Actiheart User Manual 5.1.31 (Jun 2022)
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Appendix 2: The Branched Model, Calibration Equations and
Other Equations
Estimating Energy
Expenditure (EE)
from Activity and
Heart Rate
Activity monitors use motion sensors (accelerometers) to provide an objective
indicator of total body movement. They can be used to assess the frequency,
intensity, duration and EE of physical activity. A well described limitation of
activity monitors typically worn on the hip is the inability to assess upper body
activities such as throwing, catching, carrying or lifting and lower body activity
such as cycling. Moreover they are unable to differentiate between walking on
the flat versus up or down hills or stairs. Studies have documented that activity
monitors significantly underestimate EE of common daily activities in adults for all
of the above reasons.
Heart rate (HR) monitors provide an objective indicator of physiological effect of
physical activity. They too can be used to assess frequency, intensity, duration
and EE of physical activity. HR has been shown to be linearly related to VO2 and
EE during physical activity. However the relationship between HR and VO2
depends on the amount of upper body work that is taking place. HR will be higher
for any given VO2 during arm activity in comparison with leg activity or combined
arm and leg activity. This is primarily because of the smaller amount of muscle
mass involved with arm-only activity. Therefore HR alone will lead to an
overestimation of EE. For reliable results the HR-VO2 relationship needs to be
derived for a specific individual. Numerous other factors that influence HR under
resting conditions e.g. stress contribute additional error when HR monitors are
used for extended periods of monitoring of daily activities.
Studies (1) & (2) (and others before them) have demonstrated that use of HR
along with motion data can improve prediction of energy expenditure for lifestyle
activities.
Techniques used
for Estimating
Energy
Expenditure from
Activity and Heart
Rate
The techniques used for estimating EE from either activity or heart rate alone
involve establishing calibration curves of activity vs VO2 and HR vs VO2. This is
done in a laboratory usually on a treadmill or a bike (for lower body activity).
If using a treadmill, subjects walk at a variety of incremental speeds and
gradients and HR or activity counts are measured and plotted against oxygen
consumption (VO2). A curve is fitted to the data using linear regression and the
resulting regression equation is used to predict VO2 and hence EE. 1litre of
Oxygen = 4.8 kcal= 20kJ. EE is often expressed as an average intensity in terms
of metabolic units or METS which represent multiples of resting metabolic rate. 1
MET = 3.5 ml/kg/min or 0.017Kcal/kg/min. HR Calibration curves are unique to
