Metabolic and Psychophysical Comparison of a One-Handed Lifting Task with Different Coupling Factors

In lifting tasks involving manual material handling the coupling component relates to the quality of the hand-toobject interface (grip). Good coupling is theorized to reduce lifting and grip stress, whereas poor coupling is theorized to increase lifting and grip stress. Choosing to lift with one hand is a decision often made by the worker. Understanding the metabolic demand of one-handed lifting tasks is important in job design strategies related to productivity and injury prevention. This study compared the metabolic cost and perceived exertion between a one-handed lifting task performed with identical weight but different coupling and container factors. Twenty volunteers participated (13 M, 7 F; mean 26 + 6 yr; 178 + 8 cm; 74 + 11 kg; grip strength, R = 45 + 13 kg, L = 43 + 14 kg. Participants transferred either a 12.5 kg milk crate or 12.5 kg bag of dog food individually back and forth from the floor to a table by gripping with their dominant hand. Participants performed two, 5 minute work bouts with either the milk crate or dog food bag in random order. Three minutes of rest were allowed between the bouts. Pace was constant at 8 lifts per minute. Metabolic and psychophysical parameters were monitored throughout the work bouts. Steady state data from minutes 2 – 5 was used for analysis with paired T-Test. Results: mc = milk crate; df = dog food; oxygen cost (ml.kg .min -1 ): mc = 15.0 + 3.1, df = 13.1 + 1.9; l.min -1 : mc = 1.1 + 0.2, df = 1.0 + 0.2; kcal.min -1 : mc = 5.3 + 1.2, df = 5.1 + 1.1; RER: mc = 0.9 + 0.0, df = 0.9 + 0.1; HR (bpm): mc = 104.1 + 10.3, df = 100.3 + 9.9; RPE: mc = 9.7 + 2.0, df = 9.3 + 1.7. Significant difference (p<0.05) occurred in oxygen cost (ml.kg .min -1 ) and heart rate (bpm) between the two conditions. No difference was observed in rating of perceived exertion (RPE). In conclusion, a significantly higher oxygen cost and heart rate were observed when participants performed a paced, 12.5 kg one-handed lifting task with good coupling factors (milk crate), than when lifting a 12.5 kg dog food bag with poor coupling factors. When lifting stress is measured by relative oxygen consumption and heart rate, these results are in contrast to expectations that poor coupling factors would increase stress of a lifting task. KeywordsNIOSH Lifting Equation; Grip; Work Tasks