What are the physical requirements for playing American football according to the article?

I- Strength, power, and speed can differentiate starters from substitutes and between various levels of competition.
II- During an American football game, the rate of strength development seems to remain, but maximum strength and power may be reduced. With adequate recovery and strategic substitution patterns, maximum strength and power can be maintained during a game.
III- Muscle damage markers are elevated during preseason training camps but return to baseline concentrations at the beginning of the season and remain at baseline levels until the end of the season. This represents a certain degree of desensitization to contact and has been called 'adaptation to contact'.
IV- The degree of improvement in performance decreases during the course of a college American football player's career. Improvements in strength appear more quickly, but improvements in speed and agility may take longer to occur.