Can exercise shorten your neck?

No: The neck is composed of 7 vertebra. They don't lengthen or shorten. Perception of a short neck is from added muscle mass or adipose tissue in obese people. If you are exercising to build a lot of muscle, it might appear as if the the neck shortens. On the other hand, if you are exercising to lose weight, it may appear as if your neck will lengthen.