Applied Ergon~.mics 1982, 13.4, 251-258

The effect of vibration on the skeleton, joints and muscles A review of the literature

S. Carlsi~8

Technical Work Physiology Section, National Board of Occupational Safety and Health, Stockholm, Sweden

Owing to the elasticity and plasticity of the skeleton, joints and muscles, the musculoskeletal system is capable of absorbing and damping mechanical vibration without damage as long as the vibration level is within tolerable limits. However, technical developments have led to the exposure of many people to intolerable variation levels with destructive changes as a resu It.

These injuries to the musculoskeletal system continue to be the subject of research interest. Initially, the joints and joint complaints attracted the greatest attention. Vibration damping takes place mainly in the joints. The incidence of destructive joint changes has been examined in comprehensive clinical, epidemiological and radiographic studies, mainly concentrating on the joints of the hand and arm. The response of muscles to vibration is often expressed in the form of a tonic vibration reflex (TVR) which arises as a result of stimulation of the muscle spindles and therefore resembles the classic tonic stretch reflex. There is increased muscular activation for stabilisation of the joint positions, especially during whole-body vibration. Studies have also disclosed how vibration affects body equilibrium and equilibrium control and how vibration can elicit muscle pain, cramps and reduced muscular strength.

Keywords: Vibration, skeletal system, muscles

I ntroduct ion

The human locomotor system - ie, the skeleton, joints and muscles - is designed to be capable (up to a point) of withstanding, damping and absorbing the mechanical energy generated by impacts, jolts and vibration without sustaining damage. The S-shaped spinal column with shock-absorbing intervertebral discs of cartilage, the arched shape of the foot, the design and fit of the cranial bones, the external shape of the femur and pelvis and the trabecular system within these bones are examples of elastically deformable skeletal designs.

The structure and habitual position of the joints and joint surfaces sheathed in articular cartilage also contribute considerably to the body's ability to absorb jolting and vibration. Ligaments and tendons contain collagenous fibres

The substance of this review was published as "Vibrationers inverkan p~ skelett, led och muskier", by Sven Carls~SIS. Arbete och h~lsa 1980:15. Swedish National Board of Occupational Safety and Health.

which contribute to the attenuation of tensional stress when the wavy shape of the fibres will be stretched out in response to the load. Muscle fibres themselves are not only capable of contraction. They are also elastic to some extent and therefore capable of actively and passively damping the tensional stresses generated by vibration and jolting.

The evidence suggests that the elasticity of the locomotor system is sufficient to absorb and attenuate the jolts and vibration generated during natural locomotion movement and movements made utilising the body's own muscular strength. But during both work and leisure, modern man is now being exposed to jolts and vibration which are scarcely 'natural' from the biological point of view and to which the body has not yet become adapted.

Vibration of the entire body or part(s) thereof occur in the work environment. The mechanical equipment operated by professional drivers, equipment such as buses, tractors, combine harvesters, etc, expose operators sitting on a vibrating seat to whole-body vibration. However, the load from vibrating handtools, such as chain saws, hammers and bolt guns, is essentially concentrated on the hands and arms.

0003-6870/82/04 0251-08 $03.00 ~) 1982 Butterworth & Co (Publishers) Ltd Applied Ergonomics December 1982 251

Sinusoidal oscillations and stochastic jolting in working life affect organs and organ systems in such a way as to give rise to what we now can describe as vibration damage and vibration stress, even if we are still unable to identify the tissues primarily damaged by vibration, or identify reliably the factors which give rise to the various injuries. The hypotheses proposed for causal relationships in cases of vibration damage have generally dealt with circulatory derangement. But effects on the locomotor system and on the central nervous system are becoming the subjects of increasing scientific interest.

The effects of both whole-body and segmental vibration are governed by a number of factors of which frequency, amplitude, exposure duration, vibration direction and the size of the body area in contact with the vibration source are the most important. No special vibration sense has yet been discovered in man, and the perception of vibration varies in a complicated way with the aforementioned factors.

The effects of vibration are of both short and long duration. They are manifested in the form of subjective complaints and as objectively measurable phenomena which may, of course, occur simultaneously. Thus, brief episodes of numbness and pain can be accompanied by fatigue and nausea. However, vibration effects may also occur in the form of threshold changes in sensitivity and impairment of movement precision.

Work with vibrating handtools also gives rise to heavy physical loading because the tools are heavy and work positions often awkward. Long exposure may cause bone and joint damage, in particular some changes at the joints themselves, accompanied by severe subjective symptoms.

Thus, the effects of vibration may be exerted simultaneously on different organs and organ systems of the body, but the emphasis in the following review will be on the effect of vibration on joints, ligaments and muscles. This effect will be elucidated by accounts of subjective complaints, radiographic studies, electromyographic records and animal experiments undertaken to verify hypotheses.

Skeletal and joint changes

The most frequently identified skeletal and joint changes believed to be the result of vibration are undoubtedly changes in the carpus and carpal joints. Radiographic studies have disclosed pathological changes in both the lower radio-ulnar joint and in the joints between the wrist-bones, and in particular the radio-carpal joint (Fig. 1). The joint damage often encountered in people exposed in their work to protracted vibration did not greatly differ from the degenerative and senile changes found in people not exposed to vibration. However, studies including control groups have shown that vibration and jolting may elicit joint damage, or at least accelerate and accentuate the degenerative changes normally occurring in most people. For example, vocational groups working with or without vibrating handtools have been compared.

Horvath and Kakosy (1979) studied 978 forestry workers exposed for years to vibration from chain saws and compared them with a control group of 750 people; 45% of the forestry workers displayed radiographic changes characteristic of arthrosis in the distal radio-ulnar joint but only 13% of the people in the control group. The joint surfaces were deformed as a result of osteophyte formations,

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Fig. 1 Dorsal, outer, aspect of bones of the right hand and the lower ends of the two bones of the forearm. 1-8 the wrist bones of which 3 is scaphoid, 8 lunate and 7 triquetrum, 9 radius, 10 ulna and 11 styloid process. The triangular articular disc fills up the space between ulna and lunate. The lower ends of radius and ulna are connected with each other by a joint called the lower or inferior radio-ulnar joint. The row of the three wrist bones scaphoid, lunate and triquetrum articulate with the radius and the articular disc to form the radio-carpal joint or wrist joint.

displaying an irregular, frequently sclerotic structure and a narrowed joint space owing to cartilage changes. Cystiform rarefaction was found in subchondral bony tissue. The distal end of the ulna was also frequently deformed through hypertrophy of the styloid process of the ulna. In a few cases, calcified deposits were encountered in the articular disc. The authors found that the incidence of arthroses was significantly correlated to both exposure duration and subject age. Similar effects have been found in some manual workers, and hence the phenomenon is not peculiar to vibration stress.

252 Applied Ergonomics December 1982

Suzuki et al (1978) also found that these pathological changes were common in the distal radio-ulnar joint of the 670 forestry workers participating in their study. Like Btlrkle de la Camp (1959) previously, they found that work with pneumatic tools also caused changes in the carpus and carpal joints, especially in the scaphoid and lunate bones. As a result of its shape and location, the scaphoid is constantly exposed to recurrent, minor torsional movements which lead to hairline stress fractures with attendant pseudoarthroses reminiscent of 'march fractures' of metatarsal bones. In the case of the scaphoid, vibration acts more along the long axis of the bone and does not cause any true movement of the bone. But it may produce cracks in the bone, leading in severe cases to the bone's disintegration and necrosis. Similar changes in the carpus have been reported by Schneider (1972) in miners from the Bochum area of West Germany. He described the damage as one of the symptoms of the occupational disorder he calls 'Pressluft-Erkrankung'.

Kumtin et al (1973) made a radiographic comparison of forestry workers who had operated a chain saw for years and an equally large control group with the same age distribution. The most striking radiographic finding was that 20% of the forestry workers displayed vacuoles (4-6 mm in diameter) in the carpal bones near the joint surfaces. No such vacuoles were seen in the control group but small cysts (2-3 mm in diameter) were observed in the central part of the carpal bones. The vacuoles were surrounded by a zone of sclerotic tissue. The authors felt that the vacuoles were the result of long exposure and primarily caused by vascular damage, vasoconstriction and obliteration.

Arthrosis changes, believed to be a consequence of work with compressed air tools, have also been found in and around the elbow joint and the acromioclavicular joint. However, these joint changes did not represent damage specific to vibration work. They are also found, albeit to a lesser extent, in people not exposed to vibration.

Biirkle de la Camp (1959) was unable to discover any spinal changes caused by vibration and jolting, but Freund and Dupuis (1974) felt that there was a correlation between pathological changes in the spinal column and mechanical vibration. When a person in a standing position is exposed to whole,body vibration, he/she parries or at least subconsciously attempts to parry the jolts by means of minor movements of the feet, knees and hips. These options are greatly reduced for people in a sitting position. The body's response then is generally to stiffen the joints. 71% of the tractor drivers and 80% of the truck drivers studied had pathological changes in their spines. But the changes were found in only 43% of factory workers and 29% of artisans. In the view of the authors, this shows that normal degenerative changes in the spine become more pronounced and make their debut several years earlier in groups exposed daily to protracted vibration.

Laboratory experiments on animals, common practice in medical research, were conducted in order to obtain more reliable data on the response of the skeleton and joints to vibration and in order to test prevailing hypotheses and theories.

At the aeronautical laboratory in Kentucky, USA, Jankovich (1971, 1972) studied the way in which vibration affected the growth of the femur, tibia and fibula of young rats. The vibration frequency was 20 to 25 Hz (frequencies

higher than the animals' resonance frequency). The amplitude was either 1.25 or 0.8 mm. The acceleration was 1 g which, in combination with gravity, results in acceleration ranging from 2 g to 0 g. Exposure duration was 2 h twice per day for some of the animals and 12 h daily for other animals over a period of from 35 to 180 days. The animals were placed on a vibrating plate. Their paws were in unrestricted contact with the plate but their trunks were kept relatively stationary. This resulted in constant leg flexion and extension.

No macroscopic changes were found in the vibrated animals. The shape, length and weight of their bones did not differ from parameters for the bones of rats not exposed to vibration. In other words, vibration here did not stunt normal growth. However, vibration did affect bone elasticity. This characteristic declined more than expected for the period of time in question. The longer the exposure, the greater the loss of elasticity. This was because of the vibration-induced changes in the organisation of mineralisation. The trabeculae of the vibrated animals were not concentrically arrayed around the medullary cavity or parallel to the surface of the bone, as is normally the case, but dispersed and disorganised.

The response of bone tissue to jolts appears to be decisive to the joint damage arising as a consequence of vibration. In joints, cartilage provides elastic sheathing of bone ends (Fig. 2). This cartilage consists of cells dispersed in an intercellular substance which, in turn, consists of collagenous fibrils in an amorphous ground substance (mucopolysaccharides, a protein and carbohydrate compound). This intercellular substance contains a system of interstitial channels, microscopically small channels to be sure but large enough for the passage of fluid through the matrix. This fluid (synovia) is a highly viscous lubricant coating joint surfaces. It plays an important role in the ability of cartilage to withstand stress. In joint movements, the cartilage is exposed to cycles of compression and relaxation. When the joint is loaded, fluid pressure in the most superficial layers is the same as in the joint space but lower than in the deeper intertrabecular cavities. So fluid forces its way up from deep layers of cartilage to more superficial layers, and some of it is then expressed out of the cartilage. This flow of fluid from different parts of cartilage

I

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Fig. 2 A schematic view of a joint. 1. Spongy bone 2. Subchondral bone 3. Articular cartilage

Applied Ergonomics December 1982 253

absorbs some of the stress to which the cartilage is exposed during loading. The force which is then transmitted to underlying bone is therefore less than if there were no layer of cartilage. This is what normally happens in moving joints.

But what happens in the case of vibration? No exhaustive answer to this question is a as yet available. However, Radin et al (1973) at Cambridge University in Massachusetts (USA) have made a number of valuable observations in a series of studies conducted in the 1960s and 1970s. The studies were performed on the knee joints of adult rabbits. For one hour each day the animals were exposed to periodic, recurrent jolts at a frequency of 1 jolt/s. The magnitude of the load was identical to each animal's weight, a load which must be regarded as a relatively light and physiologically tolerable load under normal conditions. It should be remembered by way of comparison that the knee in man is exposed to twice the body weight at each step in walling and 3 times body weight during running.

The changes scientists found in rabbit knee joints were about the same as are found in degenerated joints of man. After four days, there was exudation of joint fluid, six days later there was increased stiffness in the subchondral bone, ie, the bone immediately under the cartilage layer, 16 days later there was a loss of mucopolysaccharide through the surface layer and 28 days later this loss was very pronounced even from deeper layers.

The destruction of cartilage tissue was thus preceded by changes in underlaying bony tissue. Numerous healed microscopic fractures in the trabeculae plus increased stiffness were the pathological changes found in subchondral bone. Similar microfractures have also been found in man.

The studies suggest that joint degeneration may very well be a natural consequence of repetitive jolts, even when these jolts, from the force point of view, are within physiologically tolerable limits. Conditions such as these are undoubtedly found in work with compressed air-driven tools and among technicians who work for years on slowly vibrating platforms. The fact that not everyone with heavy work ultimately develops joint disorders suggests that the manner in which a load strikes a joint, rather than the magnitude of the total load, is the decisive factor. In the studies cited, the load always acted on the same part of the joint.

The view that spongy bone has shock-absorbing properties is not new. This was widely held even in the last century. Since this property does not appear to be .related to the viscous flow of blood and fat in the canals and canaliculi of bone tissue, an alternative explanation may be a limited number of trabecular microfractures. A certain number of these fractures appears to be physiologically tolerable. Since the metabolism in spongy bone is high, it is possible that there may be some measure of physiological equilibrium between fracture formation and fracture healing. But any such equilibrium is obviously susceptible to disruption.

The first tissue formed after a fracture, ie, callus, is stiffer and less dense than the bone in trabeculae, and the greater the number of microfractures, the greater the predominance of callus tissue. This also results in a decline in the intertrabecular cavities. Since cartilage obtains its nourishment and oxygen from bone marrow in medullar cavities, the supply of nutrients to cartilage could therefore

be impaired. This would, in turn, accelerate cartilage destruction.

Studies support the hypothesis that cartilage degeneration is correlated to and ensues after an increase in the relative stiffness and density of underlying shock-absorbing subchondral bone and that this stiffness is a consequence of healed micro-fractures having arisen as a result of repeated loading. Studies of autopsy materials from patients having previously displayed signs of joint degeneration have disclosed the same changes in the subchondral bone.

Muscle responses

Rood (1860)was the first person to describe the effect of vibration on human muscle. He designed a handle which could be made to vibrate at frequencies up to 60 Hz with an amplitude of around 6 ram. He found that vibration was accompanied by involuntary contractions in the hand and arms so severe that subjects had difficulty releasing their grip.

Subsequent research has also confirmed that the grip on a handle does increase in intensity when the handle starts vibrating. F/irkkil/i et al (1979) studied 89 forestry workers with years of experience of operating chain saws. A strain gauge was built into the handle of a test saw ( a Partner R 22) for measurement of grip force. The gripping force of the hands increased about 5-9 N during actual sawing compared with the force used during breaks when the engine was idling. It should be noted that people previously troubled by 'white fingers' grasped the chain saw handle harder in relation to their maximal gripping force while sawing than subjects with no such complaint. The difference was statistically confirmed.

Iwata et al (1972) studied the response of the biceps brachii muscle to vibration when subjects held on to a vibrating handtool. When a weak grip force was applied to the handle, muscle activity declined as the vibration frequency rose from 6-3 to 100 Hz. When the grip on the handle was harder, eg, 25% or 50% of maximal gripping strength, a sharp increase in activity was obtained at vibration frequencies of 10'Hz and at 50 Hz. The rise in activity level at the 10 Hz frequency was probably due to heightened demands for stabilisation of the arm within its resonance range (10-16 Hz according to various authors). It is more difficult to explain the rise in activity at 50 Hz. It may have been due to the natural resonance of certain muscle fibres or because certain nervous impulses, whose frequency is around 50 Hz during a powerful contraction of the biceps, are amplified during vibration at the same frequency. An extremely tight grip on the handle elicited great activity in the biceps but activity which was only slightly influenced by vibration.

The degree of activity in the muscles of the arms is not governed solely by the magnitude of the force with which the vibrating tool is held. The arm's position is also important. Dupuis et al (1976) and others have shown that the position of the elbow affects the response of arm muscles to imposed vibration. They found that activity was greater in the three muscles studied (the biceps and triceps brachii and the flexor carpi ulnaris) when the elbow angle was 60 or 180 than when at 90 , 120 or 150 . The force with which subjects held the handle supplying the vibration was consistently 40% of maximal gripping force. Of the three muscles, the triceps displayed the greatest reaction; the

254 Applied Ergonomics December 1982

flexor carpi,ulnaris displayed the weakest response to vibration.

The effect of vibration on muscle is governed by the muscle's degree of stretch. The more a muscle is stretched, the greater its sensitivity to vibration. This is because the sensitivity of the muscle spindles increases with the degree of stretch. The stretching to which muscles are exposed during vibration and jolting stimulates the muscle spindles, whose adequate stimulus is a stretching, into discharging impulses causing reflexive contraction in the muscle.

As early as 1938, Echlin and Fessard found that the muscle spindles of animals in their study were affected by vibration at frequencies from 7 to 32 Hz. Lance (1965) showed that the quadriceps contracted and the knee extended, in a manner similar to the pattern in the patellar reflex, when a vibrating instrument was placed on the muscle. Eklund and Hagbarth (1966) disclosed that vibration also had a strongly stimulatory.effect on human muscle spindles. They found that the flow of impulses from the stretch receptors increased as the amplitude increased up to 2 mm and the frequency increased up to 200 Hz. The more the muscle was stretched, the greater the effect of the vibration. As soon as the vibration ceased, the flow of impulses and the muscle contraction also ceased. Contractions triggered by stimulation of a muscle's spindles by vibration are referred to as 'tonic vibration reflexes'. Even when the reflex ceased when the vibration ceased, there was an after-effect in the form of more rapid muscle tensioning when vibration was repeated 10-15 s later. These muscular responses to vibration were more intense when the muscle was cold. On the other hand, vibration reflexes were depressed when the muscle was warm or the subject had taken barbiturates.

Granit and Henatsch (1956) and Brown, Engberg and Matthews (1967) found that it is the primary annulospiral endings which are stimulated by vibration whereas the secondary flower spray endings are less sensitive to vibration. This may be the reason why a muscle activated by vibration does not respond to a phasic extension reflex, such as a knee-jerk reflex. Vibration stimulation may involve the primary annulospiral endings to such a degree that additional stimulation has no effect. Flexion reflexes are also suppressed by vibration. In practical terms, this could mean that the protective reflexes in the muscles of the lower leg, which normally protect the ankle from twisting or ligament rupture, might not be triggered in, for example, a person working on a vibrating platform who stumbles.

Equilibrium and movement control

Eklund (1972) found that body equilibrium is disturbed by bilateral vibration of certain leg and trunk muscles. The vibration-induced afference from these muscles tended to derange equilibrium, not only because of local changes in the tension of stimulated muscles but also as the result of action on the supraspinal structures important to equilibrium. Even here, stimulation of the muscle spindles by vibration is the cause of this interference in equilibrium. But here it is presumably a question of stimulation of the secondary flower spray endings rather than of the primary annulospiral endings. Authors such as Ischikawa et al (1972) have shown that impulses triggered by vibration are conducted up to supraspinal centres which are as important to equilibrium as the cerebellum.

However, the vibration of calf muscles at a frequency of 150 Hz and an amplitude of 1-8 mm (vibration amplitudes of 0.5 to 1 mm have been shown to elicit the same response) caused all the subjects to sway backwards around the. transverse axis of the ankle. The sway was so great that many subjects were forced to take a step backwards to keep from falling. When the subjects were aware of this effect, they could force themselves to remain in place during the vibration. This voluntary response was always accompanied by increased, dynamic muscular activity suggesting a certain measure of instability. When vibration ceased suddenly, there was always a tendency for subjects to tip forward.

Vibration of the pretibial, hamstring and erector spinae muscles induced forward tipping of the body. To keep from falling forward, subjects moved or leaned back. However, vibration of the quadriceps and abdominal muscles had no effect on equilibrium.

Authors such as Goodwin et al (1972) and Eklund (1972) have shown that the vibration-stimulation of muscle affects the subject's perception of the position and motion of the joint upon which the muscle acts.

It is normally possible for a blindfolded person to keep both arms in the same position and perform the same movements with them at the same time. But when the biceps muscle of a subject's arm is exposed to vibration, the precision control is impaired, as was clearly demonstrated by, for example, Goodwin et al (1972). When blindfolded subjects were required to place one arm in the same position as the other vibrating arm, tl~e difference in the position of the forearms when the elbow joint was flexed could differ by as much as 40 , even when subjects believed the arms to be parallel. When the elbow angle of the vibrating arm was, for example, 100 , the angle of the non-vibrating arm could amount to 140 .

Eklund (1972) showed how kinesthesia in the lower extremities is distorted by muscle vibration. Subjects lay on their backs on a bench with their lower limbs dangling freely over the edge of the bench. One of the legs was then raised, traversing a knee angle of from 90 to about 150 , in three different ways, viz, voluntarily, reflexively (through vibration of the knee extensor) and assisted (leg lifted by an aide). 10 - 15 s later, each subject had to duplicate the position of his first leg with his other leg. Coincidence between the position of the two legs was perfect in the first instance. In the third instance the second leg was extended more than the first. In the second instance, ie, reflexive activation with vibration, the second leg failed to achieve the elevation of the first leg. This is because a vibrating muscle with a high level of receptor activity is perceived as being longer (and the knee bent to a greater angle) than it really is.

In finger movements, in which touch and vision play a major role in control, muscular movement is only slightly affected by vibration. However, increased central control is required to flex a finger (against gravity) when the finger extensor muscle is vibrated.

When an isometrically contracted muscle is vibrated, subjects frequently get the feeling that there is some slow movement in the involved joint, even though the joint is immobilised. Subjects also imagine that the joint is extended when there is a vibration-induced isometric contraction of the elbow flexors.

It is difficult to make a completely accurate assessment of the magnitude of a movement and of a joint's position in

Applied Ergonomics December 1982 255

the arms and legs. If an isometrically contracted muscle is exposed to vibration, it is not possible for a person to respond voluntarily fast enough to compensate fully for the increase in tension which develops in the muscle at the start of vibration or to avoid a brief period of over-compensation when vibration suddenly ceases.

So, human movement control is disturbed by vibration. Perception of the state of contraction and tension in the arm and leg muscles is especially distorted.

In a study of certain postural muscles, Bjurvald et al (1973) found that whole-body vibration elicited a general increase in activity in the soleus, erector spinae and trapezius (superior part), muscles active even prior to vibration, when subjects stood in an upright position. The quadriceps and hamstring muscles were also activated. Activity in these latter muscles, as well as in the trapezius, was intermittent and appeared at a pace which coincided with the vibration frequency to which subjects were exposed (Fig. 3). With subjects in an upright sitting position, the trapezius responded sharply to vertical vibration and the gluteus medius to lateral vibration (Fig. 4). This activity also appeared periodically in step with the applied vibration

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frequency. A certain amount of activity was al~o seen in the erector spinae and abdominal muscles during both vertical and lateral vibration. In whole-body vibration, both tonic vibration reflexes and more continuous muscle involvement for stabilisation of the body were found.

In experiments with rats, Hettinger (1956) found that vibration initially had a stimulating effect on the growth of muscle fibres and thereby represented a type of transient strength training. However, protracted vibration had a completely different effect. Joint pain, muscle pain, muscle weakness, reduced muscular strength and muscle cramp are not uncommon symptoms in people working with pneumatic tools (Stepanek and Kandus, 1970; Lukas and Kuzel, 1971 ; and F/lrkkil~[, 1978).

Concluding remarks

The vibrations to which people are exposed in various contexts must surely create medical problems. But the nature and extent of these problems are in many respects unknown. Judging by the literature, the problems have become the subject of medical research only in recent years. So it is hardly surprising that the results of various epidemiological studies have differed. That is why it is still impossible to draw any definite conclusions regarding the effect of vibration on various tissues and organs.

This also applies to studies of the response of the skeleton and joints to vibration. Our joints are always subject to changes caused by the ageing process. At present, it is virtually impossible to distinguish these changes - degeneration of articular cartilage, areas ofosteoporosis, bone cysts, vacuoles, etc - from changes induced by vibrations with any degree of reliability. That vibration does induce changes is evident from some experimental studies, but what other factors are present, and apparently absent for other studies, are not yet clear. Hence, as the mechanisms through which vibration causes skeletal change are not defined, protective measures cannot be specific, but are inevitably broad in nature.

Whether or not the changes found in animal experiments are pertinent to Man is still the subject of contention. So we have to admit that our knowledge about the link between vibration and changes in our joints is still very poor.

Studies of the response of muscles to vibration have yielded more reliable conclusions. This is because, inter alia, these studies can be conducted on Man directly. The effects of vibration on stability, and on grip strength and the increase in muscle tension, seem to be established. But many question marks still remain even in this field of research, such as the important question of limits for the level of tolerable vibration of different muscles as well as of the whole locomotor system.

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256 Applied Ergonomics December 1982

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Jankovieh, J.P. 1971 Structural development of bone in the rat under

earth gravity, simulated weightlessness, hypergravity and mechanical vibration. NASA CR-1823.

Kumlin, T., Wiikeri, M., and Sumari, P. 1973 Brit JlndustrMed, 30, 71-73. Radiological changes

in carpal and metacarpal bones and phalanges caused by chain saw vibration.

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reflex irradiation.

Lukas, E., and Kuzel, V. 1971 Int Arch Arbeitsmed, 28, 239-249. Klinische und

elektromyographische Diagnostik der Sch/ldingung des peripheren Nervensystems durch lokale Vibration.

Radin, E., Parker, H., Pugh, J., Steinberg, R., Paul, I., and Rose, R. 1973 J Biomechanics, 6, 51-57. Response of joints to

impact loading - I I I . Relationship between trabecular microfractures and cartilage degeneration.

Rood, O.N. 1860 Am J Sci Arts, 24, 449. On contraction of the muscles

induced by contact with bodies in vibration.

Schneider, I. 1972 Die Presslufterkrankung: Mikrotrauma-Anthrose.

Hefte zur Unfallheilkunde, 110, 153-155.

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of the wrist joint among chain saw operating lumber- jacks in Japan.

Glossary

Annu[ospiral endings - sensory nerve terminals within the muscle spindle. Muscle spindle is a sense organ situated in the muscle stimulated by stretching of the muscle.

Arthrosis, pseudoarthrosis - atrophic degeneration due to lack of nutrients to a joint.

Collagenous fibrils - thin wavy fibres. Cystiform rarefaction - bladder like expansion. Cysts - bladders or abnormal sacs containing gas, fluid or

semi-solid material. Femur - thigh bone. Fibula - calf bone'. Flower-spray endings - flower-spray-shaped terminations of

nerve ceils. Hypertrophy - increase in size. Interstitial channels - channels or space in a structure. Intrabecular cavities - hollow spaces between supporting

fibres in a structure. Isometric contraction - increased tension in a muscle without

change of its length.

Applied Ergonomics December 1982 257

Kinaesthes ia - the perception of movement. March f rac tures - a break of the thigh bone or the shin bone

without obvious displacement, sometimes occurring during marching of a fatigue nature.

Medul la ry cav i ty - a hollow area inside a long bone, containing soft yellow bone marrow.

Metatarsa l bones - five bones in the front of the foot, situated behind and connected to the bones of the toes.

Necroses - death of cells. Osteophyte fo rmat ions - bony outgrowths.

Osteoporos i s - a disease of bone characterised by, increased porosity and softness.

Sc lerot i c s t ruc ture / t i ssue - hardening of tissue. Subchondra l (bony t issue) - underneath a cartilage. Tibia - shin bone. Trabecula-u lae - supporting fibres traversing the substance

of a structure. Vacuo le - an empty space. Vascular damage - damage to small blood vessels. Vasoconst r i c t ion - narrowing of the blood vessels.

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