As soon as a person begins to exercise, the heart begins beating more rapidly in an effort to pump oxygen-containing blood to the muscles. In order to contract, which they must during exercise, muscles require more oxygen than they do when at rest. A person's heart rate continues to increase with more exertion until it reaches its maximum capacity.
Other factors contribute to an increased heart rate during exercise. High environmental temperatures elevate the heart rate by requiring the heart to pump blood to help cool the skin in addition to contracting the muscles.
Yet another factor relating to increased heart rate and exercise is duration. The longer a person exercises in one session, the more likely his heart rate is to increase as a result of what is referred to as cardiovascular drift. This occurs as perspiration causes the body to lose water and the heart pumps more blood to the skin to lower the body's temperature. Cardiovascular drift occurs whether or not a person remains hydrated while exercising.
Many personal trainers urge their clients to practice interval training as a means of losing weight more efficiently by increasing the metabolism. This causes a person to experience a spike in heart rate, followed by a dip, followed by another spike. A person does this by exercising in bursts of intensity.