Balance is all about being in control of the aspects of your life.
Balance is not necessarily spending the same amount of time on various aspects of your life, it is about considering the appropriateness of each need: physical, mental, emotional and spiritual, and looking after each facet, as and when required, without neglecting the other.
An athlete will focus primarily on the need of physical strength or endurance however; failing to consider the other three can cause imbalance that might have a detrimental effect on the pursuit of physical excellence.
Likewise a student pursuing an academic course must consider their emotional, physical and spiritual self for fear of burning out.