SUBJECTIVE, PHYSIOLOGICAL AND BIOMECHANICAL RESPONSES TO PROLONGED MANUAL WORK PERFORMED STANDING ON HARD AND SOFT SURFACES

The aim of this laboratory study was to examine the subjective, physio logical and biomechanical responses to prolonged light repetitive manu al work during standing on soft (polyurethane standard mat) and hard ( aluminum casting) surfaces. The subjects stood on the hard (10 subject s) and on the soft surfaces (11 subjects) for 2 h. Intensity of unplea santness, shank circumference, electromyograph (EMG) activities from t he right soleus and tibialis anterior muscles, mean amplitude and tota l angular displacement around the left and right ankle in the saggital plane, centre of pressure (CoP) displacement in the frontal and saggi tal planes, calf surface temperature, and pain intensity in experiment ally induced muscle pain were recorded. Maximal voluntary contraction and fatigue tests were performed before and after the 2 h experiment. Standing on a soft surface caused a lower intensity of unpleasantness. During standing on a hard surface compared to a soft one the results showed an enhanced swelling of the shank, an increased EMG activity (r ight soleus muscle) of the lower leg, a greater amplitude and total an gular displacement, and a larger CoP displacement in the frontal plane . Indications of more pronounced muscle fatigue while standing on the hard surface were also noticed. After 105 min, experimental muscle pai n was elicited by injecting hypertonic saline. The intensity of the in duced pain was lower when standing on the soft surface. Amplitude, ang ular distance and CoP displacement showed a tendency to be greater aft er injection of the hypertonic saline. It was found that the experimen tally induced pain influenced postural activity, underlining central i nteractions between proprioceptors and nociceptors. The results highli ghted a higher feeling of comfort when standing on the soft surface. I n addition, postural activity was lower when standing on the soft surf ace, but the activity was sufficient to prevent swelling of the lower legs.