"Work capacity refers to the general ability of the body as a machine to produce work of different intensity and duration using the appropriate energy systems of the body" (Siff, 2003).

If your goal is to become more fit, show up and do the work.  If during your workout you find yourself not sweating, having long conversations with friends (yes, this counts if your on your phone too) or yawning, there is a good chance you are not working at a high enough intensity.

Today I completed 16,720 pounds of work in 10 minutes.  It was not easy.

http://www.youtube.com/watch?v=dZmBba11yrs