Validation of the Fitbit One activity monitor device during treadmill walking

Abstract

Objectives

In order to quantify the effects of physical activity such as walking on chronic disease, accurate measurement of physical activity is needed. The objective of this study was to determine the validity and reliability of a new activity monitor, the Fitbit One, in a population of healthy adults.

Design

Cross-sectional study.

Methods

Thirty healthy adults ambulated at 5 different speeds (0.90, 1.12, 1.33, 1.54, 1.78 m/s) on a treadmill while wearing three Fitbit One activity monitors (two on the hips and one in the pocket). The order of each speed condition was randomized. Fitbit One step count output was compared to observer counts and distance output was compared to the calibrated treadmill output. Two-way repeated measures ANOVA, concordance correlation coefficients, and Bland and Altman plots were used to assess validity and intra-class correlation coefficients (ICC) were used to assess reliability.

Results

No significant differences were noted between Fitbit One step count outputs and observer counts, and concordance was substantial (0.97–1.00). Inter-device reliability of the step count was high for all walking speeds (ICC ≥ 0.95). Percent relative error was less than 1.3%. The distance output of the Fitbit One activity monitors was significantly different from the criterion values for each monitor at all speeds (P < 0.001) and exhibited poor concordance (0.0–0.05). Inter-device reliability was excellent for all treadmill speeds (ICC ≥ 0.90). Percent relative error was high (up to 39.6%).

Conclusions

The Fitbit One activity monitors are valid and reliable devices for measuring step counts in healthy young adults. The distance output of the monitors is inaccurate and should be noted with caution.

Introduction

Sedentary behaviour is a strong risk factor for obesity, which in turn increases the risk of chronic diseases (for example, cardiovascular disease and cancer1, 2), disability, and death.³ Increasing physical activity levels have been recognized as an important method of prevention and treatment for chronic disease and quality of life. Many governments recommend a minimum level of physical activity per week for healthy living, including activities such as brisk walking.⁴

In order to quantify the effects of physical activity such as walking on chronic disease, an accurate measurement of the amount of physical activity is needed. Activity monitors such as pedometers are popular tools used by clinicians and patients to measure physical activity because they provide an objective measure of step counts or distance walked, and are simple to use and inexpensive. However, many activity monitors do not accurately estimate physical activity levels,⁵ particularly at low walking speeds.⁶ Given that older adults and those with musculoskeletal or neurological impairment often walk at a slower pace than younger individuals, this may result in inaccurate measurement of physical activity in these at-risk populations. Inaccurate measures of physical activity levels can affect the ability to monitor health status and adherence to physical activity prescriptions during treatment. Therefore, to ensure applicability across age groups and patient populations, it is critical to assess the validity of an activity monitor at multiple speeds prior to use.

Assessment of activity monitors in multiple positions of wear is also necessary. Individuals can choose to wear the activity monitor in different placements on the body, including on the hip or in a pocket. Device placement may affect the accuracy of the activity monitor. For instance, in one study assessing pedometer validity, the percent error produced by placing the pedometer in the pants pocket was nearly five times greater than the error from wearing the pedometer on the hip (5.8% compared to 1.2%).⁷ Thus, when assessing the validity of an activity monitor, multiple placements of the device should be used in addition to multiple speeds to increase the generalizability of the findings.

A new lightweight, portable activity monitor that contains a tri-axial accelerometer has been made commercially available. The device claims to track multiple measures of physical activity – including step count and distance travelled – and to have the ability to upload activity data wirelessly to a website in order to track activity levels over time. The advantage of wireless data uploads allowing real-time data connectivity may be an attractive feature to researchers and patients alike for tracking physical activity such as walking. To date, no research on the validity or reliability of this device exists. Accordingly, validation of this new activity monitor is needed to verify its use as a measure of physical activity.

Thus, the purpose of this study was to assess the validity and reliability of the step count and distance outputs of a new activity monitor in a population of healthy adults during treadmill walking at multiple speeds and placements of wear.