I was recently doing some 5-week follow-up performance tests in The Lab and began noticing some interesting trends in the data. Some of the athletes were showing a decrease in aerobic capacity or in their ability to clear lactate from their system at certain paces. A review of their training logs held a deceptive key: they were consistently running too fast on their aerobic-focus running days. So how could running faster lead to a deteriorating aerobic system? The answer, once again, is in the science.

The body basically uses 3 energy systems to perform work: aerobic, anaerobic lactic, and anaerobic alactic. For our purposes, we can ignore the anaerobic alactic as it is only useful for 5-10 seconds in a race. In our training plans we are attempting to balance the aerobic capacity and the anaerobic capacity (and to a lesser extend the aerobic power and anaerobic power). Let’s take a look at how these two systems operate:

Aerobic Capacity – This is what we test with the VO2 Max in The Lab. The aerobic system does double-duty. It provides energy to the body as well as removes lactate as it is produced. At slow enough paces you are creating many changes in the body that allow you to quickly remove this lactate while still powering the body to complete the work at that pace. The better the aerobic capacity, the faster you can go without accumulating excess lactate in the muscles and bloodstream. This is important because the excess lactate eventually becomes too much for the aerobic system to remove and you will have to slow down.

Anaerobic Capacity – This is part of what we test with the Blood Lactate measurements in The Lab. The anaerobic system is a fast-acting and powerful source of energy. Because of this, it will help you produce higher speeds in training and racing. The trade-off is that as more is produced for the faster speeds, the aerobic system eventually cannot clear the lactate fast enough and (as mentioned above) you will have to soon slow down.

Think of the aerobic system like a vacuum cleaner and the anaerobic system like cheerios being spilled on the floor in front of you while you clean. The larger the vacuum (aerobic system) the more cheerios you can have on the floor in front of you while not getting bogged down. The more cheerios that begin to fall on the floor in front of you (lactate production from the anaerobic system) the harder it is for the vacuum to keep up, eventually falling so far behind that it is no longer possible to continue to vacuum.

The trick in all of this is that training can fall into 3 categories: aerobic focused, mix of aerobic and anaerobic, and anaerobic focused. This is where your pacing becomes critical. Remember the title of this blog. This is where running too fast in a workout may actually be making you slower. When you are working on an aerobic focused workout you are trying to build the size of the vacuum. When you are doing an anaerobically focused workout you are working on creating more cheerios to dump. The more you mix the two, the less of each you are increasing (more to come on this in the next blog). So, if you are prescribed an aerobic day you must be careful to not go too fast. If you do, you will not be focusing all of your bodies energy on growing the vacuum. So even though you may feel really good at 6:40 pace, if your test data indicates you begin producing extra lactate (cheerios) at 7:00 pace, the 6:40 pace becomes more of a mixed workout than an aerobic focused workout. The same goes for anaerobic workouts. If you do too many reps and are slowing down the paces, you may be drifting away from an anaerobic focused workout and more into the mixed zone as well. None of these 3 categories are better or worse than the other, but you need to follow the correct paces to ensure you are improving the correct system for the prescribed workout.

So, stop feeling like the only way to prove you are in good shape is to go faster than the paces indicated by your performance tests. This will inevitably lead to stagnation in your growth as a runner and you will be confused as to how you were running faster in some workouts but your performance was declining. The body is a complex organism and you cannot out-smart it simply by going faster everyday.


If you are curious as to whether or not you are currently running the right paces and getting the most out of your workouts (you all should be), then fill out the quick survey to see how you can ensure you are not wasting great training opportunities through incorrect pacing.



Ben Tilus – Owner/Head Performance Coach – XLR8 Performance Lab