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6 Applications and Methods in Biosignal Processing
eral ways to find points of interest in our data. Each way has their own advantages
and disadvantages. MATLAB algorithms will be shown for the use in finding maxima,
minima, and inflections in the course of the signals of the force and the knee joint
angle. In the discussion of the results the focus is set on the second and third stand
phases of the three knee bends which the patient had to execute. As one can see in
the Figure 6.9 the signals of the ground reaction force and the knee joint angles are
more or less oscillating. The second and third stand phases are represented as local
maximums in the signals of the force and the knee joint angle.
First approach
The MATLAB command findpeaks(), is a function that allows the determination of
local maxima of a vector data set. However, function options to find the correct values
of the two stand phases have to be found. To find the peaks in the force and the knee
joint angle data options like “MinPeakProminence“ and “MinPeakHeight” are used.
In the Listing 6.2.2 an example code determines peaks in both force and knee joint
angle data. The results are shown in the Figure 6.12 (a and b). In the angle data the
two relevant peaks of the stand phases are found but in the force data four peaks are
detected., i.e. no significant prominence and other characteristic of the force signal
the peaks of the stand phases could be isolated from the other peaks.
Second approach
A further approach to overcome the above problem, is to apply MATLAB functions
for the derivation. As the measured data is discrete values instead of analytical func-
tions are given Listing 6.2.2. For that reason the MATLAB the functions diff() and
gradient() are used to find maxima, minima, and inflections of the force and joint
angle data. This kind of approach is called numerical derivation.
Listing 6.2.2: Cut-out of the discrete measured data of the force and the knee angle; the time
columns reveal the different time steps of the force and the joint angle data.
time_force
force_value time_angle
angle_value
0
361.4984
0.00
161.1090808
0.017
361.1752
0.03
161.4235965
0.033
360.7442
0.07
161.6854311
0.05
360.3492
0.10
161.6854311
0.067
359.6668
0.13
161.6854311
0.083
359.1999
0.17
161.6854311
0.1
358.733
0.20
161.6854311
0.117
358.2661
0.23
161.6854311
0.133
357.9429
0.27
161.6854311
0.15
357.7274
0.30
161.7405815