The stiff neck that occurs in meningitis is often striking--it is really
stiff, almost boardlike, but not so painful as it is stiff. The stiffness is
caused by reflex spasm of the neck muscles due to traction on inflamed
cervical nerve roots. It is greatest with flexion, less with extension or
rotation. Of course, a lot of older people have necks that are quite stiff
due to osteoarthritis, and if they have a fever, this may occasionally lead
to diagnostic concern for meningitis. Usually their necks are stiff with
both rotational and flexion/extension movement. Associated with the stiff
neck are two other classic "meningeal signs", the signs of Kernig and
Brudzinski. Brudzinski's sign is involuntary flexion of the hip and knee
when the examiner flexes the patient's neck. Kernig's sign is limitation of
straightening of the leg with the hip flexed. Meningeal signs occur not only
in infectious meningitis, but in subarachnoid hemorrage and chemical
meningitis. Unfortunately, meningismus occurs only in about 50% of cases of
bacterial meningits, so the sign is neither highly specific nor highly
sensitive.

http://www.msevans.com/cnsinfections/cnsinf.html