An increase in activity causes the heart to beat faster. The magnitude of this increase is dependent upon body type, physical condition, activity type and intensity, and it can be changed by manipulating any of these variables. Variances in the rate response are an indicator of which fuel (fat or carbohydrate) is feeding your muscles.
Types
Everything you do changes your body's demand for oxygenated blood, therefore, everything you do changes your heart rate. Not all forms of activity, however, cause equal amounts of change. Heart rate from cardiovascular exercise is an accurate measure of how hard your body is working. On the other hand, increase caused by resistance training is a mechanism the American Council on Exercise recognizes as the "pressor response, which is governed by the autonomic nervous system and occurs reflexively from the contraction of the skeletal muscles."
Significance
As muscles work harder, they need more fuel to function. The most abundant source of fuel is fat, which requires oxygen to be used by the body. In cardiovascular activity, the heart increases both its rate and contraction strength to provide more oxygenated blood to the muscles in need. Without this increase, the muscle would fail to receive its much needed oxygen and the system would switch over to using a much less efficient and low-energy source known as carbohydrates.
Considerations
Heart rate increase with physical activity is a necessary response. After longer bouts of exercise (greater than 45 minutes), the heart slowly begins to beat faster, even in the absence of increased intensity. This is a response called cardiac drift, and is a result of increased core temperature and dehydration.
Time Frame
According to the American Heart Association, after intense physical activity the heart rate should slow by at least 18 beats per minute within the first minute of recovery. Check your recovery heart rate by monitoring it immediately after exercise, a minute after and then subtracting the latter from the former.
Medication
Certain medications can alter the heart rate's response to physical activity. Examples include beta blockers like atenolol, calcium channel blockers like Norvasc and anti-arrhythmics like Rhythmol. Hypertension specialist Stephen Sheps explains that "no matter how hard you exercise when taking a beta blocker, you may never achieve your target heart rate."
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