It may seem obvious but stamina, or the ability to put off extreme fatigue, is an extremely important part of any athletic endeavor especially Martial Arts.
It is commonly understood, the first thing to go when a person is tired is technique. The body and the muscles within become so incredibly fatigued that the person is not able to mobilize that body part in the specific way it has been trained to perform the action. This is easily observed in any professional fight. When the fight starts to get into the later rounds you start to see the fighters "fade". I.E. He may not have as much bounce in his step or his takedown attempts become sloppy.
This is why conditioning is stressed in all professional fight gyms. More often than not the fighter with the better endurance will be able to perform in the fight better than the fighter who can barely keep his hands up because he is so tired.
Here, at New York Martial Arts Academy, we develop stamina in various ways. I.E. Jumping rope, or doing multiple live rounds.