The Hand in Manual Work - Physiological and Psychophysical studies

Doctoral thesis, 2004

This thesis focuses on the hand in manual work. In particular, we have studied what happens to skin and muscles when gripping a hand tool. Different aspects of gripping that will be discussed are: pressure on the hand, effects on the skin blood flow, muscle activity and the use of wrist orthoses. Manual work is often associated with gripping, which results in pressure on the hand. The relationship between applied and perceived pressure and determined discomfort and pain thresholds was studied. Pressure was applied with an algometer. Free modulus magnitude estimation of the subjective pressure levels was made on the finger, the palm and the thumb muscle. The pressure was rated higher at the thumb muscle than on the finger and palm. The pain and discomfort thresholds were lower at the thumb muscle than at the two other application sites. Pressure is important when discussing tools. Preferences regarding power tool handles and the pressure distribution in the hand when drilling were considered. The pressure in the hand was highest on the web between the index finger and the thumb. The use of a supporting handle, gloves, or handles covered with compressible rubber, decreased the pressure on the hand. Pressure on the hand will also affect skin blood flow. The relationship between external pressure and skin blood flow on the palm and fingers was investigated. A special probe was designed for measuring the skin blood flow with a laser Doppler flowmeter and the pressure with an algometer simultaneously. Results show that the skin blood flow in the hand stops at about 50 kPa. Besides pressure on the hand, gripping is associated with flexor and extensor muscle activity. Fatigue effects are more often seen in the extensor muscles and wrist orthoses have been suggested to reduce the load on the extensor muscles. The effects of wrist orthoses on flexor and extensor muscle activity in gripping and manual tasks were studied. Surface EMG from two flexor and two extensor muscles in the forearm was recorded. The results show that the wrist orthoses tested did not reduce the EMG activity. In conclusion this thesis provides new information which may be of importance for the future design of hand tools.