Biomechanical modeling of the control of trunk muscles

This paper presents 2 models postulating specific control mechanisms for trunk movement production. The first model represents the trunk as an inverted pendulum with two degrees of freedom (trunk inclination and flexion/extension in sagittal plane) controlled by 6 mono- and bi-articular muscles. Dynamic equations for trunk extension included gravitational, passive elastic and muscle forces. The equilibrium point hypothesis (λ model) was used to simulate the control processes underlying the movement based on 3 control parameters (reciprocal and co-activation commands, and a damping factor). This model was able to simulate the empirical relationship between trunk inclination and flexion/extension as well as EMG patterns during load lifting. The second model is characterized by a more geometrically elaborated representation of the spine, rib cage and pelvis and uses a finite elements approach. It includes 160 muscle fascicles represented by non-linear springs with threshold lengths modified according to the λ model to produce the muscle recruitment patterns for specific motor tasks. This model, personalized to the shape of two subjects (one scoliotic and one healthy), was used to simulate a lateral bending test. Preliminary results showed the coherent behavior of the multiple muscle structure.