Muscle denervation after stroke and SCI has also been shown to greatly affect the relative relationship between fast and slow twitch muscle fibers.
While study results have varied there is a general consensus that a transition from slow to fast twitch fibers and an increase in fibers that show both fast and slow twitch properties arise after paralysis from stroke and SCI [
30-
33].
This transformation process was demonstrated early on by Grimby et al. as they found that several individuals from seven months to ten years post traumatic SCI were estimated to have approximately 90% fast twitch muscle fibers in vastus lateralis, gastrocnemius and soleus muscles [
30].
Likewise, over 3 decades ago Landin and colleagues found that after CVA, there was a shift from slow twitch oxidative muscle fibers to fast twitch anaerobic based muscle fibers of the paretic vastus lateralis muscle [
34]. Since then other researchers have discovered similar results in various paretic muscle groups [
32,
35,
36].
This process typically shows manifestation between 4-7 months post injury and can continue up to 70 months post injury before plateauing into a new steady state of predominant type IIx fast-glycolytic twitch muscle fibers [
22].
The fact that muscle atrophy driven by upper motor neuron injuries show skeletal muscle shifts from slow oxidative twitch muscle fibers to fast glycolytic twitch muscle fibers and muscle atrophy related to the aging process shows a shift in the reverse direction may indicate that neurogenic properties are involved in the former.
