Comparison between two dynamic methods to estimate triaxial net reaction moments at the L5/S1 joint during lifting

OBJECTIVE: The purpose of the study was to evaluate the validity of two biomechanical dynamic tridimensional (3D) multisegment models. DESIGN: An experimental study was conducted for this purpose. BACKGROUND: The models that were compared are commonly used to analyse lifting tasks. The upward model used the measured external forces at the feet, whereas the downward model used an estimation of the forces at the hands. The downward model approach is more practical to study manual materials handling (MMH) tasks but needs improvement to produce valid estimates. METHODS: One male subject participated in the study. Four cameras and two force platforms were used to get the data from 66 static and 108 dynamic trials. The triaxial net moments at L5/S1 of both models were compared. RESULTS: Good agreement was observed between the two models for the moments of the static tasks but great disparity was observed for the moments of the dynamic tasks (up to 78 Nm in extension). Both models were sensitive to different task parameters (movement or load asymmetry, movement rate, load magnitude). CONCLUSIONS: The direct measurement of the external forces at each hand is recommended. The validation should be performed with more subjects for generalization.