Treadmill gait speeds correlate with physical activity counts measured by cell phone accelerometers

Abstract

A number of important health-related outcomes are directly related to a person's ability to maintain normal gait speed. We hypothesize that cellular telephones may be repurposed to measure this important behavior in a noninvasive, continuous, precise, and inexpensive manner. The purpose of this study was to determine if physical activity (PA) counts collected by cell phone accelerometers could measure treadmill gait speeds. We also assessed how cell phone placement influenced treadmill gait speed measures. Participants included 55 young, middle-aged, and older community-dwelling men and women. We placed cell phones as a pendant around the neck, and on the left and right wrist, hip, and ankle. Subjects then completed an individualized treadmill protocol, alternating 1 min rest periods with 5 min of walking at different speeds (0.3-11.3 km/h; 0.2-7 mi/h). No persons were asked to walk at speeds faster than what they would achieve during day-to-day life. PA counts were calculated from all sensor locations. We built linear mixed statistical models of PA counts predicted by treadmill speeds ranging from 0.8 to 6.4 km/h (0.5-4 mi/h) while accounting for subject age, weight, and gender. We solved linear regression equations for treadmill gait speed, expressed as a function of PA counts, age, weight, and gender. At all locations, cell phone PA counts were strongly associated with treadmill gait speed. Cell phones worn at the hip yielded the best predictive model. We conclude that in both men and women, cell phone derived activity counts strongly correlate with treadmill gait speed over a wide range of subject ages and weights.

Figure 1. Activity counts from cell-phone accelerometers provide an accurate measure of treadmill gait speed regardless of where the sensor is worn

The top 4 traces depict raw data from a representative trial (43 y/o man) showing acceleration magnitude versus time for sensors worn at the chest, right arm, right hip, and right ankle (1^st through 4^th traces from top, respectively). For all traces the baseline is centered at 64 (midscale between sensor output of 0 for −2g, and 128 for +2g), the amount of deflection from this baseline is per the common scale provided left of these traces. The bottom 4 traces show activity counts versus time for the sensors worn at the chest, right arm, right hip, and right ankle, respectively. Counts were calculated over one minute nonoverlapping bins. Treadmill speed is given at the top of each epoch bar.

Figure 2. Relative contributions of each age cohort to the study data

At each tested speed, the average (over all tested sensors) total duration of locomotion is shown. A. Contributions to all treadmill speeds (leftmost bar) and to walking versus jogging/running speeds (right most pair). B. Contributions to treadmill speeds by 0.5 mph increment. The 0.5–1.0 bin, for example, covers all speeds above 0.5 and up to 1.0 mph. Green shading indicates young (19–35 y/o) cohort, blue shading indicates middle-aged (36–65 y/o) cohort, and red shading indicates aged (65+ y/o) cohort. Error bars are ± 1 standard error of the mean for the given treadmill speed range and given cohort.

Figure 3. Activity count versus treadmill speed relationships for all sensor locations

For all figures, the solid red line shows the linear regression between treadmill speed and activity counts (fit for all data between 0.0 and 6.4 km/hr (0–4 mph) gait speeds); the thin surrounding black lines are 95% confidence boundaries on this regression. The thick black line connects mean activity count values for each of the evaluated treadmill speeds; bars surrounding this point are ± 1 standard error of the mean. Individual observations of activity counts are shown as open colored circles. Subject age is color coded as circle color; refer to colorbar at right side for key. The dashed lines at gait speeds of 2.35 km/hr (1.46 mph) and 4 km/hr (2.5 mph) highlight system performance at two critical functional thresholds. These relationships come from cell phones placed at the right wrist (A), left wrist (B), right hip (C), left hip (D), right ankle (E), left ankle (F), and neck (G).