Taken from Variables for Weight Lifting and Exercise © 2003 Anthony Sell - All Rights Reserved
Another strategy in weight training is to vary the amount of work per set, over a three to four set period.
This is known as pyramiding the sets, and is used to regulate the intensity of a particular exercise. This is one great way to fight muscle adaptation, and to ensure that you are pushing the muscle closer to the point of failure.
There are several ways to pyramid your sets. Variables that can be increased or decreased with each set include rest interval, number of reps per set, and the amount of weight used in each set.
For example, if you had an exercise routine that consisted of 4 sets of curls; in the first set the weight is 20 lbs, 25 lbs for the second, 30 lbs for the third, and 35 for the fourth; this would be an ascending weight-based pyramid.
Likewise, if you did 15 reps the first set, 12 reps the second set, 10 the third, and 8 for the last, this would be an example of a decreasing reps-based pyramid.
Combining methods is not uncommon, such as combining the two pyramids listed above. When done correctly, pyramids can spur a great deal of muscle growth, and help to break you out of a plateau in performance.