B1127 Physiology and Biomechanics Exercise testing lecture Accuracy of step detection using a customized mobile phone app David Rowe1, Allan Hewitt1, Campbell Reid2, Arlene McGarty3 1 Physical Activity for Health Research Group, University of Strathclyde 2 Bioengineering Unit, University of Strathclyde 3 Physical Activity Research Center, University of Edinburgh

B1127 Physiology and Biomechanics Exercise testing lecture Background Walking – the “perfect exercise” Health benefits Convenience Parameters of walking: Volume (total steps) Intensity (speed, steps per minute) Pattern (gait symmetry, phase timing) Measurement Pedometer vs. accelerometer Research grade vs. applied/commercial

Dose-response – step “volume” B1127 Physiology and Biomechanics Exercise testing lecture Dose-response – step “volume” Source: Ewald et al. (in press), Journal of Physical Activity for Health

B1127 Physiology and Biomechanics Exercise testing lecture Mobile phone apps Cost Ubiquity Convenience Uses Quality control

Evolution of the mobile phone B1127 Physiology and Biomechanics Exercise testing lecture Evolution of the mobile phone

B1127 Physiology and Biomechanics Exercise testing lecture The App(s) Developed using Objective C (Apple) Adapting for Android Accelerometer function Step-detection algorithm Music playlist “Activity points” algorithm

B1127 Physiology and Biomechanics Exercise testing lecture Protocol 32 adults (53% female; 29±13 yr) Six 90-s treadmill walking trials 53, 67 and 80 m·min-1 (≈ 2.0, 2.5 and 3.0 mph) 0% and 5% gradient All trials video recorded

B1127 Physiology and Biomechanics Exercise testing lecture Protocol iPod touches (iTouches) worn in pouches on right midline thigh (waist), mid-back, and in the right pocket 3rd generation, 8GB memory

B1127 Physiology and Biomechanics Exercise testing lecture Protocol Parameters of step detection algorithm (signal processing) Sampling rate 50 Hz Smoothing algorithm Peak detection Time censoring window (0.33 s) Equivalent to > 180 steps/min (specific to walking)

Accelerometer signal

Measurement and data analysis B1127 Physiology and Biomechanics Exercise testing lecture Measurement and data analysis Criterion step counts determined by hand-counter using time-stamped video recording iPhone step counts compared to criterion (video count) Repeated measures t-tests Cohen’s d Discrepancy plot (modified Bland-Altman)

B1127 Physiology and Biomechanics Exercise testing lecture Results Pocket position: Steps were significantly (p < .05) and meaningfully (d = 0.5-0.9) over-counted in all trials Waist and mid-back: Steps were significantly (p < .05) and meaningfully (d = 0.3 and 0.6) under-counted at 53 m·min-1 Accurately counted at 67 and 80 m·min-1, at level and 5% gradient (d = 0.0-0.1).

Discrepancy plot – Waist 54 m·sec-1, 0% grade 67 m·sec-1, 0% grade 80 m·sec-1, 0% grade 54 m·sec-1, 5% grade 67 m·sec-1, 5% grade 80 m·sec-1, 5% grade

Discrepancy plot – Mid-back 54 m·sec-1, 0% grade 67 m·sec-1, 0% grade 80 m·sec-1, 0% grade 54 m·sec-1, 5% grade 67 m·sec-1, 5% grade 80 m·sec-1, 5% grade

Discrepancy plot – Pocket 54 m·sec-1, 0% grade 67 m·sec-1, 0% grade 80 m·sec-1, 0% grade 54 m·sec-1, 5% grade 67 m·sec-1, 5% grade 80 m·sec-1, 5% grade

B1127 Physiology and Biomechanics Exercise testing lecture Discussion Similar to traditional pedometers, steps are under-counted by a mobile phone app at slow speeds, but accurately counted at moderate speeds and higher, when worn securely. When carried in the pocket, steps are over-counted regardless of speed and gradient. Further analysis of the raw acceleration signal and the time-stamped video recording will help identify reasons for inaccuracy and inform future signal-processing decisions in mobile phone accelerometer uses.

Limitations/future research B1127 Physiology and Biomechanics Exercise testing lecture Limitations/future research Processor capability/dual-task processing Apple problem Battery life Slow technology development Distraction (it’s a phone!) Mobile phone habits Non “wear”

B1127 Physiology and Biomechanics Exercise testing lecture Thank you!