SIGNAL PROCESSING - Biomechanics and Motor Control of Human Movement - page 33

FORWARD ACCELERATION OF THE RIGHT FOOT

10.

TRIAL CODE: WP03H
50 HERTZ
25 HERTZ

5.

0.

− 5.

− 10.

− 15.

0.0 0.1 0.2 0.3 0.4 0.5

TIME [SECONDS]

0.6 0.7 0.8 0.9 1.0 1.1

Figure 2.13 Comparison of the forward acceleration of the right foot during walking
using the same data sampled at 50 Hz and at 25 Hz (using data from every second
frame). The major pattern is maintained with minor errors at the peaks.

another walking trial, shown in Figure 2.14. The minor differences in no way
interfere with the general pattern of joint torques over the stride period, and
the assessment of the motor patterns would be identical. Thus, for movements
such as walking or for slow movements, an inexpensive camera at 24 frames
per second appears to be quite adequate.

2.2.4.3 Deciding the Record Length. The duration of record length is
decided by the lowest frequency present in the signal. In cyclical events such
as walking, cycling, or swimming the lowest frequency is easy to determine;
it is the stride frequency or how often each segment of the body repeats itself.
For example, if a patient is walking at 105 steps/min the step frequency is
105 / 60 0 . 875 strides / s. Thus, the fundamental frequency is

0.875 Hz. However, there are a number of noncyclical movements, which do
not have a defined lowest frequency. One such “movement” is standing either
quietly or in a work-related task. In quiet standing, we model the total body as
an inverted pendulum (Gage et al., 2004), which simplifies the total body into
a single weighted-average center of mass (COM) and which can be compared
with the center of pressure (COP) measured from the force plate. Figure 2.15
presents a typical FFT of the COP and COM in the anterior/posterior direction

= 1 . 75 steps / s =