If you missed the introduction to Training Zones click here.
With Zone 4 we literally cross the threshold into a new physiological system that we are targeting. With Zone 4 training we are working to raise our Lactate Threshold. As we gradually exercise harder and harder we start to build up lactate and Hydrogen Ions (H+) as byproducts of breaking down carbohydrates to produce energy. The lactate is recycled to form more glucose (energy) but the H+ decreases the pH of the working muscle causing acidosis which can interfere with muscle contraction. This acidosis in your working muscles is the burning sensation you get in your muscles when working hard. The harder you work, the more this cycle occurs, and the more your muscles burn and tell you to stop until ultimately you do stop. This occurs in a fairly linear fashion as you continue to work harder and harder until a point where the lactate and H+ begin to increase exponentially. This point is called your Lactate Threshold. This occurs because, as your body creates lactate (and H+ too but will just reference lactate from here on for simplicity), it is able to then remove it from the working muscles but at a certain point you create more than your body is able to keep up with and it builds up. This Lactate Threshold where you create as much lactate as you remove is important as it’s a key determinant of your endurance ability. Since your ability to exercise above this threshold is limited, then raising the level of this threshold is clearly quite important.
The best way to determine where your Lactate Threshold is by performing Functional Threshold Testing which can be done for running, cycling, and swimming. The exact protocols and more details on this can be found elsewhere, but the test will result in you knowing the maximum cycling power, running/swimming pace, and/or heart rates you could sustain for approximately an hour of that particular sport. This is known as your Functional Threshold and approximates your Lactate Threshold. It is essentially the power, pace, or heart rate you can produce before lactate starts its exponential rise in you working muscles. If you try to go work harder than this threshold you will not be able to maintain for longer than an hour and if you work easier you will be able to maintain for longer than an hour.
Training in Zone 4 and improving your Functional Threshold is fairly simple. Spending time in this specific zone gets your body used to dealing with more lactate and improves your body’s ability to remove and recycle it. Since you can tolerate it better, your threshold moves up and you can work harder before you hit it. Working harder and fatiguing less if obviously beneficial to your performance. While in theory, we could spend around 60min in Zone 4, this is a very hard workout mentally and physically so it is not really practical to do. The better way to utilize Zone 4 training is medium-length intervals usually between 8 and 20min. Recovery between intervals should be slightly longer (5-10min) for the harder (high Zone 4) intervals and slightly shorter (~4-6min) for easier (low Zone 4) intervals. There are benefits to performing slightly longer intervals (20min) at the lower end of Zone 4, just at or below your threshold, as you accumulate more time where you are flooding your body with lower amounts of lactate over a longer time. There are also benefits to performing shorter intervals (8min) at the higher end of Zone 4, just above your threshold, where you are flooding your body with higher amounts of lactate over a shorter time. There are also benefits to criss-cross workouts where you spend a few minutes slightly below threshold, a few minutes above and just alternating between the two to get your body used to small bursts of lactate without ever completely stopping its production during the easier intervals. There are countless other ways to structure these workouts which can all be beneficial, but these are the basic principles for these workouts.
There are a few other logistical considerations for these intervals that I should mention. These intervals are on the longer side and the training zone is somewhat narrow so it can be difficult to do these to perfection especially if tired that day from previous training or other reasons. So first off, just try to do your best to perform them to the best of your ability. Spending 80% of the interval actually in Zone 4 is fine if that is the best you can do and is better than abandoning them all together. Second, set yourself up for success with these intervals. Ideally pick a route that is fairly flat or has only a few uphills or downhills that can make it tough to stay in that zone. A long gradual climb may be good option as well. Also pick a route that doesn’t have too many stop lights/signs that can interrupt the interval midway. You can cover a lot of ground going this hard for this long so plan accordingly.
Running ~100% of FTP 100-101% of FTHR 5-6 RPE
Cycling 91-105% of FTP 95-105% of FTHR 5-6 RPE
Swimming FTP Pace 8-8.5 RPE
Interval Duration: 8-30min w/ 4-10min Recoveries
Continue reading about Zone 5 here.