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As in many games, humanity in Blood Bowl serves as a simple baseline average of what other races are capable of. Humans are reasonably fast, strong, and agile. Their passing game is decent and their running game is decent. They're strong enough to do some damage but they don't
excel at it. They are agile enough to get out of some sticky situations but they still are tripped up from time to time.
Human linebackers are durable but expendable, which means that they can be thrown into tough situations and beat the crap out of many opponents not because they're tough, but because losing one isn't the end of the world. Blitzers are wonderful for terrifying an enemy's back line. Catchers are horrible at anything but catching, which makes their usefulness extremely situational. The human big guy, the Ogre, is great on the line like most big guys, but the speed of the human offense usually leaves him behind.
The basic strategy of any human offense should be to use a lot of linemen to tie up opponents while blitzers and a few catchers take advantage of holes in the enemy defense. The human passing game is great if a catcher can be penetrate enemy lines, but blitzers can also catch well if a re-roll is available. Because both types of players have the threat of catching, it is possible to overwhelm the enemy by simply presenting them too many things to defend against. This is called the Option Offense (because the human player has so many options) and it is the reason why the Ogre is often left behind on Human teams.
Defensively, the human team relies heavily on using the wealth of blitzers and linemen, both of which are reasonably strong but also reasonably quick, to lock down any enemy which has the ball. Blitzers in particular are fast and strong, and should be used to swoop in on enemies quickly. This will cause them to feel the need to cage in order to advance, which bogs down their entire offense.