Muscle Fibers and Polio
Question: When looking at people from the African nations we see a significant number of medal-winning long distance runners . . . obviously due to their genetic makeup. So, this brings my thoughts to PPS: Did the poliovirus attack slow twitch, i.e., Type 1 endurance muscles more often than it attacked Type II fast acting muscles? Could one’s muscle type leave one more genetically predisposed to contracting poliomyelitis?
Dr. Bruno’s Response: MUSCLE FIBERS and POLIO: SURPRISING CONNECTIONS
When we talk about polio and PPS, the focus is almost always on the damage poliovirus did to motor neurons, damage that prevented their stimulating muscle fibers and causing weakness or paralysis. But there's more to be said about post-polio muscle fibers. Let me try to give an oversimplified, but I hope understandable, description.
MUSCLE FIBER TYPES
Muscles contains two types of fibers that contract when stimulated by motor neurons:
• Type I fibers are metabolically suited for sustained muscle activity that provides endurance.
• Type II fibers are metabolically suited for rapid movements, like sprinting, rather than running a marathon.
So while motor neurons activate all the fibers in a given muscle, the fiber types themselves have their own agendas: to provide endurance or speed.
For example, typically the anterior tibialis (the muscle in the front of your lower leg that, when its motor neurons are damaged by polio, causes foot drop) has about 75% of the "slow" Type I fibers and 25% of “fast” Type II fibers. When walking, the anterior tibialis’ Type I fibers contract many times with each step, while Type II fibers contract only during rapid walking and then usually just a few times.
POLIO AND MUSCLE FIBERS
The poliovirus did not attack muscle fibers. It attacked motor neurons. Yet muscle fibers were affected.
The above description of how Type I and Type II fibers function in walking is not the same for affected -- but still working -- post-polio muscles. Surviving motor neurons sent out “sprouts”, like little telephone lines, to activate muscle fibers that were orphaned when their motor neurons died. This process of sprouting causes what's called "fiber type grouping". One study of the anterior tib muscle in polio survivors found that Type I "endurance" fibers predominated. Polio survivors had nearly 21% more Type I fibers as compared to nondisabled controls.
This Type I fiber "grouping" is seen in most other polio affected muscles where sprouting occurs. And grouping makes functional sense. Apparently the Type I muscle fibers “knew” that polio survivors needed endurance more than they needed the ability to sprint.