Human Muscle Fatigue

Craig Williams, Sébastien Ratel, Craig Williams, Sébastien Ratel

• Publisher: Routledge
• Year: 2009
• Print ISBN: 9780415453271
• eBook ISBN: 9781134053513
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Human Anatomy & Physiology
• Index: Biological Sciences

When human muscle fatigues, athletic performance becomes impaired. For those individuals suffering muscle or metabolic diseases the effects of muscle fatigue can make everyday tasks difficult. Understanding the scientific processes responsible for skeletal muscle fatigue is therefore central to the study of the physiology of sport, exercise and health.

Written by a team of leading international exercise scientists, this book explores the mechanisms of muscle fatigue and presents a comprehensive survey of current research on this important topic. Examining the wide variety of protocols, assessment methods and exercise models used to study muscle fatigue, the book explores the differential effects of fatigue as influenced by:

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• age

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• gender

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• fitness and training

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• the use of ergogenic aids

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• medical conditions including cerebral palsy, muscular dystrophy and glycogenosis.

Human Muscle Fatigue covers both clinical and applied approaches in sport and exercise physiology and devotes an entire section to the conceptual framework underpinning research in this area, helping readers from a wide range of backgrounds to engage with the topic. Accessible and detailed, this book is a key text for students and practitioners working in exercise and sports science, medicine, physical therapy and health.

PART I
THE CONCEPTUAL FRAMEWORK OF MUSCLE FATIGUE

CHAPTER 1
DEFINITIONS OF MUSCLE FATIGUE

Craig A.Williams and Sébastien Ratel

INTRODUCTION

The aim of this chapter is to clarify the meaning of muscle fatigue, indicate how fatigue can be identified, measured and classified, and thus applied in the context of exercise, disease and sport. In the literature there exist many definitions for fatigue, and although measurement techniques have significantly improved over the last century to observe this phenomenon, its aetiology is still a matter of considerable debate. Whether in the context of athletic competition, manual labour or patients with various neuromuscular diseases or chronic fatigue-type syndromes, fatigue is a commonly experienced phenomenon with important consequences. The study of fatigue has a long and illustrious research history, but advancements in its understanding have been made, particularly when fatigue has been investigated in different populations, under differing sport, ergonomic and disease conditions. For example, findings in neuromyopathies, i.e. myasthenia gravis, have presented scientists and clinicians with the opportunity to understand factors controlling force generation and to improve habitual performance.

In this chapter, to understand more fully the context and aetiology of fatigue, three important parameters are considered. These include:

1 the status¹ of the individual;

2 the task² required of the individual; and

3 the location of the fatigue.

Although the consequences of fatigue will differ depending on the three above parameters, the eventual and observable decline on performance allows clinicians and exercise scientists to explain the causal factors. This challenge cannot be underestimated because, despite several centuries of research into fatigue, the complexity of the problem is one of the few agreements amongst researchers investigating fatigue.

The search for the causality of fatigue can be divided into investigations which examine predominant factors at the central nervous system, the neuromuscular junction or the muscle unit. The former can be termed as central fatigue, while the latter two can be termed as peripheral fatigue. The complexity of establishing fatigue mechanisms is highlighted when causal factors can occur as a result of alterations at the peripheral as well as central areas, often synergistically, thus preventing the explicit identification of a single causal mechanism. The majority of research has tended to focus on peripheral factors and will predominantly be the focus of this book; this does not, however, discount the importance of central mechanisms.

DEFINITIONS OF FATIGUE

The following box represents a variety of definitions for fatigue, and the list is by no means comprehensive. Critical to the definition are three unifying points:

1 There is a decline in one or more of the biological systems.

2 The decline is reversible.

3 The decline may or may not occur before an observable performance or task failure occurs.

The first point usually refers to the decline in force, velocity or power of the biological system, usually with reference to muscle performance. The second point, reversibility, distinguishes fatigue from injury or disease when muscle performance might be impaired for a period of time or significantly debilitated. The latter point is an important consideration as it establishes the decline in performance often observed during a maximal “all-out” effort compared to the fatigue which might be progressively experienced during a prolonged exercise task.

Of course, the definition of any variable is important because without agreement on the definition, the observation and measurement of fatigue becomes extremely difficult. Part of this difficulty can be explained because fatigue occupies multiple roles and mechanisms, which can operate at many levels, from the CNS to the intrinsic muscle fibre itself (Westerblad et al., 1991; Fitts, 1994; Enoka, 1995).

Definitions of fatigue

A reversible state of force depression, including a lower rate of rise of force and a slower relaxation.

(Fitts and Holloszy, 1978)

The failure to maintain a required or expected force.

(Edwards, 1981)

Muscle fatigue is a decline in the maximal contractile force of the muscle.

(Vøllestad, 1997)

The inability to maintain of a physiological process to continue functioning at a particular level and/or the inability of the total organism to maintain a predetermined exercise intensity.

(Fifth International Symposium on Biochemistry of Exercise, 1982)

Reduction in the maximal force generating capability of the muscle during exercise.

(Miller et al., 1995)

Any reduction in the force-generating capacity (measured by the maximum voluntary contraction), regardless of the task performed.

(Bigland-Ritchie and Woods, 1984)

A loss of maximal force generating capacity.

(Bigland-Ritchie et al., 1986)

A condition in which there is a loss in the capacity for developing force and/or velocity of a muscle, resulting from muscle activity under load which is reversible by rest.

(NHLBI, 1990)

Any reduction in a person’s ability to exert force or power in response to voluntary effort, regardless of whether or not the task itself can still be performed successfully.

(Enoka and Stuart, 1992)

Any exercise-induced reduction in the maximal capacity to generate force or power output.

(Vøllestad, 1997)

Intensive activity of muscles causes a decline in performance, known as fatigue.

(Allen and Westerblad, 2001)

Performing a motor task for long periods of time induces motor fatigue, which is generally defined as a decline in a person’s ability to exert force.

(Lorist et al., 2002)

The development of less than expected amount of force as a consequence of muscle activation.

(McCully et al., 2002)

Fatigue is known to be reflected in the EMG signal as an increase of its amplitude and a decrease of its characteristic spectral frequencies.

(Kallenberg et al., 2007)

Other terms, often used interchangeably with fatigue, are exhaustion and weakness. However, these two terms are describing different functions and should not be used synonymously. Exhaustion refers to “the moment in time when the expected force level cannot be maintained” (Vøllestad, 1995: p. 186). Fatigue, as defined as a decline in the maximal force of the muscle, differentiates itself from exhaustion in that it is possible for fatigue to be observed during submaximal levels without a noticeable effect on performance. This interchanging of terms is partially due to the negative ramifications often associated with fatigue. However, the consequences of fatigue should also be viewed from a positive or protectionist perspective, as much as a negative one, as the body prohibits a metabolic crisis and conserves the integrity of the muscle fibre. As such, fatigue can be viewed as a consequence of one or several “fail-safe” mechanisms in the organism that call for temperance before damage occurs (Edwards, 1983). Another term associated with fatigue and often thought to represent a modified form is “plasticity”. This term is particularly important to rehabilitative therapists in that fatigue is a manifestation of plasticity that modifies contractile properties in order to improve the efficiency of contractions (Sargeant, 1994).

Interestingly, in exercise and sports science there has been some interest in exhaustive tests, often in the guise of timed rides or runs to exhaustion. This endurance time is then associated with the fatigability. In such studies, interventions are often applied and the time-to-exhaustion exercise repeated, and consequently findings with a demonstrated longer time to exhaustion have been used to imply improvements in fatigability. However, there are two weaknesses in this approach. First, tests which use time to exhaustion are notoriously unreliable and possess a high coefficient of variation (Jeukendrup et al., 1996), as well as being ecologically invalid. It is rare in a sport or clinical situation that an individual would need to perform a task “for as long as possible”. Second, this approach assumes there is a relationship between the decline in maximal performance and the time to exhaustion. Several studies have shown a large variability between time to exhaustion and declines in maximal voluntary contractions (MVC) (Vøllestad et al., 1988).

Weakness, which often results in observable decrements in force or power generation, is more symptomatic of maintenance of low force or power over a sustained period of time, but is independent of exercise. Weakness may occur due to the sustained atrophy of muscles experienced during ageing. Under these circumstances the inability to delay muscle fatigue during a task requiring absolute power or force is reduced. Interesting however, is the observation that relative to the maximal force or power, atrophied muscle shows a greater resistance to fatigue (Larsson and Karlsson, 1978; Overend et al., 1992). Although the mechanism for this observation is not entirely clear, increases in capillary density and the concentration of mitochondrial and metabolic enzymes and shifts in fibre types are thought to be responsible.

Therefore, we are generally left with two commonly accepted definitions. The first is that fatigue is an exercise-induced reduction in the ability of muscle to produce power or force, irrespective of task completion (Big-land-Ritchie and Woods, 1984). And second, fatigue is considered as the inability to maintain a required or expected force or power output (Edwards, 1981) under maximal or submaximal sustained contraction conditions, i.e. the time to task failure or muscle endurance. As Enoka and Duchateau (2008) concisely describe, the distinction between these two...

Table of contents

• CONTENTS
• FIGURES
• TABLES
• CONTRIBUTORS
• PREFACE
• PART I THE CONCEPTUAL FRAMEWORK OF MUSCLE FATIGUE
• PART II FATIGUE AND EXTRANEOUS FACTORS
• PART III FATIGUE AND PATHOPHYSIOLOGY
• INDEX