Overload creates a particular tension level in a muscle. The greater the weight you lift, the greater the tension level:
Greater overload (more weight) » Higher tension level
The speed with which you perform your reps and the time between sets creates a particular muscular fatigue level. The faster you work, and the less time between sets, the higher the level of fatigue:
Greater rep speed,
» Higher fatigue level
In combination, these two factors determine your fatigue/Tension Level. This is a way of expressing that it is the combined effect that reflects how "hard" you have worked. We can say ...
fatigue plus tension »
Amount of work done
Increase the fatigue level (faster reps, less time between sets), and it takes less tension (less weight) to achieve the same Fatigue/ Tension level. Decrease fatigue (slower reps, more time between sets), and it takes more tension (more weight) to achieve the same Fatigue/Tension level.
Now for the interesting part. It turns out muscle growth depends not only on overloading, not only on timing, but on surpassing a particular fatigue/Tension level called the fatigue/Tension Threshold. Unless your exercise scheme (overload plus timing) bumps you over this threshold ... no growth!
This has some interesting implications for structuring an optimal workout. It means overall workout speed is important, as well as length of rest between sets and speed of reps within sets!
You see, the fatigue level of any muscle is constantly changing. Before you start a set, the muscle's fatigue level is low; during the set, it rises; as soon as you finish, it begins to drop as the muscle recovers from the effort. In fact, if you rest long enough, the muscle's fatigue level will drop back to the level from which it began
Fig. 3-1 Fatigue level during 1 set
A series of sets, with long rests in between, has a fatigue curve that looks like this:
Fig. 3-2 Fatigue level during 3 sets: Long rests
Notice that as a result of letting the muscle's fatigue level drop so low in between sets, it doesn't get more tired during the second set than during the first, or more tired during the third than during the first.