Effects on efficiency in repetitive lifting of load and frequency combinations at a constant total power output

Sumary

Boxes were lifted and lowered repetitively at three different combinations of load and frequency. These combinations were chosen such that the total mechanical power generated was constant. Effects of the varying load or frequency conditions (but constant total mechanical power) on the rate of energy expenditure (M) and on the mechanical efficiency (ME) were measured. Mechanical power was determined from film analysis and separated into external power (generated to lift the load) and internal power (to raise the lifter's body mass). The M was determined from oxygen consumption measurements. The ME was calculated in two ways, depending on the definition of mechanical power, including either the external power only (ME_ext) or the total power output (ME_tot). Despite a constant total mechanical power, M increased at higher loads and lower frequencies. This might be explained by the increasing isometric force required in postural and load control. The M increase resulted in a decrease of ME_tot. However, at higher loads and lower frequencies ME_ext increased, indicating that more external work can be done at the same energy costs at higher loads or lower frequencies, which could be of interest from the point of view of occupational physiology. It would seem that at higher loads or lower frequencies the increased costs for isometric muscle action do not outweigh the benefit of raising the body less frequently. Furthermore, it was found that the ME,, in lifting was much lower than the values reported for other kinds of activity. This was due to the large proportion of total power output that was internal power in repetitive lifting [e.g. 83.1% (at a load of 6 kg) in the present study].