Causes of Muscle weakness. Muscle weakness may be caused by a variety of diseases and conditions, including low sodium or potassium, amyotrophic lateral sclerosis, cerebral palsy, multiple sclerosis, pinched nerve, muscular dystrophy and poisoning.uscle weakness may be caused by a variety of diseases and conditions, including low sodium or potassium, amyotrophic lateral sclerosis, cerebral palsy, multiple sclerosis, pinched nerve, muscular dystrophy and poisoning.