Is it better to brace yourself for a car accident or remain relaxed and unknowing?

There isn't any way to test this out on live subjects. Researchers have used spines from human cadavers to study the forces of impact on the neck and back. They are able to come close to the response of an unwarned occupant using this type of model. They can't mimic the active forces of muscles contracting to brace for impact.

Even so, this kind of research gives some good information. Scientists think it may be impossible to build up enough muscle contraction before impact to make a difference. Studies of simulated rear-end collisions show that peak muscle tension occurs between 149 and 924 milliseconds. Strain on ligaments occurs at around 150 milliseconds. Some people may not respond quickly enough to prevent injury.

There are two factors here: the speed of response and the amount of muscle strength needed to protect the neck. Studies so far show the muscles of the neck can't respond quickly enough or with enough strength to protect the neck.

Again, these studies aren't done with live models so there may be some factors scientists can't account for.