From a physiological standpoint, practicing an instrument and practicing a sport are entirely different endeavors.
Playing football, say, requires one to learn to perform a skill while physically exhausted, as would happen in a game situation. So a coach tiring out the players before practicing the skill is appropriate.
Playing one's instrument requires learning fine coordination between the nervous and musculoskeletal systems. When those systems tire, our bodies recruit different or larger muscles than those we are trying to develop. Once that happens, we are no longer making progress in learning to play a particular passage properly.
Our bodies can learn over time to increase the endurance of the neuro-muscular link, but once our play starts to get at all sloppy during practice it is fruitless to continue.
From this standpoint, it is better to practice more often for shorter periods of time.
Tom
I can't disagree with you, you basically spoke the truth.
But, when you get tired, you don't have to play on like normal. Maybe you can slow things down since you are tired, then focus hard to be "perfect" at that slower speed. I think if we can do that than we would all improve.
I would have no problem stopping for a few minutes and going again to build endurance. Maybe do a combination of the two, or just one or another.