Basic Notions on Mobility Function

Bones, joints and muscles are the components of the motor apparatus, and their construction can be understood by referring to the basic principles of biomechanics. A bone mineralisation servomechanism explains the plasticity of the system in relation with the specific role of mechanical compression factors applied to the skeleton and generates a piezoelectric microcurrent that stimulates the osteocytes. This mechanism also acts as the technical support for bone regeneration in case of fracture. The joints can be classified in two categories: immobile, like cranial sutures, and mobile, like the intervertebral amphiarthrosis and all the diathroses with different types depending on the articular surface shape and synthesis means. Muscles are viscoelastic actuators, non-reversible and non-linear able to create forces by shortening their contractile part no more than the third of their length which explains the particular mode of construction of the diverse mobile body segments. Command and control of muscles are of great clinical interest. The motor part concerns the functional “motor unit” corresponding to the number of muscular fibres activated in on/off mode by a motoneuron with a variable recruitment explaining a possible proportional control. The sensitive part (proprioception) is given by two transducers: the muscle spindles measure the stiffness of a muscle in order to detect their three different states—relaxed, contracted, stretched—and the Golgi organs measure the force. The position and movement control require a goniometric information given by the skin, thanks to the Ruffini transducers. The conscious and voluntary mobility control applies to movements and not to muscles.