Suppose a person is lifting heavy weight slowly, then what will it activate, red or white fibers?
Whenever you move, your body will use all of one muscle. If I lift my arm out to the side, I'm using the entire deltoid. If I do push ups, I'm using the entire chest. However, I'm only using a % of the muscle fibers. Your red fibers are the first to be recruited in any activity. If you're eating, talking, walking, whatever, you're using the red fibers. They get used all day long, which is why they have the characteristics that they do - they need to with stand use again and again without failing or giving out.
When you do a really fast movement, you're using ALL of the muscle fibers. They are all activated to complete the activity. When doing a slow and light movement, only the red fibers are involved - until of course, they become exhausted. At which point, white fibers will get involved. But the point here is, lifting something heavy uses all muscle fibers. Lifting something heavy and slow still uses all muscle fibers due to the amount of weight involved, and the white fiber will always give out first, since the red fiber can simply last longer. You cannot target 2 separate muscle fibers by this means alone.
Let me be extra clear:
Heavy weight = all fibers are used.
Lifting fast = all fibers are used.
Lifting slow = red are used first.
Lifting light = red are used first.
Lifting heavy and slow = all fibers are used.
Lifting light and fast = all fibers are used.
You get the idea now? You can emphasize one fiber GROUP over another by lifting light and slow one day, then heavy and fast the next... but fast movements, or heavy movements, always require all fibers to come into play (or almost all fibers anyway).