Authentic high quality practice can only be sustained for limited periods of time. Why? Because it’s hard. Not always physically, but mentally. The formula outlined in Geoff Colvin’s book “Talent is Overrated” tells us that “perfect” practice involves performing highly specific activities at the upper limit of your abilities, and paying close attention to errors. These activities require intense concentration, and we can only stay in the optimum zone for learning as long as this mental intensity is maintained.
