As has been mentioned....spasticity. Spasticity can affect mobility but there are meds that can help.
http://www.nationalmssociety.org/abo...ity/index.aspx

Vibrations/buzzing is a separate symptom known as an abnormal sensation (normal for MS) or a Sensory symptom. Many will notice an increase of vibrations after or during exercise. Once you have relaxed a bit the vibrations will decrease or completely go away.

You can also have vibrations/buzzing when doing nothing.
http://www.mult-sclerosis.org/mssymptoms.html