Heart rate

Heart rate is considered one of the four vital signs. Usually it is calculated as the number of contractions (heart beats) of the heart in one minute and expressed as "beats per minute".
When resting, the average adult human heart beats at about 70 bpm (males) and 75 bpm (females); however, this rate varies among people and can be significantly lower in endurance athletes. The infant/neonatal rate of heartbeat is around 130-150 bpm, the toddler's about 100–130 bpm, the older child's about 90–110 bpm, and the adult's about 80–100 bpm. 75 beats per minute translates to 4500 beats an hour, 108,000 beats per day, or about 39,420,000 beats in a year.
The pulse is the most commonly used method of measuring the heart rate. This method may be inaccurate in cases of low cardiac output, as happens in some arrhythmias, where the heart rate may be considerably higher than the pulse rate.
Listening to heart beats using a stethoscope, a process known as auscultation, is a more accurate method of measuring the heart rate.
==Maximum heart rate==
See also: stress test
Maximum heart rate (also called STD, or HRmax) is the highest number of times your heart can contract in one minute, or the heart rate that a person could achieve during maximal physical exertion. It is not the maximum one should obtain often during exercise. MHR is used as a base number to calculate target heart rate for exercise (see below).Physical Activity for Everyone. Department of Health and Human Services, Centers for Disease Control and Prevention. Retrieved on May 1, 2007. 
The heart beats about 60 to 80 times a minute when we're at rest. Resting heart rate usually rises with age, and it's generally lower in physically fit people. Resting heart rate is used to determine one's training target heart rate. Athletes sometimes measure their resting heart rate as one way to find out if they're over trained. The heart rate adapts to changes in the body's need for oxygen, such as during exercise or sleep.
===Measuring HRmax===
The most accurate way of measuring HRmax for an individual is via a cardiac stress test. In such a test, the subject exercises while being monitored by an ECG. During the test, the intensity of exercise is periodically increased (if a treadmill is being used, through increase in speed or slope of the treadmill), or until certain changes in heart function are detected in the ECG (at which point the subject is directed to stop). Typical durations of such a test range from 10 to 20 minutes.
A less costly way to calculate is to warm up thoroughly on a bike (maybe 15 minutes on the flats). On a long, steady hill (doesn't have to be steep) increase effort every minute for at least 5 minutes until you can't go any faster (sitting, not standing). Then full-out sprint for 15 seconds (it is OK to stand at this point). Stop, get off the bike (this is for safety reasons - not mandatory) and immediately check your heart rate at its maximum for a full 60 seconds. This is because checking for 30 seconds and doubling it is inaccurate.
Many exercise machines (stationary bikes, treadmills, etc.) have a built-in heart rate monitors. To ensure you are getting an accurate MHR reading, it's best to do this test on different days. Also, if you are doing this at a health club, try using various machines.
Conducting a maximal exercise test can require expensive equipment. If you are just beginning an exercise regimen, you should only perform this test in the presence of medical staff due to risks associated with high heart rates. Instead, people typically use a formula to estimate their individual Maximum Heart Rate. The most common formula encountered is:
:HRmax = 220 − age (caution: can vary significantly!)
This is attributed to various sources, often "Fox and Haskell". While the most common (and easy to remember and calculate), this particular formula is not considered by some to be a good predictor of HRmax.
A 2002 study of 43 different formulae for HRmax (including the one above) concluded the following:
#No "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of V_{\mathrm{O}_2 max}, and prescription of exercise training HR ranges)
#The most accurate formula of those examined was:
:HRmax = 205.8 − (0.685 × age)
This was found to have a Standard Deviation that, although large (6.4 bpm), was still deemed to be acceptable for the use of prescribing exercise training HR ranges.
Other often cited formulae are:
:HRmax = 206.3 − (0.711 × age)
::(Often attributed to "Londeree and Moeschberger from the University of Missouri–Columbia")
::
::HRmax = 217 − (0.85 × age)
::Often attributed to "Miler et al. from Indiana University")
::These figures are still dependent on physiology and fitness; for example an endurance runner's rates will typically be lower due to the increased size of the heart required to support the exercise, while a sprinter's rates will be higher due to the improved response time and short duration. Also, population averages are just that. Two 40-year-old males with same height, weight, strength, etc. may each have predicted heart rates of 180 (= 220-Age), but these two males could have actual Max HR 20 beats apart (e.g. 170-190). It's important not to guess.
==References==