In the biomechanical field the constraint and loading conditions are due to the muscular action to the articular reactions and to the external actions that constrain or solicit the biological structure under examination. The identification of the most significant loads is not trivial and is fundamental, since it is impossible to simulate all possible and real physiological stresses; in particular, it is necessary to distinguish the cases respectively in which one wants to make a verification of the resistance of the structure or if one wishes to study its behavior over time (fatigue or wear analysis); in the first case it is good to consider the most onerous condition from the structural point of view; in the second case, it is instead advisable to consider the most recurrent loads. Often for the choice of module, direction, way and point of application of constraints and loads reference is made, when available, to literature data.