Effects of antagonistic co-contraction on differences between electromyography based and optimization based estimates of spinal forces

Estimates of spinal forces are quite sensitive to model assumptions, especially regarding antagonistic co-contraction. Optimization based models predict co-contraction to be absent, while electromyography (EMG) based models take co-contraction into account, but usually assume equal activation of deep and superficial parts of a muscle. The aim of the present study was to compare EMG based and optimization based estimates of spinal forces in a wide range of work tasks. Data obtained from ten subjects performing a total of 28 tasks were analysed with an EMG driven model and three optimization models, which were specifically designed to test the effects of the above assumptions. Estimates of peak spinal forces obtained using the different modelling approaches were similar for total muscle force and its compression component (on average EMG based predictions were 5% higher) and were closely related (R > 0.92), while differences in predictions of the peak shear component of muscle force were more substantial (with up to 39% lower estimates in optimization based models, R > 0.79). The results show that neither neglecting antagonistic co-contraction, nor assuming equal activation of deep and superficial muscles, has a major effect on estimates of spinal forces. The disparity between shear force predictions was due to an overestimation of activity of the lateral part of the internal oblique muscle by the optimization models, which is explained by the cost function preferentially recruiting larger muscles. This suggests that a penalty for active muscle mass should be included in the cost function used for predicting trunk muscle recruitment.

Underestimation of object mass in lifting does not increase the load on the low back

Sudden, unexpected loading on the low back is associated with a high incidence of low back pain. Experiments in which sudden loading was applied during standing revealed increased compression forces on the spine and increased trunk angle, which may cause injury to the spine and hence explain this association. During a more dynamic daily activity, i.e. lifting, this could not be demonstrated, which may be due to experimental constraints. We therefore reinvestigated the loading of the low back when subjects were lifting an unexpectedly heavy object. Ten males lifted boxes, weighing 1.6 or 6.6 kg, at a self-selected lifting velocity. In some trials the mass of these boxes was unexpectedly increased by 10 kg. The ground reaction forces, body movements and trunk muscle activity were measured and from these, the L5/S1 torques and compression forces were estimated. Underestimation of the mass did not lead to an increase in low back loading. This finding was independent of the mass the subjects were expecting to lift. In conclusion, no evidence was found to support inference regarding causality of the association between sudden loading and low back pain during whole body lifting movements.