When practicing, most individuals play/practice until they get tired. This is with the belief that they will build endurance and strength, much like exercising or lifting weights. Musicians will play the entire exercise without resting and may feel fatigued. This causes the build up of lactic acid in the muscle tissues as well as the swelling of the tissues associated with trumpet playing. This causes the related muscles to become tense. The brain learns to relate playing the trumpet to being tense. This causes the body to react before playing the trumpet by what it learned from the previous experience of practicing until becoming tired. This is also known as muscle memory.
So how does one practice without become tense and what are the benefits of this? The answer is playing for short periods of time and resting that same amount of time. For example - when playing a page from the Arban's book, would you try to play the entire page without resting? Or, would you play one line, rest equal amount of time, then proceed? The body and brain will react to the stimuli you present it with. If you practice how to play relaxed, you will get better at it. The interaction between the brain and the muscles associated with playing the trumpet react to the level of stress created each time you play. If the body can remain in a relaxed state, the performance will only become more efficient. This is also true for martial arts, Golf, Tennis, etc...
A balance or relaxed state can be maintained at all times in time.