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Lean mass is total weight minus the weight of fat on a person.
We calculate the weight of fat by multiplying total weight by the 'body fat percentage'. For example, if an athlete weighs 90kg and has 20% body fat (18kg of fat) then his lean weight is 72kg.
These days trainers and clubs don't talk about 'percentage body fat'. They measure skin thickness at four places (triceps, suprailiac crest, para-umbilical, and anterior thigh) and add these measurements (each in mm) to give a 'sum of skinfolds'.
The body fat percentage in male athletes aged 15-19 is about one-fifth of the sum of these four skinfolds. So, if your sum of four skinfolds is 80mm, you have around 16% body-fat. This is OK. Anything less than 100mm is OK, unless you have a noticeable pot belly.
Rugby league players, especially props, can use a bit of fat. A layer of fat beneath the skin and over muscles protects those muscles from bruising during collisions in a game. Bleeding into a muscle (muscle bruising or haematoma) can keep you on the sidelines for two or three weeks. Bruising visible on the skin is normally just bruising of fat. It is sore but unimportant. Momentum, force, and energy transferred are all directly related to total body weight. If you are 20% heavier, then you contribute 20% more to the collision (before taking account of speed and acceleration).
Fat is only a problem when it impacts performance. When you hit the gym and put on weight in the off-season it is quite likely that you will also add a few kilos of fat to your body. But you are stronger, more powerful, and maybe even faster with the extra training.
Tip You can be bigger, stronger, faster, fitter, and fatter.
Many trainers, probably most trainers, even at NRL level, certainly at U20 level, manage the sum of skin folds and not the athlete. This is stupid. If the athlete is bigger, stronger, faster, and more confident with a bit more padding, where is the problem? The worst possible outcome has a trainer putting a player on a weight reduction diet with the result that the player has insufficient energy, and performance goes down. A complete idiot of a trainer (and there are a number around) will decide that the poor performance is due to the player carrying too much weight, and will restrict the diet even more, so the player has even less energy....
On the other hand if on-field performance is down, weight is stable, but sum of skinfolds is up, then it may be that some muscle has been replaced by fat with a loss of strength and power. The trainer should check strength and power because reasons for the sum of skin folds being different include - a different person performed the measurement, or the usual person performed the measurement differently. If on-field performance is down, but strength and power are stable, then any change in the sum of skinfolds has nothing to do with the (temporary) loss of form.
measuring four skinfolds
This is surprisingly easy to do, but you can't measure your own triceps skinfold (you can't reach the spot). However, if you have someone who can do this for you, then measure you own 'sum of skinfolds' but not more often than once a month. You need three or four measurements over three or four months to detect a trend.
The key is to get consistent measurements. The first few times you try, do the sum of skinfolds two or three times in the one day. Do you get the same result? Almost certainly not. The difference should give you a fair idea of the error in doing this. If it is only 5-10 mm, that's pretty good. If it is more than 20mm your measurer needs more practice!!
When you have your sum of skinfolds measured officially, write the result down. Have your measurer do your sum of skinfolds again as soon as possible. What is the difference between your measurement and the 'official' one? Do this again the next time. If the 'official' result is sometimes more, sometimes less, than your one, believe yours. If the difference is small but consistent, then there is nothing to worry about. That's just different people doing the measuring.
The four sites are (it is conventional to measure skinfolds on the right hand side of the body with the athlete standing relaxed, arms at his/her sides, palms facing front.)
If the 'official' tester doesn't use these four sites, then you can't use those results.
You will need a set of skin-fold calipers to do this at home. They shouldn't cost more than twenty dollars.