I woke up today with fairly heavy CNS (central nervous system) fatigue from the past several days. This has become a familiar feeling but has usually come about after just one to two days of hard training, this time I got in many before it hit me which shows improvement. Before I started training for sprinting, CNS fatigue was never really an issue. It was there for sure but with ultrarunning it isn't as severe an inhibitor to improvement as it is with sprinting. That's because of the expectation of performance in training. With ultrarunning you can wake up exhausted but that's typically OK, your performance for the day is usually just logging more miles or doing more moderate efforts. With sprinting however there are finer expectations and for a noob like myself those come down more to neurological development than muscular or metabolic. I liken it to sprinting down a ~4% grade hill. It's very unlikely that you'll approach Bolt's world record in the 100 even with the downhill because you can't move your legs fast enough. Or spin a bicycle up to 200 revs. That doesn't take superior strength or power or Vo2 max. It simply takes coordination.
Refined movement patterns or muscular coordination is not as critical when you're running easy or slow. Not even if you're running tempo. Get into higher intensities or nearer your absolute max though and it becomes more critical to have every motor unit in every muscle group working in coordination to drive you forward. In order to develop this economy you can't be too fatigued to do the very specific and technical work required. Yesterday's workout was aimed at developing that coordination but if I were to attempt the same workout today I wouldn't be able to hit the required speeds and therefore wouldn't be training the appropriate system.
For the 400 there are other systems or aspects that are just as important as economy of movement. Strength, power, lactate tolerance, endurance (endurance for a 400 looks quite different than what most people think of endurance), these are all important and I could go out today and probably pull off one of these workouts, but looking at the past six days I've done plenty of work and see little benefit to not being patient and resting.
CNS fatigue is a somewhat general term and like any "fatigue" there is a spectrum of severity. At it's most severe you are essentially over trained but that can take weeks and weeks to achieve and also involves other systems including endocrine. If you're approaching true over training you'd be a complete wreck on every level. A fatigued CNS takes a bit more time to recover than simple muscle tissue. Again, there are varying degrees of fatigue but at even the mild end, where I am, it'll take around 72 hours to freshen up enough to do another speed session. In that recovery period it's important to avoid stressing the CNS with exercise that requires immediate/ fast movement under load. Heavy lifting and of course sprinting are the main ones. More things to avoid are stress and lack of sleep. Unfortunately I have "sleep maintaining insomnia" which means I wake up early. These days it's between 2 and 3 am. Sleep is huge when it comes to healing or resting our CNS for obvious reasons so I'm going suffer a little with this. Stress is a tricky one. Emotional stress is far more detrimental to our health than physical yet it's not an easy fix like just going to bed early. Learning to let go of things you can't control or changing your perspective of a situation is useful and not that tough to do. Be positive rather than negative. Avoiding simple sugars or inflammatory foods like gluten can help and also supplementing vitamin B's. If you're a distance runner then avoiding excess volume is a good idea. Reducing stimulant use, like coffee, will help. You really only need to reduce it, not eliminate it. Unless you smoke meth. Then you should probably just eliminate it.
Symptoms of CNS fatigue can be tricky to recognize if you don't know what to look for. Start off with recognizing your previous training loads as having the potential to bring it about. If you're jogging 20 miles a week then I'd guess you won't have issues. But if you have high stress in your life, lift heavy and/or have a high volume of intensity then keep an eye out. I would very much liken how I feel today to a hangover. I quit drinking alcohol over 3 years ago and thankfully haven't felt a hangover since, but I remember it. I can feel fatigue in my legs of course but it doesn't feel profoundly different than after most solid workout days so I don't think you want to use that as your gauge. I'm talking about the central nervous system which means brain and spinal cord. One test I've read about is to tap your finger on your leg or desk as fast as possible for ten seconds and count the taps. You of course need to establish a base line number but a drop in that number shows a certain level of CNS fatigue. Measuring grip force is another but how do you do that? The best way really is to look at your log book and watch for changes in mental state.
One last thing. I wrote that for ultrarunning you can wake up exhausted but that's typically OK. What I'm talking about is performance specifically, not that CNS fatigue isn't an issue or a detriment, I'm more saying you can get away with it and still develop specific fitness like endurance and metabolic economy. With sprinting and even fast intense race distance like 5k and 10k you can't take it nearly as deep and expect to improve. Yesterday I wanted 100's in under 13 seconds yet when I trained for Leadville I wanted to run 12:00 miles for 24 hours. That's a massive disparity in every way. Not even the same sport. The main thing you must do is avoid fatigue levels that limit performance expectations.