1. 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

2. 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

3. 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

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

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

6. 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

7. 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

8. 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

9. 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)

10. Accelerometer signal

11. 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)

12. B1127 Physiology and Biomechanics
Exercise testing lecture
Results
Pocket position:
Steps were significantly (p < .05) and meaningfully (d = ) 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 = ).

13. 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

14. 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

15. 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

16. 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.

17. 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”

18. B1127 Physiology and Biomechanics
Exercise testing lecture
Thank you!