Page 136 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMECHANICS OF THE RESPIRATORY MUSCLES 113

a device with a weighted plunger, which requires development of enough pressure to lift the plunger
out of its socket in order to initiate inspiration (Nickerson and Keens, 1982). The endurance is
expressed as the time the subject can endure a particular load or as the maximum load tolerated for
a specific time (Fiz et al., 1998). In the resistive inspiratory load the subject breathes against a vari-
able inspiratory resistance at which the subject had to generate a percentage of his maximal mouth
pressure in order to inspire. Endurance is expressed as the maximal time of sustained breathing
against a resistance (Reiter et al., 2006; Wanke et al., 1994). The postinterruption tracing of mouth
pressure against time represents an effort sustained (against an obstructed airway) over a length
of time and can serve as a pressure-time index of respiratory muscle endurance. The area under this
tracing (i.e., the pressure-time integral) represents an energy parameter of the form PT, where P is
the mean mouth pressure measure during airway obstruction and T is the time a subject can sustain the
obstruction (Ratnovsky et al., 1999). Similar to the strength of respiratory muscles, large dependence
on lung volume was found for their endurance. The endurance of expiratory muscles decreased as
lung volume decreased from TLC, while the endurance of inspiratory muscles decreased as lung volume
increased from RV (Ratnovsky et al., 1999).

5.3.3 Electromyography
Electromyography (EMG) is a technique for evaluating the electrical activity of skeletal muscles at
their active state (Luca, 1997). Intramuscular EMG signals are measured by needle electrodes inserted
through the skin into the muscle, while surface EMG signals are recorded with surface electrodes
that are placed on the skin overlying the muscle of interest. Typically, EMG recordings are obtained
using a bipolar electrode configuration with a common ground electrode, which allows canceling
unwanted electrical activity from outside of the muscle. The electrical current measured by EMG is
usually proportional to the level of muscle activity. Normal range for skeletal muscles is 50 μV to
5 mV for a bandwidth of 20 to 500 Hz (Cohen, 1986).
The raw EMG data resembles a noise signal with a distribution around zero. Therefore, the data
must be processed before it can be used for assessing the contractile state of the muscle (Herzog,
1994). The data can be processed in the time domain or in the frequency domain. The EMG signal
processing in the time domain includes full wave rectification in which only the absolute values of
the signal is considered. Then in order to relate the EMG signal to the contractile feature of the mus-
cle it is desired to eliminate the high-frequency content by using any type of low-pass filter that
yields the linear envelope of the signal. The root-mean-square (RMS) value of the EMG signal is an
excellent indicator of the signal magnitude. RMS values are calculated by summing the squared values
of the raw EMG signal, determining the mean of the sum, and taking the square root of the mean

tT
+
1
RMS = T ∫ EMG 2 ()tdt (5.1)
t
The study of EMG signals in the frequency domain has received much attention due to the loss
of the high frequency content of the signal during muscle fatigue (Herzog, 1994). Power density
spectra of the EMG signal can be obtained by using fast Fourier transformation technique. The most
important parameter for analyzing the power density spectrum of the EMG signal is the mean fre-
quency or the centroid frequency, which is defined as the frequency that divides the power of the
EMG spectrum into two equal areas.

5.3.4 Electromyography of the Respiratory Muscles
Using EMG for assessment of respiratory muscles performance, in addition to the methods described
in the above paragraphs, enables differentiation between different respiratory muscles. The EMG
signals can detect abnormal muscle electrical activity that may occur in many diseases and conditions,