This is representative of the sum of the surface detected motor unit action potentials from muscles innervated by that nerve.
Electrical stimulation of nerves reverses the recruitment order, due to the lower resistance of the larger motor neuron axons.
The number of motor units per muscle can change due to aging, disease, or injury.
In neuropathies, motoneurons die off, reducing the number of motor units progressively.
In a similar vein, normal aging also reduces the number of motor units but not to the same degree as disease.