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Training Fatigue for Endurance Athletes

Runner mentally and physically tired.jpeg

One of the biggest misses in training across the board is the ability to recognize fatigue. For many endurance athletes, feeling fatigued is mistaken for being weak or under-trained. This leads to more training when the answer is to rest. The general adaptation theory describes it best. You start training at a certain level of fitness and as you train you build strength and fitness. As you build fitness, you accumulate fatigue. At a certain point you need to unload the fatigue by reducing training load and gaining more rest. This is where your body will super compensate, leading to a greater gain in strength and fitness. Post rest period, you will train with less fatigue and more fitness and thus this process starts all over again.

Training fatigue is not a one-dimensional thing. Over the past decade working with hundreds of athletes, analyzing upwards of a hundred thousand training files, we recognize three main categories of training fatigue. Internal aerobic system fatigue, leg muscular fatigue, and mental fatigue. There are times when all these fatigues are present and times when only one of these will dominate. Regardless of what types of fatigue are present, it’s important to recognize it and plan for a period of rest to unload it, recover, and gain strength in that area.

Heart rate response and internal aerobic system fatigue

Training with heart rate provides more than just a tool that can be used for training intensity. Heart rate is directly related to what’s happening on the inside of the body, such as with the heart and lungs. Heart rate has a unique relationship with power. When you are well rested with low training fatigue and heart rate is very responsive – meaning that when you go hard, like in a race, heart rate will respond easily into the upper zones of 4-5A plus ranges, while PE is relative or low. When heart rate is responsive, power is usually in good ranges or above normal ranges. On the flip side sometimes heart rate is not responsive. Let’s say you’re at the end of a 3- or 4-week block of hours and heart rate is remaining low on all harder efforts, such as remaining in zone two or lower zone three. Usually this will be accompanied by lower power, but not always.

Let’s use the end of a 3- or 4-week block of hours again, where you can assume you have built a good amount of training fatigue. Let’s say at the end of this block of hours, you can still produce good to higher power numbers, into the power zones 4, 5 and above but heart rate remains low. There will always be a day here and there where things are just off but when this trend continues for multiple days in a row, or for most of a week, then it’s a sign of internal aerobic system fatigue. This fatigue could originate from the muscles in the lungs, to the heart itself but regardless, it should be recognized. Continuing to train will simply overload these systems and lead to a plateau.

Power response and leg muscular fatigue

A power meter is a great tool and has many more uses than just simply raw power data and a way to train in certain zones. When combined with heart rate, you get a full picture of what’s happening inside the body while training. We will keep using the same scenario where you are at the end of a 3- or 4-week block of hours, where you know fatigue has been built. This time you go for efforts, and you can hit the heart rate ranges in the upper zones 4-5A plus, but power is 30-40 watts down and you can’t generate any more regardless of how hard you push. This will be more evident for longer efforts, such as your 2-5 plus minute efforts in zones 4-5A plus. This signifies muscular fatigue and could be related to the muscles in the hips, quadriceps, calf, etc., or all combined. Continuing to train without taking a period of rest to recover will only overload these muscles and lead to a plateau or worse, a decline in power over time. 

Mental fatigue

Your mind is your greatest training tool. It’s where motivation starts, where motor control happens and where stimulation post a great workout takes place. But let’s face it, when you train to get fit and strong, it takes work, continuous consistent work and that at times can become mentally challenging. Like heart rate and power, many times mental fatigue will accompany both low power numbers, non-responding heart rates and higher perceived exertions but not always. You may be feeling good on the bike but just lack the focus or motivation to train. When this feeling persists for a week or more, that’s a sign you need a break, usually 7-10 days of easier training, to let the mind recover, to regain that motivation to train again.


Training fatigue can happen at any time and it’s important to take note and listen to it. Depending on the time of year or the level of fitness you start with, training fatigue can arise after a week or two of training. If you listen to this fatigue, and recover from it, you will continue to build these individual systems stronger and that will lead to more productive training with greater time between bouts of fatigue.


Mike Schultz CSCS

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