This is a "my opinion" statement about my experience in training and finding thresholds or limitations that can lead to fatigue, injury or illness.
I believe we all know that training is "typically" progressive especially for competitors. It is a building process or a detraining process. One may either be going up or going down and this can be so minute that one may actually feel as if they are in a maintenance status, but it is my belief that you are either progressing or detraining to some degree.
However, some of the most common mistakes I see from many unseasoned athletes is the over eagerness to progress and not taking the incremental steps upward to the peak condition. I can only speak from strength sports, but I know that the steps to reaching peak conditions start with an offseason, but it is when preseason hits that things begin to change. A powerlifter that has a goal of squatting 800 lbs. at a competition that has never squatted 800 before has to work up to it over many many months. The training is cycled allowing for these incremental steps upward in weight. The focus is to reach a peak condition and yet the incremental steps are there to reduce injury. There is nothing more demoralizing to an athlete to spend months of training and sacrifice only to get a serious injury or illness in the last few weeks. So the steps have to be incremental and the principle applies to almost everyone.
Keeping a diary was probably one of the most helpful tools that I reffered to from one year to the next. I would look at the previous year and make adjustments always hoping to progress. By keeping a diary I would begin to determine how much load I could put on my body before it would fail or succumb to injury or illness. The point of most athletes is pushing (incrementally) the limit upward to find that threshold. Over a period of years of intense training your body will adjust to handle the load, but even then there are limits that have to be respected.
The picture of this upward progression is like a stock investment on a graph. Hopefully if you were to chart your efforts and your gains the trend would be an upward trend, but the looking at the graph there are some small dips along the way. Those dips are the bouts where the athlete pushed a little too far and fell prey to illness or injury and there was hopefully only a small downward trend on the graph. Yet the overall trend is upward toward the peak condition.
Some athletes do not progress enough and sometimes that is related to fear of pushing that threshold. A few months back I had a very serious crash on a road ride that required several visits to the doctor to have a hematoma drained and yet I never stopped riding despite the pain and the injury. I did not want to let my mind have time to let fear set in and yet I am still a little aprehensive in fast declines on winding roads. I have torn/strained muscle fibers 5 separate times in my quads from training legs over the past 5 years. I can still remember the last one it felt like paper ripping inside my quad and it took 6 weeks to recover. It took me about 6 months to overcome the mental fear of getting back into a progressive training routine. I see many athletes that have too much fear of pushing the threshold because something happened to them or someone they knew and now they don't push hard enough to keep up with competition. It is a mental challenge to go through the burning pain, open wounds, chronic illness or other negative impacts, but the challenge is to mentally rise above and push yourself up to that threshold and then push that threshold incrementally upward.
Knowledge is key and like Dr. Morbius is finding that pushing that threshold has a slight "temporary" downward trend he can now set a mental marker that he knows his threshold and now incrementally push that limit as his body and mind adapts and become ready for the next upward step.
Knowledge for the athlete is knowing what their body is capable of doing at that specific moment in time and knowing how to use tools and resources to accomplish those goals. What are those tools and resources?
A strategic plan - a guideline that is planned out before preseason starts. This doesn't mean that the plan is rock solid. Sometimes the plan has to be tweaked in the middle of training if results are not progressing upward.
Those plans typically have these features (only a few listed)
coaching/trainers/consultants/honest knowledgeable friends - for constructive critism and guidance in things that you may overlook.
Nutrition - key to recovery
Rest - key to recovery
Injury Treatment - annnoyances like tendinitis may need ice treatment to allow continued training despite the annoyance.
I could continue with my opinion and observations, but there is just too much.
To summarize my thoughts the athlete needs to absent of fear in pushing the limits, but not so eager that the steps are too big that there is a risk of serious injury or down time.
One last statement is that most athletes do need to have a down time or off season, but that creates a whole lot more discussion as to why and how.
disclaimer: all of this is just my opinion and obeservation through the years of training myself and others in strength related sports.