Found this interesting article fm Running Times, Jan... .....
Runners need to keep in mind that their anaerobic thresholds (AT's) are not static, but are a fluid condition. AT's are not always experienced at the same numbers of beats per minute or minutes per mile. To put it another way, AT's change with different levels of fitness. While many runners, coaches and experts typically identify AT's at around 85% of max effort (i.e., of max O2 uptake), your anaerobic threshold can range from the low 70% to the high of around 90%. It all depends on what kind of shape you're in.
For example, let's consider a runner who has had a great season with several peak performances of new PR's. He then took a couple of weeks of complete rest in order to recharge his physical, emotional and psychological batteries. Next, he spent 8 weeks building a new base foundation of endurance with just easy, aerobic mileage in the zone of 60-75% effort. As a result, he would find that his cardiovascular system is in surprisingly good shape, as evidenced by his very low resting heart rate, probably just 4 or 5 bpm's above his all-time low. However, due to the very aerobic nature of his workouts, he would also find his anaerobic conditioning to be quite lousy. Why? Because his respiratory system had done nothing except easy breathing at these highly conversational levels of effort and his leg muscles were not used to cycling through a full range of motion. Thus, with a change of training from Phase 1 to Phase 2, our example would now find, as he increased the effort and ran faster paces, that he would start some serious huffing and puffing, but at surprisingly low heart rates and disappointingly slow paces.
Well, what are workouts for if not to get in better shape? And, sure enough, over the next several weeks by pushing the effort harder a couple times per week doing tempo runs or long repeat interval workouts, it will take faster and faster paces to reach the huffing and puffing stage. And another surprising adjustment will occur: heart rates will get higher and higher until they reach that 85% target HR. Now, there will finally be a much stronger correlation between the expected number of heart beats and heavy breathing and the desired paces.
Ultimately, another improvement of several more percentage points can be realized from a combination of Phase 3 workouts and racing. At this point I would advise our sample runner planning to run a 5K race that he should find his AT at race pace somewhere between 85 and 90%. Hello. It's PR time!!
To conclude, let me point out that the three systems that contribute to one's max O2 uptake―the cardiovascular, the respiratory and the muscular―are all independent systems that can be affected separately by different types of workouts. Therefore, each can actually have its own AT. However, when all are fully conditioned to take in, distribute and absorb O2, you're ready and off to the races.
– Coach Roy BensonHere is another good infos site on AT( LT)... http://lactate.com/lactate_threshold.html