Comparison between EMG-based and optimisation-based approaches for back-muscle forces and intervertebral efforts

In biomechanics, computing muscle forces and joint efforts with mathematical optimisation copes with the muscle-redundancy problem, i.e. an infinity of possible muscle forces for a unique configuration. Achievements have been made to develop cost functions that reflect physiologically more correct muscle strategies and to validate them with experiments. It has also been proposed to use experimental input such as electromyography (EMG) in the model to guide the optimisation computation. In line with that, the present study proposes an EMG-based approach to compute back-muscle forces and the resulting intervertebral efforts in a horizontal static configuration of the trunk. This approach is based on EMG signals of three back muscles, lumbar and thoracic paravertebral muscles and the quadratus lumborum (QL), recorded on 19 healthy male subjects. Results of this approach were compared with those from optimisation computations involving four cost functions, classically used in the literature for the trunk and the spine. Our approach showed that muscle forces and intervertebral efforts were in line with these computed by mathematical optimisation, but muscle forces obtained with our approach were more representative of the measured EMG signals compared to muscle forces computed by optimisation. Indeed, three of the four cost functions completely missed to recruit the QL, while the latter was clearly activated during the experiment. This result highlights that EMG and experimental input should be more considered when using a musculoskeletal model and optimisation tools. Since the EMG-based approach used in this study was based on a pure deterministic distribution of a global equivalent force, future work will focus on involving EMG input in the optimisation process to guide its solution in a more physiological manner.