This post is superseded by a later post by Dan Partelly. It has been preserved for thread history integrity. Click on the link to go directly to his post. Giovanni Ciriani.
I may be incorrect, but I think we should distinguish between muscle plasticity and muscle adaptation.
Muscle adaptation is the faster change in muscle fiber strength, which starts occurring within two weeks from the beginning of training. I think this is what Charlie was referring to in a previous post in this thread. In muscle adaptation, the number of fibers remain the same. Their cross section increases, which explains the strength increase and the bulk increase.
Muscle plasticity is different from muscle adaptation; it’s a longer-term change in muscle composition, which probably takes at least 6 weeks (according to research). It involves a change of muscle fiber numbers, as well as their size (cross section). This causes a much larger increase in muscle strength.
It was greatly debated in the past, whether healthy muscles were capable or not of plasticity. Most practitioners and researchers believed that muscles were not plastic. That is, they believed an athlete’s muscle endowment was genetically determined, and would not change with training. In other words, the number of muscle fibers would remain the same, and the athlete would only be able to increase the size of individual cells. According to this belief, an endurance athlete would never become a sprinter, and vice versa.
However, research performed during the current decade has shown positive proof that slow-twitch fibers can be transformed into fast-twitch fibers and vice versa. Although a marathon runner would not be able to become a sprinter, being able to transform a percentage of slow-twitch fibers into fast-twitch fibers makes a difference at professional level.
Changing muscle composition, takes a longer training course, and it requires an exact understanding of what the goal is. That’s why it’s important that we use the right terminology, so that we are not misunderstood.