Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces

Abstract

The aim of this laboratory study was to examine the subjective, physiological and biomechanical responses to prolonged light repetitive manual work during standing on soft (polyurethane standard mat) and hard (aluminum casting) surfaces. The subjects stood on the hard (10 subjects) and on the soft surfaces (11 subjects) for 2 h. Intensity of unpleasantness, shank circumference, electromyograph (EMG) activities from the right soleus and tibialis anterior muscles, mean amplitude and total angular displacement around the left and right ankle in the saggital plane, centre of pressure (CoP) displacement in the frontal and saggital planes, calf surface temperature, and pain intensity in experimentally 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 (right soleus muscle) of the lower leg, a greater amplitude and total angular 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 pain was elicited by injecting hypertonic saline. The intensity of the induced pain was lower when standing on the soft surface. Amplitude, angular distance and CoP displacement showed a tendency to be greater after injection of the hypertonic saline. It was found that the experimentally induced pain influenced postural activity, underlining central interactions between proprioceptors and nociceptors. The results highlighted a higher feeling of comfort when standing on the soft surface. In addition, postural activity was lower when standing on the soft surface, but the activity was sufficient to prevent swelling of the lower legs.