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I think a level of fitness you need is to do a lot of practical sports in order to be a qualified tennis lime umpire.
Referees are needed to see if the match is fairly played and to help if someone is injured.
Age, gender, fitness level, level of physical activity are the main contributors.
Around 18 would be a starting age for high level matches. But junior referees can learn the game. The reason most World Cup/ top league referees are quite old is because they need to prove themselves at every level before they move up.
To Be Very Honest I Dont Know
because you need to maintain a high level of stamina and performance throughout the game
You can measure your strengths and weaknesses in terms of your fitness level using measurable performance parameters such as the number of games you have won.
If you have that endurance you will be able to run fast for longer.
http://www.everydayhealth.com/fitness/measuring-personal-fitness-level.aspx is a good website too look at to see how to measure your fitness level. There's information and tips on the website.
we need fitness so you dont gain alot of weight and we need fitness you can exercise and get fit.
A linesman, or Assistant Referee as one would properly be called if it is a neutral match official, needs various physical fitness skills in order to effectively do the job. These include strength, endurance, flexibility, and fast-twitch reaction. Assistant Referees at the professional level are elite athletes, with athletic skills and training comparable to most players. At the recreational, amateur, and youth levels, an Assistant Referee or club/volunteer linesman does not need to have this level of fitness, though some amount of athletic prowess is recommended and desired for the good of the game. The Assistant Referee needs to be able to run with sudden bursts of speed, change direction quickly, shuffle side-to-side, and get out of the way of incoming balls and players. An AR might run several miles during the course of the match, depending on the level of play.