Not true. Posted this before:
Here are some key takeaways from Dr. Andy Galpin's research surrounding the role of genetics vs training in performance.
www.crossfitinvictus.com
4. Fiber Type Plasticity (aka Yes, You Can Change Your Fiber Type Composition Through Training)
For many years, there was a lot of debate over whether our fiber type composition was determined mostly by our genetics or our training. Most people have a fairly even split of slow-twitch and fast-twitch fibers and it was thought that this ratio was a fixed trait like your height. However, Dr. Galpin’s team examined a pair of monozygotic identical twins who had vastly different training habits. One who had been training for marathons for 35 years while the other was a sedentary truck driver. They found that the endurance trained twin had 90% slow-twitch fibers and 10% fast-twitch fibers while the untrained twin had 50% slow-twitch, 30% fast-twitch, and 20% ultra-fast twitch. Dr. Galpin cited other studies on identical twins that all found that training had a dramatic effect on fiber type composition despite genetics. To answer the titular question of “genetics versus training”, Dr. Galpin asserted that training was far more important than genetics
There’s more to it than just fast and slow twitch muscle fibers.