Lift Angle and its Effect on Stresses to the Low-Back

A laboratory study was conducted to study the angle of lift from the trajectory of lifting motion. Six male college students were required to lift three different load levels (40, 70, and 100% of maximum acceptable weight) from the floor to a height of 81 m. Four different box sizes (25, 38, 51 and 63.5 cm) and two different body postures (squat and free style) were employed. Maximum acceptable weights were determined using a psychophysical method. Motion trajectory was investigated by using stroboscopic photography. A biomechanical model was used to study the effects of lift angle on forces and moments imparted to various body joints.

Analysis showed that lifting motions are not performed vertically up but rather at an angle to the vertical. The lift angle increased with an increase in the weight of the load (p < 0.01). Biomechanical evaluation showed that the lifting of a load at an angle from the vertical resulted in decreases in both the compressive force at the L₅/S₁ disc and moments at various body joints (except knee and ankle). The net effect of lifting at an angle was an increase in the percentage of the population capable of lifting a given load. Effects of lift angle on biomechanical stresses to the body and the differences between the psychophysically determined maximum weights and the NIOSH recommended “maximum permissible limits” are discussed.