A comparison of the effects of mixed static and dynamic work with mainly dynamic work in hot conditions.

Current physiological criteria for limiting work in hot conditions are frequently based on responses to mainly dynamic work (eg treadmill walking). Their applicability to industrial situations containing mixed static and dynamic work is questioned, since the physiological responses to static work are different from those of dynamic work. Each of eight subjects attempted a one hour uphill treadmill walk (mainly dynamic work), and an uphill treadmill walk whilst intermittently carrying a 20 kg weight in the arms (mixed static and dynamic work). The external work rates in the two conditions were equal, effected by lowering the treadmill gradient in the loaded condition. Experiments were conducted in a hot climate (33 degrees C dry bulb, 25 degrees C wet bulb). Oxygen consumption, minute ventilation, sweat rate and rated perceived exertion were all significantly higher (p less than 0.001) for the mixed static and dynamic work than for the dynamic work. This was also the case for heart rate and forearm skin temperature (p less than 0.01), and for auditory canal temperature (p less than 0.05). There was no significant difference between the two types of work for mean skin temperature, calf skin temperature and chest skin temperature. These results show that for the same external work, physiological strain and perceived exertion are greater for mixed static and dynamic work (carrying a load in the arms) than for mainly dynamic work (walking on a treadmill). They suggest that it is not appropriate to make direct comparisons of laboratory studies based on dynamic work, with practical situations containing mixed static and dynamic work in the heat.