Online Recognition with Postural Transition Awareness - Smartphone-Based Human Activity Recognition - page 116

7.4.1 The HAR Dataset with Postural Transitions

From the D 3 T dataset, relevant information regarding the duration of PTs against

the other activities was extracted. During the experiments, every patient performed
twice each PT producing a total of 60 labels for each PT. This comprises in time
9% of the entire recorded experimental data. Figure 7.4 depicts the average duration
of the 6 PTs. Overall, they all have a duration of 3.73

1.17 s but it can be clearly
noticed that their duration is different for each PT. Even inverse transitions (e.g. LiSi
and SiLi) have different durations. Some PTs, such as StLi which has the longest
average duration (4.9 s), are actually a sequence of other two (StSi and SiLi) as it can
be observed from the experimental video data. These figures were useful for defining
conditions that allow the filtering of transitions that are described in Sect. 7.3.3 .
In the particular case of SiSt and StSi PTs, the duration results are slightly different
to the ones found in (Najafi et al. 2002 ) where the transition duration between these
two PTs was not very significant and supported their finding that the type of transition
did not have a specific role for the fall risk evaluation. Although the mean duration of
these two transitions all together in our experiment is very close to their estimations
in low fall-risk patients (2.98 s for us against 2.92 s for them), we found that StSi takes
in average considerably more time than SiSt. In comparison to their approach, we
selected a higher number of participantswhich seems to provide amore representative
statistical measurement.
On the other hand, the rest of activities (BAs) took instead longer times (17.3 s

±

±

5.7 in average). This is important given that, in general, the execution time of
activities in real life takes longer than PTs which have nearly-fixed duration. This
element is also fundamental for the filtering of activities of our system.

Fig. 7.4 Transition times in seconds for the 6 studied PTs