ball as far in order to prevent the advance of any baserunners. The left fielder still requires good fielding and catching skills, and tends to receive more balls than the right fielder due to the fact that right-handed hitters, who are much more common, tend to "pull" the ball into left field. Each outfielder runs to "back up" a nearby outfielder who attempts to field a ball hit near both their positions. Outfielders also run to back up infielders on batted balls and thrown balls, including pick-off attempts from the pitcher or from the catcher. Defensive strategy Pitching Effective pitching is critical to a baseball team, as pitching is the key for the defensive team to retire batters and to prevent runners from getting on base. A full game usually involves over one hundred pitches thrown by each team. However, most pitchers begin to tire before they reach this point. In previous eras, pitchers would often throw up to four complete games (all nine innings) in a week. With new advances in medical research and thus a better understanding of how the human body functions and tires out, starting pitchers tend more often to throw fractions of a game (typically six or seven innings, depending on their performance) about every five days (though a few complete games do still occur each year). A single game often requires multiple pitchers, including the starting pitcher and relief pitcher(s). Pitchers are substituted for one another like any other player (see above), and the rules do not limit the number of pitchers that can be used in a game; the only limiting factor is the size of the squad, naturally. In general, starting pitchers are not used in relief situations except sometimes during the post-season when every game is vital. If a game runs into many extra innings, a team may well empty its bullpen. If it then becomes necessary to use a "position player" as a pitcher, major league teams generally have certain players designated as emergency relief pitchers, to avoid the embarrassment of using a less skillful player. In baseball's early years, squads were smaller, and relief pitchers were relatively uncommon, with the starter normally remaining for the entire game unless he was either thoroughly ineffective or became injured; today, with a much greater emphasis on pitch count, over the course of a single game each team will frequently use from two