Hypertonicity: increased resistance to passive lengthening of a muscle. The underlying reason for the hypertonicity is not part of the definition. Hypertonicity could be due to a neural drive problem such as spasticity or rigidity or it could be due to increased resistance due to non contractile element changes in the musculotendinous unit such as contracture.
Spasticity is velocity dependent increased resistance to passive lengthening of the muscle: The faster you stretch the muscle the greater the resistance. Spasticity is clearly neural in nature and is associated with the upper motor syndrome (UMN). Involvement of the corticospinal tract is often associated with the upper motor unit syndrome and spasticity. There are a number of clinical features that are also associated with spasticity that are part of the upper motor syndrome and some authors consider these features part of spasticity: associated reactions, hypereflexia, flexor synergy in the upper limb and extensor synergy in the lower limb. However including these phenomena as part of spasticity muddies the water so it is better to limit spasticity to velocity dependent resistance.
Rigidity is increased neural activity throughout the range of muscle excursion and is not velocity dependent: present in both agonist and antagonist. This neural impairment is often associated with basal ganglia disease such as Parkinson’s disease. Lead pipe rigidity describes a constant resistance where when moving a joint resistance is felt and maintains the joint in the altered position. In cogwheel rigidity one feels the resistance rhythmically vary when applying a passive movement. It is thought to be the product of an underlying resting tremor which is masked by the rigidity but can be felt on passive movement.
In TBI you can get all sorts of combinations of neural impairments including spasticity and rigidity. Spasticity in TBI can also be very severe and may well be a somewhat different phenomena to what one sees in conditions like stroke or MS.
NB: Many thanks to gcoe from the physio forum for these definitions!