Physical Activity Monitoring

MVPA or Advanced PAEE in Free Living


To maintain a healthy lifestyle, people should take regular exercise as inactivity has been linked to many major health issues. There is a huge scientific focus on examining Activity Energy Expenditure (AEE – the energy used beyond the Base Metabolic Rate – energy required to keep us alive) and sedentary lifestyles in the context of a wide variety of physical, psychological, social and environmental factors.

Measuring physical activity in free-living conditions is can be difficult to achieve with any level of accuracy. There are many factors that will affect the measurement and these will be discussed below in the drop-down sections for each method.

Activity trackers have rocketed in popularity in recent years and can provide valuable personal feedback to the wearer about their own relative physical activity levels over time. However, none have any scientific validation and make claims to accurately measure Energy Expenditure despite known limitations with their principle of operation.

The ‘Gold standard’ laboratory assessment for Energy Expenditure is Doubly Labelled Water. However this method has its own limitations and is very costly to administer.


Doubly Labelled Water

Doubly-Labelled Water (DLW) is the ‘gold standard’ for assessing Total Energy Expenditure (TEE). The subject drinks water labelled with isotopes of hydrogen and oxygen and as they expend energy carbon dioxide and water leave the body. The isotope displacement rates measured in blood, urine or saliva are used to calculate TEE.

However Whilst DLW is very good at measuring TEE, one cannot look in greater depth at the breakdown of energy expenditure; Activity Energy Expenditure (AEE) cannot be accurately assessed because there is no way to determine distribution, the intensity, or the duration of physical activity from the DLW technique alone.

Additionally the displacement of isotope enrichment is not a smooth exponential but is highly variable making it difficult to draw conclusions from the data other than the overall expenditure between the first and last sample. DLW is also extremely expensive to administer, the price per-dose could cost in the region of $5000 for a single subject.


Measuring Energy Expenditure

There are numerous methods for the measurement of Energy Expenditure and Physical Activity in free-living conditions. The sections below examine the key methods and their relative strengths and weaknesses.



Activity Diaries

The most popular method for collecting data on activity is the activity diary. Activity diaries are often used because they are relatively inexpensive to initially administer and are considered easily understood by the subject. However, diaries are at best a rudimentary form of estimating energy expenditure because the data is subjective, prone to recall bias, coding errors in collation and overall non-compliance. Even ‘benefits’ can be misleading: The low cost of creating diaries can be counteracted by time/ wage cost of the data-entry and collation once they are completed, and data gaps due to skipped or misunderstood questions can also be a significant issue. In short: diaries should only be considered for estimating energy expenditure when there is a low requirement for accuracy or an enormous sample group that significantly limits the cost-per-subject.


Pedometers (or step counters) have been commercially available for around 45 years; originally promoted as interventional aids to encourage activity there are a huge variety of units available. The purpose of a pedometer is to measure steps; in theory a pedometer records vertical acceleration and looks for a pattern to determine if it is a step. There is no scientific background for the 10,000 steps per day recommendation – this was simply a marketing slogan invented for the first pedometers in 1965. The technology can be broadly split in to two groups: the simplest use a spring-suspended lever arm that moves up and down opening and closing a circuit with each step. The second use an accelerometer which typically measures ‘zero crossings’.

Pedometers are typically used by those with large sample groups because they are the most inexpensive digital solution, however they are also considered inaccurate. One study comparing coiled spring-levered pedometers to accelerometers demonstrated that the pedometer detected 23.5% fewer steps (75.4% vs 98.9%, P < 0.05)13 another comparing actual steps to a spring levered pedometer’s estimation found 34% error, and measurements of children or those with slow walking speed, gait disorders, high BMI, or mobility issues may be less accurate still. Fundamentally pedometers only work well at certain walking speeds as walking either too quickly or too slowly can cause artefacts or missed steps. However even working at optimum efficiency “steps” are somewhat of a subjective unit of measurement as they vary greatly from person to person; although some can be individually calibrated, accuracy is complicated by any unit that converts “steps” to a quantifiable measurement unit (e.g.  kJoules).

Pedometers can be useful in specific circumstances with careful placement where heavy steps are taken in a regular pattern at specific times (such as on a treadmill at a regulated speed) or as an interventional aid to encourage activity. However their inaccuracy in free living conditions, where the standardisation of laboratory conditions is difficult or impossible, limits the use to either very large studies or to those that just need a very general impression of daily activity.


Many instruments designed to calculate energy expenditure use accelerometers as the primary device for collecting data. These devices measure acceleration by converting the energy of movement into an electrical signal. Actigraphs then interpret this data using algorithms to calculate the intensity (and sometimes the type) of exercise taken. Such devices will also include low pass filters to attempt to remove “background noise” and only record actual physical movement.

Accelerometers have been growing in popularity in physical activity assessment because they are small and light; as microchips have reduced in both size and price and batteries can last longer, accelerometer-based systems can be relatively inexpensive and much more accurate than traditional pedometers.

Wrist-worn actigraphy can provide useful insight into sleep analysis where one is recording overall movement to establish if the subject is asleep; however wrist worn actigraphs are not considered accurate enough to record detailed energy expenditure in free living conditions. Although the hip (or lower back) is seen as being the best location for accelerometer placement,  there are doubts over its accuracy at estimating free living EE. Central to this issue is the difficulty of using accelerometers alone to detect activity; exercise that is particularly demanding on the upper body (e.g. rowing, boxing or swimming) or strength training and activity that is demanding for the subject but not seen as “vigorous” by the device will not be detected by an accelerometer alone.

The choice of device is ultimately a matter of convenience vs accuracy; use of accelerometers, particularly on the wrist, is simple to administer and less obtrusive than other methods at the expense of overall accuracy. Our own MotionWatch device is a convenient method for monitoring physical activity, but for the reasons stated above we provide an indication of MVPA rather than an measure of absolute Energy Expenditure.

Heart Monitors

A number of systems rely on heart rate monitoring to estimate energy expenditure. These devices usually monitor Heart Rate Variability (HRV) and use heart rate to estimate EE. Heart rate monitors obviously have none of the issues that accelerometers have with placement; as long as the skin is adequately prepared and the unit is placed close enough to the heart to get a good signal the system will work. More recently there has been a move towards optical plethysmography on the wrist which introduces further issues with movement artefacts. Using heart rate alone tends to overestimate energy expenditure. One explanation for this is that heart rate alone is not necessarily an indication of exercise taken; for example stress, excitement and stimulants are all known to increase heart rate without AEE taking place. The over estimation of EE by heart rate monitors and underestimation by accelerometers has led some to try using both systems to gain a more accurate calculation. This approach uses the benefits of both methods and allows one to counteract the shortcomings of the other. However, in practice synchronising the two data sources accurately can be difficult. These issues led CamNtech Ltd. to develop the Actiheart in 2003.


The CamNtech Actiheart was the first combined, wearable Heart Rate and Accelerometer device when launched more than 15 years ago. Now in it’s third generation, the Actiheart uses the latest advances in technology to provide validated Energy Expenditure in free living conditions. The small, waterproof device is worn on the chest by means of standard ECG electrodes or optional elastic chest strap. Data may be recorded for up to 14 days with IBI and full waveform, tri-axial accelerometry. Sampling rates of up to 1024Hz may be used for ECG monitoring in short-term mode.

The Actiheart BT model offers full live data streaming of ECG and accelerometer data via Bluetooth.

Click or tap here for more information…


  Key Features

Full Waveform Recording in All Modes

Tri-Axial Acelerometer

Up to 1024Hz ECG Sample Rate

Up to 100Hz ACC Sample Rate

Raw Data Output – Open Data Format

1Gbit Memory

Record up to 14 days (IBI)

Record 1 week full ECG+ACC

Compact Size: 39.7 x 30.2 x 9.25mm

Plug-in Replacement Lead

Direct USB – No Reader Required


The CamNtech MotionWatch provides a convenient, unobtrusive wearable accelerometer combined with advanced software analysis tools for quantifying Moderate to Vigorous Physical Activity (MVPA). The MotionWatch can be worn with the standard wrist strap or optionally in other body locations such as the waist or ankle by means of our soft elastic mounting straps.

The analysis software allows cut points to be set based upon established group calibrations or by means of individual calibration for the subject. The analysis provides deatils of time spent in Sedentary, Low, Moderate and Vigorous physical activity levels.

Click or tap here for more information… 


  Key Features

Light-weight and waterproof

Fast direct USB transfer

No reader required

Ambient Light Sensor

Event marker button

User replaceable battery

Records for up to three months

Analysis of MVPA levels

Raw data export options

Software includes NPCRA

Legacy .AWD compatible

Research Publications

Click or tap below to view peer-reviewed scientific research publications using CamNtech products:

Energy Expenditure

Physical Activity

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