Besides my daily training for performance I use to do once in two days intense workout (training to exhaustion) only callisthenics and with small weight dumbbells (4kg).
Many do not agree to this type of training as it is risky:
1. You can easily over train if you do not have a well planned schedule.
2. Muscle fibers can get damaged if you do not have a proper nutrition.
3. Vitamins with energizing role are required.
Pluses:
1. Fast results (I don't know if this is available for everybody, I am a mesomorphous).
2. It helps me for endurance (an essential characteristic for a fighter)
3. It doesn't grow muscle mass that much as a powerlifter, for e.g., but it defines each muscle group that you workout.
So instead of looking like this:

You will rather look like this:

What does it consist of?: "Work until you can't rise your hand, and then do 3 more repeats." (In this case if you work for arms, but it applies to every muscle)
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