Mountain Ultra-Marathon Causes Sarcomere Disruptions of Slow Fibres For longer running loads, there is a damage to the muscles. A Spanish group suggests whether the slow or more the fast fibers are harmed in an ultramarathon.
The proof and course of certain parameters of the muscle damage concluded that the slow fibers are more likely to harm. You can read the work under https://pubmed.ncbi.nlm.nih.gov/25848839/