Gym Fighting
The gyms of San Andreas are very useful places. You can lose weight, build up stamina and muscle and learn brand new fighting moves!
While all of this may seem like an added extra, learning the fighting moves is actually a requirement towards 100% game completion.
How To Fight At The Gym
The gym is initially unlocked following a phone call from Sweet after his mission 'Drive-Thru', pretty early in the story.
In order to "complete" the gym objective, you'll need to enter the gym, marked by a 
dumbbell icon in each city (Los Santos, San Fierro and Las Venturas), then stand in the red marker next to the trainer of the gym and accept a fight against them.
The gym leaders won't agree to fight you unless you've put on a bit of muscle, so you may need to use the weights over the course of a few separate days until you're strong enough to engage in combat.
Carl will need at least 35% muscle before the trainers will agree to fight him. If CJ doesn't eat enough throughout general gameplay, he'll eventually lose fat and muscle.
Once you're muscly enough to accept a fight, you'll head into the boxing ring or fighting area and take on the trainer in hand-to-hand combat. If you win, he'll teach you some new moves.
Fighting isn't actually difficult at all and isn't too different to fist-fighting pedestrians on the streets, although the gym trainers can take a fair bit more punishment.
Workout Limits
Each day in the gym you're allowed to lose 40% fat, or gain 20% stamina or muscle. Once that limit is hit, you won't be able to work out again for 12 hours.
Hints & Tips
To speed up the process, you can head to a safe house and save your game twice - advancing time by 6 hours each time - then head back to the gym to work out again.
Tactics
Target your opponent then mash the punch or alternate attack buttons until he goes down, then keep hitting or stomping on him as he tries to get up and eventually he'll run out of health and you'll win. It really is that simple. Just chase him around the ring and don't let up until he's done. Each fight should take no more than 30 seconds.
New Fighting Moves
Each time you win a fight against a gym trainer, your unarmed combat move set will be replaced with the style of the trainer you just defeated.
Hints & Tips
If you have a preferred fighting style, you can fight your chosen gym trainer again to switch your moves to that style.
Completion
Visit all three gyms and beat the trainers in each one in order to learn all of the moves and complete this challenge.
Rewards
Defeating each trainer awards roughly 0.53% towards 100% completion, providing 1.60% in total.
San Fierro Gym
"The Cobra Marital Arts Gym" - Kung Fu Trainer
Did You Know...
The San Fierro gym trainer uses the same character model as "The Snakehead" who Carl must sword-fight on a ship during Wu Zi Mu's mission 'The Da Nang Thang'.
