Lumbar Muscle Forces and Spinal Loads Derived from External Loading by Using a Muscle Constitutive Model

Introduction Spinal muscles transmit most of the kinematical forces to the spine tissues and therefore, careful modelling of muscle activity is critical for an adequate estimation of spinal loads. However, current lumbar spine numerical models overlook such muscle contribution or suggest simplifications that do not allow thorough studies of load transfers within the deforming spine. As such, this study proposes a novel active lumbar spine muscle model in order to estimate the muscle forces exerted during upper-body inclination and explore the influence of the muscles elements on the loads transferred to the vertebra and intervertebral discs. Methods A quasi-incompressible transversely isotropic hyperelastic model was adopted for the constitutive behaviour of the passive and active behavior of the lumbar muscles based on [1,4]. The constitutive model [7] accounted for the passive matrix (1) and active and passive muscle fibre strain energy terms (2), where , the stretch ratio and , a strain-like active parameter based on the rest fascicle length.