Why does multiple sclerosis (MS) often cause muscle weakness?

Muscle weakness can occur in multiple sclerosis (MS) as a result of slowing or loss of nerve impulse conduction. It may also evolve from disuse, especially if lifestyle changes have made an individual more sedentary. Weakness contributes not only to mobility complications, but also to safety hazards. For instance, foot drop is common in MS and can cause abnormal walking patterns, placing strain on muscles and joints that are not meant for walking. This can result in pain syndromes and create a risk of falling.