At the core of a runner’s body is a variable pump that manages to spectacularly manipulate it’s blood flow output. At rest, the heart pumps roughly 250 ml/min of blood (the idling state), but this can increase two orders of magnitude to upto 22,000 ml/min during maximal exercise (redline). In highly trained athletes, maximal flow volume is of the order of 40,000 ml/min. If you look at the ratio, 40,000/250 is nearly160x times the value at rest.
Cardiac output is the product of heart rate (HR, beats per minute) and stroke volume (SV, ml per beat) expressed as :
Cardiac Output = HR x SV
The amount of oxygen that can be removed from circulating blood and used by the working tissues in a given time period is called “VO2”. An individual’s maximum utilization capacity is represented by maximum oxygen consumption or what is commonly known as “VO2max” in exercise literature. When exercise intensities exceed this aerobic capacity, sources of energy outside of the aerobic system (glycolytic, alactic etc) have to be utilized to support the movement task.
Mathematically, VO2 is the product of cardiac output and the amount of oxygen extracted from the blood. The difference between the amount of oxygen within the arterial blood and that within the venous blood returning to the heart is termed the arteriovenous O2 difference (a-VO2diff). This constitutes the extraction capacity of blood.
Combining these variables yields the famous Fick’s equation :
VO2max = HRmax x SVmax x a-VO2diffmax —- EQUATION 1
Research indicates that despite increasing age, maximum HR is more or less stable. Therefore, little benefit can be obtained by heart rate increase and any endurance training benefits is derived from the second and third terms in EQUATION 1. In other words, the more stroke volume your heart has and the more oxygen extraction is possible between arterial and venous blood return, the more is VO2max which can then be used to extract more speed out of your running.
II. Characteristics of HR Benefiting it’s Field Application
HR has proven beneficial for normal day-to-day athletes because it shows a satisfactory correlation with physiological variables such as oxygen consumption rate (VO2) and blood lactate accumulation Due to it’s correlation with VO2, HR been used to estimate VO2max as well.
It is also somewhat correlated to metabolic substrate use during exercise (the “Am I using more of Fats or More of Carbohydrate” question) and has been used to estimate energy expenditure in field conditions.
However, two caveats to the use of HR as a surrogate measure of physiological capacity :
1) The prediction of VO2max from HR is said to rely upon several assumptions and it has been shown that the results can deviate up to 20% from the true value.
2) There appears to be general consensus that this method provides a satisfactory estimate of energy expenditure on a group level, but is not very accurate for individual estimations.
3) HR by itself only answers the “how many beats per minute” question but not the “how much stroke volume per beat” question. Therefore, specific questions about adaptation to training may not be directly answered by just HR alone.
4) HR is affected by variables – such as weather conditions, hydration status and day-to-day variations such as fatigue. For example, scientific literature states an approximate variation of 3 beats/min in HRmax from day to day.
5) There appears to be a steady increase in HR during activity, a phenomenon termed “cardiac drift”.
Due to these reasons, it is a satisfactory variable to use in the field mainly for the following :
A) To monitor exercise intensity by quantifying time spent in demarcated HR zones. The demarcation of zones is subject to various schools of thought, some being more representative or less representative of athlete physical condition. Generally in practice, HR zones coincide with the accumulation of lactic acid in the blood, with HR associated with low lactate values assigned to low intensity and higher lactate values assigned to high intensity.
B) It is valid in correlating internal load, such as Training Impulse (a metric generated with the knowledge of HR) to training outcomes such as fitness, fatigue or performance. For example, a study done in cyclists found that a weekly accumulation of individualized TRIMP of 650 units was necessary to maintain improvements in aerobic fitness (power output at 2 mmol/ L).
C) It is used to inform the daily state of an athlete through measurement of a resting pulse. For example, an over-reached state of fatigue may be accompanied by a higher-than-normal resting HR or sleeping HR.
HR is not an objective measure of work rate. Some good examples why this isn’t so :
1) A sudden increase (or decrease) in work-rate, i.e running or cycling power, may not coincide with an immediate rise in HR. Due to the “laggy” non-linear response of HR, it is not ideal to use to inform about work rate changes.
2) At the same work rate, HR is known to slowly drift to higher values despite working at the same external load. Scientific studies show that this is partly co-related to dehydration.
3) In hot environments, cardiac drift has been shown to correlate with core body temperature increase. It has also been shown that in hot environments, VO2max can also be lowered. Therefore, a prediction of VO2max using HR becomes baseless.
4) In high altitudes, HR increases inspite of little to no change in VO2 or external load. Therefore, the HR-VO2 curve “right-shifts” and makes sea-level HR zone methodologies suspect. Recovery characteristics of heart rates due to acclimitization are very individual.
III. Monitoring Immediate Training Effects Using HR
Resting HR after a night’s sleep is an excellent and research proven way to monitor immediate training effects following a workout. My method of monitoring resting heart rate which I have followed for sometime is the following :
- After a 15 minute ‘stabilization’ phase of remaining still on the bed in supine position, record resting HR using a chest strap for 5 minutes. Breathing rate should be 10-12 breaths per minute strictly, and not any lower.
- Following this, get up to standing posture and measure heart rate for another 5 minutes. Again, breathing rate is maintained between 10-12 breaths per minute.
Stop recording and analyze the readings using the following methodology.
- Compare average HR in either standing or supine mode to the respective averages from the last 7 days during a period of easy training load. A week of baseline data is sufficiently long to eliminate any short term effects. If the new resting HR increases are well below 6 bpm compared to baseline, it indicates satisfactory recovery with minor fatigue. HR increases greater than 10 bpm to baseline indicates high level of fatigue. Greater than 16 serves as an alarm signal.
- The difference Average Standing HR – Supine HR (HRdiff) within 10-15 beats/minute is normal since standing HR is typically higher than supine. Any differences higher than this may indicate high sympathetic mediation and/or state of fatigue which, again maybe normal considering training cycle or a cause for precaution.
Athletes in training can rate themselves based on daily HR values as per the following table :
|Component||HR Increase Range||Points|
|Resting HR after night’s sleep||0-6 bpm|
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