That's a really good point. It reminds me of a principle of teaching, which is that growth is encouraged by not letting students become too comfortable. By keeping students off balance, students are forced to draw on other skills and experiences to deal with different situations than they might otherwise choose to. This principle can be applied to something like exercise as well. Doing only pushups will help you become better at pushups, but will not help you to increase your overall fitness. Also, repetition will eventually cause stress and the plateauing that you mentioned. By hitting the muscle fibers from different angles, more profound growth and strength will be realized. To return to your example of playing bass, how do you recognize when to change up your practice regimen or make a decision on how to change it up?