What is spasticity?
Spasticity is a contraction of muscles which can not be let go.
Spasticity is caused by the absence of inhibitory influences from the cortex. Our nervous systems have two types of signals: excitory and inhibitory. A balance between excitation and inhibition is crucial for healthy movement, cognition and behaviour. Our higher brain functions (cortex) are responsible for neurological inhibition to counterbalance the excitatory signals from the spinal chord. When a part of the brain is damaged, the spinal cord reflex of tensing the muscles increases when the inhibition from the cortex falls away. This results in spasticity. The severity of the spasticity depends on the number of nerve pathways and connections which are damaged.
One can see non-spasticity as the result of activity in the brain. When that activity falls away, this results in spasticity.
Spasticity and stretching
Stretching does not help to remove spasticity. After all, spasticity is caused in the nervous system, not in the muscles.
In addition, muscles have an automatic stress reflex to protect them from tearing. When tension is applied to a muscle, it automatically reacts by contracting. For example, think of the knee reflex. This is why it is painful to stretch a contracting muscle. If you try and stretch a muscle which is contracted, it will tend to pull even harder. This does not reduce the spasticity. To the contrary, the spasticity will tend to get even stronger.