hi there. Two things happen to the muscle but it's not clear which is first or if they both happen at once. This is important because if one causes the other we need to look to the first for a treatment to prevent the second.the cells of the muscle develop abnormal clusters of protein that interfere with the normal operation of the cell. These clusters contain several different types of protein including tau protein and beta amyloid protein. For the past 10 or 20 years IBM research has heavily focused on these proteins. The newest research shows that defects in a different protein called TDP are actually much more prominent in IBM muscle cells then are the other protein defects and this gives researchers a new avenue of research to look at.
For some reason, the immune system sees flags on the muscle cells and destroys them. These flags may be displayed because of the protein abnormalities or there may be some other reason the muscle cells come to display these flags and the protein problems may be another secondary result. either way, the immune system sees the muscle cells as abnormal and attacks and kills them.
The more muscle cells that are affected and killed, the weaker the muscle becomes, eventually there are not enough muscle cells to contract the muscle and it becomes simply fibrous tissue.
For some reason that is not understood, not all types of muscle are affected equally. It appears that long striated muscle is affected first-- that's why the quadriceps muscles are usually the first effective as they are the largest and longest striated muscles in the body, followed by the biceps triceps and deltoid muscles. involvement of the smooth muscle of the diaphragm and intestine are usually far less involved and finally, cardiac muscle appears to be the least affected.