The biomechanical model of upper extremity (UE) is demonstrated as a construction, consisting of lever arm and the joint connection in articulation humeri. The required parameters for this model were obtained by measuring one human subject (male, 56 years old, 74 kg weight, 176 cm height, shoulder length 27 cm, forearm 24 cm, hand langth 21 cm). Objective: Modeling of the effect of abduction and adduction of UE on the load exerted upon the spine by mans of the laws of theoretical mechanics and the application of the results in clinical medicine. Based on three general conditions of balance of planar system of forces in theoretical mechanics the authors calculated the dimensions of reaction forces (HA, HB, VA) originating in the shoulder joint during abduction of upper extremity (UE). The reaction forces (HA, HB, VA) originating in the load exerted upon the shoulder joint, are transferred via mm. spinohumerales on the spine as so called action forces, which are equally high, but act in opposite directions. The case is so called equivalent system of forces of shoulder joint acting on the spine. The HB strength is transferred onto spine as a horizontal traction force, the HA force as a horizontal pressure force. In the abduction of one UE the sum of force moments M1A’ and M1B’ (-2119.32 N cm) and the pressure vertical strength VA2 (-42 N) is exerted upon the spine. In abduction of two UE, the moments of the forces M1A (l.dx.), M1A (l.sin.), and M2A (l.dx.), M2A (l.sin.) are mutually abolished, since these are equal, having opposite direction and common sphere of activity in point A’ and vertical pressure forces VA1 (l.dx.) and VA1 (l.sin.) are summarized (-84 N). Conclusion: The results of calculating the forces acting upon spine indicate that during abduction of both UE the stress on the spine is the sum of two vertical power forces of the 84 N size, in contrast to abduction of one UE only, where the unfavorable effect of rotation force moments (M1A + M2A) of the size M =-2199.32 N cm and the vertical power force VA1 of the =42 N (Newton) take place. Application of the results of modeling of load exerted upon the spine in relation to load upon arms: A) composition of the method of locomotor therapy aimed at exercise of upper extremities for patients with osteoporosis; B) implementation of basic principles of motion activity in every day life; C) recommendation of rehabilitation remedy for patients with acute painful fractures of vertebrae in thoracic region.

        Key words: osteoporosis, biomechanics, spine, upper extremities, motion therapy