Increasing the load/weight is not the only way to increase intensity. Since your body, as a sum of complex body systems, registers intensity only as an effort in overcoming training resistance, it does not differentiate among the factors involved in the intensity increase. The intensity of your training can be increased in three ways.
The first and the most popular method of increasing intensity is to increase the training weights. By steadily increasing the weights that you are using over time your training intensity will increase, therefore muscles will grow bigger. The problem with this method is that so many people rush into lifting heavy weights, often sacrificing the form of exercising and increasing the risk of injuries. Furthermore, not enough reps are performed when the weight is increased too quickly.
The second method is even more effective, but is not used often as it dents the ego… This is to slow down the pace of training and perform the set in perfect form for at least 40 seconds! If you lift and lower the weight for 2 seconds in each direction, a single repetition will last 4 seconds. Performing 10 reps using this pace will give you the desired 40 seconds. But unfortunately, you will not be able to impress people in the gym and your own ego with the amount of weights that you will be using this way… Nevertheless, muscles will respond very well to this method, increasing in size.
The third method of increasing intensity is used mainly by experienced and competitive bodybuilders, and consists of reducing the resting period between the sets down to 30 seconds for the upper body and 40-60 seconds for legs. Unlike most trainees, experienced bodybuilders don’t socialise between the sets, but rather knock set after set with minimum rest between them. This is an excellent method for increasing intensity.
It is now down to you to try the aforementioned methods and discover which one will work best for you. My advice, for the best results, is to incorporate all three methods in your training sessions.