A 63-year-old woman was referred because of decreased strength of her right leg manifesting with buckling of the knee for the past five years. Sometimes this led to falls, which made her feel insecure while walking. She experienced some aching in her right heel and in her right knee after long walks. She was able to walk for two hours. She and her husband loved to walk in the mountains, and during those hikes she used a cane. The previous history is relevant because at age 5 years she had suffered from poliomyelitis anterior acuta, which had affected both legs. She had a partial recovery in the sense that she regained normal strength of her left leg and was left with residual weakness of her right leg. She underwent surgery at age 10 years (ankle arthrodesis on the right and epiphysiodesis of the left leg).

When playing slow passages, a 55-year-old professional accordionist noticed painless cramping of the right finger flexors. Treatment with botulinum toxin for suspected dystonia had no effect. In the following months, cramps also occurred at rest, and he had to give up his profession. He also noted twitches in the limb muscles.

An 18-year-old man had noticed progressive painless weakness and a decrease in size of his left forearm and hand for about one year. He was not able to lift objects, and at the gym he had difficulty handling the dumbbells. He had noticed involuntary contractions not only of his forearm muscles, but also of his chest and once in his legs. Previous medical history and family history were unremarkable.

Disease of the lower motor neuron causes a decrease in strength and myotatic reflexes. Involved muscle shows weakness and twitching. The latter is of two types, fibrillation and fasciculation. Fibrillation is due to spontaneous depolarization of single muscle fiber. Fasciculation is due to spontaneous depolarization of the many muscle fibers of a motor unit. As a rule, fibrillation is pathological. Fasciculation may be pathological or benign.