Not yet investigated
What we have so far
Slow and fast are real and largely set by the nerve, not the muscle. Cross-innervate them in an animal and the fibres convert — the nerve is giving the orders.
This star is on the map because the show intends to take the question apart properly — the trials, how it is actually tested, whether the thing sold is the thing studied. That work is not done yet, and nothing here should be read as a conclusion. When the investigation is published it will appear on this page.