What is 'rippling muscle disease' that bodybuilder Jo Lindner suffered from before death?

German bodybuilder and YouTuber Jo Lindner, who recently died aged 30 after suffering from aneurysm, had recently revealed he was diagnosed with rippling muscle disease (RMD), a rare condition primarily affecting muscles. It's a neuromuscular disorder characterised by muscle hyper-irritability triggered by stretch, percussion or movement with fatigue, cramps, muscle stiffness as common symptoms.