Dynamic versus static analyses of lifting a box from the floor

Lifting objects from below knee height has been implicated as a source of low back pain and injury. Static models have often been used to assess forces produced in the lumbar region by lifting; however, inertial forces generated by acceleration may be significant. Therefore, the goal of this investigation was to assess differences between static and dynamic analysis methods. Sagittal plane kinematics were collected on 21 men and 22 women of college age while lifting a milk crate (men = 25 kg, women = 15 kg) from the floor to standing knuckle height on level as well as sloped (facing uphill and downhill at 10 degrees and 20 degrees) ground conditions. Both static and dynamic top-down inverse models were utilized to assess net muscular moments at L5/S1 as well as the posture of the person at the time of static max (TSM) and dynamic max (TDM) moments. The TDM moment was significantly later than the TSM in the level through uphill conditions (p < 0.001). The dynamic max moment was significantly greater than the static max moment in all conditions (p < 0.001). Torso angles at TSM exhibited a significantly greater forward lean (by < 2 degrees) in the level through uphill conditions (p < 0.001). Overall low-back curvature, hip angles, knee angles, and ankle angles were not affected by the type of model (p > 0.05), though several minor differences occurred at conditions other than the level (most dramatic in the downhill 20 degrees condition). Therefore, if moments are of interest, a dynamic model should be utilized. However, body position is very similar at TSM and TDM.