Weight lifting is often referred to as progressive resistance training, as you need to increase the resistance to see continued progress. The question of when to increase the weight is a very valid one, but the answer depends on your goals and other factors. Consult your physician before beginning any exercise program.
If your goal is to add muscle, improve strength or develop power, you need to train heavy. "Heavy" in this case means at least 75 percent of your one-repetition maximum on the exercise you are performing. This will cause your endocrine system to respond by producing more testosterone, which is the hormone primarily responsible for muscle growth. If you are training with considerably less intensity, such as weights in the 50 percent range, gradually add weight to allow your body to become accustomed to greater intensities.
If your goal is to increase the amount of weight you can lift in a single repetition of an exercise, spend time training with weights above 90 percent of your one repetition maximum. An increase to 90 percent or more should only occur after you have become comfortable training in the 75 percent range, and you should gradually increase the weights to 90 percent. This will improve your body's ability to recruit additional muscle fibers; greater recruitment results in greater strength. If you are training a lift multiple times per week, only one workout should be performed at 90 percent or more.
Another method of deciding when to increase training weights is called periodization. This is a method of gradually increasing your training percentage on a weekly basis until you have exceeded your previous one-repetition maximum. An advantage of this method is that it gradually allows you to increase intensity while becoming accustomed to heavier training weights. An example would be starting a cycle by training with 60 percent of your one-repetition maximum and increasing by 5 percent a week until you have achieved 105 percent, or even 110 percent of your previous maximum.
Use Good Judgment
One of the greatest mistakes in training is increasing weight too soon, which can result in injury. Use proper judgment on increasing weight, do not let your ego decide. Never sacrifice good technique to add a few pounds to the bar. Weightlifting is one of the safest sports in existence, with a lower injury rate than badminton. It is never an exercise itself that causes an injury; it is poor judgment by the lifter.