Under normal conditions when training the neck variation in movement between individuals and intra-individual varies each time one exercises. Range of motion is not consistent either in time or according to the direction of motion. Variability of range of motion can occur even in the prescribed postural position as much as 10-30 degrees.

Based upon cervical intervertebral kinematics, full range of motion is assumed to be the sum of a fixed percentage of the total motion of the skull and the first thoracic vertebrae. The assumption is that relative motion is pure. The complexity of movement is much more complicated - an example: 

The contributions of the middle cervical levels of the vertebrae C3–C4 and C4–C5 are greater near a neutral posture and the lower cervical levels C5–C6 and C6–C7 increase their contributions toward the end ranges of motion. Thus slight changes in form change muscle force as joints affect muscle length and joint angles.

Range of motion of the cervical vertebrae are affected not only by posture, but head position such as retraction, protrusion, chin position, shoulder elevation, seat position and many other subtle movements.

There are >25 pairs of muscles in the neck and all need to be recruited to maximize development. Neck strength is equal to the moment generating capacity of the neck muscles. That is, the sum of moments of all muscles. This, as discussed, means that neck strength is influenced by intervertebral kinematic variation, acquiring a repeatable well postured seat height, replicating form during each repetition and pausing at the top of each movement to ensure maximum participation of the movement associated musculature are keys in Getting the neck Strong.

Pendulum Neck Machine
Shoulder Elevation Changes Ranges of Motion and Muscle Participation