I'm reading the paper "The Problem of the Interrelation of Coordination and Localization", which is written by N. Bernstein. The paper says,
The degree of tension of a muscle is a function, in the first place, of its innervational (tetanic and tonic) condition $E$, and, in the second place, of its length at a given instant and of the velocity with which this length changes over time.
I don't know why the tension of a muscle is the function of those values and I can't understand what $E$ means. Does anyone know?