Download 1 2 General Properties of Fatigue the Mechanic of Muscle Fatigue

Chapter 8 Fatigue
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Contents
1
General Properties of Fatigue
2
the Mechanic of Muscle Fatigue
Section 1 General Properties of
Fatigue
•
Physical fatigue can be defined as a state of disturbed homeostasis
attributable to work and to work environment (Christensen 1960 ). This can
give rise to subjective as well as objective symptoms, but so far very little
is known about the nature of this disturbed homeostasis. Obviously, the
decrease in blood sugar observed in a fasting subject engaged in prolonged
submaximal work can lead to a feeling of general fatigue as one of the
symptoms of hypoglycemia ，and lactic acid accumulated in muscles
engaged in intense work can cause local fatigue .
Section 1 General Properties of
Fatigue
• The subjective symptoms of fatigue range from a slight feeling of tiredness
to complete exhaustion. Attempts have been made to relate these subjective
feelings to objective physiological criteria such as the accumulation of
lactate in the blood. Although such a relationship often is observed in
connection with strenuous athletic events, this relationship is not usually
present in prolonged light or moderate work. Subjective feelings of fatigue
usually occur at the end of an 8-h workday when the average work exceeds
30%-40%of the individual maximal aerobic power, and certainly when the
load exceeds 50% of the maximal aerobic power. Subjective feelings of
fatigue also can be associated with hypohydration in heat-exposed workers
who do not consume enough water during their working shift.
• We define muscle fatigue as “any exercised-induced reduction in the
maximal capacity to generate force or power output”(table 15.1;vollestad
1997)
Section 1 General Properties of
Fatigue
Section 2 the Mechanic of Muscle
Fatigue
• In the 19th century ,the prevailing view was that fatigue was
caused by processes in the center nervous system. Mosso (1892)
has been quoted as advocating this view, but actually he clearly
demonstrated that muscle fatigue could be entirely peripheral.
Mosso was the first to record the decrease in muscle force by
means of an “ergograph” during fatiguing contractions in
humans, and he found s comparable decline in force both during
voluntary contractions and during stimulation of the muscle
(figure 15.1). ”One must conclude, ” he wrote, ”that the mental
factor does not exercise a preponderating influence, and the
fatigue may even be a peripheral phenomenon.” Despite this ,the
view that fatigue was central in origin prevailed until the middle
of the 20th century, when Merton(1954) and Naessa and StormMathisen(1955) showed that processes within the muscle could be
the only mechanism. ”Fatigue peripheral, ” Merton wrote, “for
when strength fails, electrical stimulation of the motor nerve
connot restore it”.
• In the 1970s, Asmussen and Mazin(1978) reintroduced a central
component as a factor in muscle fatigue, but the relative
contribution of central factors is not yet settled and probably
Section 2 the Mechanic of Muscle
Fatigue
•
Muscle fatigue inevitably decrease performance in activities
requiring muscular force , and most people experience an
increasing difficulty in maintaining a certain activity level. If
the level is high, or more specifically, higher than what is
defined as “the habitual level of physical activity” for the
individual, the problems arise early and increase rapidly. If the
activity level is lower, the problems come later and are more
protracted. Although the symptoms of fatigue can appear late
during the fatiguing activity , the processes underlying the
fatigue begin to develop as soon as the activity begins.
• The chain of command from volition to measurable force is a long one
(figure 15.2), and during the exercise, changes occur at all steps in force
production from upstream of the motor cortex to the myofibril. A schematic
view of the chain of command with possible feedback loops is shown in
Section 2 the Mechanic of Muscle
Fatigue
Section 2 the Mechanic of Muscle
Fatigue
Section 2 the Mechanic of Muscle
Fatigue
1
2
Central and
Peripheral
Fatigue
Reduced
Excitation
of the
Motoneurno
ns
3
Factors
Affecting
Generation
of an
Endplate
Potential
4
Fatigue
Attributable
to Processes
Beyond the
Neuromuscul
ar JunctionPeripheral
Fatigue
1 Central and Peripheral
Fatigue
• In the fatigue literature, the distinction between central and peripheral
fatigue is very common, but as we shall see, it is not always easy to
distinguish between the two. A convenient definition of central fatigue
could be “any force decline caused by a reduction in the firing frequency of
the motoneurons involved.” This would include all kinds of reduced
excitatory drive to the motoneurons, but it dose not include failure in the
propagation of the action potentials from the motoneuron to the muscle
fibers. This possibility has to be considered because the main method for
assessing central fatigue is the twitch interpolation technique, where a
single or double supramaximal electrical stimulus is delivered to the
muscle(or to the nerve) during a fatiguing contraction(figure15.4). The
force increment caused by the stimulus shows the part of the muscle force
potential not evoked by the nervous system and represents the force deficit
attributable to central fatigue (figure 15.5). In table 15.1, the maximal force
is defined as “the force generated by a muscle or group of muscles when
addition electrical stimulation does not augment force .” In according with
this, central fatigue can be defined as ”any exercise-induced reduction in
maximal voluntary contraction force, which is not accompanied by the
reduction in maximal evocable force”. Peripheral fatigue , in the other hand,
can be defined as “a force or power deficit occurs despite optimal
activation of the muscle fibers by their motoneurons”.
1 Central and Peripheral
Fatigue
1 Central and Peripheral
Fatigue
1 Central and Peripheral
Fatigue
• Thus , although a distinction between central and peripheral fatigue seems
logical at first, such a distinction in many ways is arbitrary, and the terms
central and peripheral are not used in a strictly anatomical meaning of the
words. The reason for this is that the definition of central fatigue is an
operational one (see table 15.1). In addition, it is evident that several sites
that can give rise to “central fatigue” definitely are located in the peripheral.