fatigue

the altered physical and mental state that results from exertion and causes a temporary reduction of human or animal operating capacity. The subjective sensation of fatigue is called tiredness.

Dynamics. The body’s operativeness is a dynamic process that includes several phases: activation, or mobilization for action; the initial reaction, representing the process of quantitative equilibration; overcompensation, or the search for an appropriate decision; compensation, or the maintenance of a level of efficiency adequate to the demands of the activity engaged in; and undercompensation, decompensation, and breakdown—that is, the gradual exhaustion of the body’s reserves and loss of operating capacity. Fatigue commonly occurs—beginning at the stage of undercompensation and persisting through the phases that follow—when the physiological reserves are considerably reduced and the body shifts to less energy-efficient types of reactions. One such reaction is the maintenance of the rate of blood flow by means of an increased heart rate rather than the more efficient reaction whereby the stroke volume is increased; another one is the activation of a greater number of motor units to effect a motor reaction when the power of contraction of individual muscle fibers is reduced—that is, when the alternating sequence of muscular contraction and relaxation is disrupted.

The initial stages of fatigue in man are marked by the reduced effectiveness of his activity; in other words, greater expenditures of physiological and psychic energy are required to perform the same work, and consequently work productivity declines. The effect of fatigue is, first of all, to impair the stability of the autonomic functions, the force and rapidity of muscular contraction, the regulation of bodily functions, and the formation and inhibition of the conditioned reflexes. As a result, the tempo of operation is slowed down; there is impairment of the rhythm, precision, and coordination of movements, and a greater expenditure of energy is needed for the performance of a given action. The thresholds of the sensory systems are raised, readily accessible stereotypic forms become dominant in the decision-making process, and there is loss of capacity to focus and transfer attention. Fatigue is generally accompanied by an increase in the number of errors and by a change in the type of errors made. Quantitative errors predominate in the initial phases, and qualitative errors in the later ones.

The overall development of fatigue can be described as impairment of the body’s ability to respond adequately to the demands placed on it by a particular activity. The response is inadequate in this case by failing to satisfy the following three criteria: optimal quality and coordination of the specific reactions underlying the activity, a bodily response that both quantitatively and qualitatively corresponds to the demands of the task at hand, and minimal expenditure of the physiological reserves.

Work may cease altogether when fatigue is pronounced. The subjective signs of fatigue in man are disagreeable sensations in the muscles and joints involved in the task and, when the body is in a static position, pain and numbness in the muscles of the back, abdomen, and neck, pain in the front and back of the head (especially in sensory and mental fatigue), loss of ability to concentrate, and easy distractibility; at first there is a slight increase in the individual’s contacts with others in the immediate environment, followed by a sharp reduction of such contacts; and finally, an unconscious drive to take longer and more frequent work breaks is observed in human subjects.

Men and animals share several common mechanisms that are characteristic of fatigue—namely, those associated with biochemical changes at the cellular level and with impairment of the conditioned reflexes. On the other hand, several fundamental differences have been identified in the dynamics as well as in the structural mechanisms of human fatigue—these being determined and regulated by the motivation, the goals, and the social nature of human activity. One such difference is the absence of clear-cut phases in the development of fatigue in animals; rather, the latter show a steady decrease in the quantitative indicators of fatigue and a less pronounced modification of performance. Furthermore, fatigue in animals is virtually unaffected by volition.

As a dynamic process, fatigue reflects the nature of the activity engaged in, and particularly its intensity, duration, and tempo. With optimal intensity, fatigue sets in later; with an increase or decrease of such optimal intensity, the onset of fatigue is accelerated. Fatigue develops rapidly when the individual is engaged in monotonous or static activity or is subjected to sensory deprivation—for example, when performing the same operation over an extended period during which a limited range of movements is required. Such monotonous activity as the narrowly specialized work performed on the assembly line results in lowered attention, diminished motivation, and the rapid onset of fatigue. This is especially true of static activity, when work is performed in a fixedly tense position, or when there is limited exposure to such stimuli as acoustic or light signals that convey information about the work environment. The work environment includes such major external factors as the microclimate, and particularly the air’s temperature, humidity, rate of movement, composition, and content of chemical impurities, as well as noise, vibration, and intensity.

Other factors in the development of fatigue are the individual’s state of health and physical condition; if these are satisfactory, they not only build up greater physiological reserves but also help activate the bodily functions more rapidly and contribute to their operational effectiveness. The time at which fatigue sets in and the rate at which it develops also depend on such individual psychological traits as the subject’s level of anxiety and volitional qualities, including persistence; these qualities, which may be called the parameters of activation, include those functional characteristics of human beings that enable them to realize their potential in a specific activity. For example, attention is an activation parameter that stimulates the efficient operation of memory; another example is a high level of volition, which makes it possible to maintain the required level of activity in spite of marked tiredness. The higher mental faculties, which shape the individual’s ideals and world outlook, play the main role.

Types. Depending on the type of work performed, fatigue may be mental or physical, with corresponding variations in the metabolic indicators—for example, changes in body temperature or in the bioelectric potentials. It has been found that physical and mental fatigue have a common base; thus the classification system that is commonly used has identified the nervous system, which directs human activity, as the primary site of fatigue. A distinction is made between sensory fatigue (including its varieties, such as the perceptual and the informational) and effector fatigue. In addition, there is the separate category of generalized fatigue. Any such classification, however, rests on an underlying physiological theory.

Sensory fatigue results from prolonged or intensive exposure to a stimulus, such as light or a loud noise, which causes a primary change in the sensory systems—a change that begins in the receptor and terminates in the cortical ending of the analyzer. Perceptual fatigue, chiefly localized in the cortical ending of the analyzer, is associated with loss of ability to detect a signal, as in the case of a high degree of interference, a low-intensity signal, or poor differentiability. Informational fatigue arises from an insufficiency of information or from information overload, when the greatest load falls on the dynamics of transaction between the nerve centers. This transactional process consists of a formation of the temporary connections between the various components of the central nervous system and an activation of the associative connections by means of which the objective environment is correctly perceived.

Effector fatigue results from the changes that take place chiefly in the sections of the central nervous system where motor activity originates. Mental fatigue arises from the changes produced by one of the following intensive operations: (1) reproductive activity that involves the mere processing of information according to hard-and-fast rules, such as counting or classifying; (2) productive activity, including data reduction, judgment-making, conceptualizing, and drawing conclusions; and (3) heuristic activity, or creative activity that is governed by individually determined implicit algorithms. Since all the changes mentioned above are usually found in combination, one may speak of generalized fatigue as a condition associated with all working activity, while distinguishing those cases in which impairment of the central nervous system is most pronounced.

Theories. Of the many theories on fatigue, those that have a purely historical interest include the “toxicity” theory of the German scientist E. Pfluger (1872), the “exhaustion” theory of M. Schiff (1868, Switzerland), and the “metabolic” theory of the Englishman A. Hill (1929). The two leading sets of theories generally accepted today are concerned with the changes that take place in the nerve centers. According to one such group of theories, fatigue is linked to hypoxia (that is, a deficiency of oxygen), impairment of the homeostatic control mechanisms, and especially changes in the metabolic mediators and in the chemical processes of excitation. Proponents of the second set of theories reject the association of fatigue with the operation of any single mechanism. In their view, fatigue may be determined by a number of factors or combinations thereof, ranging from inadequate blood circulation in the case of local muscular fatigue to protective inhibition in the case of general fatigue—namely, the structural changes effected in the homeostatic regulatory system by the higher divisions of the central nervous system.

Major contributions to modern theories of fatigue were made by I. M. Sechenov, I. P. Pavlov, N. E. Vvedenskii, A. A. Ukhtomskii, and L. A. Orbeli. Orbeli, for example, associated fatigue with the breakdown of the autonomic nervous system’s function of adaptotrophic regulation. Other Soviet physiologists who have made studies of fatigue include G. V. Fol’bort and S. A. Kosilov. Recent investigations have revealed a set of fine mechanisms associated with fatigue; these are the mechanisms involved in the impairment of the metabolism of high-energy compounds, decrease in the activity of the oxidizing enzymes, and changes in the hypothalamic function of endocrine regulation. For example, it was found that the functioning of the adrenal gland is lowered, the secretion of corticotropin by the pituitary gland is reduced, and there is an initial increase and subsequent decrease in the activity of the islands of Langerhans. The result is an increase of incompletely oxidized products and hyperglycemia, which in turn cause secondary changes to take place in the afferent impulse function, followed by an even greater disruption of homeostasis and a breakdown in the balance between autonomic and motor reactions.

In its initial stages, fatigue has a beneficial effect on the body’s resistance, contributing to the body’s more rapid and complete marshaling of its reserves and compensatory functions as well as mastery and reinforcement of skills. Pronounced fatigue, on the other hand, has the negative effect of reducing productivity, and it may lead directly to the prepathological phase of a breakdown; in the absence of proper rest, it may cause pathological exhaustion. Extreme fatigue may result in neuroses and vascular diseases.

Means of control. Fatigue can be prevented and controlled by introducing sound work and rest routines, improving the work environment, implementing biotechnological recommendations with respect to workplace organization and the arrangement of control panels or consoles, and adopting a rational system of distribution of human and mechanical operations. A major weapon in combating fatigue is work training, which makes it possible to develop optimal functional systems whereby work is performed at the prescribed rate with minimal expenditure of physiological reserves; at the same time, such systems ensure the reinforcement of skills and provide correctly planned pauses and work breaks.

Emotion and motivation are important factors in any activity. The more powerful the motivation, the later fatigue sets in, particularly when the motivation has high social significance and is competitive in nature. This type of motivation arouses interest in and a creative attitude toward one’s work. Positive emotions enhance the worker’s adaptation to an even rhythm of work, prolong optimal efficiency, and serve to more fully activate the body’s physiological reserves.

the change in the mechanical and physical properties of a material induced by stresses and strains that vary cyclically over time. The change in the condition of a material subject to fatigue alters the material’s mechanical properties, macrostructure, microstructure, and substructure. The changes occur in stages and are dependent on the original properties, the type of stressed state, the history of loading, and the effect of the environment. Irreversible phenomena, characterized as fatigue damage, occur at some stage and lower the breaking strength of the material. Microcracks first form within the structural components of materials and along the boundaries of their junctions, for example, the grains of a polycrystalline metal, the fibers and matrix of composites, and the molecular chains of polymers. In later stages these grow into microcracks or lead to the final failure of a structural member or a specimen undergoing mechanical testing.

The fatigue process may be described quantitatively by the relationship between the cumulative damage and the number of cycles or the duration of the loading with respect to the magnitude of the cyclic stress or strain. The corresponding relationship between the number of cycles and the stages of damage, including the onset of cracks or the final failure, is called a stress-endurance, or endurance-limit, curve and is the primary characteristic of fatigue in materials.

The accumulation of cyclic damage reflects the strain in a material as a macroscopically and microscopically inhomogeneous medium (for metals, as a polycrystalline conglomerate; for polymers, as a conglomerate of molecular chains; and for composites, as a regular structure consisting of a matrix and fibers). This process in the field of a uniformly stressed state, for example, simple tension and compression, may be described by a mechanical model in which the sections create an inhomogeneous strain in the structural components of the material; the inhomogeneity is characterized by probability distributions for the values of the microstrains and microstresses, including residual values.

A cyclic load on such inhomogeneous structures produces irreversible elastoplastic and viscoelastic strains in the most highly stressed structural sections, which accumulate as the number of cycles and the amount of time under cyclic loading are increased. These increase up to critical values characteristic of the material and the environment in which the material is located, resulting in the onset of macroscopic cracks as the limiting state of the first stage of fatigue failure. The kinetics of the change of state of the material at this stage is manifested submicroscopically (by a change in the density of dislocations and in the concentration of vacancies), microscopically (by the formation of slip lines, extrusions, and intrusions on the free surface of microstresses), and mechanically (by changes in hardness, the parameters of the elastoplastic hysteresis loop, the cyclic elastic modulus, and such ma-crophysical properties as density and the electrical, magnetic, and acoustic resistances).

Figure 1. Stress-endurance curves in the region of high-cycle fatigue

In the second stage of fatigue failure, the accumulated damage may be evaluated by the rate of growth of macrocracks and by the decreased resistance to failure (quasi-brittle or brittle fracture), which depends on the change in the material’s static strength, including the fracture toughness characteristics as critical values of the stress intensities at the edge of a fatigue crack.

Stress-endurance curves in the region of high-cycle fatigue (where the number of cycles at failure is over 10⁵) are caused by repeated elastic strains; they are plotted in amplitudes (or maximum stresses) of the cycle with logarithmic (log σ and log N) or semilogarithmic (σ and log N) coordinates (Figure 1). Depending on the characteristics of the material, the ambient temperature during testing, and the physical and chemical activity of the environment, stress-endurance curves may have either an asymptotic character (curve 1) or a continuously decreasing character with a convexity turned toward the origin of coordinates (curve 2). The value of the stress amplitudes σ_–1, which are the asymptotes in curves of the first kind, is called the endurance limit of the material; the value of the stress amplitudes

(σ_–1) N_f

for which failure occurs with the number of cycles N_f along a curve of the second kind, is called the fatigue strength. Type 1 curves are typical of materials having very stable structures and for very low temperatures; type 2 curves apply to materials having less stable structures, to higher temperatures, and to active environments.

Figure 2. Typical stress-endurance curve in the region of low-cycle fatigue

Curves in the region of low-cycle fatigue (where the number of cycles at failure is 10⁴ or less) resulting from repeated plastic strains are plotted in amplitudes of these strains with logarithmic coordinates log ∊_af and log N (Figure 2).