Muscle weakness.  When it's not just lack of fitness.
D002493:Central Nervous System Diseases / Q000150:complications*; D015444:Exercise / Q000502:physiology*; D006801:Humans; D018908:Muscle Weakness / Q000209:etiology* / Q000503:physiopathology / Q000628:therapy; D009135:Muscular Diseases / Q000150:complications; D010523:Peripheral Nervous System Diseases / Q000150:complications*