Instrumented Usability Analysis for Mobile Devices - Human-Computer Interaction and Innovation in Handheld, Mobile and Wearable Technologies - page 14

cal acceleration sensed through the device will
complete one phase cycle at every step the user
takes. The lateral acceleration can also be seen to be
oscillatory. However, one oscillation in the lateral
direction will now correspond to a combination
of one left foot step and one right foot step. The
dominant frequency of the lateral oscillation is
therefore half that of the vertical oscillation. The
device will therefore undergo consistently differ-
ent disturbances depending on whether the user
is stepping with the left or right foot.
The data gathered from the accelerometer was
analysed to separate left and right foot steps. The
vertical acceleration was used to delineate the steps
with the lateral acceleration used to determine right
and left foot steps. Using this method ensures that
valid comparisons can be made between the one
step-one cycle data and the one step-two cycle
data. Figure 14 shows the distribution of tapping
through the phase of the left and right foot steps.
The first eight bins correspond to a left foot step,
and the second eight bins to the right foot step. It
can be seen from this figure that the tapping pattern
for the two step per phase cycle data follows the

one step per phase cycle data. There are distinct
interactions visible between the tapping and the
stepping in each step. The pattern for each step
is consistent, with the peak tapping phase values
occurring at around the foot down phase of the
step for both left and right foot. There were no
significant differences detected between the tap
errors for left foot steps and right foot steps. The
median magnitude of error for taps occurring dur-
ing the left foot phase was 2.9 pixels compared
to 2.8 pixels for taps during the right foot phase
section. There were no significant differences
with either the separated x error or the y error for
taps during the left or right steps.

Walking Speed Analysis

Analysis of the participants'walking speed
throughout the experiment showed that the step
rate during the study was relatively consistent
for all users. Figure 15 shows the estimated step
rate for five typical participants over the dura-
tion of the study. The task chosen for the study
was consistent throughout the task (tapping on a

Figure 14. Box plot visualising the distribution of tapping times. Median phase in which the user taps
(split into 16 sections). Unlike Figure 10, one phase cycle includes both a left and a right step.