If you keep going, your body keeps moving, exchanging and fatiguing.  It keeps finding ways to get fuel where you needs it most.  And if you stick with it over time, your body begins to believe that moving is part of living; it begins to understand that you will need lots of fuel and it stops storing it for later.

If you could describe some of your favorite side effects of a great workout, would fatigue be one?  Or, do you dread getting winded, feeling your muscles tight and in pain from movement?  What if you embraced those side effects as positive feedback.  If fatigue was your friend, would you ask it to stay around for a while?

Physiologically, muscular fatigue only occurs when the energy needed to produce movement is greater than what's available.  It's not exactly like withdrawing from an empty bank account, but you may feel the same pinch of discomfort.  This feedback is one way your body talks to you.  As you continue exercising with a little discomfort, your body finds ways to keep the fuel coming...it borrows from surrounding tissue, it uses different sources of energy, and it keeps the muscle moving.  If you keep going, your body keeps moving, exchanging and fatiguing.  It keeps finding ways to get fuel where you needs it most.  And if you stick with it over time, your body begins to believe that moving is part of living; it begins to understand that you will need lots of fuel and it stops storing it for later.  Engaging with fatigue is how you communicate back to your body that you mean business about being fit.

Not everyone enjoys getting tired.  Exercising to exhaustion can feel like an incredible accomplishment, but it's not the only means to the end, especially if you're just starting or starting over.  If you're a seasoned exerciser and not making progress, you have to come to terms with the fact that fatigue will be an ever present companion.  Getting tired while you're moving is a sign that your intensity is climbing, and it's the intensity that you need to see your body get fit. If you don't like fatigue, can you at least be it's acquaintance?  

The ironic truth about exercise fatigue is that it's the best guest at the workout party.  You give it priority, and your body will respond with renewed energy...you give and you'll get it back.  That's pretty friendly if you ask me.