Chapter 8 Fatigue

Contents 1 2 General Properties of Fatigue 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 Storm- Mathisen(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 never will be,because their importance is bound to vary with the task at hand.

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 figure Admittedly, most causes of muscle fatigue are located beyond the neuromuscular junction, but processes with the nervous system also contribute.

Section 2 the Mechanic of Muscle Fatigue

1 Central and Peripheral Fatigue 2 Reduced Excitation of the Motoneurno ns 3 Factors Affecting Generation of an Endplate Potential 4 Fatigue Attributable to Processes Beyond the Neuromuscul ar Junction- Peripheral 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

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.