Why does your heart rate increase when you run?

Your heart beats faster when running as In order to move faster the muscles in your legs require more energy. Energy is released to the muscles by a process known as respiration, which requires oxygen to occur. Oxygen is delivered to the muscles for respiration via the blood pumped by the heart. In conclusion your heart beats faster whilst running as your body's muscles require more oxygen for respiration to deliver the necessary energy required for running.