Analytic joined statistical - thermodynamic expression for entropy as a function of state for measuring disorder of the corresponding micro state and the mechanical parameters which feature in Hooke’s law on elasticity as a measure of macro state was used in the situation of isothermal elongation of viscoelastic ligament biostructure simultaneously with external force of constant intensity to determine the change of entropy and the resultant reactive elastic force in the function of relative elongation. The sample used for the analysis and testing of the original joined theory shown with adequate equations is tested on linear biostructure approximating the data of Lig. collatelare fibulare which strengthens the lateral side of the knee joint. The obtained results for the tested linear biostructure according to which the minimal value of the difference in dS corresponds to the state of maximal entropy and the minimal value of elastic force indicates the acceptable level of elongation at which a reversible process is still possible, with low probability. This leads to the conclusion that, according to the results obtained here, the biostructure under examination can withstand the elongation which is approximately equal to twice the initial length L0. The relation between the friction force Ftr and the elasticity force Fe for the initial state is 1.0417, which indicates that the friction force is to a certain extent higher than the elastic force. With elongation this ratio changes in favour of the friction force and becomes more prominent with advanced age.