Anterior-posterior Shear Forces Research Articles

The Effect of Variations in Trunk Models in Predicting Muscle Strength and Spinal Loading

A unified modeling technique is proposed to predict the strength contour of trunk muscles during isometric exertions, which can identify the feasibility of task performance and quantify the utilization ratio for assessment of risk of development of muscular fatigue and over-exertion injury. In addition, the proposed model can estimate the joint reaction forces during performance of the feasible complex loading task in upright standing. Six anatomical models and six cost functions suggested by various research groups were considered in determining their effects on the predicted three-dimensional strength and lumbar intervertebral joint reaction forces obtained using linear and nonlinear optimizations. The present investigation has shown the importance of the fidelity of anatomical models used in estimation of spinal loading and trunk strength. The effect of cost functions on estimated compression and anterior-posterior shear forces was statistically significant. These models can assist in identifying the injurious tasks that must be ergonomically modified to prevent the risk of occupationally related low back injuries.

The Effect of Lifting vs. Lowering on Spinal Loading

In industry, workers perform tasks requiring both lifting and lowering. During concentric lifting, the muscles are shortening as the force is being generated. Conversely, the muscle lengthens while generating force during eccentric lowering. While research on various lifting tasks is extensive, there has been limited research performed to evaluate the lowering tasks. Most of the research that does exist on lowering has investigated muscle activity and trunk strength. None of these studies have investigated spinal loading. The current study estimated the effects of lifting and lowering on spinal loads and predicted moments imposed on the spine. Ten subjects performed both eccentric and concentric lifts under sagittally symmetric conditions. The tasks were performed under isokinetic trunk velocities of 5, 10, 20, 40, and 80 deg/s while holding a box with weights of 9.1, 18.2, and 27.3 kg. Spinal loads and predicted moments in three dimensional space were estimated by an EMG-assisted model which has been adjusted to incorporate the artifacts of eccentric lifting. Eccentric strength was found to be 56 percent greater than during concentric lifting. The lowering tasks produced significantly higher compression forces but lower anterior-posterior shear forces than the concentric lifting tasks. The differences in the spinal loads between the two lifting tasks were attributed to the internal muscle forces and unequal moments resulting from differences in the lifting path of the box. Thus, the differences between the lifting tasks resulted from different lifting styles associated with eccentric and concentric movements