BIOMECHANICS AS AN INTERDISCIPLINE - Biomechanics and Motor Control of Human Movement - page 5

1.1.2 Analysis
The measurement system yields data that are suitable for analysis. This means
that data have been calibrated and are as free as possible from noise and
artifacts. Analysis can be defined as any mathematical operation that is per-
formed on a set of data to present them in another form or to combine the data
from several sources to produce a variable that is not directly measurable.
From the analyzed data, information may be extracted to assist in the assess-
ment stage. In some cases, the mathematical operation can be very simple,
such as the processing of an electromyographic signal to yield an envelope
signal (see Figure 1.3). The mathematical operation performed here can be
described in two stages. The first is a full-wave rectifier (the electronic term
for a circuit that gives the absolute value). The second stage is a low-pass

Figure 1.3 Processing of raw electromyogram (EMG) signals to present the variable
in a different form. Traces 1 and 3 show the full-wave rectified EMG of the medial
hamstrings and soleus muscles during walking. A cutoff frequency ( f c = 100 Hz) is

indicated for the rectified signal because this is the bandwidth of the pen recorder. In
traces 2 and 4, the linear envelope signal (low-pass filter with f c = 3 Hz) is presented.