Biomechanical Modelling of Cart Pushing and Pulling.

Abstract

A dynamic biomechanical model is formulated. With this model, slip potential and body postures which minimize slip potential for given h and forces when pushing or pulling a cart can be studied. In addition, back muscle and vertebral column loadings can be predicted as a function of task and subject variables. For validation of the model, various cart pushing and pulling situations were simulated in a laboratory. The dynamic foot forces, h and forces required to move the cart, body motions at various speeds, and back muscle actions were measured while six subjects pushed and pulled a specially constructed cart. The foot forces predicted by the model were compared to foot forces measured by the force platform (r('2) = .56 for the horizontal foot forces and r('2) = .65 for the vertical foot forces). The muscle forces predicted by the model were converted to equivalent IEMGs and then compared to IEMGs measured during the dynamic cart pushing and pulling (r('2) = .31 for the erector spinae muscle and r('2) = .44 for the rectus abdominis muscle). The peak coefficients of friction predicted by the model were also compared to the peak coefficients of friction measured by the force platform (r('2) = .64). Simulations of the model on task variables, such as h and load, velocity of motion and cart h and le height as well as anthropometric variables such as subject's weight, were performed to find their effects on slip potential and back loading. Some practical observations from the application of the model are: (1) The predicted levels of the coefficient of friction required to stop the foot from slipping were decreased by using a high h and le in pushing and a low h and le in pulling; however, these h and le heights resulted in adverse effects on the predicted loads on a subject's low-back. (2) A speed of pushing or pulling over approximately 3.6 Km/hr caused higher predicted compressive forces than the NIOSH recommended limit, even when relatively light (approximately lower than 98 newtons) h and loads were required to move the cart. (3) Heavier subjects' body weight decreased the required level of the coefficient-of-friction in both the pushing and pulling tasks studied.