Liver and muscle biopsies reveal that glycogen content in the liver is normal but muscle glycogen is elevated. the biochemical structure of glycogen in both tissues appears to be normal. what is the problem

What happens is that If the structure is normal, this indicates branching and debranching enzymes are normal. But the reality is that this person must have an issue with a phosphorylase enzyme specific to muscle. One of them could be the one called McArdle's disease which is a metabolic disease affecting skeletal muscle also known as Type V glycogen storage disease.