After about 4 months of using my Polar F6 heart rate monitor, i have learned that working out at different heart rate ranges affect the body differently and that working yourself out at maximum exertion is not necessarily beneficial to your body. In fact, there is an optimal target depending on your personal goal. I also learned that working out on the upper zones is not cumulative (i.e., that you DO NOT gain the benefits of the other zones below it).
In the book Heart Zones Cycling by Sally Edwards and Sally Reed, they describe two methods for determining these heart zones: either by figuring out the maximum heart rate (HRmax) or aerobic treshhold (aka VT1). These are practically identical except for Zone 5 where the treshhold based range is further subdivided into 3 subareas (5a, 5b and 5c). For simplicity, I wil just outline the one based on maximum heart rate here which divides the heart zones from 1 to 5.
Zone 1 (50% to 60% of HRmax)
Zone 2 (60% to 70% of HRmax)
Zone 3 (70% to 80% of HRmax)
Zone 4 (80 to 90% of HRmax)
Zone 5 (90% to 100% of HRmax)
So what do these numbers mean? I guess the bottomline here is that how hard you need to work out depends on your primary goal. Is it to loose weight? Is it to increase your athletic performance (like ride faster when cycling) or extend your endurance (increased distance when biking)? One can also deduce from this that there is such a thing as a sweet zone when working out. If you overdo it (aka overtrain), you risk working so hard that it will take longer for your body to recover or you could be in pain (and with very little gain at that). This could be among the reasons why people who try gym memberships eventually give up after a few months when they think feel they've been working very hard at the gym and yet don't get the results they expect.
By the way, in case you're wondering how to figure out your maximum heart rate, a quick and easy formula for HRmax is: 210 - [your age] (this is in beats per minute). I also found a formula for calculating HRmax on wikipedia but the quickie version works similarly to what my Polar F6 calculates.