Computation of the muscle forces parameters in the case of a force-length
 Muscle model : F = Fmax*(A*fa(lm_norm)+fp(lm_norm))

   Adapted from:
       - Sarshari, E., 2018
       A Closed-Loop EMG-Assisted Shoulder Model, Ch. 5: An initialization
       technique for Hill-type musculotendon models, 75-87. EPFL

   INPUT
   - Lm_norm : Muscle length ratio to optimal length
   OUTPUT
   - Fl : Vector of force-length relationship
   - Fp : Vector of passive forces relationship
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