Reliability of the psychophysical approach to manual lifting of liquids by females

The psychophysical method for setting lifting standards was evaluated by having seven, female college students lift at four diiferent frequencies (1, 3, 6 and 12 lifts min−1). Only one lifting session was performed in a 24 hour period. During the 4 hour lifting task, subjects were asked to select the amount of water that they believed they could lift comfortably for 8 hours. Subjects were encouraged to make as many weight changes as they needed. Each time the weight was changed the carton was weighed and the time was recorded. At 15 min intervals, subjects were asked about their degree of confidence (DOC) that the current weight was the maximum acceptable one for an 8 hour shift. Oxygen consumption was measured at 30,120 and 240 min of the task. The weights chosen by the subjects at 30 min did not differ significantly from the 4 hour values for frequencies of 1, 3 and 6 lifts min−1. However, at 12 lifts min−1 the weight decreased with time such that the 4 hour value was 23% lower than the weight chosen after 30 min. DOC increased over time and did not differ significantly between frequencies. Oxygen consumption was unchanged over time and accounted for 19, 25, 35 and 45·5% of VdotO2 max for frequencies 1, 3, 6 and 12 lifts min−1, respectively. It was concluded that the psychophysical method in its present form should not be used to set lifting standards for frequencies higher than 6 lifts min−1.