Terminology. Not sure whether you are asking about an MS "hug", which is a tightness over the trunk due to spinal cord lesion. Muscle cramping in legs can occur in normals, but may also be a manifestation of spasticity. Are you wondering about muscular fasciculations, which could feel like a twitch?
CNS vs muscle. Ms twitching comes from brain and/or spinal cord abnormal electrical signals telling muscle fibers to contract, then relax. Overuse twitching usually involves muscle mechanisms only, stemming from local metabolic conditions.