Like smooth muscles, cardiac muscles contract in a constant rhythmic pattern. This creates and controls the heart's constant beating at a constant pace. They have the ability to increase or decrease this rate. Heart rate often increases after a workout or shock. High intensity workouts result in a faster contraction rate because as muscles work they require more and more oxygen. Blood carries oxygen to the muscles and, as a result, causes the pump to be stronger and faster to provide tired muscles with the oxygen they need. When a person is afraid, the heart rate also increases. This is caused by sympathetic nerves, which trigger an impulse in the brain to release adrenaline from the adrenal glands located at the top of each kidney. This hormone makes the heart beat faster and delivers oxygen to the body's muscles more quickly. This causes a person to be more alert and prepared for a fight-or-flight response. A fight-or-flight response is an immediate process that the nervous system undergoes when frightened. When this is activated, nerve fibers are activated and hormones are released which increase muscle tension. Unlike smooth and skeletal muscles, cardiac muscles are destined to never tire due to the terrible importance of the heart. If the heart stops working, asphyxiation, the deprivation of oxygen, can cause death within minutes. Like skeletal muscles, the heart can also be subjected to resistance training. Aerobic exercise is the preferable way to achieve this as it increases your heart rate. These exercises include activities such as jogging or cycling. Similar to smooth muscles, cardiac muscles are also involuntarily controlled by the autonomic nervous system