches we can conclude:

--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;

--the 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;

--the influence of the position of the eyes are also the most important because the instability will be bigger in the open eyes position than the closed eyes position. This situation is due of the visual or audio external stimulus.

--the medical rehabilitation treatment affect the stability of the person with neuromotor disabilities and these are the most important aspect in our research.

Acknowledgements

This work was a part of a Research Grant A1088/2005-2007 financed by CNCSIS Romania.

5.REFERENCES:

Kent Van der Graaff (1998).Atlas anatomie, V-th edition, McGraw-Hill, 1998

Baritz M. (2006)--Studiul profilelor speciale prin metode complementare (The study of the special shape by compementaire methods), Ed. Infomarket 2002, Brasov;

Manual for force plate Kistler, (2006)