Inertial effects from single body segments in dynamic analysis of lifting

Biomechanical calculations of loads on the locomotor system in lifting tasks are often based on static models. Several investigators have shown, though, that in a dynamic act like lifting, inertial effects are not generally negligible. A complete dynamic analysis includes the calculation of linear and angular accelerations of body segments and is rather complicated to carry out. So it was of interest to find out whether the contribution of single body segments to the dynamic effects of the whole body might be negligible. Ten male subjects lifted a 12.8 kg box using two different techniques and two speeds. Ground reactions and body segment movements were recorded and reaction forces and net moments at different joints were calculated with static, dynamic and 'semidynamic' methods. The latter method incorporated the ground reaction forces as measured from a force plate but was otherwise equal to the static method. The results from semidynamic analyses agreed well with those from complete dynamic analyses for the ankle, knee, hip, and L5/S1 joints. For example, the semidynamically calculated peak moments at L5/S1 differed on average less than 3% from the dynamically calculated values. The contributions of the lower extremities and the pelvis to the dynamic effects of the whole body seemed to be quite small. The less complicated semidynamic method offers a good alternative to a fully dynamic analysis for estimations of peak moments in the lower back, at least for the studied lifting techniques.