Because Rugby contains a lot of running back & forth, for which you need endurance.
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Added to this are the demands of the tackle which requires the tackler to move the opposing player off the ball. lineout put great demand on the forwards as they have to jump and support to get to the ball. Such elements as rucking, mauling and scrummaging put high levels on demand on the body.
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Physical strength is a vital part of the game in both codes. It requires high levels of stamina and muscle strength. Areas in highest demand are in pushing such as scrummaging, rucking and mauling, running and tackling.
Rugby is very physically demanding. You have to be able to run for 80 minutes straight with very few breaks in the play and you must also be able to sprint at any given moment. Due to rugby being a contact sport you must also have the ability to take hits and deliver them all game.