about the muscle's tension's variables

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?

The full citation is:

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. intact organism the length of a muscle is in its turn a function of the angle of articulation $\alpha$; for this reason we may write that the momentum of a muscle with respect to a joint is: $$F = F(E, \alpha,\frac{d\alpha}{dt})$$

In more detail, $E$ is a level of innervation of the muscle,

$\alpha$ is an angle of articulation,

$\frac{d\alpha}{dt}$ - angular velocity,

$F$ - momentum of the muscle.

So this is a basis of Bernstein's theory which connect momentum of the muscle $F$ with its innervation $E$. His further developments on the subject allow to draw an equation of for the movement of a single limb in a gravitational field under the influence of a single muscle.