The pathophysiology of spasticity is a complex subject and one frequently avoided by clinicians. Spasticity and the other features, positive and negative, of the upper motor neurone (UMN) syndrome arise from disruption of certain descending pathways involved in motor control. Hyperexcitability of spinal reflexes forms the basis of most of the positive clinical signs of the UMN syndrome, which have in common excessive muscle activity. These spinal reflexes may be divided into two groups, proprioceptive reflexes and nociceptive/cutaneous reflexes. The clasp-knife phenomenon combines features of both groups, at least in the lower limbs. Contractures are a well known and feared complication of the UMN syndrome, reducing the range of motion of a joint. There has been a recent investigation of the relationship between the stretch reflex hyper excitability of spasticity and contractures. Spasticity does not appear to exist in contracting agonists and would not really interfere with movement.