Human muscles are reduced, due primarily to various stimuli. This process is accompanied by thickening or shortening of muscle fibers, and thus the entire muscle as a whole. How can you call it reduction?For example, you can spend a short electric shock into the body, which will cause immediate muscle contraction. It will not last more than 0.1 seconds. Another way to cause muscle contraction to be for a long time in a certain position. That is, while lying in bed or sitting at the computer. In these cases are prolonged muscle contractions. All this is due to the position of the body in which almost no movement.So why shrink the muscles?This process occurs in vivo under the influence of excitement, which enters the muscle from the nervous system by centrifugal nerves. You can perform two simple experiment to visually understand how this happens.To begin sit on a chair and put one foot to the other. Ask you to hit the edge of the palm on the knee. This will entail the stimulation of the receptors of the tendons of the knee. The process of excitation arising in them, are transmitted via the spinal cord through the nerves to the muscles of a person. They will be reduced for some time, leading to the "jumping" of the tibia. This process is also called the patellar reflex. It is caused by excitement, which came from the CNS to skeletal muscles.The second is a simple example of muscle contraction. Hail any passing person on the street. You will see how he will turn his head, despite the fact that you can get it and not know. Moreover, he turns in the direction of the sound. This is a reflex process, which is caused by nervous excitement. It enters the receptor organs of hearing and goes through the brain to the muscles of a person. Thus there is a reduction of muscles.