Increasing supraspinal output, is one in the same with effort. As you intend to apply more force, the signals from the CNS, to the muscles, are increasing. Activation increases, both recruitment (number of motor units) and rate coding (frequency the motor units are firing). After recruitment is full, rate coding still continues to increase.
At the lowest effort, the smallest slowest motor units are twitching at lower rate coding/low force. Mid range and larger motor units are not active at all. As you increase effort, rate coding increases and more MU's are recruited, etc. The tension a fiber creates is increased at higher frequencies of rate coding. This is important and the crux of the whole effort idea. As rate coding increases, the fibers controlled by that motor unit put out more and more force until the frequency is high enough that they are in tetany. (No relaxation between twitches, the cell is saturated with calcium and all crossbridges are attaching.)
Say your at mid effort, the first recruited are at a relatively high rate coding level (high for them as slower units fuse at lower frequencies) and putting out close to, or even max force. The later recruited units are at a lower relative rate coding frequency (even though the frequency is similar betwen the two, the later recuirted start higher and end higher) and putting out a lower percentage of maximum force. The largest last recruited motor units may be intermitantly twitching at relatively low force. The only time the largest last recruited motor unit is putting out it's max force and staying fully on, is during an MVC (maximum voluntary effort), aka your max effort.
1) The percentage of maximum tension increases orderly just like with recruitment. As effort increases, supraspinal output increases and therefore activation increases from smallest to largest.
2) Your maximum voluntary strength (whether fresh or fatigued) is the sum of all motor units firing at maximum.
During a 1RM attempt, all motor units are recruited and fired at maximum rate coding. Every fiber is putting out as much force as you can voluntarily ask of it. During the first rep with your 3RM, all motor units are recruited, but the largest last recruited are not 'on solid' but most likely are at full intermitant force. A 'full always on force' is not needed yet. During the third rep with your 3RM, all motor units are recruited and fired at maximum rate and 'on' solid.
The fatigue of the two previous reps has limited your strength so the final rep now requires the full strength from all available motor units. Every fiber is putting out as much force as you can voluntarilly ask of it. In other words, fatigue from the first two reps has limited your strength, so at that moment, your max effort = the weight on the bar.
The fibers are most likely however, putting out maximum tension even on the first rep, but are in a 'rotational recruitment' pattern. The only difference is whether they are 'on solid' and fatiguing rapidly or not.
Repeat reps with a pause allows the fibers to reload ATP and Pcr so they maintain their ability to generate a higher tension repeatedly. Continuing a set, such as 'to failure', forces the fibers to remain in tetany and continually lose force due to high levels of fatigue and energy depletion.
What does all this mean? It shows both ends of the spectrum for the two main hypertrophic paths. (There is ample evidence for both extremes, and everything in-between; mTOR ATP depletion, mTOR in relation to MGF/IGF1,The Tenson/Time Index,P70 with higher rate coding, mechanocytes, ect.)
If you believe a fiber must experience both of the following, 1) create and experience full tension for a short period that decends rapidly with fatigue and 2) Work deep into fatigue with the highest A.T.P. depletion, then the only place the largest motor units will experience this, is 'at max effort'. (if a fiber is not 'kept on' it will have time to reload ATP and Pcr)
If you believe a fiber needs to create it's highest tension over and over intermitantly, with less fatigue and lower A.T.P. depletion, then 'hard' but not 'maximum' effort is all you would require.
And as I said above, both of these factors have been shown to initiate protein synthesis. What is not known is how much A.T.P depletion is requried, if it's variable or just a 'permissive switch', how much tension (percentage of maximum) is needed, and if it's also variable with stimulation or a 'permissive switch'.