ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

induces a vertical alignment of the trunk and the legs. “Lying” means “to be in a
horizontal, recumbent, or prostrate position” which corresponds to a horizontal
alignment of the trunk and the legs. Finally, “sitting” is defined as “to rest with the body
supported by the buttocks or the thighs” and can be characterised by horizontal thighs
and a vertical trunk. According to these definitions, the differentiation of these postures
depends then on the relative orientation of the thighs and trunk with the vertical.
When the subject is moving, the algorithm tests first whether the subject performs a
cyclical activity. For that purpose, an autocorrelation on the accelerations of the body
segments is computed. Below the threshold T2 of the autocorrelation result, a cyclical
movement is considered.
As aforementioned, each PA corresponds to a specific body-segments coordination that
can be characterised by a coordination of kinematics data such as accelerations. The
identification of the cyclical physical activity is obtained from the comparison of
accelerations of the body segments (Cacc) with the same sets of the accelerations
representative of each PA category (here Caccwalk, Cacccycle, and Caccrun). This
comparison is quantified by the computation of the Hausdorff distances (HDs) between
Cacc and each Cacc of reference namely Caccwalk, Cacccycle, and Caccrun. The Hausdorff
distance measures how far two subsets of a metric space are from each other [7]. This
distance can be generalised to space of n-dimensions, allowing a comparison of the
coordination of n accelerations.
The cyclical PA is considered as indeterminate if these HDs are superior to the
threshold T3. If not, the PA is classified according to the minimal HDs obtained.
The parameters of the proposed algorithm are then the thresholds T1, T2 and T3, and
the specific reference n-space curves Caccwalk, Caccrun, Cacccycle. The n accelerations
have also to be determined. This is the subject of the study proposed in the next section.
3.2 Determination of the n accelerations for the PA classification
16 subjects of different ages (8