When they don't even approach it they experience boredom or depression. The key for all of us is to identify and stay within our zone. Think about your own natural velocity. When you get above your zone, when you're pushing yourself too hard, the first thing you experience is stress, a feeling of tension. Now, realize that stress can be a good thing.
Stress is just pressure, resistance.
When people handle this pressure in unhealthy ways,stress gets a negative connotation. If you continue to push harder against the pressure of stress and if you stay anxious, you can start losing effectiveness. You start losing motivation and need a time of recovery to get back into your zone. On the lower side, if you don't push yourself hard enough, if you don't take on a big enough challenge, the first thing you experience is boredom. You'll find that you become disinterested or bored if you continue without experiencing enough challenge to get yourself going.