Description of activity categories

The SENS web app provides a detailed analysis of physical activity by categorizing data from the sensor into nine distinct activity types. This articles offers a comprehensive guide for users seeking a deeper understanding of the data presented on the platform and the categories in the exported CSV-files. 

This article consists of the following sections:

• Background
• Description of Activity Categories from Web App
• Description of Categories (columns) in Exported CSV Files
  • Including an explanation of units

For a description of sleep parameters, see this article: Description of sleep parameters

Background

Data are analysed in 5 second intervals and each interval is estimated to belong to a specific activity category. Different types of physical activity are displayed in unique colours. A measurement may appear on the web app like this:

The data ia analyzed according to the intensity and frequency of the activity. Each bar in the graph represents 15 minutes. The activity categories are represented in the graph vertically with colours depending on the type of activity.

Description of Activity Categories from Web App

Measured activity is divided into 9 categories for data visualization on the web-app. Below you will find a brief explanation of these categories. For details, refer to the next section.

Description of Categories (columns) in Exported CSV Files

This section contains a detailed description of the activity categories as seen on the column headers in an exported CSV file. You will also find an explanation of the used units of measurement here.

Click below to go directly to a category

Time categories:

• UTC
• Local
• Unixts

Activity categories:

• Resting (lying_sitting_rest)
• Lying or Sitting Movement (lying_sitting_movement)
• Standing (upright_stand)
• Sporadic Walking (upright_sporadic_walk)
• Walking (upright_walk)
• Moderate Intensity (upright_moderate)
• High Intensity/Running (upright_run)
• Biking (cycling)
• Steps taken (steps, steps2, steps3)

Time categories

UTC

Stands for "Coordinated Universal Time". It is the primary time standard by which the world regulates times. It is not time zone specific and is therefore used as a reference point for timekeeping. The format is YYYY-MM-DD T hh:mm:ms

Local

Shows time in local time zone. The format is YYYY-MM-DD T hh:mm:ms

Unixts

Stands for "Unix timestamp". It is a system for tracking time as a running total of seconds. It represents the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970. It is used to manage time across all the different platforms of SENS motion. 

Activity categories

Resting (lying_sitting_rest)

The sensor is in a horizontal position (+- 45 degrees) and the intensity count is less than 2. At higher intensities, the movement may be classified as 'lying or sitting movement'.

This category should be seen as an indication of the patient being sedentary, sitting or lying down.

Lying or sitting movement (lying_sitting_movement)

All movements in lying or sitting position (sensor is horizontal +- 45 degrees) and where the intensity count is above 2. Biking might also fall into this category, as the sensor often is horizontal. The signal is further processed, see description of biking.

Standing (upright_stand)

The sensor is in a vertical position (+- about 45 degrees) and the intensity readings are lower than a minimum movement threshold of about 0.1 G. This category should be seen as an indication of the patient standing still with only minor insignificant movements recognized.

Sporadic walking (upright_sporadic_walk)

All movements in upright position where the vector magnitude is above the movement threshold, but below the moderate intensity and where the movement is not assigned to other categories. This will include epochs where an activity is starting towards the end of the epoch or taking a few steps before again standing still. Also biking slowly for shorter periods will fall into this category.

Walking (upright_walk)

Assigned when a repeating pattern is detected with:

• An intensity above the measurement threshold, but below the threshold for moderate activity
• A frequency above 0.2 Hz 
• Asymmetrical movement, as only one leg takes a step forwared at a time

This category should be seen as an indication of the patient walking for a period of 5-10 seconds continuously. Smaller walking intervals (trips) below 3-9 seconds will likely be detected as sporadic walking.

Moderate Intensity (upright_moderate)

Assigned to any walking activity above the walking intensity interval for a period of at least 3-9 seconds continuously. The intensity is higher than regular walking, but lower than high intensity activities such as running.

High Intensity/Running (upright_run)

The high intensity category is used for any walking activity above the moderate intensity interval for a period of 3-9 seconds continuously (which is not recognized as biking). This category should be seen as an indication of the patient running or doing high intensity training involving their legs. Smaller running intervals (trips) below 3-9 seconds will likely be detected as moderate intensity or walking.

Biking (cycling)

A repeating pattern is recognized with a frequency above 0.2 Hz, where both legs move continuously, and the measured movement is symmetrical. This is recognized based on pattern recognition. Also, the intensity must be above the measurement threshold. This category should be seen as an indication of the patient doing a cyclic leg movement for more than 1-2 minutes (with pauses). Smaller biking intervals below 1 minute are not seen often and will likely be detected as walking or sporadic walking.

Steps taken (steps)

The number of steps is counted during sporadic walking, walking or high intensity activities. The "Steps" category is further divided into 3 categories depending on the frequency of steps taken.

Comparison to wrist band activity trackers: Steps taken with a very low intensity are included in the step count, and as wrist band activity trackers are usually less sensitive, the number of steps taken during low intensity activities such as cooking, or cleaning will be larger with SENS compared to specially wrist band worn activity trackers which tend to underestimate the steps taken during such activities. To compensate for this, using only “Step 1” will provide a number comparable to wrist band activity trackers.

Units of measurement for activity categories 

This section details the categorization and definitions for each category, as seen in the column headers in the exported CSV file. The unit of measurement for each column is specified in the header after the second slash, e.g., 'activity/upright_stand/time'.

When exporting a CSV file the categories are measured in two units: "count" and "time".

The intensity count is calculated as the average magnitude of movement based on high pass filtered 3 axis accelerometer measurements at 12 Hz sampling frequency, subtracted the noise present on each measurement axis, done on each 5 second epoch. This measurement gives a good indication of the intensity of the measurement. The intensity count in a 5 second interval is normally between 0-100, where 100 is very high intensity.