1. health and fitness
2. disorders

UMEÅ UNIVERSITY MEDICAL DISSERTATIONS
N ew Series N o 136 - ISSN 0346-6612
From the Department o f Environmental Medicine, University o f Umeå,
Umeå, Sweden and the N ational Board o f O ccupational Safety and H ealth,
Technical unit, Box 6104, Umeå, Sweden
Vibration Exposure of the Glabrous
Skin of the Human Hand
by
RONNIE LUNDSTRÖM
Umeå University
VIBRATION EXPOSURE OF THE GLABROUS SKIN
OF THE HUMAN HAND
av
Ronnie Lundström
Fil. kand.
Institutionen för Hygien och Miljömedicin, Umeå Universitet,
901 87 Umeå och Arbetarskyddsstyrelsen, Forskningsavdelningen
Tekniska enheten, Box 6104, 900 06 Umeå.
Akademisk avhandling som med vederbörligt tillstånd av
Medicinska Fakulteten vid Umeå Universitet för avläggande av
doktorsexamen i medicinsk vetenskap kommer att framläggas
för offentlig granskning i Sal A 5, Farmakologiska institu­
tionen, Umeå Universitet, fredagen den 29 mars 1985, kl 09.00
Avhandlingen baseras på följande rapporter:
I.
Local vibrations - Mechanical Impedance of the Human
Hand's Glabrous Skin.
R Lundström.
Journal of Biomechanics, 17:2(1984) pp. 137-144.
II.
Responses of mechanoreceptive afferent units in the
glabrous skin of the human hand to sinusoidal skin dis­
placements .
R S Johansson, U Landström & R Lundström.
Brain Research, 244(1984) pp. 17-25.
III. Sensitivity to egdes of mechanoreceptive afferent units
innervating the glabrous skin of the human hand.
R S Johansson, U Landström & R Lundström.
Brain Research, 244(1982) pp. 27-32.
IV.
Acute impairment of the sensitivity of skin mechano­
receptive units caused by vibration exposure of the
hand.
R Lundström & R S Johansson.
Ergonomics (submitted for publication).
V.
Effects of local vibration on tactile perception in the
hands of dentists.
R Lundström & A Lindmark.
Journal of Low Frequency Noise and Vibration, 1:1(1982),
p p . 1-11.
VI.
Effects of local vibration transmitted from ultrasonic
devices on vibrotactile perception in the hands of
therapists.
R Lundström.
Ergonomics, (1984) (In press)*.
ABSTRACT
Lundström, Ronnie, 1985. Vibration exposure of the glabrous skin
human hand. Umeå University, Medical Dissertions, New Series, 136.
of
the
An occupational exposure to hand-arm vibration can cause a complex of neu­
rological, vascular and musculo-skeletal disturbances, known as the 'vibra­
tion syndrome'. However, the underlying pathophysiological mechanisms are not
at all clear. Early signs of an incipient vibration syndrome are often inter­
mittent disturbances in the cutaneous sensibility of the fingers, i.e. numb­
ness and/or tactile paresthesias. At later stages, a vasoconstrictive phe­
nomenon appears, usually as episodes of finger blanching.
When using a vibratory tool, all mechanical energy entering the body has
to be transmitted through, or absorbed by, the glabrous skin in contact with
the handle. Therefore, the aims of this study was to investigate: (i) mecha­
nical responses of the skin to vibrations, (ii) the response properties of
cutaneous mechanoreceptors to vibrations, and (iii) influences of vibration
exposure on touch perception.
It was found by measuring the mechanical point impedance (0.02-10 kHz)
that the skin is easy to make vibrate within the range of 80 to 200 Hz.
Within or close to this range are the dominant frequencies of many vibratory
tools. Thus, strong mechanical loads, such as compressive and/or tensile
strain, can appear in the skin which, in turn, may induce temporary or per­
manent injuries.
Recordings of impulses in single mechanoreceptive afferents, while the
skin as exposed to vibrations, were obtained using needle electrodes inserted
into the median nerve. The 4 types of mechanoreceptive afferents (FA I, FA
II, SA I, and SA II) in the glabrous skin exhibited different response cha­
racteristics to vibrations. The FA I units were most easily excited at vibra­
tory frequencies between ca 8 and 64 Hz and the FA II units between ca 64 and
400 Hz. The SA units were most sensitive at lower frequencies. At high sti­
mulus amplitudes, such as may occur while using vibratory tools, a consi­
derable overlap existed between the frequency ranges at which the units were
exited. Evidence was also provided, that mechanical skin stimuli produced by
edges of a vibrating object, compared to flat surfaces, more vigorously
excited the FA I and particularly the SA I units. Thus, a marked edge en­
hancement, essential for tactile gnosis and precision manipulation, seems to
exist already within the peripheral nervous system.
Acure impairment of tactile sensibility caused by vibrations, proved to be
due to a reduced sensitivity of the mechanoreceptive afferents. A loss of
manual dexterity a*vi an increased risk for accidents may therefore appear,
both during and after a vibration exposure.
Percussive tools, high speed drills and ultrasonic devices are known to
generate mechanical energy at frequencies above 1 kHz, i.e. frequencies
usually not felt. At these frequencies, it is known that most of the energy,
entering the body, is absorbed by the skin. Therefore, it was investigated
whether a long-term exposure to high-frequency vibration may have a detri­
mental effect on the cutaneous sensitivity. One group of dentists and one of
therapists, professionally exposed to high-frequency vibrations, were studied
with regard to vibrotactile thresholds in their hands. The study showed that
deleterious effects on tactile sensibility, at local exposure to high fre­
quency vibration, can not be excluded.
Key
words:
Vibration, hand, skin, mechanical
tactile perception, high-frequency.
Ronnie Lundström
Dept. Environ. Med. Univ. Umeå, S-901 87 Umeå,
impedance,
Sweden.
mechanoreceptors,
UMEÅ UNIVERSITY MEDICAL DISSERTATIONS
New Series N o 136 - ISSN 0346-6612
From the Department of Environmental Medicine, University of Umeå,
Umeå, Sweden and the National Board of Occupational Safety and Health,
Technical unit, Box 6104, Umeå, Sweden
Vibration Exposure of the Glabrous
Skin of the Human Hand
by
RONNIE LUNDSTRÖM
K • u
m
X e*
Umeå University
1985
2
ABSTRACT
Lundström, Ronnie, 1985. Vibration exposure of the glabrous skin
human hand. Umeå University, Medical Dissertions, New Series, 136.
of
the
An occupational exposure to hand-arm vibration can cause a complex of neu­
rological, vascular and musculo-skeletal disturbances, known as the 'vibra­
tion syndrome'. However, the underlying pathophysiological mechanisms are not
at all clear. Early signs of an incipient vibration syndrome are often inter­
mittent disturbances in the cutaneous sensibility of the fingers, i.e. numb­
ness and/or tactile paresthesias. At later stages, a vasoconstrictive phe­
nomenon appears, usually as episodes of finger blanching.
When using a vibratory tool, all mechanical energy entering the body has
to be transmitted through, or absorbed by, the glabrous skin in contact with
the handle. Therefore, the aims of this study was to investigate: (i) mecha­
nical responses of the skin to vibrations, (ii) the response properties of
cutaneous mechanoreceptors to vibrations, and (iii) influences of vibration
exposure on touch perception.
It was found by measuring the mechanical point impedance (0.02-10 kHz)
that the skin is easy to make vibrate within the range of 80 to 200 Hz.
Within or close to this range are the dominant frequencies of many vibratory
tools. Thus, strong mechanical loads, such as compressive and/or tensile
strain, can appear in the skin which, in turn, may induce temporary or per­
manent injuries.
Recordings of impulses in single mechanoreceptive afferents, while the
skin as exposed to vibrations, were obtained using needle electrodes inserted
into the median nerve. The 4 types of mechanoreceptive afferents (FA I, FA
II, SA I, and SA II) in the glabrous skin exhibited different response cha­
racteristics to vibrations. The FA I units were most easily excited at vibra­
tory frequencies between ca 8 and 64 Hz and the FA II units between ca 64 and
400 Hz. The SA units were most sensitive at lower frequencies. At high sti­
mulus amplitudes, such as may occur while using vibratory tools, a consi­
derable overlap existed between the frequency ranges at which the units were
exited. Evidence was also provided, that mechanical skin stimuli produced by
edges of a vibrating object, compared to flat surfaces, more vigorously
excited the FA I and particularly the SA I units. Thus, a marked edge en­
hancement, essential for tactile gnosis and precision manipulation, seems to
exist already within the peripheral nervous system.
Acute impairment of tactile sensibility caused by vibrations, proved to be
due to a reduced sensitivity of the mechanoreceptive afferents. A loss of
manual dexterity and an increased risk for accidents may therefore appear,
both during and after a vibration exposure.
Percussive tools, high speed drills and ultrasonic devices are known to
generate mechanical energy at frequencies above 1 kHz, i.e. frequencies
usually not felt. At these frequencies, it is known that most of the energy,
entering the body, is absorbed by the skin. Therefore, it was investigated
whether a long-term exposure to high-frequency vibration may have a detri­
mental effect on the cutaneous sensitivity. One group of dentists and one of
therapists, professionally exposed to high-frequency vibrations, were studied
with regard to vibrotactile thresholds in their hands. The study showed that
deleterious effects on tactile sensibility, at local exposure to high fre­
quency vibration, can not be excluded.
Key
words:
Vibration, hand, skin, mechanical
tactile perception, high-frequency.
impedance,
Ronnie Lundström
Dept. Environ. Med. Univ. Umeå, S-901 87 Umeå, Sweden.
mechanoreceptors,
3
ORIGINAL REPORTS
This thesis is based on the following papers, which in the text
are referred to by their Roman numerals:
I.
Local vibrations - Mechanical Impedance of the Human Hand's
Glabrous Skin.
R Lundström.
Journal of Biomechanics, 17:2(1984) pp. 137-144.
II.
Responses of mechanoreceptive afferent units in the glabrous
skin of the human hand to sinusoidal skin displacements.
R S Johansson, U Landström & R Lundström.
Brain Research, 244(1982), pp. 17-25.
III. Sensitivity to egdes of mechanoreceptive afferent
innervating the glabrous skin of the human hand.
R S Johansson, U Landström & R Lundström.
Brain Research, 244(1982), pp. 27-32.
IV.
units
Acute impairment of the sensitivity of skin mechano­
receptive units caused by vibration exposure of the hand.
R Lundström & R S Johansson.
Ergonomics (submitted for publication).
V.
Effects of local vibration
hands of dentists.
R Lundström & A Lindmark.
on tactile perception
Journal of Low Frequency Noise and Vibration,
pp. 1-11.
VI.
1:1
in the
(1982)
Effects of local vibration transmitted from ultrasonic
devices
on
vibrotactile perception
in the hands
of
therapists.
R Lundström.
Ergonomics (In press).
4
CONTENTS
1.
GENERAL BACKGROUND OF THE STUDY
1.1 Vibration syndrome
1.2 Risk assessment
5
5
9
1.3 Mechanical response of the hand-arm system
1.4 Tactile sensibility
12
14
1.5 Acute impairment of tactile sensibility
following vibration exposure
19
1.6 Effects of high frequency vibration on
tactile sensibility
20
2.
SPECIFIC AIMS OF THE PRESENT STUDY
21
3.
RESULTS
22
3.1 Mechanical response of the glabrous skin
to vibration
3.2 Neural responses to vibration
3.3 Neural sensitivity to vibrating edge contours
3.4 Acute impairment of tactile sensibility
following vibration exposure
3.5 Effects of high frequency vibration on tactile
perception threshold
22
25
27
29
31
4.
DISCUSSION AND CONCLUSIONS
34
5.
ACKNOWLEDGEMENTS
38
6.
REFERENCES
39
5
1.
GENERAL BACKGROUND OF THE STUDY
1.1. Vibration syndrome
The vibrations in many electrically or pneumatically powered
handheld tools or workpieces may cause a complex of neurologi­
cal, vascular and musculo-skeletal disturbances. These pheno­
mena, occurring together or independently, are becoming widely
recognized as an important occupational disease known as the
vibration syndrome (among others Hamilton 1918, Dart 1946,
Agate and Druett 1947, Pyykkö 1974, Taylor 1974, Griffin 1980,
Taylor and Brammer 1982, Gemne and Taylor 1983).
The best documented and most easily observed condition connect­
ed to hand-arm vibration exposure is a vasoconstrictive pheno­
menon, appearing as episodes of fingers blanching together with
tingling and numbness in the exposed hands. However, similar
symptoms of "white fingers" were first described by Raynaud in
1862 on healthy non-vibration exposed individuals (Primary Ray­
naud's Disease). Since then many causes of "white fingers" have
been recognized, including vibration, and are referred to as
"Secondary
Raynaud's
Phenomenon".
Vibration-induced
white
fingers were first described about stone cutters and rockminers
(Loriga 1911). The literature relevant to this injury has ever
since increased tremendously and it has been denoted by diffe­
rent names, "Raynaud's Phenomenon of Occupational Origin",
"Traumatic Vasospastic Disease" (TVD) and "Vibration-Induced
White Fingers" (VWF) are the most commonly used (for refs, see
Taylor and Pelmear 1975, Taylor and Brammer 1982).
The degree of severity of VWF are often assessed according to a
scale presented by Taylor and Pelmear 1975 (Table 1). In the
early stages of VWF, a person ususally reports symptoms of neu­
rological disturbancies,
such as slight tingling and numbness
of his fingers. Later, after regular prolonged exposure, the
tips of one or more fingers suffer attacks of finger blanching.
The episodic vasoconstrictions are usually precipitated by
cold. With further exposure, in the magnitude of years, the
area of finger blanching extends proximally to the base of
those
fingers
exposed
to
vibration.
During
the
attacks
the
6
Table 1. Grading scale for the severity of VWF according to Tai­
lor and Pelmear (1975).
Symptoms and signs
Stage of
disorder
0
Oip
Condition of digits
Work and social inter­
ference
No blanching
No complaints
Intermittent tingling
No interference
without blanching
°N
Intermittent numbness
No interference
without blanching
°TN
Intermittent tingling
No interference
and numbness without
blanching
1
Blanching of one or
more fingertips without
tingling and/or
numbness
No interference
2
Blanching of one or
more complete fingerS/
Slight interference at
home or in social activi­
usually during winter
ties but not at work
Extensive blanching,
usually all fingers bi­
laterally. Frequent epi­
Definite interference at
work, at home and in so­
sodes during all seasons
tions of hobbies
Extensive blanching of
all fingers, both winter
and summer
Occupation changed be­
cause of severity of
signs and symptoms of VWF
3
4
cial activities. Restric­
7
fingers feel swollen, painful and inflexible, and there is a
reduction in tactile sensibility, in manual dexterity and in
grip
strength
1972,
Färkkilä
(Taylor
and
et.al.
Brammer
1982).
The
1982,
Bannister
vasoconstriction
and
Smith
terminates
after some minutes up to about one hour being due to the degree
of severity of the VWF attack and the possibility of warming of
the
hands.
During
this
period
a reactive hyperaemia
occurs,
usually painful and unpleasant.
The mechanisms behind these vasomotor reactions
are still not
known but several theories have been suggested, based primarily
on changes in peripheral bloodvessels and/or in the peripheral
nervous system (e.g. Ashe et.al. 1962, Magos and Okos 1963,
Stewart and Goda 1970, Gurdjian and Walker 1945 and Hyvärinen
et.al. 1973). One theory,
suggested by Hyvärinen and coworkers
(1973), is that a sympathetic reflex, triggered by exposition to
cold,
regards
loud
noise
vibration,
or
vibration,
the activity
entails
vasoconstriction.
originating
in Pacinian
As
cor­
puscles was suggested to elicit the reflex. These mechanoreceptors are particularly sensitive to vibrations at frequencies be­
tween ca. 50-500 Hz (see below).
Apart from vasomotor reactions, other types of injuries seem to
be associated with the use of vibratory hand-held tools. Peri­
pheral neuropathy (Seppäläinen 1972), changes in nerve conduc­
tion velocity (Kumlin and Seppäläinen 1969) and changes in tac­
tile perception threshold of the hand (e.g. Lidström et.al.
1982) have earlier been reported. Musculo-skeletal changes such
as unnormally strong muscle fatigue, and vacuoles, cysts and
décalcification of bone tissues, are somewhat less commonly
reported (Färkkilä et. al. 1982, Kumlin et. al. 1973).
The correlation between generated injuries and the physical cha­
racteristics of the vibration source has proved to be very com­
plex. Man's response to vibration appears to be determined by
several physical, operational, ergonomical, environmental and
personal factors (Guignard 1979, Taylor and Brammer 1982,
ISO/DIS 5349.2 1984). Some of them are listed in Table 2.
Table 2. Some factors known or believed to influence the severi­
ty of vibration syndrome.
EXTERNAL AND OCCUPATIONAL FACTORS
Physical factors
Intensity, directionality and durability of vibration
entering the hand.
Dominant frequencies and amplitudes that enter the hand.
Total duration and temporal pattern of exposure each
work day.
Operational and ergonomic factors
Work cycle and method of using the tool.
Weight of tool.
Configuration of handle.
Hand grip force required to guide tool.
Working posture.
Use of gloves of soft texture.
Surface area in contact with the tool.
Environmental factors
Prevailing climate (temperature, humidity etc) at
worker's home and workplace.
Surrounding noise.
INTERNAL AND PERSONAL FACTORS
Years of employment involving vibration exposure.
Entry age.
Level of training, skill and familiarity with the tool.
Body constitution and biological susceptibility.
Vasoconstrictive agents (smoking, medication etc).
1.2
Risk assessment
The proposed international standard ISO/DIS 5349.2 (1984) speci­
fies methods for measuring and reporting hand-transmitted vibra­
tion in three orthogonal
axes within the
frequency
range
of
about 5 to 1500 Hz. All measurements and assessments are to be
based on 'broad-banded frequency weighted acceleration values'
determined with a specified frequency weighting network or by
conversion of one-third or octave band data. The nominal gain of
the network is to be zero from ca 6-16 Hz, and further on, up to
1250 Hz, the acceleration signals will be attenuated by 6 dB per
octave. The attenuation at even higher frequencies should be at
least 12 dB per octave. The risk of getting vibration injuries
is thus considered to decrease with increasingfrequency,
*
assuming that man is less sensitive to high frequency compo­
nents. If the characteristics of the weighting network, on the
other hand, is expressed in terms of vibration velocity, the
hand-arm system is
range of 16 to 1250
considered as equally
Hz (cf. Figure 1).
sensible
within the
The guidelines for the assessment (ISO/DIS
5349.2 - Annex A),
specified in terms of frequency weighted acceleration for the
dominant axes and daily exposure time, is based on the present
knowledge
on the dose-effect relationship
(Brammer 1982).
The
exposure times in years for different percentiles of a popula­
tion before the onset of finger blanching may thus be estimated.
A recent study, where the vibration of 110 tools were measured
(Lundström and Burström 1984), shows that large variations, as
regards the time interval from vibration exposure to the onset
of vascular disorder is to be expected both within and between
different types of tools (Table 3). The shortest estimated time
intervals were found,
nibblers.
for instance,
among chipping hammers and
Mean 1/3-octave band spectras expressed in both vibration acce­
leration and velocity have also been calculated for each type of
tool, and the result is shown in Figure 1. For the grinders and
the nibblers
the dominant
frequencies,
which were related to
their rotational speeds, were near 100 and 63 Hz, respectively.
For percussive tools,
such as chipping hammers
and wrenches,
10
the frequency spectras appeared to be more broadbanded. As can
be seen, the shape of the acceleration spectras differs from
those expressed in terms of velocity. The high-frequency com­
ponents are more dominant in the acceleration spectras.
Table 3. Mean frequency weighted acceleration for various types
of hand-held tools and corresponding estimated exposure
times before the onset of vascular symptoms for diffe­
rent percentiles of an exposed population in accordance
with ISO/DIS 5349.2 (1984). The dose-effect relation­
ship given is based on a daily exposure of 4 hours.
Data gathered from Lundström and Burström (1984).
Type of
tools
Number
of
tools
Grinders
54
Weighted accelera­
tion in m/s^(rms)
for dominant axes
Mean
± SD
3.3
2.4
Years before the onset
of finger blanching
Percentile of population
± SD
10%
50%
± SD
‘ 13
5
20
>25
>25
2.5
7.2
Chipping
hammers
4.7
12
7
Wrenches
27
2.8
1.7
11
>25
2.5
Nibblers
6.8
4.7
.
14
9
f 16
24 i
1L> 25
[ 6
10 I
1L >25
4.5
Others
15
4.2
2.3
..
15
6
L 25
1[
^ 10
i
1.>25
According to guidelines based on 1/3-octav band-spectras given
in a previous proposal for international standard, ISO/DIS 5349
(1979), the daily use of both riveting hammers and nibblers
should, on average, not exceed 4 hours. A daily exposure of be­
tween 4 and 8 hours is acceptable for grinders. No limitations
in exposure time seems to be required for wrenches. It should be
noted that large variations in vibration levels exist between
each group of tools, why any generalisation as regards recommen­
ded daily exposure time is difficult. The assessment in a parti­
cular case should therefore be based on the actual vibration en­
vironment .
11
V E L O C I T Y , mm/s
ACCELERATION, m/s
100
100
31.5
GRINDERS
31.5
n«20
10
10
3.1
1
0 .3
0.1
T
I I I
I I
i
i
i
i
100
CHIPPING
HAMMERS
31.5
n =8
100
100
31.5
WRENCHES
31.5
n =26
10
3.1
1
0.3
0.1
100
NIBBLERS
31.5
n =6
F R E Q U E N C Y , Hz
Figure 1. One-third octave band spectras expressed in both vib­
ration acceleration and velocity for four different
types of hand-held tools frequently used in industry.
The upper and lower limits in the frequency spectras
indicate the maximal and minimal average level, res­
pectively, for any of the three directions. The fi­
gures also show exposure guidelines for hand-trans­
mitted vibration, for 4 to 8 hours' daily exposure,
continuous or not regularly interupted. The graphs are
in accordance with a previous draft of international
standard, ISO/DIS 5349 (1979). For more information
see text.
12
1.3. Mechanical response of the hand-arm system
Due to its mechanical complexity,
the response characteristics
of the human hand-arm system to different dynamic inputs, such
as vibration, is difficult to describe, both in quantitative and
qualitative terms. This system can be regarded as consisting of
a large number of separate mechanical elements, such as masses
held together with springs and viscous dampers. However, more or
less sophisticated hand-arm models can be found in literature,
and a fairly good idea how the system responds to vibration can
therefore be obtained under specific conditions with regard to
hand-arm posture,
squeeze force etc.
(Dieckman 1958,
Reynolds
and Keith 1977, Mishoe and Suggs 1977, Wood et.al. 1978, Byström
et.al. 1982).
The mechanical
characteristics
of man's hand-arm
system have
proved to be affected by several factors, for example bodily
constitution, vibration direction, muscle tension, arm's posi­
tion and squeeze force. The resonance frequency,
(or fre­
quencies) is influenced by these factors. Impedance measurements
taken in earlier studies, by among others Abrams and Suggs
(1970), Zaveri (1974), Mishoe and Suggs (1977), Reynolds and
Keith (1977) indicate that it is below 500 Hz.
It is known that vibrations entering the body, through the hand,
are highly attenuated as they are propagated through the hand
and arm. According to Bekesy (1939), at 50 Hz an attenuation of
the vibration amplitude of approximately 15 dB occurs from the
handle to the elbow and another 25 dB to the head. Furthermore,
it is also known that a vibration input is more heavily attenua­
ted the higher the frequency. As can be seen from Figure 2, dif­
férencies of up to about 60 dB between a low and a high frequen­
cy, 31.5 Hz and 2000 Hz, respectively, have been reported as re­
gards the attenuation taking place in the hand (Suggs, 1970).
Thus, at frequencies above ca. 50 Hz most of the vibration ener­
gy entering the hand-arm system has dissipated in the hand. This
is more pronounced the higher the frequency (see also Reynolds
and Angevine 1977, Reynolds and Keith 1977). Up to about 500 Hz
the mechanical energy is propagated as shear waves within the
skin. Higher frequencies are probably propagated as bulk shear
waves through underlying tissues (Potts et.al. 1983).
Handle,
input to hand
20
6.3
CO
-1 0
pi
2.0
-20
.63
-30
.20
-40
.063
-50
ATTENUATION,
. 020
-60
dB
cr
Radius,
< .0063
-70
O)
z
O
-
<
£
I
LU
o
O
.0020
31.5
125
500
FREQUENCY,
2000
-80
Hz
Figure 2 . Vibration attenuation in the hand and arm.
from Suggs 1970).
(Modified
14
1.4. Tactile sensibility
In addition to provide a mechanical protection of the deeper
tissues of the hand,
the glabrous skin has a number of func­
tions. One of them is to lodge the huge number of sense organs,
which provide the central nervous system with information about
tactile, thermal and noxious events on the skin surface. Such
information is necessary when using the hand in explorative
tasks, but is also essential for the motor control of the hand.
The tactile information clearly plays the most important role
for the more or less automatic processes within the central ner­
vous system, and are of paramount importance in precision move­
ments carried out by the fingers (e.g. Moberg 1962; Johansson
and Westling 1984). The tactile signals provide, at every mo­
ment, the central nervous system with an image of the mechanical
state of the skin and thereby also information related to mecha­
nical properties of hand-held objects. The term tactile unit is
applied to a single mechanoreceptive neuron, including its affe­
rent nerve fibre and all its peripheral branches (terminals),
connected to the appropriate end organs in the skin. This unit
responds to skin deformations within a specific skin area de­
noted the unit'sreceptive field. The signals
arising in the
nerve terminals
are transmitted through the afferent nerve
fibres to the central nervous system as nerve impulses, ca. 1 ms
in duration. The tactile units have large-diameter myelinated
fibres (Aq£) with conduction velocities of ca 30-70 m/s.
With the introduction of the microneurographic method for re­
cording
impulses
from
single
afferent
nerve
fibres
in
awake
human subjects (Vallbo and Hagbarth 1968), it became possible to
study the peripheral tactile neuronal mechanism in man. This
method also provided the possibility to combine directly neurophysiological and psychophysical responses. After Knibestöl and
Vallbo (1970) first explored the tactile units in the glabrous
skin of the human hand using this technique, several investiga­
tions of their functional properties have been carried out.
Their main results are briefly reviewed in a following section.
The reader may consult some recent reviews for more detailed
information
1984).
(Johansson and Vallbo 1983,
Vallbo and Johansson
15
There are about 17000 mechanoreceptive units of four different
types supplying the glabrous skin area of the hand (Johansson
and Vallbo, 1979). They have been classified into two major ca­
tegories on the basis of their response to a sustained indenta­
tion of the skin. Furthermore, within each of these categories
two different types of units can be distinguished on the basis
of the size and structure of their receptive fields (Figure 3).
ADAPTATION
Fast ,
no st at i c
r e s po n s e
Slow,
st ati c
r e s po n s e
Small
_l
LU
LL
shar p
bor ders
LU
>
I-
Q_
LU
O
LU
CC
Large
obscur e
borders
Figure 3. Types of mechanoreceptive afferent units in the gla­
brous skin of the human hand classified on the basis
of their adaptation and receptive field properties.
The graphs show schematically the impulse discharge
(lower trace) to ramp indentation of the skin (upper
trace). The black patches and dashed areas of the
drawings of the hand show the extent of typical re­
ceptive fields for type I and type II units respec­
tively. For further information, see text.
16
Two types are fast adapting (FA I and FA II) and respond only
when the stimulus is in motion. The FA I units seem to be most
dependent on the velocity of the skin indentation, whereas the
FA II units are more sensitive to higher derivatives,
acceleration
(Knibestöl
1973).
The
fast
adapting
such as
units
can
respond during both the increasing and the decreasing part of
the skin indentation. The FA I units constitutes about 43% of
the total number of tactile units within the glabrous area of
the human hand and are characterized by small, usually round or
oval, and well defined receptive fields (Johansson 1978). The
median size of the fields, when measured with von Frey hairs at
an intensity of 4-5 times the threshold force of the unit, is 13
mm2 (Johansson and Vallbo 1980). The second type, FA II, has
larger receptive fields with obscure borders. The median size
has been approximated to about 101 mm2, determined with the same
technique (Johansson and Vallbo 1980). They constitute about 13%
of the total number of tactile units in the hand. An interesting
feature of these units is their extreme sensitivity to mechani­
cal transients
even when the
stimulation
is applied remotely
from the receptor.
The two types of slowly adapting units, SA I and SA II, respond
only during increasing skin indentation but, in addition, with a
sustained discharge during a constant deformation of the skin
(Knibestöl 1975). The receptive fields for the SA I units are
small with sharp borders while the SA II units have large field
with no distinct borders. The median fields for the SA I and SA
II units are 11 and 59 mm2, respectively, estimated with the
same method as for the FA units (Johansson and Vallbo 1980). SA
I and SA II units constitutes about 25% and 19%, respectively,
of the unit population in the glabrous skin area of the hand
(Johansson and Vallbo 1979). Some characteristic features of the
SA II units are their exquisite sensitivity to remote lateral
skin strech, and many of' them are also continously discharging
in the absence of skin indentation (Johansson 1978).
In Figure 4,
the
structure
and
location of the proposed
end
organs are schematically indicated. The end organs of the FA
I units are most likely Meissner corpuscles, located in the der­
mal
papillae.
The
Pacinian
corpuscles,
located
in
the
deep
17
dermis
and subcutaneous
tissues,
and the smaller
and simpler
lamellated Golgi-Mazzoni bodies in the dermis are probably the
endings of FA II units. The endings of SA I units are Merkel
cell
neurite
complexes which
intermediate epidermal
ridges.
are
located
Finally,
at the
tip
of
the
the SA II units most
likely terminate with the spindle shaped Ruffini endings located
in the dermis.
I
FA I
Epidermis
SA I
Dermis
S A II
F A II
i
Subcutis
Figure 4. Vertical section through the glabrous skin of the hu­
man hand schematically showing the location of the
end organs of four types of mechanoreceptive units.
(Adapted from Johansson & Vallbo, 1983).
It has been shown that the density of tactile units in the
glabrous skin differs between unit types as well as between dif­
ferent regions of the glabrous skin area (Johansson and Vallbo
1979). As shown in Figure 5, the density is highest in the
distal part of the finger tip. The density decreases stepwise
towards the wrist. There are two steps; between the distal and
proximal half of the distal phalanx and between the palm and the
bases of the fingers. It has been suggested that type I units,
with
small
and
well-defined
receptive
fields,
together
with
their high innervation density particularly at the finger tips,
account for the high spatial acuity of the tactile sense. Thus,
18
a strong correlation between the density of these units and the
spatial discrimination capacity has been shown (Vallbo and
Johansson 1978). For all unit types there is a huge overlap be­
tween receptive fields, especially on the finger tips (Johansson
and Vallbo 1980).
FA I
FA II
SA I
21
70
37
10
30
25
9
S A II
Figure 5. Estimated average densities of mechanoreceptive affe­
rent units in three different areas of the glabrous
skin of the human hand innervated by the median nerve.
Figures indicates the average unit density per cm^ for
each region based on the results from Johansson and
Vallbo 1979.
It should be noted that in the present summary, a modified ter­
minology for the fast adapting units have been used compaired to
paper II and paper III, where the FA I units are denoted RA, and
the FA II are denoted PC. The motives for a change to the pre­
sent names have been further discussed by Johansson and Vallbo
(1983).
19
1.5. Acute impairment of tactile sensibility following vibration
exposure
As previously pointed out (cf. Table 1) early signs of an in­
cipient vibration syndrome are neurological symptoms primarily
affecting the sensibility of the hand,
e.g. paraesthesias and
impairment of the tactile sensibility (Taylor and Brammer 1982,
Lidström et.al. 1982). Vibration exposure is known to account
for a temporary increase in vibrotactile thresholds on healthy
human subjects, not previously exposed to vibrations (e.g. Tominaga 1973, Verrillo and Schmeidt 1974, Nishiyama and Watanabe
1981). The magnitude of the temporary threshold shifts (TTS) as
well as the time for recovery depend on several factors, such as
the frequency, amplitude and duration of the vibration exposure.
In general, longer duration and/or higher amplitudes increase
the threshold shifts and the time for recovery. How the fre­
quency effects the thresholds is not yet that clearly documen­
ted. Neither are the physiological mechanisms accounting for the
depression of the tactile sensibility known.
Interestingly, the TTS elicited by a given vibration exposure is
higher and the duration of the recovery is longer for subjects
showing a previous history of work with vibrating tools
(Lid­
ström et.al. 1982). This is true even for workers with normal
absolute thresholds. With increased exposure, a permanent de­
crease in tactile sensibility may appear. Thus, it appears as if
the mechanisms accounting for the TTS and the permanent impair­
ment of the sensibility would somehow be related. Therefore, it
might be fruitful to study further the TTS in terms of under­
lying physiological mechanisms since it may provide clues to the
pathophysiology of the vibration syndrome.
20
1.6. Effects of high frequency vibration on tactile sensibility
At the end of the 1970s complaints about numbness and stiffness
in the fingers were heard from dentists. Some of them had been
forced to resign due to reduction in tactile sensibility and in
motor skill. Thus, the complaints resembled those to be expected
among industrial workers using vibratory hand-held tools. Most
handpieces used in modern dentistry have high rotational speed,
over 100000 rpm. The outcome of a vibration measurement showed
vibration levels as high as about 100 m/s^ (rms) within the fre­
quency range of 1-50 kHz (Lundström and Liszka 1979) . Similar
results, have been obtained in later investigations as well
(Andersson et.al. 1983). High levels of high frequency vibra­
tions have also been measured in grinders, drills, riveting ham­
mers and at pedestal grinding (Lundström and Liszka 1979, Dandanell 1984, Starck 1984). A high prevalence of VWF has been re­
ported for personal using pedestal grinders and percussive tools
(Dandanell 1984, Starck 1984).
It is known that the greater part of mechanical energy, at high
frequencies,
is absorbed by superficial tissues in direct con­
tact with the vibrational source
(cf.
section 1.3). Cutaneous
mechanoreceptors may therefore be affected with a loss in sensi­
bility and motor precision performance as a consequence. Distur­
bances of the tactile sensibility indicate influences on the
nervous system and may be an early sign in the development of
more serious injuries, such as VWF (cf. Table 1). However,
according to ISO/DIS 5349.2 (1984) all assessments shall be
limited to vibrational frequencies below ca. 1.5 kHz, assuming
that man is regarded as being not adversely influenced by higher
frequencies. Against this background it seems important to exa­
mine whether a reduction in tactile sensibility can be objecti­
vely established among people, proffessionally exposed to vibra­
tions only containing high-frequency components.
21
2.
SPECIFIC AIMS OF THE PRESENT INVESTIGATION
The scientific literature relevant to the injurious effects of
hand-transmitted vibration probably amounts to hundreds of pub­
lications. Despite of that, still a paucity of firm experimental
and epidemiological data exist as regards the causal relation­
ship between vibration exposure and associated injuries.
When using a vibratory hand-held tool, all mechanical energy
entering the hand-arm system has to be transmitted through, or
absorbed by, the skin in contact with the vibrating handle. To
fully understand how man is influenced, basic knowledge of how
the skin responds, both physically and physiologically, is thus
needed. Therefore the principal aims of the present investiga­
tion were to answer the following questions:
- How does the human hand's glabrous skin, in contact with the
vibrating surface, mechanically behave when exposed to vibra­
tions?
- How does the mechanoreceptive afferent units in the glabrous
skin of the human hand respond to vibrations of different fre­
quencies and amplitudes?
- Is there a temporary reduction in sensitivity among cutaneous
mechanoreceptors after a powerful vibration exposure? If so,
how does it relate to the reduction in tactile sensibility
observed with phychophysical methods?
- Does a long-term exposure to vibration with very high fre­
quencies, above 1 kHz, have a deleterious effect on tactile
sensibility?
22
3.
RESULTS
3.1. Mechanical response of the glabrous skin to vibration (I)
When using a vibratory tool all mechanical energy entering the
hand has to be transmitted through, or absorbed by, the glabrous
skin in contact with the vibrating handle. In the present study,
the mass-like, the spring-like and viscous properties in diffe­
rent areas of the glabrous skin of human hand were investigated
by measuring the mechanical point impedance (I).
The mechanical impedance (Z), defined as the ratio between force
(F) and velocity (v), describes the mechanical structure's re­
sistance to follow an applied vibration. If, as in this inves­
tigation, force and velocity is measured at the same point is is
called point impedance.
Sinusoidal vibrations within the frequency range of 20 to 10 000
Hz were delivered to the glabrous skin of the human hand through
an impedance head mounted on a small vibrator whereas the force
and the velocity applied to the skin were continously recorded.
The magnitude of the point impedance could then be calculated.
The vibrations were superimposed on two static loads (0.5 and 1
N). Two cylinder-shaped test probes, with plane surfaces in con­
tact with the skin were used (0.5 and 1 cm^). The vibration amp­
litudes varied between 1 and 10 m/s^ (rms). In each of 6 adult
subjects the skin impedance were determined on ten different
points located on the right hand (Figure 7).
It was found that the skin behaved as a mechanical system con­
sisting of a spring, a damper and a mass element coupled in
parallel. For such a system, the magnitude of the impedance de­
creases with increasing frequency down to a minimum value, after
which it increases again. The frequency at the turning point
defines the system's resonance frequency, at which the magnitude
of the impedance is mostly dependent on damping properties of
the system. At lower frequencies the skin mainly acts like a
spring element causing the impedance to decrease with increasing
frequency. At higher frequencies, at which the mass element do­
minates, it will increase with the frequency.
23
Figure 6 shows separate impedance curves referring to two test
points (LI and L10, see Fig. 7) and an average curve referring
to
all
testpoints
(L1-L10).
As
can be
seen
there
are
small
differences between testpoints as regards the skin as a damper
and a mass element (right limbs of the curves). More pronounced
regional différencies were found regarding the skin's spring­
like properties (left limbs of the curves), i.e. the skin of the
fingers showed a higher stiffness than the palmar skin.
-1 0
UJ - 3 0
L1-10
L 10
-40
< - 5 0
-60
-70
FREQ U EN CY, Hz
Figure 6. Mechanical impedance for the skin and the hand-arm
system. DX and DZ are hand-arm impedance curves deriv­
ed by Dieckmann (1984). The lower triplet of graphs
show the
hand's skin impedance for two individual
test-points (LI and L10, 6 subjects pooled) and an
average for all points (Ll-10, 6 subjects pooled).
The locations of the testpoints are indicated in
Figure 7.
As
a consequence,
the resonant
frequency varied between
test
points which are illustrated in Figure 7. The highest and the
lowest resonance frequencies, ca 80 and 200 Hz were found at the
distal part of the finger and at the thenar eminence, respecti­
vely. No difference between male and female subjects could be
discerned.
24
m oo|?o &
*00&|Sd *
co
0
°*°j® *
o£
I
20
50
100
200
Frequency,
Figure
7.
500
1k
2k
Hz
Mechanical resonant frequencies at different skin
areas on the hand's inside for three female ( ) and
three male ( O ) subjects. Straight lines indicate
the position of an average resonant frequency.
A biodynamic model of the hand-arm system has recently been
constructed, based on impedance data from several investiga­
tions. This work has been initiated by the member bodies within
ISO TC 108/SC 4/WG 5 (Biodynamic modelling) and carried out by
Dieckmann (1984). The results from these calculations are shown
in Figure 6 for two directions (DX and DZ ) together with the
present results on the impedance data for the skin. It may be
noted that for frequencies above ca 100 Hz the shapes of the
impedance graphs of the hand-arm system as a whole and of the
skin show some similarities. For this reason, it seems resonable
to assume that the mechanical impedance of the total skin area
in contact with a handle is close to the impedance of the whole
hand-arm system. This supports the idea that vibrational ener­
gies of frequencies above ca 100 Hz is mostly dissipated into
the skin and the superficial tissues of the hand.
25
3.2. Neural responses to vibration (II)
The purpose of this part of the study was to investigate syste­
matically the responses in tactile afferent units innervating
the glabrous skin of the human hand to vibrations with different
frequencies and amplitudes (II).
Single unit nerve impulses were recorded from adult human sub­
jects using the microneurographic technique developed by Vallbo
and Hagbarth (1968). The tungsten needle electrodes were insert­
ed in the median nerve on the upper arm about 10 cm proximal to
the elbow. The recording electrode was then adjusted manually in
small steps until the activity from a single unit could be dis­
tinguished from the background activity of the nerve fascicle.
After isolation and classification of the unit sinusoidal skin
displacement were delivered through a small perspex probe (dia­
meter:
6-8 mm)
(Westling
et.al.
driven by
1976).
a feed-back moving coil
A
set
of
computer
stimulator
controlled
test
sequencies, consisting of sinusoidal vibrations with different
frequences (0.5-400 Hz), and amplitudes (0.001-1 mm, peak to
peak)
were
executed.
The
neural
responses,
expressed
as the
average number of impulses elicited per sine wave cycle and de­
noted as the cycle response, were then analysed.
It was showed that the cycle response varied with unit type,
frequency and amplitude of the stimuli (Figure 8). The FA II
units were most easily excited at high frequencies, above ca 50
Hz. At 256 Hz the stimulus amplitude, required for evoking a
discharge, could be as low as 1 ,pmp_p, which corresponds to a
vibration acceleration amplitude of ca 0.9 m/s^(rms).
The FA I units were most easily excited at frequencies between
ca 8 and 50 Hz. The lowest amplitude, ca 8 ^imp_p(0.11 m/s2(rms))
for evoking discharges were at 32 Hz. A striking finding as re­
gards both FA I and II units was that the frequency range, with­
in which the units could be excited, quite rapidly increased
with the stimulus amplitude. Furthermore, as the stimulus ampli­
tude was increased the frequency for the maximal cycle response
decreased.
26
7
FA
FA
SA
6
SA
5
4
3
2
1
0
2n
1
0
0.5
2
8
32
128
512
S i n e w a v e f r e q u e n c y , Hz
Figure 8. Relation between the sine wave frequency and the aver­
age number of impulses described per sine wave cycle
for the four types of mechanoreceptive afferent units
innervating the glabrous shin of the human hand. Pool­
ed data from 8 FA 1, 4 FA II, 4 SA I and 8 SA II
units. The graphs represent two different stimulus
amplitudes; 500 and 62 ^im (peak). For more details,
see text.
27
The slowly adapting units were particularly sensitive at
low
stimulus frequencies. As to the SA I units they could be excited
most easily within the frequency range of 2-32 Hz and especially
at 8 Hz with an absolute threshold of about 16 jamp_p (0.014 m/s^
(rms)). The SA II-units were most easily excited at frequencies
below ca 8 Hz, with the maximal cycle response at the lowest
test frequencies.
As can be seen in the upper graph of Fig. 8, at high stimulus
amplitudes, well above the thresholds of the units, a big over­
lap existed between FA I, FA II and SA units, with regard to the
frequency ranges at which they responded. Therefore it was con­
cluded that a selective stimulation of a particular type of unit
by selecting its "best" frequency is
mulus amplitude is fairly
only possiblewhen the sti­
close to the absolute thresholds of
that unit population.
3.3. Neural sensitivity to
vibrating edge contours (III)
The handle of a vibrating
order to avoid
slipping with
tool is often grooved basically in
a risk for accidents as a con­
sequence. It is therefore of great interest to know how rough
and/or grooved surfaces influence the ability of the tactile
units
to respond to stimuli.
Thus,
the purpose of the third
paper (III) was to examine whether edges of vibrating objects in
contact with the skin can alter the response patterns compared
to flat surfaces.
By using similar experimental technique as previously described
(paper II) the cycle response from FA I and SA I units to sinus­
oidal skin displacements were studied with regard to two diffe­
rent locations of the stimulus probe in relation to the recep­
tive field (Fig. 9A): (1) The stimulus probe completely covered
the receptive field (No edge condition); and (2) the probe
covered only a part of the receptive field (Edge condition),
i.e. the edge of the probe cuts through the field.
The outcome showed that the FA I units, and especially the SA I
units, responded more readily during the edge condition (Figure
28
9 A ) . The edge response was stronger within the entire ranges of
stimulus frequencies and amplitudes studied and this tended to
be most pronounced at lower stimulus frequencies. Furthermore,
for most units the threshold was lower in the edge condition.
This implies, as schematically illustrated in Figure 9 B, that
afferent signals from the population of FA I and SA I units con­
tains distinct information about the presence and location of
edge contours. A similar edge enhancement has also been demon­
strated for the SA I units in the monkey (Phillips and Johnson
1981). Hence, mechanisms accounting for enhancement of spatial
contrast and our distinct awareness of edges of objects seem to
occur not only in the central nervous system due to lateral
NO EDGE
□ p ro b e
■ r e c e p tiv e
fie ld
Skin
in d en ta tio n
EDGE
)
t /\/\/\/\/V_ /\/\/\/\/\_
4
A fferen t
S tim u lu s
A fferen t
Ï
Perip h eral
nerve
R esp on se
m a g n itu d e
Figure 9. Edge sensitivity of SA I units. (A) The afferent res­
ponses are shown for two different stimulus conditions
(Edge and No edge condition). The skin displacements
were mechanical sinusoids (16 Hz, 0.2 mm (peak) super­
imposed on a static preindentation (0.5 mm). The dia­
meter of the flat stimulus probe was 6 mm. As can be
seen a stronger response is obtained when the edge of
the stimulus probe cuts the receptive field. (B) A
schematic illustration of the response profile when a
group of SA I units is stimulated by a block indenta­
tion. A marked edge enhancement would appear. (Adapted
from Johansson and Vallbo, 1983).
29
inhibitory mechanisms
as
earlier proposed
(Bekesy
1967),
also at the level of the tactile afferent units. Moreover,
but
it
may be inferred that the vibration transferred to the hand via a
grooved handle on a vibrating tool will account for a stronger
excitation of the type I units than would be the case with a
smooth handle provided that the intensity of the vibration would
be the same.
3.4. Acute impairment of tactile sensibility following vibration
exposure (IV)
The use of vibratory hand-held tools is known to cause an acute,
but reversible, impairment of the tactile sensibility as mea­
sured by psychophysical methods (cf. 1.5)*. A fundamental quest­
ion adressed in the present study is whether the tactile loss is
caused by an influence on mechanoreceptive afferents. If not, it
has to be explained by processes taking place within the central
nervous system.
The microneurographic method previously mentioned was used to
study the thresholds of tactile units in the glabrous skin be­
fore, and after a powerful vibration exposure (IV). Correspond­
ing psychophysical threshold determinations were also carried
out while recording the responses from single afferent units.
Three frequencies were used during the threshold determinations,
2, 20 and 200 Hz. They were chosen on the basis of their capa­
city to activate preferentially SA, FA I and FA II units, res­
pectively. The unit thresholds, as well as the psychophysical
thresholds were determined by a modified version of a threshold
tracking method first introduced by von Bekesy (1939). The ex­
perimenter or the subject, respectively,
exerted control of the
stimulus amplitude through a pushbutton, making it to decrease
while the button was pressed and to increase while the button
was released. During the determination of the unit threshold,
the experimenter pressed the button as soon as one nerve impulse
was evoked per sine wave cycle and released it again as soon as
the impulse discharge disapperared, i.e. the atunal interval
(cf. Talbot et. al. 1968) was tracked. During the psychophysical
30
measurements the subject was asked to press the button as soon
as a vibration could be perceived (upper limen) and not release
it until the sensation of movement completely disappeared (lower
limen). The perception threshold and the unit threshold were
defined as the midpoint of the atunal interval and the midpoint
between the upper and lower limen, respectively.
The frequency of the vibration exposure was either 2, 20 or 200
Hz and the amplitude was preset to 1.5, 1 and 0.25 mm(p_p) at
these frequencies, respectively. These amplitudes, which cor­
respond to accelerations of ca 0.08, 5.6 and 139 m/s^ (rms),
were approximately 20-40 dB above the normal perception thresh­
old at 2 and 20 Hz and 30-50 dB above it at 200 Hz.
The results showed that the vibration exposure could account for
an acute depression of the sensitivity of tactile units. This
depression was temporary and the recovery rate was initially
rapid but it declined with time. With the exposure parameters
used in the present study, a nearly complete recovery took place
within one or a few minutes. Moreover, the magnitudes and the
time courses of the recovery of the encountered neural threshold
shifts were approximately the same as observed for the corres­
ponding psychophysical threshold shifts, provided that the
threshold was measured at the appropriate frequency, i.e. 200
Hz, 20 Hz and 2 Hz for the FA II, FA I and SA I units, respec­
tively. Thus, it was concluded that the acute impairments of the
tactile sensibility caused by vibration exposure as observed in
psychophysical studies most likely is explained by an influence
on the peripheral nervous system,
excitability of the tactile units.
i.e. by a depression of the
31
3.5. Effects of high frequency vibration on tactile perception
threshold (V, VI)
In the light of the suspicions that hand-transmitted vibrations
at frequencies above 1 kHz may have a detrimental effect on
man's peripheral nervous system, (cf. 1.6) two groups, one con­
sisting of dentists (V) and the other of therapists (VI), using
high-speed handpieces and ultrasonic therapy devices, respecti­
vely, were tested with regard to the vibration perception in
their hands. For the sake of comparison two reference groups,
one for the dentists and the other for the therapists, were also
examined. The experimental procedure and apparatus used in the
study of dentists respective therapists were quite similar
except that two different methods for perception threshold
determination were used.
The first,
used on dentists
(V), was based on a continous in­
crease of the vibration amplitude (10 dB/s) up to the perception
level. As soon as the vibration was perceived the subject pres­
sed a remote control button after which the amplitude momentari­
ly dropped to zero and started to increase again when the button
was released. The perception threshold was defined as the ampli­
tude at which the button was pressed. Ten test frequencies with­
in the range of 40 to 400 Hz were used. At each frequency six to
eight separate threshold determinations were made and their
average
formed
the
basis
for
the
data
analysis.
The
second
method used on therapists (VI) was based on the same technique
as used in paper IV. In this study twenty test frequencies with­
in the range of 5 to 400 Hz were used, each during a period of
twenty seconds.
For the dentists a 3 dB higher vibration amplitude in average
was needed for perception for the exposed hand compared to the
other. For the corresponding controls no difference between
hands could be found. When comparing the non-exposed hands of
either group somewhat higher perception thresholds, ca 3 dB,
were observed among the dentists, probably due to an age diffe­
rence of 7 years between the tested groups (cf. Verrillo 1980).
An analysis of variance (Anova, SPSS) has recently been carried
out as regards the threshold data with respect to both groups
32
(Table 4) . As can be seen a significant difference between
controls and dentists
(Source: G) and between hands existed
(Source: H ) . There was also a significant interplay between
groups and hands (Source: G by H) implying that the significant
difference between hands could have existed within any of the
tested groups. However, as no difference was found between the
hands of the controls it was concluded that the difference be­
tween hands has to be within the group of dentists.
For the therapists a slightly higher perception threshold was
seen for their right hands compared to their left hands (all
therapists were right-handed). The difference was about 2-6 dB
and most pronounced at test frequencies above 40-50 Hz. As ex­
pected there was no or little difference between hands of the
reference group. It could, however, be established that two
thirds of the therapists also used their left hands on tiring
occations why both hands have to be considered as exposed to
vibrations. This might explain the slight difference of about
1-2 dB between the therapists left hand and the hands of the
reference group. Thus, it seems likely that the reduction in
tactile sensibility has developed due to contact with ultrasonic
devices.
It can be concluded, that both the dentists and the therapists
showed a reduced tactile sensibility for vibration exposed
hands. Even if these reductions in sensitivity have to be con­
sidered as quite small, they support the idea that vibrations
with high frequencies (> 1 kHz) have a detrimental effect on
man.
Table
<N
VO
CO
00
CO
VO
II)
00
(Tv
(Error
4. Analysis of variance (Anova SPSS) of the vibrotactile threshold data for individual
test frequencies with respect to dentists and corresponding controls. Effect names:
G (Group), S (Subject) and H (Hand). For further information, see text.
33
34
6.
While
DISCUSSION AND CONCLUSIONS
using
entering
the
vibratory
hand-arm
hand-held
tools
system has
all
to be
mechanical
transmitted
energy
through,
and/or dissipated in, the glabrous skin in contact with the vib­
rating handle. In the hope of getting closer to an understanding
of the pathophysiological mechanisms behind the development of
the vibration syndrome, the present study has been addressed
to problems related to the interplay between the vibrating sur­
face and the mechanical properties of the skin as well as the
responses properties of cutaneous mechanoreceptors while exposed
to vibratory stimuli. Furthermore, possible deleterious effects
on tactile sensibility at long-term exposure to vibrations at
frequencies above 1 kHz were also considered, i.e. at fre­
quencies usually not felt.
From the frequency spectras presented in Figure 1, it is clear
that many vibratory hand-held tools used in industry have fre­
quency components within the range of 40 to 160 Hz, i.e. within
or close to the range where the skin's resonant frequencies are
to be found (80-200 Hz) (I). During excitation at resonances the
skin is fairly easy to make vibrate, why strong mechanical
loads, such as compressive and tensile strain, will appear.
Therefore, such tools may have particularly harmful mechanical
influence on the skin tissues. Moreover, the extremly low
psychophysical threshold to vibration within this frequency
range
(Verrillo 1980,
Lidström
et.
al.
1982,
Löfvenberg
and
Johansson 1984) indicate that tactile afferent units are easily
excited by the vibrations produced by such tools. From a compa­
rison between the vibrational intensities produced by the tools
and the sensitivity characteristics and densities of tactile
units in the glabrous skin, it can readily be inferred that a
huge number of mechanoreceptive afferents are strongly excited.
Thus, it is not surprising that these tools evoke intense vibra­
tory
sensations.
While directly recording the
signals
in the
tactile units, it appears that the 4 unit types exhibit diffe­
rent response characteristics to vibrations (II). An interesting
thing is, that one type of units, the FA II units connected to
Pacinian
corpuscles,
is
particularly
easy
to
excite
within
approximately the same frequencies as where the resonance fre­
35
quencies of the skin are found. Indeed, experiments with isola­
ted Pacinian corpuscles
(from the cat's mesentery)
have shown
that the corpuscle respond to vibration within a wide range, but
the optimal frequency being between 150-300 Hz depending on the
temperature (Sato 1961). Thus, under the assumption that the
Pacinian corpuscles of the mesentery and those of the glabrous
skin have
similar properties,
the
findings
suggest
an almost
perfect impedance match between the skin tissue and the relevant
receptors in the high frequency range. In contrast to this, the
mechanical impedance of the skin is high at the frequency ranges
in which the FA I units (ca 8-50 Hz), the SA I units (ca. 4-16
Hz) and the SA II units (< 8 Hz) are most easily excited. Their
regions of best sensitivity must therefore be the results of
mechanical properties of the receptors themselves, and not the
transmitting properties of the skin (cf. Talbot et.al. 1968).
Moreover,
due
to
the
considerable
overlap
of
the
frequency
ranges at which the units respond at higher stimulus amplitudes,
it can be inferred that all types of mechanoreceptive afferents
most likely will be excited while using most vibratory hand-held
tools.
It
has
been
shown
that
vibration
causes
a
vasoconstrictive
reflex among intact blood vessels which is most easily provoked
within the frequency range of 60-200 Hz (Färkkilä and Pyykkö
1979, Welsh 1980), i.e. within the range where the FA II units
are most sensitive. Pyykkö and Hyvärinen (1973) have suggested
that this reflex is sympathetically triggered by afferent acti­
vity from the Pacinian corpuscles, i.e. the FA II units. A repetetive activation of these vasoconstrictive reflexes,
like
during long-term daily use of vibratory tools, probably produces
a hyperthrophy of the muscle layers of the vessel walls which
makes them react more strongly to the sympathetic drive (Ashe
et.al. 1962). At this stage an exposition to vibration, noise
and/or particularly to cold, accounting for a vasoconstriction,
the level for the critical opening pressure of the vessel may be
more easily reached,
triggering a strong vasoconstrictive
attack, such as a VWF attack. Hence, it seems likely that the FA
II units may play an important role in phatophysiology of VWF.
For more detailed information about this hypothesis, the reader
may consult a recent review (Pyykkö et. al. 1982).
36
The present result provide evidence that the temporary losses in
tactile sensibility appearing after vibration exposure, pre­
viously shown in psychophysical studies, to a large extent de­
pends on a depression of the excitability of the tactile units.
(For a brief discussion of possible mechanisms, see paper IV).
During the post-exposure depression of the sensibility, the
control of the precise movements of the hand may be impaired
together with disturbancies in the regulation of grip forces
(cf. Johansson and Westling 1984). An impaired control of the
hand is not only a serious handicap in most activities, but may
also imply increased risks of accidents. Consequently, it cannot
be excluded that the depression of the tactile sensibility
occuring after work with a vibrating tool account for many acute
injuries usually not connected to the vibration, e.g. knife cuts
due to grip slip. The risk for such injuries
may be particular­
ly pronounced during tasks requiring alternations between use of
non-vibrating (knives, hatches, screw drivers etc) and vibrating
tools.
Most previous studies concerning temporary or permanent losses
in vibrotactile sensibility have appearently been limited to
frequencies above 40-50 Hz, i.e.
frequencies normally detected
by the FA II units (e.g. Wedell and Cummings 1938, Hahn 1966,
Gescheider and Wright 1969, Streeter 1970, Tominaga 1973, Lid­
ström et.al.
1982,
Lundström and Lindmark 1982).
Possible im­
pairments of the sensibility at lower frequency detected by FA I
and SA units has thus been neglected. The present findings have
showed that these units also can be depressed by a vibration
exposure. Vibrotactile measurements as an index for vibration
damages should therefore include test frequencies not only
focussing on the FA II units, but also on other types of mechanoreceptive units, i.e. they should be carried out at fre­
quencies below ca 50 Hz as well.
The present results provide evidence that the distinct awareness
about the presence and location of edge contours in contact with
the skin may be due to a pronounced edge sensitivity of the FA I
and particularly the SA I units (III). Thus, a marked edge en­
hancement seems to occur already at the level of afferent
response patterns of the type I units and not only due to late­
37
ral
inhibitory
processes
within
the
central
nervous
system
(Bekesy 1967). It is tempting to speculate that grooved handles
improve the accuracy in the precision control of hand-held
tools.
In addition,
such handles increase the frictional force
against the skin which, in turn, reduces the squeeze force need­
ed for a firm grip. Consequently, with a vibrating tool, the
dissipation of vibrational energy into the hand may be lowered,
at least for vibration components above ca 100 Hz (cf. Mishoe
and Suggs 1977).
On the other hand,
a more vigorous response
from the type I units may increase the risk of a more rapid sen­
sitivity depression (IV). At the present stage, it is difficult
to argue whether this negative effect overweights the advantages
of a grooved handle.
Finally, the present findings support the idea that a long-term
exposure to very high-frequency vibrations
(> 1 kHz) may also
have an untoward effect on man's tactile sensibility. However,
the sensory loss seems to be quite small and the affected per­
sons probably fully adapt to the loss. This might explain that
non within the exposed groups had any subjective complaints
indicating an injury. The mechanisms behind the observed sensory
losses are not at all clear.
According to the guidelines given in ISO/DIS 5349.2 (1984) man's
hand and arm is considered to have an uniform sensibility be­
tween 16 to 1250 Hz when the vibration amplitude is expressed in
velocity and
very little attention is paid to vibration at
higher frequencies (cf. 1.2). Considering the present results it
seems resonable to conclude that there are strong reasons for
changing the
shape of the frequency weighting network. In the
author's opinion, the frequency ranges for low mechanical impe­
dances of the skin (ca 80-200 Hz) and for high tactile sensiti­
vity (ca 100-300 Hz) should be taken into more serious conside­
ration. Moreover, further studies concerning the exposure to
vibration at
high frequencies (> 1kHz) are also of utmost
necessity to obtain reliable basic data for setting the upper
limiting frequency of future guidelines.
38
7.
ACKNOWLEDGEMENTS
This work has been carried out at the Department of Environmen­
tal Medicine, University of Umeå,
in cooperation with The Swe­
dish National Board of Occupational Safety and Health, Technical
unit in Umeå, and with the Department of Physiology, University
of Umeå. I express my most sincere gratitude to these depart­
ments for providing the outmost satisfactory facilities at my
disposal and to all employers who in many ways have contributed
to this thesis.
I wish to thank Prof. Axel Ahlmark and my supervisor Doc. Ludwik
Liszka for their guidance, support and interest throughout this
work.
Special thanks are due to Doc. Roland Johansson, for his super­
vision, guidance and for many fruitful discussions within the
fascinating field of neurophysiology.
Special thanks are also due to Mrs Asta Lindmark and Mr Bertil
Nordström for their technical assistance. Excellent typing work
has been done by Miss Barbro Johansson.
This work has financially been supported by grants from the Swe­
dish Work Environmental Fund (Project ASF 78-156 and ASF 79-104)
and is greatly acknowledged.
Finally, I .am greatly indebted to my wife, Agnetha, and my
children, Anders and Anna, for their support, encouragement and
patience throughout this work.
39
8.
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UMEÅ UNIVERSITY MEDICAL DISSERTATIONS
N ew Series N o 136 - ISSN 0346-6612
Editor: the Dean of the Faculty of Medicine
Centraltryckeriet Umeå, 1985