Effect of attempted lifting speed on forces and torque exerted on the lumbar spine.

Factors affecting compressive and shear forces and torque at the L5/S1 vertebral joint were examined for nine different load/lifting-speed combinations during performances of a dynamic lifting task by 10 subjects. Loads were 40, 60, and 80% of each subject's maximum lifting capability. Target lifting times were 1.5, 3.5, and 7 s. A rigid link model of the human body was used to derive estimates of axial force, shear force, and torque acting at the L5/S1 vertebral joint from high-speed film data. Variables examined empirically were myoelectric activity at the fifth lumbar level of sacrospinalis and intra-abdominal pressure. The respective effects of load and attempted lifting time were assessed through two-factor ANOVA's and subsequent Scheffé tests. Heavier loads were found to result in significantly higher (P less than .001) values of mean compressive force, mean shear force, and maximum myoelectric activity. Faster target lifting times resulted in significantly higher mean and maximum values of all the variables quantified except intra-abdominal pressure.