Depending on speed, endurance or strength needs, the training stages modify the muscular fibers in quantity and quality (quality is generally more important than quantity).
There are two main types of fibers:
Slow twitch fibers (type I)
These fibers have a low power but a high endurance. They have a small cutting diameter and a high capillary density (red colour) because they are adapted to aerobic efforts and request the cardiovascular system.
Fast twitch fibers (type II)
These fibers have a high power but a low endurance. They have a great cutting diameter and a low capillary density (white colour for the fastest or pink) because they are adapted to anaerobic efforts.
The "slow" fibers are always requested first. The "fast" fibers are only requested in strong efforts, during a short period of time.
More generally, speed requests the "fast" fibers, endurance requests the "slow" fibers and strength requests the whole fibers.
For each muscle, there are various proportions of two fiber types. Training consists essentially to develop the fibers to the right proportions.
For example, into some leg muscles, the fibers proportion is 80% of "fast" fibers and 20% of "slow" fibers for a 100 m sprinter and at the opposite, 20% of "fast" fibers and 80% of "slow" fibers for a marathon man.
For a sedentary adult, the proportion is 48% of "fast" fibers and 52% of "slow" fibers.
A muscle can work in three ways:Concentric contraction
The muscle works with shortening (ordinary movement).Eccentric contraction
The muscle works with lengthening (released movement). This contraction is the most traumatic for the fibers.Isometric contraction
The muscle works without shortening or lengthening (blocking).
The contraction allows getting, at a given time, a strength F and a speed V.
When the strength is at max the speed is at min, and inversely, when the speed is at max the strength is at min.
Consequently, the power P = F . V is at max for a particular strength and speed.
Do not confuse power (P = F . D / T) and energy (W = F . D).
For example, to move up more quickly less heavy weights (weightlifting, etc.), represents obviously a lower energy but can represent a greater power. This is important for the speed sports contrary to the strength sports.