A. There are a few reasons for lifting lighter weight for higher reps before attacking the heavy "work set". First and foremost, for safety. In order to lift weight that is heavy, your muscles generate a lot of force to move the weight. The force generated by the fast twitch muscle fibers may place a lot of stress on the connective tissues, namely the tendons and ligaments which connect the muscle to bone and joints. Most powerlifters who are known for lifting maximal weights, that is one rep maximum, will do an "opening" set with lighter weight and progress with heavier weight until their one rep max is reached. Bodybuilders as well, perform a warm up set and progress to their heavier weights. This is known as the Pyramid Principle.
So how does this apply to a non-powerlifter or bodybuilder? By performing one or more sets of progressively heavier weight with each exercise, you draw blood into the muscles and joints being used. This will not only warm up the muscles and joints but will allow you to perform the specific exercise through a full range of motion stretching the muscles and joints to prevent them from being tight and thus straining them while lifting the heavier weights.
Finally, research has shown that the body responds well to progressive resistance. If you perform your maximal lift at the first set, you will have to "guess" what your weight is for the heavier set. By progressing or pyramiding your weight and reps, you can not only increase your weight at safe intervals, but more effectively choose a weight that will allow you to progressively add resistance to stimulate muscle growth and strength.
PYRAMID PRINCIPLE
The pyramid principle is not complex at all. It is a very basic systematic approach designed to create a structure of reps and sets to provide the optimum setting for muscle growth and strength. It is set up by performing a set with light to modest weight for higher reps. After completing the first set, you would add weight and perform fewer reps. Upon completing the second set, you would again add more weight and perform fewer reps. This is known as ascending pyramids. An example of pyramid sets using the chest press would be as follows;
Set one: 100lbs for 12 reps
Set two: 110lbs for 10 reps
Set three: 120lbs for 8 reps
It makes sense that you will decrease the reps as the weight is increased as the muscles will be placed under a greater load and will fatigue faster.
If you're looking to shake up your regular routine, give pyramid sets a try! Staying with the same weight for each workout will eventually lead to a plateau and halt in progress. You can adjust the rep scheme and resistance to suit your fitness needs and fitness level. When you can perform the 8 reps easily, increase the weight for each set. *If in doubt, contact myself or another Certified Personal Trainer for a demonstration and to help set up a program using the pyramid principle.