Thanks for the great questions!
(1) How does splitting a workout into two (one early in the day, one late) affect post-workout central fatigue?
This is essentially the same question as finding the optimal training frequency across the week (1, 2, or 3 etc. times for each muscle group per week). Increasing frequency (and simultaneously reducing volume) does lead to less muscle damage in each workout, but whether the reduction is linear or non-linear is not clear. If linear, then splitting the workout in half wouldn’t make any difference, but if non-linear then it might.
However, there are two other problems with using a higher frequency. One problem is that it is much harder to achieve progressive overload in a workout (adding a rep to at least one of the work sets) when only doing 1 or 2 sets than when doing at least 3 or 4 sets of an exercise. The second problem is that oxidative stress can impair the signaling effects of a second workout when that workout is done very soon after the first.
(2) How do these definitions account for the massive fatigue one experiences after maximum efforts?
They don’t really account for it very well.
This is something I want to examine in more detail and write about soon, in addition to the different fatiguing mechanisms that occur during isometric, concentric, and eccentric contractions.
(3) Why is it hard to get the necessary reps for hypertrophy with very high weights?
Central nervous system (CNS) fatigue increases with increasing rep range (number of contractions). As an exercise bout starts to look less like a strength training set and more like endurance exercise, CNS fatigue increases.
This is why *very light* load strength training doesn’t cause as much hypertrophy as light, moderate, or heavy load strength training with the same number of sets to failure, but *very light* load strength training in combination with blood flow restriction (BFR) does. Using BFR together with *very light* loads reduces the number of reps involved to a similar number as when lifting light loads without BFR.
