Muscular modelling: relationship between postural default and spine overloading.

The objectives of the study are to describe and use a muscular model to compare spinal loads and muscles recruitments between an unbalanced subject (patient) and a normal volunteer. Data collection was performed and imputed into the muscular model: from sagittal X-rays, together with plantar foot pressure measurements, external loads for the L3/L4 level were calculated. Using MRI of the thoraco-lumbar region and muscular testing, a personalized muscular model was constructed. The main results are as follow: external loads for the unbalanced subject were higher because of the postural default, especially for flexion moment. Running the model, simulations showed a higher erector spinae group activation for the patient. This induced a significant difference in joint compression. Setting the maximum admissible stress of the extensor muscles of the patient to an equivalent level as the one found for the volunteer to maintain the posture, a second simulation was performed. Joint compression was reduced, but postero-anterior shear and flexion moment increased drastically. The model suggests that either the muscular system needed a stronger activation, yielding a higher joint compression and probably a muscle fatigue in such an activation level, or the spinal loads increased to a higher and probably dangerous level.