A new method for evaluating the electromyography data obtained for transversal striated muscles of human lower extremities has been developed. The new method is based on nonlinear dynamics and thermodynamics and can be used for the identification of patholodies. The main parameter that describes the coherence of muscle work is the distance between two trajectories of the potential, which is measured for the similar muscles of two extremities. Based on these parameters, the Lyapunov index and the initial conditions forgetting time were obtained, which in turn, reflects, in general, the chaotic dynamics of muscle activity. The application of the theory of deterministic chaos to the analysis of electromyograms allows one to improvise the quality of diagnosis of diseases of the peripheral nervous system and to control more effectively the course of their treatment. The advantages of the new method are the pictorial presentation of the data, self-descriptiveness of the Lyapunov index determination, and the nonlinear characteristics of the phase-plane portraits in pathology risk evaluation. The results obtained confirm the important role of the nonlinear balance of the in systems normal and pathologic.