The semispinalis and splenius are strong neck extensors. The semispinalis consists of three parts: the semispinalis capitis, the semispinalis cervicis and the semispinalis thoracis. The splenius is a two part muscle the splenius capitis and splenius cervicis. The capitis and cervicis portions of these muscles have very little, almost zero electrical activity at rest.
With very little tightening and contracting during time away from activity neck extension on a regular basis is important to prevent atrophy. Make neck extension an important part of your exercise regime. Keep and Get the neck Strong.
Neck Extension on the Pendulum 5-Way Neck Machine