No idea. But if I understand what you’re asking, the the following may shed some light:
One way to look at it is that performance is a result of training the body through exertion and then recovery. Recovery, not training, is when the body makes advancements. Figuring out the balance between the two for each athlete, and corresponding to the development of both fast twitch and slow twitch muscle fibers, is the science, and it’s not necessarily apparent without knowing how to correctly analyze the situation.
(Small but relevant tangent):
One way athletes have beaten training science is by using pharmaceutical science ala PEDs. PEDs allow for the athlete a *smaller* recovery period thus allowing more sessions of exertion without physically breaking down, which creates a higher peak. Without using PEDs, the athlete has a smaller window of getting the training “right” because of the symbiotic nature of exertion/recovery. This is why athletes on PEDs routinely beat clean athletes - they have a bigger window to “get it right” since they recover so much faster.
Too much exertion with too little recovery and the body breaks down. Too little exertion and the body doesn’t max out. To achieve the right balance, the athlete has to understand how to push themselves to their limits of the specific exertion while also being able to accurately relay how they “feel” to the coach. The coach has to be able to analyze the info they’re given (by the athlete and mechanical data) and be able to recognize if it corresponds to what the training should be achieving or if it’s above/below anticipated results. If the athlete isn’t comfortable with the training exercise, it could feel harder to them because of the unfamiliarity, thus not stressing the body enough, or the flip side is that they may not recognize that they’re pushing too hard which in turn overstresses the body which over time breaks down from not enough recovery. But getting it right, the training, could produce percentage points of increased returns, and for elite athletes, even fractions of a percentage point can mean the difference in competition. Look at Maxi - he’s ridiculously fit and can run non-stop, but if he doesn’t max out his training, and his fitness goes down a percent, that could be the difference between max running for 90min verse maybe needing a sub at 80-85min.
Sorry if that’s a long response, and hopefully it addresses what you were asking regarding diminished returns.