Clonus is commonly seen in the ankle but may exist in other distal structures as well, such as the knee or spine. Diagnosis
The clinical underpinnings of two of the most common spasticity conditions, spastic cerebral palsy and multiple sclerosis, can be described as follows: in spastic diplegia, the upper motor neuron lesion arises often as a result of neonatal asphyxia, while in conditions like multiple sclerosis, spasticity is thought by some to be as a result of the autoimmune destruction of the myelin sheaths around nerve endings—which in turn can mimic the gamma amino butyric acid deficiencies present in the damaged nerves of spastic cerebral palsy children, leading to roughly the same presentation of spasticity, but which clinically is fundamentally different from the latter.Spasticity is assessed by feeling the resistance of the muscle to passive lengthening in its most relaxed state. A spastic muscle will have immediately noticeable, often quite forceful, increased resistance to passive stretch when moved with speed and/or while attempting to be stretched out, as compared to the non-spastic muscles in the same persons body (if any exist). As there are many features of the upper motor neuron syndrome, there are likely to be multiple other changes in affected musculature and surrounding bones, such as progressive malalignments of bone structure around the spastic muscles (leading for example to the scissor gait and tip-toeing gait due to ankle equinus or ankle planter flexion deformity in spastic cerebral palsy children.
Spasticity  (from Greek  spasmos- drawing, pulling) is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles. Clinically, spasticity results from the loss of inhibition of motor neurons, causing excessive velocity-dependent muscle contraction. This ultimately leads to hyperreflexia, an exaggerated deep tendon reflex. Spasticity is often treated with the drug baclofen, which acts as an agonist at GABA receptors, which are inhibitory. Spastic cerebral palsy is the most common form of cerebral palsy, which is a group of permanent movement problems that do not get worse over time. GABAs inhibitory actions contribute to baclofens efficacy as an anti-spasticity agent. Cause
Spasticity mostly occurs in disorders of the central nervous system (CNS) affecting the upper motor neurons in the form of a lesion, such as spastic diplegia, or upper motor neuron syndrome, and can also be present in various types of multiple sclerosis, where it occurs as a symptom of the progressively-worsening attacks on myelin sheaths and is thus unrelated to the types of spasticity present in neuromuscular cerebral palsy rooted spasticity disorders.The cause of spasticity is thought to be where an imbalance occurs in the excitatory and inhibitory input to α motor neurons caused by damage to the spinal cord and/or central nervous system. The damage causes a change in the balance of signals between the nervous system and the muscles, leading to increased excitability in  muscles.