Weight and frequency effect on spinal loading in a bricklaying task

In manual materials handling jobs a reduction in the weight of materials often concurs with an increase in handling frequency. The effect of weight and inversely related frequency on spinal load was studied in two bricklaying tasks: building the skin and the floor of a steel ladle. In both tasks five subjects laid bricks of varying weight and frequency (obtained from field observations). The load parameters investigated were peak values and time integrals of the compressive force on the L5-S1 motion segment and stature loss, which is assumed to reflect motion segment creep due to compression. Peak compression was found to increase at higher brick weights. No differences in integrated compression were observed among four out of five combinations of weight and frequency (both in skin and floor building). Laying bricks for a fixed period of 47 min yielded average stature losses of 2.0 3.6 mm. Differences in stature loss among weight-frequency conditions were not significant. In conclusion, at lower weights peak loads decrease, but the benefit of this should be doubted because the frequency of exposure to these peak loads was found to increase. Moreover, this increase was such that no effects were found on spinal load estimates that incorporate both magnitude and time aspects of the load, like time-integrated compression and stature loss.