Assessment of Physical Activity Intensity with Accelerometers and Oxygen Consumption

Summary

Here, we present a protocol to assess physical activity intensity levels measured with indirect calorimetry and two accelerometers (on the right wrist and waist) during an incremental walking-jogging test (from 0.84 to 2.37 m/s) on an oval track. The results show differences among the physical intensity assessed through those methods.

Abstract

The study compares objective methods of assessing the intensity of physical activity. The goal was to identify the differences in physical activity intensity measured through indirect calorimetry and triaxial accelerometry on the wrist and waist, using the counts cut-off points for the accelerometers and ventilatory threshold one (VT1) and two (VT2). This research is a cross-sectional analytical study of university women (n = 35, 22.4 ± 1.9 years old). Triaxial accelerometers and portable ergospirometry were used during an incremental standardized walking-jogging test in an oval track of 160 m length (speed increments every minute with an audible signal from 0.84 m/s to 2.37 m/s). Accelerometry (from wrist and waist), oxygen consumption, and ventilatory response data were collected during the test. VT1 and VT2 were estimated through VSlope (VO₂/VCO₂) and carbon dioxide production/minute ventilation (VCO₂/VE), respectively. An open-source statistical software was used. All the participants were able to finish the test. VT1 was found in stage 2 (n = 5), stage 3 (n = 5), stage 4 (n = 3), stage 5 (n = 8), stage 6 (n = 5), stage 7 (n = 3), stage 8 (n = 1), and in stage 9 (n = 5). VT2 was found during the incremental test only in 9 volunteers, in stage 7 (n = 2), in stage 8 (n = 3), stage 9 (n = 2), and stage 10 (n = 2). Differences between wrist and waist accelerometer measurements were observed in higher intensity stages of the test. Higher PA intensity measurements were found in the wrist, mainly above VT1. Also, there were differences between physical activity intensity levels detected by accelerometers and VT1 and VT2. The commonly used methods to assess physical activity intensity show no agreement in identifying the speed at which vigorous-intensity physical activity begins during an incremental test.

Introduction

Regular physical activity (PA) has multiple health benefits and reduces the risk of developing various non-communicable diseases¹. However, these effects depend on the intensity at which physical activity is performed (i.e., light, moderate, or vigorous)². To obtain these effort levels, they can be evaluated using a Cardiopulmonary Exercise Test (CPET); this allows us to objectify the functioning of the cardiorespiratory system to objectify its PA intensity levels based on the ventilatory thresholds (VT), which are the gold standard to differentiate the passage from light to moderate or vigorous PA³.

Accelerometers are wearable devices that measure the accelerations of the body segment on different axes to which the monitor is attached⁴. The accelerometer measures the PA intensity in an arbitrary unit named counts per minute (CPM). The amount and intensity of PA may be obtained by classifying counts, a measure that quantifies acceleration within a specific time interval (epoch) with a set of cutpoints, i.e., intensity thresholds for PA intensity classification⁴. Light PA in healthy people usually corresponds to activities such as walking at less than 3.2 km/h and warm-up games. Moderate PA includes activities such as walking at more than 3.2 km/h and aerobic games. Lastly, vigorous PA includes running and high-intensity games⁵.

Although accelerometry provides quantifiable data, these vary depending on the types of accelerometers used or their positioning on the body⁶. The traditional cut-off points to identify the different levels of intensity of PA have been proposed from data obtained in normal-weight subjects⁷, and they have even been previously used in hospitalized patients⁴. In adults, cut-off points usually used are 0-2689 CPM (light PA), 2690-6166 CPM (moderate PA), 6167-9642 CPM (vigorous PA), and 9643 and above CPM (very vigorous PA)⁸.

It is unknown whether the PA intensity identified with a triaxial accelerometer corresponds to the intensity detected through ventilatory thresholds (VT1 and VT2) during a CPET. So, the aim of this research was to assess the physical activity intensity levels measured by indirect calorimetry and wrist and waist-worn accelerometry during an incremental walking-jogging test on an oval track and examine the agreement between these assessment methods in young healthy women⁹.

Protocol

The research was performed in compliance with institutional and international guidelines and was approved by the Committee on Ethics and Research in Human Beings of the institution.

1. Recruiting the participant

1. Use posters and send an electronic flyer to student's institutional intranet to invite potential participants among the university student population.
2. Provide an informed consent form and explain the project to potential participants.
3. Assess inclusion (healthy women, 18-40 years old) and exclusion (self-reported cardiac arrhythmias, severe cardiovascular diseases, chronic kidney disease, chronic obstructive pulmonary disease, use of beta-blockers, or any musculoskeletal condition that could impair the test) criteria and apply the American Heart Association (AHA)/ American College of Sports Medicine (ACSM) Pre-Participation Screening Questionnaire.
4. Once the volunteer agrees to participate and signs the informed consent form, return one copy to the participant and store another copy with the research team leader.
5. Coordinate the date and time for the evaluation. Ask the subjects to fast for at least 3 h before testing. Ask them to wear comfortable clothes, refrain from drinking caffeinated and alcoholic beverages, and avoid performing intense physical exercise over the preceding 24 h.
6. Perform weight and height assessments of every participant in a room with a controlled temperature.

2. Preparation of measurement equipment

1. Assemble the portable ergospirometer with the harness, attach accelerometers to the velcro straps, and set up the Bluetooth heart rate (HR) monitor.
2. Calibrate gases and flowmeter of the portable ergospirometer outside of the walking-jogging track.
   1. To do that, press the button Calibrate on the software screen. Then press the button Step 1 on the gas calibration window to assess the concentration of oxygen and carbon dioxide in the air.
   2. Once step 1 is concluded, connect a gas calibration bottle to the line of the gas analyzer. Press the button Step 2 on the gas calibration window to assess the concentration of oxygen and carbon dioxide in the calibration bottle (15% oxygen, 4% carbon dioxide, and balance nitrogen).
   3. After the successful gas calibration, connect the flow meter to a 3 L calibration syringe. Press the button Start calibration on the flow meter calibration window of the software screen.
   4. Then, move the moving part of the syringe through the entire range trying to reach a stable air flow for 5 cycles. When the required flow is reached, the screen appears with a blue and a red bar showing that the correct flow was achieved.
3. Add the participant data (name, weight, height, date of birth, sex) and the size of the ergoespirometer mask to the ergoespirometer software by pressing the button Add new participant.
4. On the laptop with the accelerometer software, connect the accelerometer to a USB port, press the Initialize button, and select Regular Initialization from the submenu to add the required volunteer's data (name, weight, height, date of birth, dominance, and sex), usage time for synchronization purposes, and body placement (wrist or waist).
5. Select the device sampling rate (number of times per second that the activity monitor will record data) to 100 Hz.
6. After completing the initialization parameters form, select Enter Subject Information. Select Initialize 1 Device. A progress bar on the screen shows when the initialization process is completed (a critical part of this process is that the accelerometer must have at least a battery charged at 80%).
7. Then, unplug the accelerometer and replace the USB cap. The device will begin collecting data when the selected start time elapses and will continue to collect data until the stop time occurs.
8. Prepare clean masks in the size that best fits the volunteer, perform an air leak test from the mask, and connect to the portable ergospirometer.
9. Connect the heart rate monitor to the ergoespirometer software, accelerometers on the right wrist and waist, and portable ergospirometer. Press the button Device Setup to verify the connection of the ergospirometer and the Bluetooth HR sensor.
10. Then, press the Sensor Adjustment button, which is an ambient gas calibration step that is performed prior to each test. Finally, press the Next button to access the next screen.
11. Bring the participant to the starting point on the track, where the participant must remain seated and rest on a chair for 3 min before starting the test.
12. Check the audio of the Modified Incremental Shuttle Walking Test Modified (MISWT) on a mobile device to ensure a correct sound volume to be heard by the participant¹⁰.

3. Explanation of the test

1. Instruct the participant to remain still in the chair, without speaking, and breathe normally to mark the starting point for the baseline measurements of the portable ergospirometer and the accelerometers' inactivity.
   NOTE: Take baseline data for 3-5 min; use this time to carry out step 3.2.
2. Play the audio for the MISWT to show the volunteer the speed of walking following the sound signal to cover 10 m (the field track is marked every 10 m), synchronizing the speed of the walking with the sound signal.
   NOTE: Repeat as necessary for comprehension.
3. Then, let the participant stand up and, along with two researchers, begin the test, stepping aside from the chair to avoid obstructing the track.
   NOTE: Two researchers are required here. Researcher 1 will carry the mobile device with the MISWT audio to guide the speed of the test with increments every 1 min; Researcher 2 on the ergoespirometer laptop.

4. Execution of the test

1. Researcher 1: Have the participant start the test and help guide the participant's walking pace with the MISWT audio, following the incremental pace of the test until the end of the MISWT audio.
   NOTE: The walking-jogging test begins at 0.84 m/s with increments every 1 min, and the last stage reaches a jogging speed of 2.37 m/s. There is no rest during the test.
2. Researcher 2: Press the Start test button on the laptop with the ergospirometer software at the same time as Researcher 1, and then press the Add text button with every increase of speed as indicated by the MISWT audio.
3. Once the audio is finished, ask the participant to sit down and remain still, without speaking, and breathe normally for 2 min to mark the endpoint of the test.
4. After 2 min, press the Square symbol to stop the measurements and remove the equipment, concluding data collection on the laptop in the ergospirometer software.

5. Review and data extraction

1. Review the data on the ergospirometer software to ensure the collected data of oxygen consumption (VO₂), exhaled carbon dioxide (VCO₂), and minute ventilation (VE) was reliable for the project. After finishing the test on the laptop, press the Next button, then the Smoothing button to average the data obtained every 10 s.
2. Download the accelerometer's data to the physical activity software to verify that the collected data was reliable, having previously connected the accelerometer to the laptop using the USB cable. When downloading the accelerometer record, select an epoch length (amount of time the raw acceleration data) of 10 s.
3. Download the accelerometer and indirect calorimetry files with data and manually transcribe them to the statistical software file.

6. Equipment storage

1. After all measurements for the day are completed, properly store the equipment.
2. Separate dirty from clean materials.
3. Clean dirty materials with water and soap and let them air dry.

7. Data analysis

1. Estimate Ventilatory Threshold 1 (VT1) and Ventilatory Threshold 2 (VT2) of every participant through VSlope (the change in the slope of a scatter plot of VO₂/VCO₂) and the increase in the slope of a scatter plot of carbon dioxide production/minute ventilation (VCO₂/VE) with the Ventilatory Threshold window of the ergospirometer software respectively from every indirect calorimetry file.
2. From the data obtained in step 7.1, register the subject's speed where VT1 and VT2 were found in the database.
3. In the accelerometer software, open the log file for both the waist and wrist recordings to obtain the counts. Copy and transfer these to a spreadsheet, from which they are averaged over the last 30 s of each stage of the test.
4. Apply descriptive and inferential statistics (Paired Student's T-Test, Intraclass Correlation Coefficient, and Cohen's D) using open-source statistical software.

Results

A sample size of 24 volunteers was calculated to observe a difference of 1,000 CPM. Women (n = 35) were recruited from undergraduate female university students with an age of 22.4 (± 1.9) years, a weight of 62.2 (± 11.3) kg, and a Body Mass Index of 24.5 (± 4.3) kg/m². All volunteers with respiratory comorbidities and acute musculoskeletal injuries that influenced the performance of the activity were excluded from the study.

Regarding the data distribution, we ...

Discussion

Our results show differences between the intensity of an incremental physical activity assessed through indirect calorimetry and accelerometry. Even more, there are statistically significant differences between CPM data collected from wrist and waist triaxial accelerometers at higher speeds of the test. These findings are similar to previous reports, where it has been shown that there is not a good agreement between accelerometer data when collected from different anatomic placements of the devices¹¹

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Accelerometer software	Actigraph	Actilife 6.0	Software for the analysis of the recorded information recorded in the accelerometers
Calibration gases	Cortex	N/A	Balloon with an hypoxic and hypercapnic solution of gases. It used to calibrate the gas cells of the ergoespirometer
Calibration pump	Cortex	N/A	Syringe of 3 L, used to calibrate the turbine to assess the minute ventilation
Ergoespirometer	Cortex	Metamax 3B	Portable indirect calorimeter to register oxygen comsumption, carbon dioxide production and minute ventilation.
Heart rate chest band	Polar	H10	Bluetooth transmisor synchronized with the ergoespirometer
Metasoft studio	Cortex	N/A	Software for the analysis of the ergoespirometer
Portable speaker	JBL	Go essential	Bluetooth portable speaker used to reproduce the audio signal to control the walking-jogging speed
Statistical software	Jamovi	2.3.28	Open source statistical software built on top of the R statistical language
Triaxial accelerometer	Actigraph	GT3-X	Device used for accelerometer changes in three axis. The unit of measure is counts per minute (CPM)