The biomechanical function of individual neck muscles is difficult to characterize. This is true because we are unable to voluntarily activate isolated neck muscles. Not eliciting movement based solely on a muscles underlying biomechanics presents optimizing their strength more challenging.

The neural system that controls the neck musculature combines the biomechanics of the >25 pairs of muscles to achieve its simultaneous goals of movement and stability. As activation increases during training the biomechanical constraints based upon the direction of the movement and other factors dictate the activation of synergistic neck muscles, which contributes to the target direction. This synergistic effect elicits more musculature activation and larger net forces - an important reason to pause at the top of the movement of a repetition.

Pausing provides the recruitment time necessary to Get Strong.

Neck training on the Pendulum Neck Machine
Pausing at the Top of a Repetition