ch 2-amphoux physiopathological aspects of personal equipment for protection against falls

8/10/2019 Ch 2-Amphoux Physiopathological Aspects of Personal Equipment for Protection Against Falls

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hapter

PHYSIOP THOLOGIC L

SPECTS

OF

PERSON L EQUIPMENT FOR PROTECTION

G INST F LLS

Maurice Amphoux

INTRODU fION

When the possibility of standardizing personal equipment for protection against falls

was envisaged some years ago

t

seemed necessary to go beyond the traditional view

of the problem namely requiring that the components have mechanical strength

combined with a large safety coefficient. t was not sufficient to be sure of finding the

accident victim caught on the structure. t was apparently important to make sure

of

finding him free of injury

or

at least to try to minimize the consequences of the fall so

that wearing personal protection equipment would represent a significant and proven

benefit compared with free fall.

The problem thus raised meant finding as a first step the possible circumstances of

falls n the construction industry and the job requirements that had to be respected so

that the recommended devices could be used.

Editorial note This paper was first presented at the Specialist meeting 11 Personal Protective

Equipment Agaillst aLls from heights held

11

March 7 to 19 1982 ill Paris,

France. It was subsequently published

ill its original, French version titled

"Protectioll individuelle cOlllre les chutes - Aspects physiopathologiques" ill

the collectioll ofpapers from the meeting Proteclioll individuelle confle les

chutes de hauteur - Rencontre d'experts

17-19

Mars 1982".

Publisher: Comite NatiollaL de l'Orgallisme. Professiol1nel de Prevention du

Batiment et des Travaux Publics, Tour Amboise, 204, Rond-Poin du Ponl

de-Serres,

92516

Bou[ogne-Billancourt, France.

33


2/16

At the time, the French regulations required limiting the fall distance to 1 metre.

When analyzed, this requirement did not appear to correspond to any reality. In a

certain number

of

cases, the fall could be much shorter, but it had to be accepted that

the protective equipment permitted the worker to move upright on the highest solid

element

of

the structure.

n

this case, the lanyard can only be attached below the feet

of

the subject on the permanent part

of

the structure. The worker can only be attached

to this anchoring point by a lanyard, connected, moreover, clearly above his own center

of gravity, and preferably to the upper part of his trunk, for reasons we shall refer to

later. n these conditions, the lanyard must be at least 2 metres long Fig. 1) and the

total fall distance cannot be less than 4 metres.

t

was therefore necessary to come up

with equipment capable

of

accepting these kind

of

falls.

fig

That meant, first

of

all, that the mechanical strength

of

protective equipment must

be sufficient. But

it

also meant that the victim

of

the fall should not suffer any injury

either from his equipment

or

his fall.

Detailed analysis

of

the phenomenon showed that, from the biomechanic point

of

view, to which we shall confine ourselves

in

this article, the injuries could have three

types

of

causes.

First, the body gripping device, holding the body

of

the worker, had to rest on the

most solid parts

of

the body and

in

no way pose a threat to the vital organs, both during

the fall and when

it is

arrested and during the more

or ess

prolonged passive suspension

that may then occur until the victim

is

rescued. This equipment also had to be tolerated

4


3/16

without discomfort during work-and indispensable condition if

it is

to be worn every

time.Secondly, the maximum arrest force

(MAF)

acting on the subject at the instant

of

the arrest of fall must be compatible with his corporal integrity and the energy acquired

by the subject during the fall and which he must absorb when it

is

arrested, must not

reach the level where injuries become possible.

Finally, the arrest of the fall

is

equivalent to a negative acceleration with mainly

vertical component, and this acceleration must remain compatible with the limited

physiological possibilities

of

the subject. This acceleration itself has two rather different

aspects which are, first, the global acceleration felt by the body, and secondly, the

accelerations of the various individual parts of the body which have quite different

moments of inertia and trajectories.

Those

are the main points that

we

shall touch

on

in this article. But it

is

clear that

this analytical study could only be carried out because

we have

simultaneously studied

and experimented on all these points, as a team, with the engineers of the

OPPBTP,

the

CEBTP

and the INRS,

who

developed the aspects pertaining to their

own

dis

ciplines.

The

physiological and pathological considerations mcntioned here cannot be

isolated from this technical context without which they would hardly have any value.

The ergonomy of

a fall arresting system, like any ergonomic study, can be the result

only

of

a multidiscipline approach.

We

are pleased to point

out

here that the collabora

tion achieved enabled us to overcome the sometimes very profound contradictions and

opposition typical of this delicate subject, with the result that

it is

now possible to make

a coherent rcport on it.

BODY GRIPPING DEVICES

The

traditional

waist belt

The

waist belt used to be recommended as the only body gripping device.

t was

a

wide belt in artificial textile

or

even leather, fitted with

0

rings. Its characteristics

were covered in a German standard, DIN

7470. t

could resist stresses of several tonnes,

we

would say now several tens of kilonewtons. Providing the rope

or

the chain to which

it

was attached was of similar strength,

it

offered every guarantee from the mechanical

point of view. t could not be broken by effect of a fall, even a much higher fall than

the 1 metre stipulated in the regulations.

But the workers generally refused to use it, feeling, intuitively, that in the event of a

fall they would

be

cut

in

two . In fact, tests

of

simple suspension (Fig. 2) have

shown

that the situation was not tolerable for more than a few seconds and that the abdominal

organs, liver and spleen

in

particular, were directly threatened.

35


4/16

Fi .

Any possible fall with such equipment therefore had to be excluded. Moreover, it

was obviously over-dimensioned and straps with comparable resistance but much

lighter were available on the market. The tongue system only had to be replaced with

a fastening system based on overall tightening, which would eliminate the risk o

longitudinal tearing by the tongue. On the traditional belt the edge o the hole, even

fitted with a grommet, was by far the weakest point, and on some models even a very

weak point.

The belt with shoulder straps

We therefore turned to a much lighter device based

on

that currently used

by

mountaineers. The belt was much lighter and fitted with two short thin shoulder straps

which allowed it to be kept under the armpits, on the lower part o the rib cage. n the

event o a fall or suspension, the pressure was taken on the rib cage, which is much

more resistant than the abdominal wall Fig. 3).

Fig. 3

36


5/16

In addition, the fastening was farther away from the subject's center

o

gravity and

the final position appeared much more natural . Tests on an anthropometric man

nikin, the same one that was used for tests

o

car belts, convinced us that this device,

much lighter than the belt, was also much more comfortable and we made a few tests

on humans. Tolerance was perfect

in

falls

o

up to 50

or

60 centimetres-

or

at least it

appeared perfect until the day when, at heights and in fall conditions which had not

previously causes any trouble, two rib fractures occurred, showing that the risks

o

injury had not been eliminated, and that falls from greater height could be dangerous.

At the same time simple suspension tests lasting a few minutes showed up another

risk: there was a tendency to faint suddenly with sudden acceleration

o

the pulse and

breathing, which was clearly shown by the recording

o

heart beat and breathing carried

out both at

INRS and by us.

n practice, however, it could not be expected that the victim

o

a completely

unforeseen accident could be rescued

in

such a short time and therefore suspension by

a simple chest strap had

to

be eliminated.

The physiological explanation

o

this phenomenon was not obvious. The apparently

innocuous thoracic compression could have permitted diaphragm breathing

as

the

abdomen was not compressed. n fact, it was accompanied by a muscle blocking

o

the

whole rib cage and therefore led to a progressive asphyxia which was not felt by the

subject until the fainting trouble appeared.

We therefore tried to remedy this inconvenience by making shoulder-belts with

multiple straps and complete vests, in the hope

o

distributing the thoracic compression

better and facilitating abdominal breathing (Fig. 4). None

o

the tests were satisfactory,

even when they prevented too much local pressure on the thorax

or

the armpits.

ria. 4

7


6/16

The Location

o

the D ring

At the same time as the preceding studies there was a certain urgency about the

problem o the location o the D-ring on the body-gripping device.

The position

o

this point seemed to condition the distribution

o

forces when the

fall was arrested, among the various parts

o

the body.

Lateral fastening allowed or even favoured by the location

o

the D-rings on the belt,

was quickly eliminated.

Transverse bending

o

the spinal column appeared inacceptable, as it has to bear the

weight

o

the lower limbs and all the stress is taken by the small, not very mobile joints

which the vertebrae have between the bases

o

the transverse apophyses. In fact the

frequency

o

fracture at this not very resistant point Fig. 5) is well known. Only stresses

in the axis o the body appeared acceptable. Then the very solid posterior ligament is

stretched and the bony elements o the spine are much less threatened.

ria ,

There remained two possibilities, at the level o the thorax, D-ring in the front or at

the back.

Front location was very tempting.

In

some circumstances, e. g. mountaineers, it may

actually be convenient for the user

to

be able

to

reach

his

D-ring directly, change it

adapt it. But this location has, however, two major inconveniences.

8


7/16

First the face o the wearer s directly threatened, in the event

o

a fall, by the rope

which passes n front o him, then the inevitable stretching o the straps through the

knot and the O-ring (Fig. 6).

ria. 6

Secondly, and this seems much more serious, the head will be thrown backwards

when the fall

s

arrested and will suffer sudden deflection, which considering its own

mass, cannot be damped, either by the weak muscles at the front o the neck, or the

not very resistant posterior articulations, or by any anatomic obstacle. Therefore, there

s a great risk o whip-lash , with all the serious problems that may cause: fractures o

the posterior arches

o

the cervical vertebrae, injury to the medulla or to nerves.

We concluded that this frontal O-ring location should be avoided and that the most

favorable situation was fastening on the back. There the rope threatens only the back

part

o

the skull, and then only very slightly. Forward deflecting

o

the head, which

s

much more natural because o the position

o

its center

o

gravity, will be less sudden,

limited by the chin resting on the sternum and possibly checked by the very solid

muscles o the posterior paravertebral grooves. The risk

o

injury will therefore be

considerably reduced and we thoroughly recommend this mode o fastening. (Fig. 7).

ig

9


8/16

Parachutists whose competence in this field

is

indisputable and on whom we have

relied to a large extent confirmed that our position

is

well founded. The movement

obtained with the dorsal D-ring location is that imposed

on

parachutists at the moment

their parachute opens so that the head is not thrown backwards which could cause

fracture

o

the cervical vertebrae.

Moreover the fastening

o

a parachute consists o four straps at shoulder level which

reduces further the stresses affecting the head but makes the equipment more

complicated and heavier. This particularly favorable solution seems difficult to apply

in practice in equipment used in construction but any solution that comes close

to

it

must be encouraged.

The

arness

The above considerations led to the conclusion that the only acceptable solution was

a complete harness once there is a risk

o

falling more than about 10 cm and

o

being

suspended longer than a few minutes while awaiting rescue.

A complete harness consists essentially

o

shoulder straps and a strap under the

buttocks and other straps arranged so as

to

keep the first ones

in

the proper place

in

all circumstances.

The essential element

is

the strap under the buttocks. t rests in fact on the pelvis

the most rigid and solid element o the human frame and

is

cushioned by large are

as

o flesh.

Prolonged suspension can thus become quite comfortable and tolerable without

trouble for several hours.

No element

o

the harness must be allowed to interfere with breathing

or

create

painful pressure on any part

o

the body. That is more difficult to achieve than appears

at first sight.

In

fact buckles rings additional straps are inevitable so that the whole

device

is

kept in the proper position even when the straps have become stretched by

the tension applied to them in the fall. Moreover the morphology

o

the users varies

greatly and even assuming the harness

is

made

in

several sizes adjusting devices are

still necessary. The example

o

the parachutists is not convincing in this case as their

suspension cannot be prolonged and they have no experience

o

that.

We therefore carried out suspension tests with strict electrocardiogram and visual

control

on

a number o harness models worn by different persons. Details o these

tests are given in Chapter 1

t

appeared that each model proved dangerous for at least

one

o

the test persons after

5

to 20 minutes and inversely that each test person could

tolerate the various models differently and one at least caused trouble within the time

indicated. Therefore in spite o a considerable improvement

in

the situation the use

o present harnesses including a model for parachutists does not allow suspension

without risk for more than

5

minutes. This means that each time that a worker has

to

use this type o equipment the necessary facilities must be provided for rescue

in

the

shortest possible time in the event o a fall.

4


9/16

From the point o view o arresting falls the strap under the buttocks is also a very

favorable element. The maximum stress in the case o falling feet first will be exerted

on a specially resistant part

o

the body. However the spine as a whole will then be

subjected to compression and not

to

tension. Its mechanical strength is then not so

great. Crushing o the vertebrae injury to intervertebrae discs and fractures o the

apophyses may occur through stresses which as we shall see later are in no way

impossible and that will make additional measures necessary. The existence o curva-

tures o the vertebrae particularly in man and the possibility

o

anomalies in these

curvatures cyphosis and scoliosis which are harmless

in

persons otherwise

in

good

health aggravate the risks. However the limits that must not be exceeded do not differ

very much from those

in

the case

o

vertebral traction. Moreover the most fragile zone

is

the cervical region and this

is

affected in all cases in similar bending conditions. On

the whole therefore the situation is clearly more favorable than with chest straps.

However falls are not always feet first and the possibility

o

a fall being arrested

when the head

is

down cannot be eliminated. The shoulder straps become the essential

element in this case. Repeated measurements have actually shown that the tipping o

the body to reach its final position in suspension on the strap under buttocks only

occurs after the first shock which is the one with maximum intensity.

The shoulders are affected first and the resistance o the shoulder straps and their

dimensions must be such that the stress

is

bearable for this region which is less solid

and has less muscle than the buttocks region but which on the other hand is more

supple and more suitable for diffusing some

o

the energy.

With respect to the vertebrae the mechanisms o vertebral compression will be very

similar to those in the previous case except for the cervical column where the relative

bending movement

o

the head

is

then mainly due to the rotation

o

the rest

o

the

body around the attachment point and involves the head secondarily. The deflection

is perhaps less sudden but the movement may not be in the most favorable axis and

the local accelerations may be considerable as we shall see later.

t nevertheless seems difficult to eliminate completely such possibilities but even

though their harmfulness does give cause for concern their probability appears rather

slight.

Therefore from the point

o

view

o

making the body gripping device meet the

requirements

o

anatomy and physiology only a harness and a harness which

is

well

designed and carefully tested can answer the problem raised. The other solutions

could at the most only continue to be tolerated as

is

the case

in

many countries as

accessories for devices preventing any fall o more than some tens

o

centimetres and

in working conditions where the victim can regain his balance at once and avoid

remaining in suspension. n all other cases a harness is necessary all the more so as

the energies and accelerations in question then make it just as indispensable as do the

requirements for supporting the body.

4


10/16

INJURY THRESHOLD

The stresses suffered by the human body when a fall is arrested are the consequence

of the obligation to dissipate the energy acquired by the worker and his equipment

during the free fall.

The entire Fall Arresting System has elastic properties and, consequently, is able to

absorb part of the energy, so that the human body itself absorbs only a part of the total

energy.

We have therefore made a series of tests with a mannikin recording the maximum

stresses at the anchoring point, which gives values around 12,000 N in the above

mentioned conditions.

Similar measurements were,

in

fact, also carried out many times during parachute

drops in order to estimate the level of stress that can

e

tolerated by the body and

it

was found that 12,000 N was an upper limit which was already extremely dangerous for

a young, well-trained body, well prepared to face the shock of opening of a parachute

by contracting his muscles to the maximum and thus keeping his body in a particularly

favorable position.

In fact, the values usually recorded when a parachute opens are very much lower and

it

was calculated that the subject exerted on the straps of his harness a force

of

the

order of 3,750 N with the old techniques

of

folding the parachute as first canopy , and

that that force dropped to 2,250 N with the introduction in the 60 s of folding it s first

rigging line , for military jumps.

That force

is

applied essentially to the strap under the buttocks and, if the position

of

the parachutist

is

satisfactory, it

is

exerted more or less

in

the axis

of

the spine (Fig.

8) which will suffer a compression stress at all levels, especially at the level

of

the cervical

part and the joint between the neck and head. The latter must at that moment be bent

forward, chin resting on the chest, in preparation for the shock.

Fig. 8

42


11/16

n these conditions the risks are very limited. Fractures of the spine of the order of

1 for 10 000 jumps are usually the result of incidents on landing rather than on opening

of

the parachute. But other types

of

disabling vertebral injuries are 5

or

6 times more

frequent. The fact that the change in the mode of folding the parachute reduced the

frequency shows that even for such levels of energy injuries are still possible. f course

they are then the result very often of faulty posture technique opening when the

relative position of the body and the axis of the parachute can direct the stresses in a

less favourable direction.

That however is precisely the usual situation in the case of a worker who falls. in

fact the accident will be an absolutely exceptional unforeseeable phenomenon

psychologically an experience that should never happen. The persons involved are of

all ages and all physical constitutions and have obviously not undergone any special

training.

The

curvature of their spine and their paravertebral muscles are not neces

sarily

in

perfect conditions.

Moreover during the few tenths of a second that the fall will last the reflexes do not

have the time to react and it is totally excluded that the worker can react prepare

himself for the shock modify his position in any way and contract his muscles.

Finally the fall is not necessarily feet first and vertical to the anchoring point and

consequently the relative positions of the body and the lanyard at the moment

of

shock

can present any configuration.

We are therefore far from the usual conditions of the parachutist but the exceptional

character of the fall nevertheless allows us to be less demanding about it harmlcssness.

t was all the more important to be able to reduce these demands as the reduction of

stress when the fall is arrested can only be obtained by increasing the braking distance.

The

space available below the anchoring point may be small and it is important

therefore to limit this braking distance as much as possible a requirement which

conflicts with the previous one but is compulsory considering the usual conditions of

use

of

FAS.

A compromise

th

erefore had to be found related also to the tolerance limits for

acceleration. With respect to forces it seemed reasonable

to

propose a limit of 6 000

N

i e. half the quantities considered dangerous

in

the case

of

parachutists a particular

ly favorable case and double that commonly met in this sport with a risk considered

acceptable for a repetitive activity.

The proposal also takes into account the knowledge acquired in the special field of

mechanical strength of the various elements making up the vertebral column. There

are mainly two types of elements some rigid: the vertebrae the others more elastic:

the intervertebral discs with the cartilagious part their fibrous part and their central

liquid core. In the case of an arrest of the feet-first fall the body is supported

on

the

strap under the buttocks the shock wave is transmitted almost exclusively by this bone

system the soft parts providing gradual damping first through the buttock muscles

then from disc to disc from the first sacral vertebra up to the head. That is a relatively

favorable situation as the crushing resistance of the vertebrae gradually diminishes

from the lumbar region to the dorsal region then to the cervical region and the known

rupture limits should never be reached.

4


12/16

n

the event

of

a head-first fall - the situation

s

less favorable, but

t s

probable that

the shock wave reaches the vertebral column damped more by the shoulders/rib-cage

system than by the buttocks/pelvis system.

There remains now the slightly worrying problem of the increased stressing of the

posterior articulations

of

the vertebrae, which are much less resistant and much less

elastic than

the

intervertebral discs, in the case

of

falls

n

intermediary positions

or

when the vertebral column has accentuated curvatures

or

dissymmetries.

Therefore, the proposed compromise

of

6,000 N seems to us, from the point

of

view

of the stresses suffered by the human body in general, and by its most affected parts,

to be a limit that must not be exceeded, and that can only be achieved with a shock

absorber which also influences the accelerations.

ACCELERATION

During the free part

of

the fall, the vertical acceleration downwards, n -Gz

s

obviously that of gravity. t s only when the cause

of

the accident s an external

mechanical impulse (moving bucket, traps, etc.), that a non-vertical component can

occur, adding the risks

of

initial shock and pendulum movement.

The most dangerous moment

s

when the fall

s

arrested.

e

have mentioned above

the necessity

of

reducing as much as possible the braking distance, for reasons

connected with the circumstances in which F AS-s are used. Here too the compromise

to be found will have to come close to the reasonable limits of tolerance

of

acceleration,

which in this case are negative downwards, in - Gz, and therefore positive upwards,

n

Gz if we refer to the environment of the subject and the vertical of the place.

For the subject in suspension, at the end of the arrest of the fall t will be an

acceleration very close to his

own

vertical axis Z. This depends on how the harness

s

constructed and the position

of

his fastening point. But the maximum acceleration peak

can be directed according to any axis of the subject, depending

on

the relative position

of the body and the lanyard at the moment the latter reaches its maximum extension.

We must therefore consider limits

of

bearable acceleration for the human body,

whatever the direction

of

its application.

The effect

of

the acceleration on the body

s

complex.

t

has been studied

n

the

course

of

research on car accidents and

on

the ejection

of

fighter pilots

n

emergencies.

An

essential point

s

that the different segments and organs

of

the human body have

different inertial characteristics and that their acceleration through the force applied

at

one

point

of

the body,

s

only obtained gradually through the more

or

less rigid

connections between the parts.

Therefore, the solid internal organs, like the heart and the liver, will tend to come

loose and break their moorings under the external shock. Similarly, the brain and the

cerebral column could be destroyed by being crushed against the walls

of

the skull or

the edges

of

the occipital opening. Finally the limbs could suffer tearing stresses at their

joints.

The changes of acceleration called, the jolts, will therefore be one of the elements

determining injuries, and the data usually available n terms of acceleration will not be

enough to estimate the risk.


13/16

Another important element, however, will be the duration of the phenomena or,

what

is

the same up to a certain point, the size of the displacement. Thus, accelerations

of

several thousands

of

m/s2 have been tolerable for a thousandth

of

a second, while if

the duration approaches a second, the maximum tolerance in Gz is

of

the order of 50

m/s2 Fig. 9).

,.

,

ao

or

I ~

\

\

to

,

0 - - - - - - - -

Fi

9

10

o , 10

.

10 U lO

40

TlU, - 'UI:Cco .o t

The phenomena that we are interested

in

are relatively extended

in

time.

n

fact, the

rope, harness and human body have a certain elasticity and the arrest of the fall cannot

be immediate. n addition, to limit the mechanical force it was necessary to add a shock

absorber to the device, calibrated at 6,000 N as proposed earlier. That increases the

time that elapses between the full extension of the lanyard and the complete arrest of

the body until the whole system has reached its final extension.

5


14/16

But that increase in the breaking time

is

offset by a considerable lowering of the

accelerations imposed and their variations, at least with some shock absorber. This

means excluding any system containing too many elastic elements, which restore at the

end

of

the elongation part

of

the absorbed energy in the form of a thrust in +Gz, and

increasing accordingly the MAF, the relative accelerations and the duration of the

whole phenomenon.

In practice, however, a large acceleration peak cannot be prevented from occuring

very briefly at the moment when the lanyard

is suddenly tightened at the end

of

the

free fall and the shock absorber begins to take effect, to stop the fall. The measurements

obtained during our tests vary much more than the stresses. In fact, the position of each

element of the system, lanyard, harness straps, body and limbs of the mannikin can be

very different and the appearance of the phenomenon can only be roughly reproduced.

However, we have recorded resultant accelerations of the order of

150

to

200 m s

2

for

durations

of

the order

of

0.02

to

0.04

sec,

i

e. a jolt

of

the order

of

6 km/s3.

for T

APP

j

who is the ~ r e t expert on the subject, the maxima tolerable are 350

mls

at 5 kmls

and 120 m s at 1.3 km/s3. We are therefore quite within the area of acceptable limits

and the use of any device which would increase either the value of the acceleration, or

the duration of the phenomenon, should be prohibited.

That is a second reason, as important as the first, for requiring the use

of

a shock

absorber when the fall distance exceeds a few tens of centimetres. It is very probable

that the fracture of the ribs mentioned above, which occurred during falls of 50 em

arrested by a rope

of

the same length and therefore not very extendable, are the

consequence of very brief but very sudden accelerations, exceeding the elasticity limits

of the rib cage.

That does not solve conclusively the problem of local accelerations on certain

segments and the resulting distortions. Our most worrying observations concern

experimental falls head first at a distance from the vertical of the anchoring point. At

the moment the lanyard

is

taut we find, on the one hand, that the mannikin turns over,

pivoting around the point where the lanyard is attached to the harness, and on the

other, that a pendulum movement b e g i ~ s centered on the anchoring point Fig. to

Accelerations of more than 300

mls

have been recorded lasting 0.02 seconds,

i e

a jolt of almost to km/s3 at the level

of

the head. It can be imagined that such situations

may be particularly threatening for the cervical column of the subject. Bending

frontwards and therefore back fastening or, better, fastening on both shoulder blades

are the only acceptable methods

in

such a case.

6


15/16

\

-

Fig

10

CONCLUSIONS

I

I

I

The

conclusions given here can

of

course only be provisional. The measurements

and the experiments reported are difficult to carry out and many points remain obscure.

Extrapolation from a mannikin whose biomechanics can only be an imperfect model

of

the human body

s

always tricky. Other elements

of

information could cast doubt

on the points

or

view expressed.

However t seems to us a fact that considering the requirements

of some

of the jobs

t s necessary to limit as much as possible the consequences of the most severe falls i.e.

falls

of

the order of 4 metres.

In these conditions the probability

of

injury for the victim can only be brought within

acceptable limits by devices including:

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a complete harness ergonomically designed and carefully tested

not only for its mechanical qualitites but also for its behaviour

with respect to the subject during arrest

o

the fall and the sub-

sequent suspension.

a shock absorber simultaneously limiting the force to around the

l r e ~ d y high limit o 6 000 N and the accelerations to around 100

m s for a few hundreths o a second.

s it is

not possible to envisage regular training

o

those concerned nor strict medical

selection on hiring the use o these devices must be reserved for rare cases where

no

other solution

is

available.

It

must be realized that they are only a lesser evil and do

not exclude the possibility o injury that their use requires precautions which make us

recommend a minimum o training for users and that the further we get from the

tolerance limits

o

the body which we have mentioned the more satisfactory the

protection will be.

8

ch 2-amphoux physiopathological aspects of personal equipment for protection against falls

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