Every muscle is comprised of groups of muscle fibers called motor units. When we perform a set we fatigue only certain motor units. Then we rest between sets and allow those motor units time to recover. If they are completely recovered we continue to train the same motor units on the next set. This is why so many guys fail to see continued gains in size and strength. They’re always training the same motor units.
The key to muscular hypertrophy training is to fatigue the muscle and still continue to train. This will force us to recruit new muscle fibers to activate. To do this we must have incomplete recovery between sets. This necessitates shorter rest periods. Because the motor units are not recovered, we will recruit new motor units to fire to continue the training.
Every set must be performed until failure. Max effort. The rest between sets must be short; say 45 to 60 seconds. To complete successive sets the weight must be lowered each set. Because we can not repeat a max effort with such a short rest, the weight must be lessened to achieve the same number of reps on following sets.
For Example: Barbell Curls
1st set 115 X 10 reps
2nd set 105 X 10 reps
3rd set 95 X 10 reps
4th set 75 X 10 reps
Time to put your ego aside. The weights you will be handling on your last couple of sets will be lighter than usual. Its going to be hard work. The short rest and high reps is physically demanding. The weights may be relatively light, but you will be training with maximum effort on every set.
Like we said, every set is a max set. However with such a short rest and incomplete recovery between sets, our max effort will only allow us to complete sets with progressively lighter and lighter weights. As motor units fatigue, we will be recruiting different motor units to fire to continue the work. This greater than normal number of motor units being recruited is what will bring about increased muscular size. More fibers firing, means more fibers growing, means more fibers contributing to muscular size.