Reasons for fatigue training
In RuneScape Classic, melee-using players stake by Hits level instead of combat level. This works around Magic always raising players' combat levels. Magic is required for certain quests, for example, Family Crest and Legend's Quest.
Players with a higher Hits level will be asked to "go low", or "go bare", by taking off armour in order to allow their opponent to hit higher numbers at the start of the duel. This is to make up for the difference in stats.
Fatigue training can be used to gain the advantage of having higher melee stats than a player with the same hits level. Players will generally hide that they have fatigued in order to maintain this advantage. This is frowned upon, but has become less of a problem in recent times because the majority of staking now takes place on "flats" accounts (with 99 in all melee skills).
Attack/Defense/Strength XP is added before hits XP when training with melee combat, and if the player's fatigue is suitably high, only Attack/Defense/Strength XP will be added. Players will usually cut trees and make fires, arrow shafts, make arrows, or use a Magic spell such as high level alchemy in order to raise their fatigue to the necessary percentage for fatigue training. This makes the process of getting experience very slow, but some players consider the PvP advantage to be worth it.
$ XP = Level \times 2 + 20 $
To get the amount of XP added to the player's Hits skill from this value, divide by four. Thus, to get the amount of Hits XP obtained from killing a level 46 black knight:
$ 112 = 46 \times 2 + 20 $
$ 112 / 4 = 28 $
The amount of Attack, Defense, or Strength XP added is equal to the amount of XP added to Hits, multiplied by three. Thus, a level 46 Black Knight will add 84 XP to either of those three skills. To calculate the minimum fatigue percentage required for fatigue training, divide this amount by 100 and then subtract the divided amount from 100:
$ 28 \times 3 = 84 $
$ 100 - (84 / 100) = 99.16% $
As you can see, it may not reach an exact number lower than 99%. This can make fatigue training in an accurate and optimal way very difficult for legitimate players.