That is the definition of threshold. In other words, a threshold is a point or level that when you exceed it, something happens. This can be a good or a bad thing, but something happens.
All of you are familiar with threshold. Your teachers all have thresholds of patience, when you push them too far, they get angry. You all have thresholds of pain, when pain gets too intense, you react in a certain way (cry, quiver, etc.) There are many different types of thresholds, and they all differ from person to person, but in distance training, the most important threshold is the one that governs how our running muscles are powered.
For our intents and purposes, there are two types of running: aerobic and anaerobic. Aerobic running (jogging) burns a different type of fuel, produces a different type of waste, and can be carried out for a much longer time than anaerobic running (sprinting). However, aerobic running is usually much slower and plodding than anaerobic running. At a certain pace, your body automatically switches from powering itself like an aerobic runner to powering itself like an anaerobic runner. The difference between really good runners and not-so-good runners is the running pace or speed at which they switch over between systems.
For example, Runner A might be able to run aerobically at 5:00/mile pace. As soon as Runner A goes at 4:55/mile pace, she is running anaerobically. Then there is Runner B. Runner B runs aerobically at 5:30/mile pace, but as soon as Runner B goes 5:25 pace, she is running anaerobically.
Obviously, Runner A is a better overall runner. Runner A can run more intervals, at a faster pace, for a longer amount of distance, without totally seizing up and collapsing on the track. And if you can run more intervals, at a faster pace, for a longer distance, you are going to continue improving in your race times and become faster and faster. That's the whole goal here, to become faster on race day, right?
Not all is lost for Runner B however. Runner B's level of 5:30/mile pace is not set in stone. It is not genetic, nor is it unalterable. What we do on Mondays is designed to move Runner B's level more towards Runner A's.
By running at a pace below your aerobic/anaerobic switching level (or your lactic threshold), you actually train your body to not switch over to the anaerobic phase until you go a little faster. You can condition your body to hold off on that internal automatic switch for a little while longer. You teach your body to run at a faster pace powering itself aerobically. Which means you don't seize up as quickly, you don't feel the "burn" as quickly, and you can do more and faster training, thereby getting faster for race day. So, over time, Runner B, through deliberate and careful training, can actually push their level (lactic threshold) to a faster pace. Instead of making the switch at 5:50/mile pace, they can train themselves to internally hold off until they are running 5:15/mile or 5:00/mile or even 4:45 mile pace. It just takes discipline and patience and faith in the program. And then Runner B will kick Runner A's ass on race day! :) :)
The most important part to all of this is training at or below this level (or lactic threshold). If you train above the threshold, if you run faster than the pace at which you "switch", then you are causing your body to run anaerobically. If you remember the definition, a threshold is the point at which you cause a change. The change is going from aerobic to anaerobic. If you cross the lactic threshold on Mondays, if you run faster than you should, you cause your body to run anaerobically. And you don't want to do that. You can't teach your body to hold off on your automatic system switch if you cross the threshold on Monday. Your body will not react in the way you want it to if you go beyond that threshold . It will not learn to get rid of lactic acid efficiently, it will not learn to utilize oxygen more productively, it will not learn to move blood more effectively. When training on Mondays, you have to get right up to this level, but not above it. You can push your teachers patience just enough, but not go too far. You want to go right up to your teacher's tolerance threshold, and in training, you want to go right up to the lactic threshold. But in both instances, you don't want to go too far.
In our program, Monday practices are designed to teach the body to go faster without switching over to anaerobic. We want to teach our bodies "to endure higher intensity for a greater period of time. " (J. Daniels, Daniel's Running Formula, 1999) Going too fast on these kilometer repeats is not doing the body systems any good. Going a second or two slow is optimal, as you are still getting all the benefits, but you are not risking crossing the lactic threshold and wasting the interval. I thought today was a great day for that; as a group, all of us were running right on those times, and getting maximal benefit from these workouts. We will not only feel these benefits on race day, but also on Wednesday, Saturdays, and every other day of training. We will be able to train harder, longer and faster throughout the week, and that will propel us to becoming better racers!
However, it does not happen over night. Through a long season, we will make those gains, but it takes deliberate focused practice every day to achieve the maximal benefit. This afternoon was a great start to that. Congrats! And of course, if you have any questions/comments/observations, let me know in the comments section!