Mechanical vibration arising from powered processes or tools and entering the body at the fingers or the palm of the hands is called hand-transmitted vibration. Frequent synonyms for hand-transmitted vibration are hand-arm vibration and local or segmental vibration. Powered processes and tools which expose operators’ hands to vibration are widespread in several industrial activities. Occupational exposure to hand-transmitted vibration arises from hand-held powered tools used in manufacturing (e.g., percussive metal-working tools, grinders and other rotary tools, impact wrenches), quarrying, mining and construction (e.g., rock-drills, stone-hammers, pick-hammers, vibrocompactors), agriculture and forestry (e.g., chain saws, brush saws, barking machines) and public utilities (e.g., road and concrete breakers, drill-hammers, hand-held grinders). Exposure to hand-transmitted vibration can also occur from vibrating workpieces held in the hands of the operator as in pedestal grinding, and from hand-held vibrating controls as in operating lawn mowers or in controlling vibrating road compactors. It has been reported that the number of persons exposed to hand-transmitted vibration at work exceeds 150,000 in the Netherlands, 0.5 million in Great Britain, and 1.45 million in the United States. Excessive exposure to hand-transmitted vibration can cause disorders in the blood vessels, nerves, muscles, and bones and joints of the upper limbs. It has been estimated that 1.7 to 3.6% of the workers in European countries and the United States are exposed to potentially harmful hand-transmitted vibration (ISSA International Section for Research 1989). The term hand-arm vibration (HAV) syndrome is commonly used to refer to signs and symptoms associated with exposure to hand-transmitted vibration, which include:

• vascular disorders
• peripheral neurological disorders
• bone and joint disorders
• muscle disorders
• other disorders (whole body, central nervous system).

Leisure activities such as motorcycling or using domestic vibrating tools can occasionally expose the hands to vibration of high amplitude, but only long daily exposures may give rise to health problems (Griffin 1990).

The relationship between occupational exposure to hand-transmitted vibration and adverse health effects is far from simple. Table 1 lists some of the most important factors which concur to cause injuries in the upper limbs of vibration-exposed workers.

Table 1. Some factors potentially related to injurious effects during hand-transmitted vibration exposures

Vibration characteristics

      Magnitude (r.m.s., peak, weighted/unweighted)

      Frequency (spectra, dominant frequencies)

      Direction (x-, y-, z- axes)

 Tools or processes

Ø  Tool design (portable, fixed)

Ø  Tool type (percussive, rotary, rotating percussive)

Ø  Condition

Ø  Operation

Ø  Material being worked

 Exposure conditions

v  Duration (daily, yearly exposures)

v  Pattern of exposure (continuous, intermittent, rest periods)

v  Cumulative exposure duration

 Environmental conditions

      Ambient temperature

      Airflow

      Humidity

      Noise

      Dynamic response of the finger-hand-arm system

      Mechanical impedance

      Vibration transmissibility

      Absorbed energy

Individual characteristics

·         Method of working (grip force, push force, hand-arm posture, body position)

·         Health

·         Training

·         Skill

·         Use of gloves

·         Individual susceptibility to injury 

Biodynamics

It may be presumed that factors influencing the transmission of vibration into the finger-hand-arm system play a relevant role in the genesis of vibration injury. The transmission of vibration depends on both the physical characteristics of vibration (magnitude, frequency, direction) and the dynamic response of the hand (Griffin 1990).

Transmissibility and impedance

Experimental results indicate that the mechanical behaviour of the human upper limb is complex, as the impedance of the hand-arm system—that is, its resistance to vibrate—shows pronounced variations with the change in vibration amplitude, frequency and direction, applied forces, and orientation of the hand and arm with respect to the axis of the stimulus. Impedance is also influenced by body constitution and structural differences of the various parts of the upper limb (e.g., the mechanical impedance of the fingers is much lower than that of the palm of the hand). In general, higher vibration levels, as well as tighter hand-grips, result in greater impedance. However, the change in impedance has been found to be highly dependent on the frequency and direction of the vibration stimulus and various sources of both intra- and inter-subject variability. A resonance region for the finger-hand-arm system in the frequency range between 80 and 300 Hz has been reported in several studies.

Measurements of the transmission of vibration through the human arm have shown that lower frequency vibration (>50 Hz) is transmitted with little attenuation along the hand and forearm. The attenuation at the elbow is dependent on the arm posture, as the transmission of vibration tends to decrease with the increase of the flexion angle at the elbow joint. For higher frequencies (>50 Hz), the transmission of vibration progressively decreases with increasing frequency, and above 150 to 200 Hz most of the vibrational energy is dissipated in the tissues of the hand and fingers. From transmissibility measurements it has been inferred that in the high-frequency region vibration may be responsible for damage to the soft structures of the fingers and hands, while low-frequency vibration of high amplitude (e.g., from percussive tools) might be associated with injuries to the wrist, elbow and shoulder.

Factors influencing finger and hand dynamics

The adverse effects from vibration exposure may be assumed to be related to the energy dissipated in the upper limbs. Energy absorption is highly dependent on factors affecting the coupling of the finger-hand system to the vibration source. Variations in grip pressure, static force and posture modify the dynamic response of the finger, hand and arm, and, consequently, the amount of energy transmitted and absorbed. For instance, grip pressure has a considerable influence on energy absorption and, in general, the higher the hand grip the greater the force transmitted to the hand-arm system. Dynamic response data can provide relevant information to assess the injury potential of tool vibration and to assist in the development of anti-vibration devices such as hand-grips and gloves