Factors that influence heart rate during outdoor training
In addition to exercise intensity and your level of aerobic fitness, the following factors affect your heart rate during outdoor training :
Activity. Heart rate can change across activities, due to different muscle mass involved, level of experience and technical proficiency. Running typically elicits the highest maximum heart rate during a stress test, whereas cycling and paddling maximum heart rates can be 10-15 beats lower during a similar test. This means that you may need to adjust your training heart rate intensities by 5-10 beats for activities other than running.
Heat and Humidity. Temperature and humidity will influence your heart rate. As the environment gets warmer and more humid, heart rate will gradually increase throughout your activity, even if your pace doesn't change. This is due to your "air-conditioning" and level of hydration. You produce a lot of energy in the form of heat when you move and this heat needs to be dissipated, typically via sweating. Humidity reduces the effectiveness of sweating, resulting in an increase in body temperature, and thus an increase in heart rate. Even if the humidity is low, heart rate will still be elevated, due the extra work the heart must do to help cool your body. It's not uncommon for heart rates to be 5-10 beats above normal ranges in these conditions. Use your heart rate combined with perceived exertion and subjective feeling to set an appropriate pace.
Hydration. Failing to stay hydrated can result in an increase in heart rate, as your blood volume decreases and your body runs low on the fluids needed to maintain body temperature. Dehydration can occur in cold as well as hot environments. If you notice your heart rate increasing with no change in pace or other variables, then increase your fluid intake.
Altitude. The lower air pressure at altitude means there is less pressure to drive oxygen into your lungs. Less pressure means your heart has to work harder to deliver enough oxygen to your working muscles. The result is a higher heart rate at a given pace. Fortunately, your body adapts to higher altitude in several days to 2 weeks, but if you're only at altitude briefly, you'll need to slow your pace to keep your heart rate in the proper range. It also takes longer to recover from a hard effort at altitude, so rest periods may need to be longer.
Fuel. Your body is always using a combination of carbohydrates, fats and proteins for energy production. As the exercise intensity increases, you burn more carbohydrates and less fat (protein metabolism is always fairly small). Even at low intensities, you need some carbohydrate to burn fats (fats burn in the flame of carbohydrate). What does this have to do with heart rate? If you start to run low on carbohydrate, it will become difficult to maintain your pace at a given heart rate. Your perceived exertion and subjective feeling will increase, but your heart rate will be falling. This is informally called "bonking" and can be remedied by eating foods high in carbohydrate. As a rule of thumb, always bring along some form of ingestible energy on any outing lasting more than 2 hours.