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Measurement Technology and Definitions 

Our Clinically Validated Optical Sensor

When the heart beats, capillaries expand and contract based on blood volume changes.  Our patented optical sensor, utilizing motion-tolerant PPG technology, emits light signals that reflect onto the skin to accurately and continuously measure weak blood flow signals.  A state-of-the-art signal extraction method is used to remove optical noise from skin motion, body motion, and environmental noise (such as sunlight) in real time.  By removing this noise from the blood flow signal, we can accurately and continuously measure heart rate and heart rate variability even during extreme physical activity.

Heart Rate (HR) is the speed of the heartbeat measured by the number of contractions of the heart per minute (beats per minute or bpm). The heart rate can vary according to the body’s physical needs; activities that can provoke change include physical exercise, sleep, anxiety, stress, illness, and indigestion of drugs.


Several studies, as well as expert consensus, indicate that the normal resting adult heart rate range is 50-90 bpm.  During sleep a slow heartbeat with rates around 40-50 bpm is common and is considered normal.  Tachycardia is a fast heart rate, defined as above 100 bpm at rest.  Bradycardia is a slow heart rate, defined as below 60 bpm at rest.  When the heart is not beating in a regular pattern, this is refered to as arrhythmia.  Abnormalities of heart rate sometimes indicate disease.


Resting Heart Rate (HRrest) is defined as the heart rate when a person is awake, in a neutral temperate environment, and has not been subject to any recent exertion or stimulation, such as stress or surprise.  The normal resting adult heart rate range is 50-90 bpm although endurance athletes at the elite level have been found to have resting heart rates below 50 bpm.


Maximum Heart Rate (HRmax) is the maximum heart rate an individual can achieve without severe problems through exercise stress and generally decreases with age.  There are a number of formulas that are used to estimate HRmax, the most widely used is by Fox and Haskell:  HRmax = 220 - age


Target Heart Rate or Training Heart Rate (THR) is a desired heart rate reached during aerobic exercise that enables one’s heart and lungs to receive the most benefit from a workout.  This theoretical range varies mostly based on age; however, a person’s physical condition, sex, and previous training are also used in the calculation.  The TRH is usually calculated as 65%-80% of the person’s HRmax, also called the calculation.  The TRH is usually calculated as 65%-80% of the person’s HRmax, also called intensity.

Heart Rate Variability (HRV) is the measure of the beat-to-beat variation (the time interval between heartbeats) over a period of time.  Unlike heart rate (HR) that averages the heart beats per minute, HRV looks much closer at the small fluctuations of the heart that occur in response to internal and external events; it is therefore, a measure of the heart’s ability to accelerate or decelerate quickly and efficiently to meet the body’s immediate needs.  It is also referred to as RRi (R to R interval).  HRV is gaining popularity in biometrics as a means to create more in-depth insights including measurements of stress, sleep, training status, and much more.  In general, decreases in HRV indicate some kind of negative general stress.


HRV is a direct link to your Autonomous Nervous System (ANS) and can therefore, be used to gain insights into your nervous system, stress, and recovery activity.  Clinical studies have shown that when HRV levels are high, a person experiences low levels of stress and greater resiliency.  When HRV levels are low, a person experiences greater stress and lower resiliency.  Low HRV can be due to a number of different factors including age, gender, disease, or stress among other things.  Stress is a state of physical, mental, and emotional strain on the body.  One should not think of stress as a single event or experience but rather an accumulation of all circumstances occurring over time.  Stress from all sources have an additive effect on the body and will eventually lead to decreased mental and/or physical performance on not properly balanced with relaxation and/or recovery.

Our HRV score is calculated from the RMSSD (Root Mean Square of the Successive Differences).  RMSSD is strongly backed by research and is considered the most relevant and accurate measure of Autonomic Nervous System activity over the short-term. Furthermore, a natural log is applied to the RMSSD [ln(RMSSD)] in order to distribute the numbers in an easier to understand range.  The table below provides indicate ranges by age and gender.

HRV is not the same for all individuals and cannot be assumed that date between individuals will show the same physiological response to similar stresses.   It is helpful, therefore, to analyze individual baseline HRV metrics.  To help resolve challenges with the minute-to-minute and day-to-day variation, it is recommended to collect weekly and/or 7-day rolling averages, which have higher validity and statistical value than daily recordings.  For more information see …

VO2 or oxygen consumption, is a measure of the volume of oxygen used by your body to convert the energy from the food that you eat into the energy molecules that your body uses at a cellular level.  It is measured in liters per minute. 


VO2max (or maximal oxygen consumption) is the maximum possible VO2 that a person can achieve.  VO2 and VO2max are important in the context of exercise because they are a measure of your body’s ability to generate ATP (adenosine triphosphate) and ATP is the energy source that allows your muscles to continue working while exercising.  VO2max is a measure of your cardiorespiratory fitness level.

Breathing (or respiration) rate is defined as the number of breaths per minute.  The typical breathing rate for a healthy adult at rest is 12-20 breaths per minute.  Breathing rates vary by age and may increase with fever, illness, or other medical conditions.  Our breathing rate is calculated at rest. 

Average resting breathing rates by age groups are:

  • 10 years: 17-23 breaths per minute

  • Adults: 12-18 breaths per minute

  • Elderly ≥ 65 years old: 12-28 breaths per minute

  • Elderly ≥ 80 years old: 10-30 breaths per minute​

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