You will know when to change the exercise when the muscles no longer get a pump. The pump is when the blood fills the muscle after it has been worked. When performing the same exercise over and over you no longer produce enough stimulation to cause excess amounts of blood to rush to the area. This is because the muscle fibers have learned the exercise and figured a way to lift the weight without having to recruit new fibers. The nervous system has been tuned up as well. The nervous cell synapse is more sensitive and secretes just enough acetylcholine and growth hormone to work the muscle. In order for greater stimulation and more muscle cell recruitment the exercise has to be changed. The new exercise will produce a challenge to the CNS causing an increase in growth hormone and nerve stimulation. The new stress will force the muscle to figure out a way to handle the workload. Blood will rush into the area to help with the metabolism and breakdown. This new exercise can last a few workouts until the CNS figures it out and then you will have to change it up again because you won't have a pump.
You have to decide when it is time to change up the exercise. If you still get a pump from the exercise then it is still worthy of doing. However, once you don't feel the pump and aren't getting anything from it chances are you have adapted. Body builders will call this "going stale."
I find that I need to change up my program every two weeks. You must constantly change up the methodic to get the muscles to get stimulated. If you have been doing the same program for a while I suggest that you change it up to give the muscles a new stimulation.
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