l contains 2 solids (pelvis and lower
   trunk), exhibits 3 dofs for pelvis/lower trunk joint

    Based on:
    - Damsgaard, M., Rasmussen, J., Christensen, S. T., Surma, E., &amp; De Zee, M., 2006.
     Analysis of musculoskeletal systems in the AnyBody Modeling System. Simulation Modelling Practice and Theory, 14(8), 1100-1111.

   INPUT
   - OsteoArticularModel: osteo-articular model of an already existing
   model (see the Documentation for the structure)
   - k: homothety coefficient for the geometric