Lumbar load during one-handed bricklaying

Abstract

Lumbar load, as indicated by the moment of force and the force at the lumbosacral disc, was determined for one-handed bricklaying tasks using a dynamic 3-D model, ‘The Dortmunder’. The grasp height differed (90, 50, 10 cm). By contrast, the final postures were assumed almost upright in all cases. This resulted in considerable variance in the postures during the computer-simulated movements. The task duration varied (2.0, 1.5, 1.0 s). The lower the grasp height and the shorter the time, the higher the lumbar load (moment of force at L5-S1 up to 140 Nm, compressive force up to 6 kN), and the larger the differences between dynamic and static calculations. Increasing brick mass (0, 5, 10 kg) leads to an upward shift in the moment and compression curves (20 Nm or 1 kN per 5 kg). For the assessment of lumbar load during the analysed bricklaying tasks, the lumbosacral moment of force was first classified accoding to Tichauer (1978). Bricklaying involving a 50 cm grasp height requires ‘selection of labor, careful training and rest pauses’. Lower grasp heights of bricks of 10 kg should not occur throughout ‘the entire working day’. Lumbosacral force was then compared with lumbar strength values provided in the literature. These vary within a wide range (0.8–13 kN). Strength mean ± s. d. amounts to 5.0 ± 2.2 kN for the total sample ( n = 507), to 5.8 ± 2.6 kN for males ( n = 174) and to 4.0 ± 1.5 kN for females ( n = 132). Strength dependes primarily on age. Assuming linear regression models, strength (in kN) is 10.53-0.97/decade for males ( r 2 = 0.39) and 7.03-0.59/decade for females ( r 2 = 0.35). A strength prediction model considering 3 additional factors was developed ( r 2 = 0.62) in order to explain most of the remaining variance. Since average values may overestimate an individual's strength, the mean or regression model value should be reduced by the s. d. of the respective sample. This would result in a lumbar load limit of 5.5 kN for 25-year-old men and 2.6 kN for 55-year-old men. Corresponding values for women are 4.1 and 2.3 kN. If the brick-supply stack is 90 cm high, the lumbar load limits will not be exceeded for any person in these age groups. By contrast, all limits would be exceeded for a 10-kg 1-s brick transfer from a grasp height of 10 cm. In conclusion, to ensure that the predicted lumbar load during bricklaying remains below the limits, the brick-supply stack should be above 50 cm.