This is an excerpt from Runner's Edge, The by Stephen McGregor & Matt Fitzgerald.
Managing fatigue by reducing your training as necessary is one of your most important responsibilities as a competitive runner. Fatigue is a symptom of incomplete physiological adaptation to recently completed training. When fatigue persists, it means that your body is not benefiting from the hard training that is causing your fatigue. A day or two of soreness and low energy after hard workouts is normal and indeed much preferable to never feeling fatigued, which would indicate that you weren't training hard enough to stimulate positive fitness adaptations. Extended recovery deficits, however, must be avoided at all costs.
You can minimize the need for spontaneous training reductions simply by training appropriately. Don't ramp up your training workload too quickly (obey the guideline of 5 CTL - chronic training load-points per week), don't try to do more than three hard workouts per week, follow each hard day with an easy day (featuring an easy run, an easy cross-training workout, or complete rest), and plan reduced-workload recovery weeks into your training every few weeks. Even if you take these measures, however, you will, assuming you train as hard as you can within these parameters, find yourself sometimes feeling flat on days when you had hoped and expected to feel strong for a harder workout, or find your fatigue level building and building over several days. At these times it's important that you listen to your body and reduce your training for a day or two or three to put your body back on track.
Technology is no substitute for your own perceptions in these cases. No device can measure your recovery status and readiness to train hard any better than your own body can. When your body is poorly recovered from recent hard training, you can always feel it. And when factors outside of your training, such as lack of sleep or job stress, compromise your capacity to perform, you can always feel that. Before you even lace up your shoes, you know that you're not going to have a good day because of the heaviness, sluggishness, soreness, or low motivation you feel. Your body itself is an exquisitely crafted piece of technology whose primary function is self-preservation. One of the most important mechanisms that your body uses to preserve your health through hard training is a set of symptoms of poor recovery (those just named) that encourage you to take it easy when that's what your body needs most. It's important that you learn to recognize these symptoms and get in the habit of obeying them. Pay attention to how your body feels before each workout and then note how you perform in the run so that you can discern patterns. Through this habit you will develop the ability to anticipate when it's best to reduce workouts or take a day off and when to go through with planned training.
Technology can be an adjunct to listening to your body in making such decisions. We recommend three specific practices: monitoring your resting pulse, correlating poor workout performances with training stress balance, and performing a neuromuscular power test.
The first practice is monitoring your resting pulse, or performing orthostatic testing, as described in chapter 1. Look for patterns in the relationship between the numbers observed in orthostatic testing and how you perform in your workouts. (It will take at least three weeks for such patterns to become observable.) If, for example, you always perform poorly in workouts on days when your morning pulse is at least four beats per minute higher than normal, you can use this information to change your workout plans as soon as you observe a high morning pulse reading instead of waiting to find out the hard way that you need a recovery day (that is, by feeling lousy in the planned run).
Training Stress Balance
A second way to use technology in determining whether and when you need a rest is to note where especially poor workouts and stale patches of training tend to fall in relation to your ATL, CTL, and TSB. Specifically, on days when you have a harder run planned and you expect to feel ready to perform well but instead you feel fatigued and have a subpar performance, note your present ATL, CTL, and TSB. The next time these variables line up in a similar way, you will know to expect lingering fatigue and can alter your training accordingly. Don't expect to find 100 percent predictability through this exercise, however, because many other variables factor into your daily running performance that these variables do not capture.
These variables may be somewhat more reliable in predicting the multiday stale patches that sometimes occur during periods of hard training. For example, you might find that you always hit a stale patch when your CTL exceeds 50, or when your TSB drops below −20, or when these two things happen simultaneously. Again, once you have observed such a pattern, you can take future actions to reduce the frequency of those stale patches.
Neuromuscular Power Test
Finally, you can use a neuromuscular power test to assess your recovery status. Research has shown that when the body is carrying lingering fatigue from endurance training, maximal power performance is compromised. Your maximum sprint speed is one good indicator of your current neuromuscular power. Running a set of short sprints once a week is a good way to increase and then maintain your stride power, but it also serves as a reliable recovery status indicator. For example, each Monday, after completing a short, easy recovery run, you might run 4 to 10 × 10 seconds uphill on the same hill each time at maximum speed. After completing the sprints, note the highest speed achieved. Pay attention to how you perform in the next hard workout that follows a sprint set in which your maximum speed is lower than normal. Through this process you might locate a maximum speed threshold that indicates the need to alter your training plans for additional recovery.