Let’s get one thing straight: Muscle is muscle. Despite what many fitness influencers may have you believe, there’s no such thing as "lean muscle" or "bulk muscle." It’s all made from the same stuff.

More than 30 rare muscular dystrophy types cause progressive muscle weakness from inherited gene mutations. Combined, they affect about 1 in 5,000 to 8,000 people. Common subtypes vary by age of onset ...