We do a lot of pull ups in the gym. One of the flaws that we seem to develop with the pullup is throwing our head back to get the chin over the bar. While this is the standard for a pull up, throwing the head back to achieve it, rather than getting stronger, is a bad habit to develop.
On the simplest level throwing the head back to push the chin up over the plane of the bar, establishes a false finish line for the exercise. We pull until we get to the point where we can assume that throwing the head back will complete the movement. In effect we’re shortening the pull and training our nervous system that this is what is expected. In other words we pre-maturely turn-off the pulling required and exchange it for a throwing the head back, we move from a simple movement to a much more complex one.
The other reason we don’t want to throw the head back is the potential for damaging the structures—muscles, ligaments, nerves and blood vessels that reside in the junction between the head and neck. This motion is the same one that is characterized as Whiplash in a car accident. The repeated low grade whipping of the head causes a contraction in the sub occipitals this creates a potential for pulling on the Cervical vertebrae. Rotations in the Cervicals can cause a host of problems, of which neck pain and dizziness are common.
The repetition of this low intensity movement can result in hypertrophy of these muscles which then occupy more space.
Another more dangerous consequence of repeatedly throwing our head back is the occlusion of the Vertebral Artery from both the rotation of a vertebrae, muscle hypertrophy and the constant entrapment of artery between the skull bone (Occiput) and the Atlas. (The artery is in red in the picture.).
A less conscious effect is that of the limbic system down regulating the force required for a pull up when it senses a threat to the neck and head.
Should we ever throw our head back when doing a pull up? I would argue that the only time this may be an acceptable practice is during a competition, where the number of pull ups is contained. This containment, doesn’t happen in training the pull up over months and years.
Take your time, develop the needed strength to pull yourself up with out some aberrant movement pattern.
And remember there is not reason—for a sane person—to do something harmful to their body in the name of exercise.