Page 137 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

114 BIOMECHANICS OF THE HUMAN BODY

including muscular dystrophy, inflammation of the muscles, peripheral nerve damage, and myasthe-
nia gravis and other (ATS/ERS, 2002). The EMG of human respiratory muscles has been measured
for many years in order to investigate their activity during various respiratory maneuvers and at dif-
ferent lung volumes. Measurements of EMG signals from the diaphragm during inspiration revealed
peak values of about 50 μV during quiet breathing and maximal values around 150 μV as inspiration
effort increases (Agostoni and Fenn, 1960; Corne et al., 2000; Hawkes et al., 2007; Petit et al., 1960).
A similar electrical activity during quiet inspiration was also measured from the parasternal muscle
(De Troyer et al., 1982) and from the external intercostals that reached amplitudes of 80 to 100 μV
(Hawkes et al., 2007; Maarsingh et al., 2000; Ratnovsky et al., 2003). The EMG activity from the
accessory muscle is controversial. Several studies detected activity only at high inspiratory efforts
(Costa et al., 1994; Estenne et al., 1998) while other found activity even during quiet breathing which
increased as inspiration effort increased (Breslin et al., 1990; Ratnovsky et al., 2003).
Quiet expiration is predominantly the result of passive elastic recoil of the lung and chest wall
(Osmond, 1995), and thus the abdominal muscles (i.e., rectus abdominis, external and internal oblique,
and transverse abdominis) are not active during quiet breathing. However, as breathing effort increases,
EMG signals are observed at the beginning of expiration and they become increasingly noticeable as the
expiration proceeds and reaches maximum values of 200 μV (Abraham et al., 2002; Hodges and
Gandevia, 2000; Ratnovsky et al., 2003).
The EMG activity of respiratory muscles is also useful for assessing respiratory muscle
endurance and fatigue after muscle training or exercise. It is widely accepted that respiratory muscle
fatigue is related to the change in the power spectrum of the measured EMG (Rochester, 1988).
When a skeletal muscle is in a fatiguing pattern of contraction, the mean frequency in the power
spectrum of the EMG is decreased. An additional indicator for muscle fatigue is a reduction in the
ratio between the EMG powers in the high-frequency band to that in the low-frequency band
(H/L ratio). Using the above frequency analysis of EMG, it has been shown that inspiratory loads
higher than 50 percent of maximal diaphragmatic pressure lead to diaphragmatic fatigue (Gross
et al., 1979). In addition, an inverse relationship between the inspiratory or expiratory loads and both
the mean frequency and the H/L ratio was demonstrated in the diaphragm and rectus abdominis muscles
(Badier et al., 1993).
Respiratory muscle fatigue may also be developed in healthy subjects during high-intensity exer-
cise (Johnson et al., 1993; Mador et al., 1993; Verges et al., 2006), which may limit exercise tolerance
in both trained and untrained individuals (Sheel et al., 2001). The findings that respiratory muscles
training enhances performance in normal subjects support the hypothesis that respiratory muscle
fatigue is potentially a limiting factor in intense exercise (Boutellier, 1998; Boutellier and Piwko,
1992). Simultaneous measurement of surface EMG from respiratory muscles (e.g., sternomastoid,
external intercostal, rectus abdominis, and external oblique) and the calf muscles demonstrated sig-
nificantly faster fatigue of the inspiratory muscles (e.g., sternomastoid and external intercostal) than
the calf muscles during intense marching on a standard electrically powered treadmill (Perlovitch
et al., 2007). Progressive muscle fatigue was associated with the increase of root-mean-square (RMS)
values of the surface EMG data (Krogh-Lund and Jorgensen, 1993; Ohashi, 1993).

5.3.5 Forces of the Respiratory Muscles
The EMG of skeletal muscle provides information on the level of muscle activity during different
tasks. Accordingly, several models have been developed for prediction of the forces generated dur-
ing muscle contraction. The biophysical cross bridge model of Huxley (Huxley, 1957) is commonly
used for understanding the mechanisms of contraction at the molecular level, and to interpret the
results of mechanical, thermodynamics, and biochemical experiments on muscles.
The Hill-type muscle model was derived from a classic study of heat production in muscle and
became the preferred model for studies of multiple muscle movement systems (Hill, 1938). The
model is composed of three elements: the contractile element (representing the contractile muscle
fibers), the series elastic component (representing the connective tissue in series with the sarcomeres
including the tendon), and the parallel elastic component (representing the parallel connective tissue
around the contractile element). The relationship between the force generated by the elastic elements