Biomedical Engineering Reference
In-Depth Information
major study has been reported by Komi (1973). In it the subject did both
positive and negative work on an isokinetic muscle-testing machine. The
subject was asked to generate maximum tension while the muscle length-
ened or shortened at controlled velocities. The basic finding was that the
EMG amplitude remained fairly constant in spite of decreased tension during
shortening and increased tension during eccentric contractions (see Section
9.2.1). Such results support the theory that the EMG amplitude indicates the
state of activation of the contractile element, which is quite different from
the tension recorded at the tendon. Also, these results combined with later
results (Komi et al., 1987) indicate that the EMG amplitude associated with
negative work is considerably less than that associated with the same amount
of positive work. Thus, if the EMG amplitude is a relative measure of mus-
cle metabolism, such a finding supports the experiments that found negative
work to have somewhat less metabolic cost than positive work.
10.4.3 Electromyogram Changes during Fatigue
Muscle fatigue occurs when the muscle tissue cannot supply the metabolism
at the contractile element, because of either ischemia (insufficient oxygen)
or local depletion of any of the metabolic substrates. Mechanically, fatigue
manifests itself by decreased tension, assuming that the muscle activation
remains constant, as indicated by a constant surface EMG or stimulation
rate. Conversely, the maintenance of a constant tension after onset of fatigue
requires increased motor unit recruitment of new motor units to compensate
for a decreased firing rate of the already recruited units (Vredenbregt and
Rau, 1973). Such findings also indicate that all or some of the motor units are
decreasing their peak twitch tensions but are also increasing their contraction
times. The net result of these changes is a decrease in tension.
Fatigue not only reduces the muscle force but also may alter the shape
of the motor action potentials. It is not possible to see the changes of shape
of the individual m.u.a.p.'s in a heavy voluntary contraction. However, an
autocorrelation shows an increase in the average duration of the recruited
m.u.a.p. (see Section 10.1.4). Also, the EMG spectrum shifts to reflect these
duration changes; Kadefors et al. (1973) found that the higher-frequency
components decreased. The net result is a decrease in the EMG frequency
spectrum, which has been attributed to the following:
1. Lower conduction velocity of the action potentials along the muscle
fibers below the nonfatigued velocity of 4.5 m/s (Mortimer et al., 1970;
Krogh-Lund and Jørgensen, 1991).
2. Some
of
the
larger
and
faster
motor
units
with
shorter
duration
m.u.a.p.'s dropping out.
3. A tendency for the motor units to fire synchronously, which increases
the amplitude of the EMG. Normally, each motor unit fires indepen-
dently of others in the same muscle so that the EMG can be considered
Search WWH ::
Custom Search