ArticlePDF AvailableLiterature Review

Translating Fatigue to Human Performance

March 2016
Medicine and Science in Sports and Exercise 48(11)

DOI:10.1249/MSS.0000000000000929

Authors:

Roger Enoka

University of Colorado Boulder

Jacques Duchateau

Université Libre de Bruxelles

Despite flourishing interest in the topic of fatigue-as indicated by the many presentations on fatigue at the 2015 annual meeting of the American College of Sports Medicine-surprisingly little is known about its impact on human performance. There are two main reasons for this dilemma: (1) the inability of current terminology to accommodate the scope of the conditions ascribed to fatigue, and (2) a paucity of validated experimental models. In contrast to current practice, a case is made for a unified definition of fatigue to facilitate its management in health and disease. Based on the classic two-domain concept of Mosso, fatigue is defined as a disabling symptom in which physical and cognitive function is limited by interactions between performance fatigability and perceived fatigability. As a symptom, fatigue can only be measured by self-report, quantified as either a trait characteristic or a state variable. One consequence of such a definition is that the word fatigue should not be preceded by an adjective (e.g., central, mental, muscle, peripheral, and supraspinal) to suggest the locus of the changes responsible for an observed level of fatigue. Rather, mechanistic studies should be performed with validated experimental models to identify the changes responsible for the reported fatigue. As indicated by three examples (walking endurance in old adults, time trials by endurance athletes, and fatigue in persons with multiple sclerosis) discussed in the review, however, it has proven challenging to develop valid experimental models of fatigue. The proposed framework provides a foundation to address the many gaps in knowledge of how laboratory measures of fatigue and fatigability impact real-world performance.

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Translating Fatigue to Human Performance

Roger M. Enoka1 and Jacques Duchateau2

1Department of Integrative Physiology, University of Colorado, Boulder, CO

2Laboratory of Applied Biology and Neurophysiology, ULB Neuroscience Institute,

Université Libre de Bruxelles (ULB), Bruxelles, Belgium

Accepted for Publication: 24 November 2015

ACCEPTED

Translating Fatigue to Human Performance

Roger M. Enoka1 and Jacques Duchateau2

1Department of Integrative Physiology, University of Colorado, Boulder, CO

2Laboratory of Applied Biology and Neurophysiology, ULB Neuroscience Institute, Université

Libre de Bruxelles (ULB), Bruxelles, Belgium

Correspondence: Roger M Enoka

Department of Integrative Physiology

University of Colorado

Boulder, CO 80309-0354

enoka@colorado.edu

This work was partially supported by an award from National Institute of Child Health & Human

Development (R03HD079508) to RME. The authors declare no conflicts of interest. Understandably,

ACSM is unable to endorse either the data or its interpretation as presented in this article.

Medicine & Science in Sports & Exercise, Publish Ahead of Print

DOI: 10.1249/MSS.0000000000000929

ACCEPTED

ABSTRACT

Despite flourishing interest in the topic of fatigue—as indicated by the many presentations on

fatigue at the 2015 annual meeting of the American College of Sports Medicine—surprisingly

little is known about its impact on human performance. There are two main reasons for this

dilemma: (1) the inability of current terminology to accommodate the scope of the conditions

ascribed to fatigue, and (2) a paucity of validated experimental models. In contrast to current

practice, a case is made for a unified definition of fatigue to facilitate its management in health

and disease. Based on the classic two-domain concept of Mosso, fatigue is defined as a disabling

symptom in which physical and cognitive function is limited by interactions between

performance fatigability and perceived fatigability. As a symptom, fatigue can only be measured

by self-report, quantified as either a trait characteristic or a state variable. One consequence of

such a definition is that the word fatigue should not be preceded by an adjective (e.g., central,

mental, muscle, peripheral, and supraspinal) to suggest the locus of the changes responsible for

an observed level of fatigue. Rather, mechanistic studies should be performed with validated

experimental models to identify the changes responsible for the reported fatigue. As indicated

by three examples (walking endurance in old adults, time trials by endurance athletes, and

fatigue in persons with multiple sclerosis) discussed in the review, however, it has proven

challenging to develop valid experimental models of fatigue. The proposed framework provides

a foundation to address the many gaps in knowledge of how laboratory measures of fatigue and

fatigability impact real-world performance. Key words: performance fatigability, perceived

fatigability, endurance, critical power, multiple sclerosis

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Despite a substantial literature on the influence of fatigue on human performance, relatively

little progress has been made in translating this knowledge into practice. One of the key

impediments has been the scope of its usage, with fatigue denoting reductions in physical and

cognitive function that extend from an exercise-induced impairment of motor performance

through to the sensations of tiredness and weakness that accompany some clinical conditions.

As a consequence, knowledge on fatigue has become compartmentalized within such disciplines

as clinical medicine, ergonomics, physiology, and psychology. The resulting fragmentation of

work on fatigue has led to the absence of a consensus definition of fatigue, the emergence of

terminology that tends to exclude individuals not familiar with a particular discipline, and the

development of questionable experimental models.

Contemporary research on the physiology of fatigue, for example, is often based on the

Mosso dichotomy in which the phenomenon of force diminution is regarded as distinct from the

sensations that arise from prolonged muscular activity (19). Mechanistic studies on the

physiology of fatigue, therefore, have largely focused on identifying rate-limiting adjustments in

central and peripheral processes that limit human performance (7) (Figure 1). Despite the many

studies that have adopted the central-peripheral dichotomy, however, two key limitations with

this approach have precluded the development of a consensus understanding on what causes

fatigue.

The first limitation is the implicit assumption that the adjustments in neuromuscular activity

needed to counteract an exercise-induced decrease in force capacity and thereby sustain task

performance are independent from those involved in generating the accompanying sensations.

Although it is possible to isolate the decline in contractile function during a fatiguing contraction

by recording electrically evoked forces (6), adjustments in the activation signal discharged by

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motor neurons during a voluntary contraction begin before a detectable reduction in muscle force

and are attributable to changes occurring within the muscle (11,20). Moreover, sensory feedback

transmitted by group III/IV afferents can influence both the integration of synaptic input by

spinal motor neurons and contribute to perceptions of pain (42). Such observations indicate that

it is not possible to identify the etiology of fatigue by attempting to disentangle the decline in

muscle force from sensations about fatigue, especially during long-lasting contractions (68).

The second limitation is that most of the physiological processes involved in performing a

voluntary action, extending from the generation of the motor command down to the cross bridge

cycle, can be challenged under appropriate experimental conditions and thereby contribute to the

development of fatigue. For example, the decline in maximal voluntary contraction (MVC)

force—a classic index of muscle fatigue—that develops during sustained low-intensity activity is

largely attributable to a reduction in the activation signal generated by the nervous system,

whereas the reduction in MVC force after high-intensity activity is more likely due to a decline

in contractile function (68,72). Indeed, much of the literature on the physiology of fatigue is

focused on exploiting a range of experimental protocols to quantify the capabilities of the

neuromuscular system in various contexts (18,22,34,46,59). This work has clearly demonstrated

that the rate-limiting adjustments during fatiguing contractions vary across conditions, which has

become known as the task dependency of fatigue (19). A critical question that emerges from this

work, however, is which of these impairments limit real-world performance?

The purpose of this brief review is to present a framework that can be used to evaluate the

functional significance of fatigue-related adjustments. The framework comprises a taxonomy

that can encompass the scope of the conditions ascribed to fatigue and a general approach to

determine the influence of fatigue on human performance. To achieve this goal, we propose that

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fatigue be conceptualized as a disabling symptom in which physical and cognitive function is

limited by interactions between performance fatigability and perceived fatigability.

PROPOSED TAXONOMY

The original scheme proposed by Mosso remains relevant (Figure 1), but needs to be

broadened to accommodate the contemporary scope of conditions ascribed to fatigue during

human performance. As suggested in the taxonomy proposed by Kluger et al. (37), the concept

of fatigue should acknowledge its two attributes: (1) performance fatigability – the decline in an

objective measure of performance over a discrete period of time; and (2) perceived fatigability –

changes in the sensations that regulate the integrity of the performer (Figure 2). Fatigue is

defined in terms of fatigability to normalize the level of fatigue reported by an individual relative

to the demands of the task that produces it. Individuals who are less fatigable, for example, are

able to meet a greater demand to reach the same level of fatigue. When a person reports the level

of fatigue during ongoing activity, therefore, the value at a specific time point will depend on the

rates of change in its two attributes. As indicated in Figure 2, the attribute of performance

fatigability depends on the contractile capabilities of the involved muscles and the capacity of the

nervous system to provide an adequate activation signal derived from descending commands and

afferent feedback for the prescribed task. In contrast, the attribute of perceived fatigability is

derived from the initial value and the rate of change in sensations that regulate the integrity of

the performer based on the maintenance of homeostasis and the psychological state of the

individual.

In contrast to performance fatigability, perceived fatigability can be assessed when a person

is either at rest (21,23) or performing physical activity (57,62). Elevated perceived fatigability at

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rest is attributable to deviation of one or more of the modulating factors (e.g., core temperature,

hydration, motivation, and pain) from normal baseline values. Conversely, perceived fatigability

during onging activity is derived from rates of change in the modulating factors and are used to

regulate the pace of the performance and thereby control the development of fatigue. Although

the proposed scheme (Figure 2) suggests that the influence of psychological factors on fatigue is

mediated via perceived fatigability, it remains to be determined whether or not psychological

factors can have a direct effect on fatigue without involving perceived fatigability.

Most of the factors listed in Figure 2 are the familiar candidates that have been demonstrated

to represent rate-limiting adjustments under selected conditions (17,24,38,50,52,58,68) (Figure

1). One of the unique features of the proposed scheme, however, is the explicit inclusion of

psychological factors as significant contributors to the construct of fatigue. Although a potential

role for psychological factors in the development of fatigue is occasionally acknowledged in

studies on healthy individuals (43), they assume a more prominent role in the fatigue reported by

various clinical cohorts. For example, the level of fatigue (Fatigue Severity Scale) reported by

individuals with either multiple sclerosis or Parkinson’s disease is significantly correlated with

the level of depression (37) and represents a deviation in one of the modulating factors (mood)

from normal baseline values. Similarly, lesions in the ascending reticular activating system

suggest that reductions in arousal may mediate some aspects of fatigue in individuals with post-

polio syndrome (37). Even in healthy individuals, however, psychological factors, such as

performance feedback, can influence the average power produced by trained cyclists when

performing a time trial as quickly as possible, which presumably coincides with maximal ratings

of perceived exertion at the end of the task (66).

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Although the taxonomy illustrated in Figure 2 lists many of the factors that can influence

each attribute of fatigue (performance fatigability and perceived fatigability), the scheme

acknowledges that most voluntary actions performed by humans involve significant interactions

between the two domains. For example, several of the modulating factors that contribute to

perceived fatigability, including the levels of blood glucose (49), core temperature (50), arousal

(36), and mood (64), can all modulate the capacity of the individual to generate the required

amount of voluntary activation, which is a factor that influences performance fatigability.

Similarly, afferent feedback generated during high-intensity exercise can influence the

adjustments required to maintain homeostasis and thereby contribute to perceived fatigability

(33,59). The key feature of this scheme is that the level of fatigue experienced by an individual

emerges from the many adjustments that occur in the modulating factors within and between

each fatigability attribute. With this construct, fatigue is defined as a disabling symptom in

which physical and cognitive function is limited by interactions between performance fatigability

and perceived fatigability. As indicated in Figure 2, the level of fatigue experienced by an

individual can be modulated by challenges to homeostasis, disturbances in the psychological

state, reductions in contractile function, and limitations in the capacity to provide an adequate

activation signal to the involved muscles.

According to the proposed definition, fatigue is a single entity that does not need to be

modified by an accompanying adjective, such as central fatigue, mental fatigue, muscle fatigue,

peripheral fatigue, physical fatigue, and supraspinal fatigue. Although the descriptors are often

intended to imply the likely locus of the adjustments in the modulating factors that limit

performance, the distinctions are too vague to be meaningful. Moreover, the literature on fatigue

would be more coherent if the practice was to state the primary outcome variable and describe

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how it was influenced by the protocol, without suggesting that the study examined a particular

type of fatigue.

Studies on performance fatigability, for example, should focus on the key outcome variable,

which could include the duration that a task can be sustained (endurance time or time to task

failure), the time it takes to complete a prescribed task (walking endurance, time trial), or the rate

of change in such variables as MVC force, power production, the amplitude of evoked responses,

reaction time, rating of perceived exertion, heart rate, mean arterial pressure, and core

temperature. Critically, however, the outcome variable must be an ecologically valid measure of

human performance, such as those specified in the NIH Toolbox, indices of athletic prowess, and

standardized clinical assessments. Such an approach would facilitate the development of

guidelines on how to manage fatigue in health and disease (3,37,77). An analogous construct has

been proposed to monitor the fatigue experienced by athletes across a training cycle (26).

HUMAN PERFORMANCE AND FATIGUE

We now consider three examples of what we know about how fatigue influences human

performance. Similar approaches have been used by other groups (24,38,63). In each of our

examples, a performance criterion is identified and then what is known about the fatigue-related

adjustments that limit this performance is discussed. In addition to providing a framework that

can be extended to other activities, the examples included in the current review identify specific

gaps in knowledge that need to be addressed in future work.

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Walking Endurance of Healthy Adults

The National Institutes of Health (NIH) developed a multidimensional set of measures,

known as the Toolbox, to provide brief yet comprehensive tests of neurological health and

function across the lifespan. The NIH Toolbox has four domains: cognition, emotion, motor, and

sensation (55). Motor function—the ability to perform physical tasks—is quantified with tests of

endurance, locomotion, manual dexterity, non-vestibular balance, and strength. Endurance,

which the Toolbox assesses as the distance that can be walked in 2 min, characterizes the global

and integrated responses of the pulmonary, cardiovascular, and musculoskeletal systems. The

distance that can be walked in 2 min varies across the lifespan (Figure 3) and walking endurance

is reduced in old adults (65-102 yrs) who self-report elevated levels of fatigue (61,70).

Moreover, time to complete another test of walking endurance (400 m) is a significant predictor

of mortality in old adults (71).

Given that walking endurance provides a clinically relevant measure of human performance,

Justice et al. (30) compared the associations between walking endurance and measures of

neuromuscular function in young (22 ± 4 yrs) and old (75 ± 6 yrs) adults. Of particular interest

was the strength of the association between walking endurance (500-m walk) and a laboratory

test of leg-muscle fatigability. Due to the significant loss of function in the dorsiflexor muscles

of old adults (4,54,67), the test of fatigability involved supporting a submaximal mass (20% of

the one-repetition maximum) with the dorsiflexor muscles for as long as possible while seated

with the foot in a neutral position. The two groups (young and old) experienced a similar decline

in maximal voluntary contraction (MVC) torque at task failure (~23%), but endurance time for

this particular fatiguing contraction was longer for the young adults (15.5 ± 0.9 min) than the old

adults (8.9 ± 0.6 min) even though the young adults were stronger.

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In a subsequent analysis of the data reported by Justice et al. (30), the outcome measures

were entered into a stepwise, multiple-regression model to predict the variance in 500-m walk

time for each group of participants. Separate models emerged for young and old adults, with

more of the variance explained for young adults (R2 = 0.42) than for old adults (R2 = 0.31;

Figure 4). The three explanatory variables in the model for old adults also emerged as predictors

in the model for the young adults: MVC force for the knee flexors, 1-RM load for the

dorsiflexors, and force steadiness when supporting a 20% 1-RM load with the dorsiflexors. The

model for young adults included an additional two explanatory variables: endurance time for the

measure of leg-muscle fatigability and dorsiflexor MVC force. These findings indicate that

dorsiflexor fatigability during a sustained, submaximal contraction was moderately related to

walking endurance for young adults, but not for old adults. Additional studies are needed to

determine if more of the variance in walking endurance can be explained by other tests of

performance fatigability, such as actions that engage multiple joints and those involving the

stretch-shortening cycle (39).

In contrast to the absence of an association between walking endurance and the test of

performance fatigability used by Justice et al. (30), another measure of fatigue has been found to

be associated with walking endurance in old adults. In a sample of 1,155 adults (65-102 yrs),

Vestergaard et al. (70) examined the association between fatigue and physical function by

categorizing participants into those who self-reported elevated levels of fatigue and those who

did not. Fatigue was assessed with two questions from the Center for Epidemiologic Studies-

Depression Scale based on experience in the past week: (1) I feel that everything I did was an

effort; (2) I could not get going. Possible answers were: (a) rarely (<1 day); (b) some of the time

(1-2 days); (c) occasionally (3-4 days); or (d) all of the time (5-7 days). Those individuals who

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reported ≥3 days for either question were classified as fatigued, the prevalence of which was

greater for women (29.1%) than for men (15.3%).

In general, the health behaviors and prevalence of clinical conditions was relatively similar

for the two groups of participants for both sexes. The exceptions were that men who were

classified as being fatigued, but not women, were more sedentary, had more comorbid

conditions, reported a greater prevalence of coronary heart disease and chronic

bronchitis/emphysema, and had greater levels of C-reactive protein in blood samples.

Nonetheless, the fatigued participants of both sexes had weaker handgrip strength, greater

activity limitations (Instrumented Activities of Daily Living), and worse walking endurance. For

example, average speed during the 400-m walk was less for the fatigued groups (men: 1.29 and

1.22 m/s; women: 1.14 and 1.05 m/s) and the percentage of those individuals who could not

complete the 400 m walk was greater for the fatigued groups (men: 5.9 and 21.0%; women: 6.9

and 14.8%). These findings indicate that an elevated trait level of fatigue is associated with

reduced walking endurance (a measure of performance fatigability) in old adults.

Time Trials by Endurance Athletes

The peak power that a muscle can produce during sustained activity declines as the duration

of the task increases. The relation between power production and task duration can be

characterized with a hyperbolic function, with the asymptote corresponding to critical power and

the curvature denoting the finite amount of work that can be performed above critical power at

each level of power production (24,28). Critical power is located at the boundary between heavy

and severe exercise, and has been characterized as the greatest exercise intensity at which the

physiological adjustments can accommodate the challenge to intramuscular homeostasis.

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Exercise can be sustained for durations up to a maximum of ~30 min when performed at

intensities above critical power; that is, in the severe domain.

An individual’s critical power, which provides a measure of performance fatigability, can be

estimated with a single, 3-min test on a cycle ergometer in a laboratory (69). The test requires

participants to cycle at a maximal cadence with the ergometer resistance set midway between the

power outputs that correspond to the gas exchange threshold and maximal oxygen consumption

as determined during a ramp protocol. The participant is required to achieve maximal power

production quickly and to maintain as high a cadence as possible until told to stop. Critical

power is quantified as the average power produced during the last 30 s of the test and the

curvature of the hyperbolic function is estimated from the power-time integral above critical

power.

The validity of the method as a measure of human performance was examined by comparing

the ergometer-based estimate of critical power with the time it took 10 competitive cyclists to

complete a 16.1 km time trial (8). Such time trials are performed at an intensity similar to

critical power. The time trial was completed in 27.1 ± 1.2 min (mean ± SD) and critical power

was estimated as 309 ± 34 W (4.20 ± 0.41 W/kg). Time trial performance was significantly

correlated with critical power (r2 = –0.69, P < 0.01), indicating that faster times were associated

with greater values for critical power. A similar correlation was found between time trial

performance and total work (power-time integral) performed during the 3 min test. These

findings indicate that the critical power protocol represents a valid model for mechanistic studies

of performance fatigability during time trials, at least in highly trained individuals.

As one example of such a mechanistic study, Klass et al. (36) compared the influence of

three drug conditions on the performance of 10 cyclists and triathletes on a time trial. The drug

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conditions, one of which was administered in each experimental session, comprised a placebo or

a reuptake inhibitor for either noradrenaline (reboxetine) or dopamine (methylphenidate). The

reuptake inhibitors result in each neurotransmitter lingering longer among synaptic contacts and

thereby augmenting the actions it elicits. On each occasion, the participants performed a time

trial in which they were required to complete a fixed amount of work (the equivalent of 30 min at

75% of maximal power production) on a cycle ergometer as quickly as possibly. The rating of

perceived exertion (6-20 scale) was reported during the test. Before and after each time trial,

contractions were evoked with transcranial magnetic stimulation and nerve-muscle stimulation to

estimate the level of voluntary activation.

The average power produced during the time trial was depressed when noradrenaline

reuptake was inhibited relative to the other two conditions (Figure 5). As a consequence, the

time to complete the simulated time trial was similar for the placebo (30.8 ± 2.1 min) and the

inhibition of dopamine reuptake (29.7 ± 1.4 min) conditions, but significantly longer for the

noradrenaline condition (33.7 ± 3.6 min). Consistent with these results, the level of voluntary

activation, as assessed with transcranial magnetic stimulation, did not differ from baseline values

(~97%) to those measured at 10 min after the time trial for either the placebo (95 ± 1.5%) or

dopamine (95 ± 1.4%) groups, but was depressed for the noradrenaline group (89 ± 3.0%). In

contrast, the rating of perceived exertion at the end of the time trial did not differ for the three

groups (18.0 ± 1.4, 18.1 ± 1.1, and 18.7 ± 1.1, respectively). These results suggest that the

average power produced during each time trial was regulated by adjustments that influence the

rating of perceived exertion and that these adjustments were compromised when noradrenaline

reuptake was inhibited relative to the other two conditions.

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The association between physiological adjustments and ―volitional fatigue‖—when ratings of

perceived exertion were presumably maximal—during high-intensity exercise was demonstrated

in another study when McKenna et al. (44) compared the influence of an antioxidant compound

on cycling performance. The antioxidant administered during the study was N-acetylcysteine,

which can influence the activity of Na+,K+ pumps in muscle and prolong the duration that high-

intensity exercise can be sustained. In a cross-over design in which the influence of the drug was

compared with saline, McKenna et al. (44) examined the performance and adjustments exhibited

by eight endurance-trained athletes when they cycled on an ergometer for 45 min at 70% of peak

oxygen consumption and then as long as possible at 92% of maximal oxygen consumption. The

influence of the antioxidant compound was characterized by measuring maximal Na+,K+ pump

activity in biopsy samples from vastus lateralis and the associated changes in plasma [K+].

Infusion of the antioxidant had a significant influence on maximal Na+,K+ pump activity,

plasma [K+], and endurance time relative to the saline condition. The duration that the cyclists

could sustain the cycling task was prolonged by 24 ± 8% for the antioxidant condition and this

improvement in performance was accompanied by lesser changes in both maximal Na+,K+ pump

activity and plasma [K+] when normalized to the amount of work that was performed. The

attenuation of the decline in maximal Na+,K+ pump activity during the antioxidant condition

suggests that the accumulation of reactive oxygen species contributes significantly to the

inactivation of Na+,K+ pumps during high-intensity exercise. Because the task was performed

for as long as possible for the two conditions, the results suggest the contraction-associated

modulation of Na+,K+ pump activity can influence the neural pathways involved in generating

the maximal level of tolerable exercise (76).

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Taken together, these findings indicate that the physiological adjustments exhibited by

endurance athletes during high-intensity exercise are strongly associated with ratings of

perceived exertion. Key gaps in knowledge for this level of human performance include

information about the relative significance of the different adjustments that contribute to the

generation of perceived fatigability (1) and the extent to which tolerance of these perceptions can

be improved with exercise interventions (29).

Fatigue in Multiple Sclerosis

Multiple sclerosis (MS) is an inflammatory disease that progressively impairs the ability of

neurons to generate and conduct action potentials. Although the course of the disease varies

among individuals, it invariably involves the development of difficulties with walking and

elevated levels of fatigue (10,40,47). One large survey found that 74% of respondents (n =

9,077) reported severe fatigue, and worsening fatigue was associated with the development of

limitations in walking (25).

Several studies have examined the association between the fatigue reported by individuals

with MS and performance fatigability during standardized laboratory tests. In one such study,

Steens et al. (64) compared the characteristics of 20 persons who had one form of the disease

(relapsing-remitting MS) with a control group of age- and sex-matched individuals. Fatigue was

assessed with the Fatigue Severity Scale (FSS), which is a validated and reliable instrument that

quantifies the impact of fatigue on activities of daily living in individuals with MS (41). The

FSS is a unidimensional scale that largely focuses on the physical aspects of fatigue.

Performance fatigability was quantified as the decline in force during a sustained, isometric

MVC with a hand muscle (first dorsal interosseus). This muscle was chosen because it is less

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likely than limb muscles to be impacted by the deconditioning associated with the relatively

sedentary lifestyle of individuals with MS. In addition, mood was evaluated using the depression

subscore of the Hospital Anxiety and Depression Scale questionnaire.

Participants in the MS group exhibited significantly greater levels of fatigue (FSS: 5.3 ± 0.9)

and depression (4.8 ± 3.3) than those in the control group (2.9 ± 0.6 and 0.9 ± 1.0, respectively).

There was a moderate association between FSS score and depression for the MS group (r2 =

0.36), but not for the control group. MVC force for the right hand muscle was not significantly

different between groups, but when standardized as a Z score the MVC force was less for the MS

group relative to the control group (–0.7 ± 1.0). Normalized MVC force (Z score) was

significantly associated with the level of voluntary activation assessed during the MVCs for the

MS group (r2 = 0.33), but not for the control group. The decline in force during the fatiguing

contraction (sustained MVC for 124 s) was similar for the MS (61 ± 17%) and control (63 ±

12%) groups, and was significantly associated with MVC force (partial r2 = 0.15).

The assocation between FSS score and the measure of performance fatigability (decline in

MVC force) was examined with a multiple-regression analysis. A significant amout of the

variance in the FSS score for the participants in the MS group (R2 = 0.45), but not the control

group, was explained with a model that included the measure of performance fatigability (partial

r2 = 0.37) and normalized MVC score (partial r2 = 0.35). The model indicates that the

individuals with MS who had greater FSS scores exhibited a greater amount of performance

fatigability than would be expected on the basis of the normalized MVC score. However, more

of the variance in FSS scores for the MS group was explained by including the depression score

in the model (R2 = 0.77), which may have arisen from some of the FSS items concerning social

aspects of fatigue.

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In a subsequent study with similar groups of participants, Steens et al. (65) compared the

adjustments in the activation signal generated by the nervous system during a fatiguing

contraction with the hand muscle. The variance in performance fatigability (decline in MVC

force) was explained by different outcome measures for the two groups of participants. Much of

the variance for the decline in force exhibited by the control group was explained (R2 = 0.49) by

the initial MVC force (strength) of the hand muscle. As reported in some other studies on the

performance fatigability of healthy individuals (27), the strongest individuals were the most

fatigable. Although the participants in the MS group experienced a similar reduction in force as

the control group, the primary explanatory variable (R2 = 0.51) was the decline in voluntary

activation as assessed by nerve-muscle stimulation (–20.1 ± 20.6%). The level of voluntary

activation at the start of the fatiguing contraction did not differ between the two groups (~90%)

and declined only slightly by the end of the fatiguing contraction for the control group (–7.8 ±

11.8%). The regression model for the MS group indicated that those individuals with greater

FSS scores experienced larger declines in voluntary activation.

To assess the cortical adjustments during the fatiguing contraction, Steens et al. (65)

compared the changes in blood oxygen level-dependent (BOLD) contrast for the two groups of

participants. As observed in other studies on healthy adults (53), the control group demonstrated

a widespread increase in cortical activity during the fatiguing contraction despite a small

decrease in voluntary activation and the progressive decline in maximal force. One potential

explanation for the increase in cortical activity during fatiguing contractions is that it

compensates for the reduction in excitation provided to spinal motor neurons (35,45).

Nonetheless, the increase in cortical activity is not sufficient to overcome the adjustments

responsible for the decrease in force during strong contractions. Consistent with this

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interpretation, participants in the MS group did not increase cortical activity and this was

accompanied by a larger decline in voluntary activation. The similar decline in MVC force for

the two groups, therefore, must be explained by other adjustments for the MS group. Indeed, the

peak force elicited by a pair of stimuli—a measure of force capacity and the efficacy of

excitation-contraction coupling—declined during the fatiguing contraction and was negatively

related to the reduction in voluntary activation for the MS group (R2 = 0.25). Thus, greater

decreases in voluntary activation were associated with lesser declines in the evoked twitch forces

at rest.

Taken together, these findings indicate that the trait level of fatigue in persons with MS is

associated with a measure of performance fatigability, the strength of the muscles involved in the

fatiguing contraction, and the capacity to sustain a high level of voluntary activation during the

fatiguing contraction. The caveat, however, is that these studies were performed on a hand

muscle and future studies need to examine similar associations in limb muscles. In addition,

little is known about the influence of fatigue and performance fatigability on mobility, such as

walking endurance, in individuals with MS.

TRANSLATING FATIGUE TO PERFORMANCE

By combining the proposed taxonomy (Figure 2) with the information presented in the

preceding examples, we can complete the framework on how to translate knowledge on fatigue

into practice. The foundation of this approach is the definition of fatigue as a symptom, which

means it can only be measured by self-report. Classic measures of fatigue, such as the time to

complete a prescribed task, the reduction in MVC force, and the decline in power production, are

indices of performance fatigability, but do not provide a measure of the intensity of the

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symptom. Rather, the assessment of fatigue requires the individual to interpret relevant

physiological and psychological factors by providing responses to standardized questions

(3,5,26,56,75).

Trait and State Properties

As with many symptoms, fatigue can be assessed either as a trait characteristic or as a state

variable. The trait level of fatigue represents the average amount of fatigue experienced during

the preceding several days, whereas state measures reflect the rate of change in key adjustments

during a fatiguing task. In the example of walking endurance in healthy adults, Vestergaard et

al. (70) used a standardized instrument (Center for Epidemiologic Studies-Depression Scale) to

determine the level of fatigue experienced by old adults during the preceding week. Such a

measure indicates the trait level of fatigue, which Vestergaard et al. (70) used to assign the study

participants to either a control group or a fatigue group. The relevance of the approach was

underscored by finding significant differences in health status and performance criteria between

the two groups of participants. Similarly, there are several instruments (e.g., Fatigue Severity

Scale, Mobility-Tiredness Scale, Modified Fatigue Impact Scale, Multidimensional Fatigue

Inventory, Recovery-Stress Questionnaire, Situational Fatigue Scale) that rely on self-reported

responses about the preceding several days or weeks to quantify the trait level of fatigue

experienced by individuals with various illnesses (3,5,37,47,64) and in athletes during a training

cycle (26).

The measure of fatigue at a specific instant in time indicates the state level of fatigue. This

can be accomplished by asking the performer to answer one or more questions about the level of

fatigue ―right now‖. One instrument often used to assess the state level of fatigue is the visual

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analog scale, which can be anchored by ―Fatigue is absent‖ on the left and ―Most fatigue ever‖

on the right (12). Another option is to use the fatigue scale from the Profile of Mood States

questionnaire, which has been demonstrated to provide a reliable and valid measure of the state

level of fatigue across a wide range of cohorts (51). Participants are asked to provide a response

on a 5-point scale (0 = not at all, 1 = a little, 2 = moderately, 3 = quite a bit, 4 = extremely) to

five terms: Worn out, Fatigued, Exhausted, Sluggish, and Weary. The combined score indicates

the state level of fatigue and can be compared at selected time points during a protocol. For

example, Dishman et al. (14) used the Profile of Mood States fatigue scale to compare the

influence of cycling exercise on the state level of fatigue in individuals who reported an elevated

trait level of fatigue. Participants were assigned to either a 6-wk intervention (low- or moderate-

intensity cycling) or a control group. The state level of fatigue was compared before and after

cycling exercise at three time points during the 6-wk protocol. Participants in the low-intensity

group, but not the other two groups (moderate-intensity exercise or control), exhibited less of an

increase in the state level of fatigue after an exercise bout at weeks 3 and 6 of the study. These

findings were interpreted to indicate that low-intensity exercise can be used to manage clinical

symptoms and mood disorders, such as fatigue.

Alternatively, the state level of fatigue can be assessed with an instrument that is developed

for a specific cohort. For example, Barry and colleagues devised a Fatigue and Energy Scale to

measure the state level of fatigue after performing a bout of physical activity in individuals with

chronic fatigue syndrome (32). One of the characteristics of the syndrome is the prolonged

exacerbation of symptoms after performing physical activity. The approach involved convening

focus groups comprising afflicted individuals to inform the development of a self-report

instrument, examining the psychometric properties of the instrument in two case-control

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challenge studies, and assessing the ecological validity of the instrument. The resulting

instrument (Fatigue and Energy Scale) comprised two domains—physical and mental fatigue—

that each included three items. As intended, the Scale was able to capture the state level of

fatigue reported by individuals with chronic fatigue syndrome after performing a bout of

physical activity. With the increasing prevalence of fatigue being included in the diagnosis of

various clinical conditions, there is an urgent need for similar instruments that can enable

clinicians to quantify condition-specific levels of fatigue.

However, a word of caution is necessary about the use of self-report measures to assess the

state level of fatigue in some cohorts, such as athletes and special operations forces personnel.

Due to the competitive nature of these individuals, they are more likely than not to understate

responses to questions about fatigue (1). Nonetheless, some assessment of fatigue is essential for

these individuals to optimize the training load and lessen the likelihood of injuries and

overtraining (26).

Experimental Strategies

To determine the influence of fatigue on human performance, the proposed approach

involves three levels of analysis: (1) select a criterion measure of human performance that is

modulated by fatigue; (2) identify a laboratory test that is a strong predictor of performance on

the criterion measure; and (3) conduct mechanistic studies to determine the relative significance

of the adjustments in the modulating factors (Figure 2) that limit performance on the laboratory

test. Of the three examples discussed previously (walking endurance of healthy adults, time

trials by endurance athletes, and fatigue in multiple sclerosis), most progress has been made on

determining the influence of fatigue on the performance of endurance athletes during a time trial.

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The time it takes a trained cyclist to complete a time trial is reasonably correlated with the

absolute critical power for the individual (8,9), which depends on the capacity of intramuscular

adjustments to accommodate the challenge to homeostasis during high-intensity exercise

(24,28,69). Given a specific critical power capability, however, performance during an

individual time trial may be limited by factors that contribute to perceived fatigability, such as

those that establish the performer’s psychological state. Power production can be modulated, for

example, by interventions that manipulate homeostasis and thereby alter ratings of perceived

exertion (36,44). Future studies are needed to determine the relative significance of the various

adjustments that influence ratings of perceived exertion.

In contrast, much less is known about how fatigue influences walking endurance of healthy

adults and the fatigue reported by individuals with multiple sclerosis. Walking endurance, which

is a strong predictor of health status (61,62,70), is reduced in old adults who exhibit an elevated

trait level of fatigue, but there is not yet a validated test of fatigability that predicts performance

on tests of walking endurance. Without an appropriate laboratory measure, it is not possible to

identify the adaptations responsible for declines in walking endurance with advancing age and

thereby develop appropriate countermeasures.

Although more is known about the influence of fatigue—as quantified with validated

questionnaries—on motor function in persons with multiple sclerosis (13,48,74) than is known

about walking endurance, significant gaps in knowledge remain. The elevated trait level of

fatigue reported by individuals with relapsing-remitting MS is strongly associated with the

strength of a hand muscle, the performance fatigability of the same hand muscle during a

sustained maximal contraction, and perceived fatigability as indicated by a depression score

(64,74). Despite comparable measures of performance fatigability (decline in MVC force), the

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explanatory variables were related to muscle strength for the control group and voluntary

activation for the MS group (65). Additional studies are needed with this clinical cohort to

examine such issues as the adjustments that limit voluntary activation during fatiguing

contractions, the adjustments that establish performance fatigability on a laboratory test and how

these relate to the trait level of fatigue, and the interaction between fatigue and limitations in

mobility.

Some insight on these issues can be gleaned from randomized controlled trials of strength

training performed by individuals with relapsing-remitting MS. Consistent with the finding that

some of the variance in the trait level of fatigue (Fatigue Severity Scale) can be explained by the

strength of a hand muscle (64), a 10-wk strength-training program that increased the strength and

endurance of leg muscles also reduced the trait level of fatigue (Modified Fatigue Impact Scale)

but did not improve walking endurance (2-min walk) (15). However, the decrease in fatigue was

significantly correlated (r = 0.40) with a test of leg-muscle endurance, but not with the gains in

muscle strength. Similar strength gains were achieved in another 12-wk intervention and these

were accompanied by correlated (r = 0.30) reductions between the trait level of fatigue (Fatigue

Severity Scale) and depression (Major Depression Inventory), but there was no association

between the improvement in fatigue and increases in either muscle strength (knee extensors) or

functional capacity (cumulative score based on chair-rise test, ascending-stairs test, 10-m walk

test, and the 6-min walk test) (13). These findings suggest that some of the fatigue reported by

individuals with relapsing-remitting MS can be explained by depression and the reduced capacity

to sustain high levels of voluntary activation, and that the functional consequences are likely

attributable to an inability to compensate for the disease-related decline in muscle activation.

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PERSPECTIVES

Although there is widespread acceptance of the notion that fatigue can limit human

performance, there are considerable gaps in knowledge on the underlying mechanisms and how

these can be managed. This dilemma is largely attributable to the inability of current

terminology to accommodate the scope of the conditions ascribed to fatigue and a paucity of

valid experimental models. In an attempt to provide a more unifying rubric, it is proposed that

fatigue be defined as a symptom in which physical and cognitive function is limited by

interactions between performance fatigability and perceived fatigability. As a symptom, fatigue

can only be measured by self-report, quantified as either a trait characteristic or a state variable.

The trait level of fatigue represents the average amount of fatigue experienced during the

preceding several days and depends on the absolute values of the modulating factors that

contribute to perceived fatigability. In contrast, the state level of fatigue reflects the rate of

change in the modulating factors that contribute to both performance and perceived fatigability

during ongoing activity.

The two measures of fatigability (performance and perceived) normalize the observed fatigue

to the demands associated with specific tasks. Less fatigable individuals, for example, are able

to tolerate a greater demand to reach a given level of fatigue. Performance fatigability depends

on the contractile capabilities of the involved muscles and the capacity of the nervous system to

provide an adequate activation signal for the prescribed task. Perceived fatigability is derived

from the sensations that regulate the integrity of the performer based on the maintenance of

homeostasis and the psychological state of the individual. Perceived fatigability can be

measured at rest or during physical activity, whereas performance fatigability is quantified as the

rate of change in a criterion outcome due the adjustments made during a fatiguing task.

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To determine how fatigue influences human performance, it is necessary to identify those

modulating factors most responsible for establishing the levels of fatigability that contribute to

the observed trait and state levels of fatigue. This can only be accomplished, however, after the

development of a reliable laboratory test that is strongly associated with a criterion measure of

human performance. Once an ecologically valid laboratory test has been determined,

mechanistic studies can probe the critical adjustments and thereby suggest countermeasures to

moderate the influence of the two attributes of fatigue on human performance. As indicated by

the three examples discussed in this paper, much remains to be learned about how knowledge on

fatigue can be translated into practice. The proposed framework provides a foundation to

address these gaps in knowledge.

ACKNOWLEDGEMENTS

This work was partially supported by an award from National Institute of Child Health & Human

Development (R03HD079508) to RME. The authors thank Andrew Jones, Benjamin Barry, Inge

Zijdewind, Katrina Maluf, and Robyn Capobianco for comments on a draft of the manuscript.

The authors declare no conflicts of interest. Understandably, ACSM is unable to endorse either

the data or its interpretation as presented in this article.

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Figures

1. The physiological processes that can contribute to fatigue are classically categorized into two

domains, those that establish the level of muscle activation (central) and those that influence

contractile function (peripheral). Reprinted from Enoka (16).

2. The proposed taxonomy suggests that fatigue be defined as a self-reported disabling symptom

derived from two interdependent attributes: perceived fatigability and performance

fatigability. Each of the attributes has two domains that themselves depend on the status of

4-7 modulating factors. The list of modulating factors is not intended to be all-inclusive, but

rather it provides an initial set that can be expanded as new experimental findings emerge.

By defining fatigue in terms of fatigability, the level of fatigue reported by an individual

depends on the rates of change in the two attributes and thereby normalizes it to the demands

of the task being performed. Adapted from Kluger et al. (37).

3. Normative data (mean ± SE) for the distance walked by individuals ranging in age from 3 yrs

to 85 yrs. There were approximately 200 participants in each age group: 3, 4, 5,…16, 17, 18-

29, 40-49, 50-59, 60, 69, 70-85 yrs. Note that the distance walked by the 5-yr group was

similar to that for the oldest group (70-85 yrs). Data from Kallen et al. (31).

4. Associations between observed and predicted 500-m walk times for old (A) and young (B)

adults. The relations were derived from a multiple-regression analysis of the data reported

by Justice et al. (30).

5. Average power (% initial) produced by 8 trained cyclists during a ~30-min time trial

performed on three occasions. On each occasion, the participant was tested after oral

ingestion of either placebo or one of two reuptake inhibitors (dopamine or noradrenaline).

The goal on each occasion was to perform the prescribed amount of work as quickly as

possible. *P < 0.05 relative to the initial level of power production. Data from Klass et al.

(36).

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Sep 2023
SPORTS MED

Ultra-endurance running (UER) poses extreme mental and physical challenges that present many barriers to completion, let alone performance. Despite these challenges, participation in UER events continues to increase. With the relative paucity of research into UER training and racing compared with traditional endurance running distance (e.g., marathon), it follows that there are sizable improvements still to be made in UER if the limitations of the sport are sufficiently understood. The purpose of this review is to summarise our current understanding of the major limitations in UER. We begin with an evolutionary perspective that provides the critical background for understanding how our capacities, abilities and limitations have come to be. Although we show that humans display evolutionary adaptations that may bestow an advantage for covering large distances on a daily basis, these often far exceed the levels of our ancestors, which exposes relative limitations. From that framework, we explore the physiological and psychological systems required for running UER events. In each system, the factors that limit performance are highlighted and some guidance for practitioners and future research are shared. Examined systems include thermoregulation, oxygen delivery and utilisation, running economy and biomechanics, fatigue, the digestive system, nutritional and psychological strategies. We show that minimising the cost of running, damage to lower limb tissue and muscle fatigability may become crucial in UER events. Maintaining a sustainable core body temperature is critical to performance, and an even pacing strategy, strategic heat acclimation and individually calculated hydration all contribute to sustained performance. Gastrointestinal issues affect almost every UER participant and can be due to a variety of factors. We present nutritional strategies for different event lengths and types, such as personalised and evidence-based approaches for varying types of carbohydrate, protein and fat intake in fluid or solid form, and how to avoid flavour fatigue. Psychology plays a vital role in UER performance, and we highlight the need to be able to cope with complex situations, and that specific long and short-term goal setting improves performance. Fatigue in UER is multi-factorial, both physical and mental, and the perceived effort or level of fatigue have a major impact on the ability to continue at a given pace. Understanding the complex interplay of these limitations will help prepare UER competitors for the different scenarios they are likely to face. Therefore, this review takes an interdisciplinary approach to synthesising and illuminating limitations in UER performance to assist practitioners and scientists in making informed decisions in practice and applicable research.

The Effect of a Mental Task Versus Unilateral Physical Fatigue on Non-Local Muscle Fatigue in Recreationally Active Young Adults

Article

Sep 2023
J SPORT SCI MED

Non-local muscle fatigue (NLMF) has been attributed to both physical and mental fatigue. The purpose of this study was to investigate the effects of mental exertion versus unilateral physical fatigue on NLMF. Sixteen recreationally active participants completed a physical task (2-sets of 100-s unilateral knee extension (KE) maximal voluntary isometric contractions (MVIC) with the dominant leg with 40-s recovery between sets, mental task (4-minute Stroop task), and control condition. Before and after each condition, blood lactate was collected, and contralateral 5-s KE, flexion (KF) and bilateral lateral trunk flexors MVIC (measure of trunk stability strength) was performed. Following the post-test 5-s MVICs, participants performed 12 non-dominant KE MVICs with a work-to-rest ratio of 5/10-s. Electromyography was monitored during the MVICs. Neither the 4-minute Stroop test or the unilateral KE physical fatigue intervention adversely affected the non-dominant KE forces or EMG activity with a single MVIC or 12 repetition MVICs. Although the non-dominant KF fatigue index forces and hamstrings EMG were not impaired by the interventions, there was a significant interaction (p = 0.001) small magnitude (d = 0.42) decrease in the non-dominant KF single MVIC force following the contralateral fatigue intervention, albeit with no significant change in hamstrings EMG. This MVIC deficit may be related to the significant decrease in dominant (p = 0.046, d = 2.6) and non-dominant external obliques (p = 0.048, d = 0.57) activation adversely affecting trunk stability. In conclusion, a 4-minute Stroop test or unilateral KE physical fatigue intervention did not impair non-dominant KE single or repeated 12 repetition MVIC forces or EMG activity. The small magnitude deficit in the non-dominant KF single MVIC force following the contralateral fatigue intervention are in accord with the heterogenous findings common in the literature.

The Effect of a Mental Task Versus Unilateral Physical Fatigue on Non-Local Muscle Fatigue in Recreationally Active Young Adults

Article

Full-text available

Sep 2023
J SPORT SCI MED

Performance Fatigability and Neuromuscular Responses Are Not Joint Angle Specific Following a Sustained Isometric Forearm Flexion Task Anchored to a High Perceptual Intensity in Women

Article

Sep 2023

Objectives: To examine the effects of joint angle (JA) on maximal voluntary isometric contractions (MVIC) and neuromuscular responses following a sustained, isometric forearm flexion task anchored to a rating of perceived exertion (RPE) of 8 (RPE=8). Methods: Nine women (age: 20.7±2.9 yrs; height: 168.8±7.2 cm; body mass: 66.3±6.8 kg) performed 2,3s forearm flexion MVICs at JAs of 75°, 100°, and 125° prior to and following a sustained, isometric forearm flexion task anchored to RPE=8 to task failure (torque reduced to zero) at JA100. Electromyographic (EMG) and mechanomyographic (MMG) signals were recorded from the biceps brachii. Results: The MVIC at JA100 (collapsed across Time) was significantly greater (p<0.05) than JA75 and JA125. The pre-test MVIC was significantly greater (p<0.001) than the post-test. For EMG amplitude (AMP) and EMG mean power frequency (MPF), pre-test values were significantly greater (p<0.05) than the post-test values, with no differences between JAs. For MMG AMP and MMG MPF, there were no significant (p>0.05) differences between Time or JAs. Pre-test neuromuscular efficiency (normalized MVIC/normalized EMG AMP) was significantly greater (p=0.005) than post-test. Conclusion: Following a sustained, isometric forearm flexion task anchored to RPE=8 at JA100, the fatigue-induced MVIC and neuromuscular responses were not affected by JA.

Predictors of Physical Activity Level, Self-Reported Physical and Mental Fatigue in Sports Science Students

Article

Full-text available

Sep 2023

El objetivo de este estudio fue revelar la posibilidad de que los niveles de actividad física de los estudiantes universitarios predigan los niveles de fatiga autoinformados. Participaron voluntariamente en la investigación un total de 331 estudiantes (mujeres: 130, hombres: 201) de la Facultad de Ciencias del Deporte de la Universidad Karamanoğlu Mehmetbey ; y se utilizaron el Cuestionario de información personal, el Cuestionario Internacional de Actividad Física y la Escala de Fatiga de Chalder. Los datos recogidos en el estudio se analizaron en el programa estadístico Jamovi (2.0.0) con intervalo de confianza del 95% y nivel de significancia de 0,05. Para analizar los datos se utilizó la prueba t de muestra independiente, la prueba de correlación de Pearson, la prueba de regresión multinomial y el diagrama de dispersión. Las mujeres tuvieron puntuaciones de fatiga total y fatiga física significativamente más altas que los hombres, mientras que los hombres tuvieron puntuaciones totales de MET significativamente más altas que las mujeres (p<0,05). A medida que aumentaron las puntuaciones totales de MET de los participantes, su fatiga total y fatiga mental disminuyeron significativamente (p<0,05). La investigación demostró que el aumento de los niveles de actividad física de los estudiantes de ciencias del deporte puede disminuir los puntajes de fatiga física y mental autoinformados.

Ischemic Preconditioning, But Not Priming Exercise, Improves Exercise Performance in Trained Rock Climbers

Article

Aug 2023
J STRENGTH COND RES

MacDougall, KB, McClean, ZJ, MacIntosh, BR, Fletcher, JR, and Aboodarda, SJ. Ischemic preconditioning, but not priming exercise, improves exercise performance in trained rock climbers. J Strength Cond Res XX(X): 000-000, 2023-To assess the effects of ischemic preconditioning (IPC) and priming exercise on exercise tolerance and performance fatigability in a rock climbing-specific task, 12 rock climbers completed familiarization and baseline tests, and constant-load hangboarding tests (including 7 seconds on and 3 seconds off at an intensity estimated to be sustained for approximately 5 minutes) under 3 conditions: (a) standardized warm-up (CON), (b) IPC, or (c) a priming warm-up (PRIME). Neuromuscular responses were assessed using the interpolated twitch technique, including maximum isometric voluntary contraction (MVC) of the finger flexors and median nerve stimulation, at baseline and after the performance trial. Muscle oxygenation was measured continuously using near-infrared spectroscopy (NIRS) across exercise. Time to task failure (Tlim) for IPC (316.4 ± 83.1 seconds) was significantly greater than CON (263.6 ± 69.2 seconds) (p = 0.028), whereas there was no difference between CON and PRIME (258.9 ± 101.8 seconds). At task failure, there were no differences in MVC, single twitch force, or voluntary activation across conditions; however, recovery of MVC and single twitch force after the performance trial was delayed for IPC and PRIME compared with CON (p < 0.05). Despite differences in Tlim, there were no differences in any of the NIRS variables assessed. Overall, despite exercise tolerance being improved by an average of 20.0% after IPC, there were no differences in neuromuscular responses at task failure, which is in line with the notion of a critical threshold of peripheral fatigue. These results indicate that IPC may be a promising precompetition strategy for rock climbers, although further research is warranted to elucidate its mechanism of action.

Unraveling the interplay between daily life fatigue and physical activity after subarachnoid hemorrhage: an ecological momentary assessment and accelerometry study

Article

Full-text available

Sep 2023
J NEUROENG REHABIL

Background Fatigue is one of the most commonly reported symptoms after subarachnoid hemorrhage (SAH) and is indirectly associated with physical activity (PA). Associations between fatigue and PA are primarily examined based on conventional measures (i.e. a single fatigue score or average PA levels), thereby assuming that fatigue and PA do not fluctuate over time. However, levels of fatigue and PA may not be stable and may interrelate dynamically in daily life. Insight in direct relationships between fatigue and PA in daily life, could add to the development of personalized rehabilitation strategies. Therefore we aimed to examine bidirectional relationships between momentary fatigue and PA in people with SAH. Methods People (n = 38) with SAH who suffer from chronic fatigue were included in an observational study using Ecological Momentary Assessment (EMA) and accelerometry. Momentary fatigue was assessed on a scale from 1 to 7 (no to extreme fatigue), assessed with 10–11 prompts per day for 7 consecutive days using EMA with a mobile phone. PA was continuously measured during this 7-day period with a thigh-worn Activ8 accelerometer and expressed as total minutes of standing, walking, running and cycling in a period of 45 min before and after a momentary fatigue prompt. Multilevel mixed model analyses including random effects were conducted. Results Mean age was 53.2 years (SD = 13.4), 58% female, and mean time post SAH onset was 9.5 months (SD = 2.1). Multilevel analyses with only time effects to predict fatigue and PA revealed that fatigue significantly (p < 0.001) increased over the day and PA significantly (p < 0.001) decreased. In addition, more PA was significantly associated with higher subsequent fatigue (β = 0.004, p < 0.05) and higher fatigue was significantly associated with less subsequent PA (β=-0.736, p < 0.05). Moreover, these associations significantly differed between participants (p < 0.001). Conclusions By combining EMA measures of fatigue with accelerometer-based PA we found that fatigue and PA are bidirectionally associated. In addition, these associations differ among participants. Given these different bidirectional associations, rehabilitation aimed at reducing fatigue should comprise personalized strategies to improve both fatigue and PA simultaneously, for example by combining exercise therapy with cognitive behavioral and/or energy management therapy.

Effectiveness of physical activity interventions on reducing perceived fatigue among adults with chronic conditions: a systematic review and meta-analysis of randomised controlled trials

Article

Full-text available

Sep 2023

Fatigue is barrier of physical activity participation in adults with chronic conditions. However, physical activity alleviates fatigue symptoms. This systematic review and meta-analysis aimed to (1) synthesise evidence from randomised controlled trials (RCTs) exploring the effects of physical activity interventions on fatigue reduction and (2) evaluate their effectiveness. Medline/CINAHL/EMBASE/Web of Science and Scopus were searched up to June 24th, 2023. Two reviewers independently conducted study screening and selection (RCTs), extracted data and assessed risk of bias (RoB2). Outcome was the standardised mean difference (SMD) with 95% confidence intervals in fatigue between experimental and control groups. 38 articles met the inclusion criteria. Overall, physical activity interventions moderately reduced fatigue (SMD = 0.54, p < 0.0001). Interventions lasting 2–6 weeks demonstrated a larger effect on fatigue reduction (SMD = 0.86, p < 0.00001). Interventions with 18–24 sessions showed a large effect on fatigue reduction (SMD = 0.97, p < 0.00001). Aerobic cycling and combination training interventions had a large to moderate effect (SMD = 0.66, p = 0.0005; SMD = 0.60, p = 0.0010, respectively). No long-term effects were found during follow-up. Physical activity interventions moderately reduced fatigue among adults with chronic conditions. Duration, total sessions, and mode of physical activity were identified as key factors in intervention effectiveness. Further research is needed to explore the impact of physical activity interventions on fatigue.

Endurance Training

Chapter

Aug 2023

Carson Patterson

This chapter begins by providing an explanation of how each energy system within the human body operates and their relevance to alpine ski racing. Secondly, it debunks several myths about “traditional” endurance training for alpine ski racing and provides more effective tactics for meeting those demands while remaining complimentary to the vast array of training demands one requires at a high level of the sport. Several unique research studies are discussed at length in addition to numerous anecdotal experiences from a high-level coaching practitioner himself. Lastly, several factors that affect one's ability to reach a high level of alpine ski racing-specific endurance are discussed at length including but not limited to: genetics, altitude, gender, and training experience.

Kadın yüzücülerde akut yorgunluğun denge performansına etkisinin incelenmesi

Article

Full-text available

Sep 2023

z Bu çalışma kadın sporcularda maksimal tüketici bir egzersiz sonrası oluşan akut yorgunluğun denge performansı üzerine etkisinin incelenmesi amacıyla yapılmıştır. Çalışma grubunuz Bingöl Üniversitesi Spor Bilimleri Fakültesinin Antrenörlük eğitimi bölümündeki 4. sınıf yüzme uzmanlık kadın (n=12) öğrencilerinden oluşturulmuştur. Araştırmada zayıf deneysel araştırma modeli (ön test-son test) tercih edilmiştir. Yüzücülerin denge performanslarını belirlemek için Stabilometrik platform, akut yorgunluk oluşturmak içinse koşu bandı üzerinde Bruce protokolü uygulanmıştır. Yüzücülerden alınan ön test ile son test verilerini değerlendirilmesi SPSS 23 paket programı kullanılarak yapılmıştır. Elde edilen bulgulara göre yüzücülerin hem gözleri açık hem de gözleri kapalı ön test ve son test statik denge sağa-sola standart sapma, öne-arkaya standart sapma, sağa-sola salınım hızı, öne arkaya salınım hızı, basınç merkezi çizim analizi ve salınım alanı değerleri karşılaştırıldığında anlamlı farklılık olmadığı görülmüştür (p>0,05). Sonuç olarak, kadın yüzücülere uygulanan tüketici bir egzersiz protokolü sonrası oluşan akut kas yorgunluğunun vücut denge performans değerleri üzerinde istatistiksel olarak anlamlı farklılıklar görülmese de denge performansını üzerinde olumsuz etkisinin olduğu görülmüştür. Anahtar Kelimeler: Akut, denge, performans, yorgunluk Investigation of the effect of acute facilitation on balance performance in women swimmers Abstract This study was carried out to examine the effect of acute fatigue on balance performance in female athletes after a maximally exhausting exercise. Your study group consisted of 4th grade swimming specialization female (N=12) students in the Coaching Education Department of the Faculty of Sports Sciences of Bingöl University. A weak experimental research model (pretest-posttest) was preferred in the study. Stabilometric platform was used to determine the balance performance of swimmers, and Bruce protocol was applied on the treadmill to create acute fatigue. The evaluation of the pre-test and post-test data taken from the swimmers was made using the SPSS 23 package program. According to the findings, both eyes open and eyes closed pretest and posttest static balance right-left standard deviation, forward-backward standard deviation, right-left swing speed, forward-backward swing speed, pressure center drawing analysis and swing area values When compared, it was seen that there was no significant difference (p>0.05). As a result, although no statistically significant differences were observed on body balance performance values of acute muscle fatigue after a exhausting exercise protocol applied to female swimmers, it was observed that it had a negative effect on balance performance.

Aerobic Training Increases Pain Tolerance in Healthy Individuals

Article

Full-text available

Feb 2014
MED SCI SPORT EXER

The hypoalgesic effects of acute exercise are well documented. However, the effect of chronic exercise training on pain sensitivity is largely unknown. To examine the effect of aerobic exercise training on pain sensitivity in healthy individuals. Pressure pain threshold, ischemic pain tolerance and pain ratings during ischemia were assessed in 24 participants before and after 6 weeks of structured aerobic exercise training (n=12) or following 6 weeks of usual physical activity (n=12). The exercise training regimen consisted of cycling 3 times per week for 30 min at 75% of maximal oxygen consumption reserve. Significant increases in aerobic fitness (p = 0.004) and ischemic pain tolerance (p = 0.036) were seen in the exercise group following training, while pressure pain threshold and pain ratings during ischemia were unchanged (p > 0.2). No change in aerobic fitness (p > 0.1) or pain sensitivity (p > 0.1) was observed in the control group. Moderate-vigorous intensity aerobic exercise training increases ischemic pain tolerance in healthy individuals.

Critical Power: Implications for the Determination of VO2max and Exercise Tolerance.

Article

Full-text available

Oct 2010

For high-intensity muscular exercise, the time-to-exhaustion (t) increases as a predictable and hyperbolic function of decreasing power (P) or velocity (V). This relationship is highly conserved across diverse species and different modes of exercise and is well described by two parameters: the 'critical power' (CP or CV), which is the asymptote for power or velocity, and the curvature constant (W') of the relationship such that t = W'/(P-CP). CP represents the highest rate of energy transduction (oxidative ATP production, V? O2) that can be sustained without continuously drawing on the energy store W' (composed in part of anaerobic energy sources and expressed in kilojoules). The limit of tolerance (time t) occurs when W' is depleted. The CP concept constitutes a practical framework in which to explore mechanisms of fatigue and help resolve crucial questions regarding the plasticity of exercise performance and muscular systems physiology. This brief review presents the practical and theoretical foundations for the CP concept, explores rigorous alternative mathematical approaches, and highlights exciting new evidence regarding its mechanistic bases and its broad applicability to human athletic performance.

What limits performance during whole body incremental exercise to exhaustion in humans?

Article

Full-text available

Aug 2015
J PHYSIOL-LONDON

To determine the mechanisms causing task failure during incremental exercise to exhaustion (IE) sprint performance (10 s all-out isokinetic) and muscle metabolites were measured before (control) and immediately after IE in normoxia (Nx, PI O2 :143 mmHg) and hypoxia (Hyp, PI O2 :73 mmHg) in 22 men (22 ± 3 years). After IE, subjects recovered for either 10 or 60 s, with open circulation or bilateral leg occlusion (OC, 300 mmHg) in random order. This was followed by a 10 s sprint with open circulation. Post-IE peak power output (Wpeak) was higher than IE-Wmax (P < 0.05). After 10 and 60 s recovery in Nx, Wpeak was reduced by 38 ± 9 and 22 ± 10% without OC, and 61 ± 8 and 47 ± 10% with OC (P < 0.05). Following 10 s OC, Wpeak was 20% higher in Hyp than Nx (P < 0.05), despite similar muscle lactate accumulation ([La]) and phosphocreatine and ATP reduction. Sprint performance and anaerobic ATP resynthesis were greater after 60 than 10 s occlusions, despite the higher [La] and [H(+) ] after 60 s than 10 s OC recovery (P < 0.05). The mean rate of ATP turnover during the 60 s OC was 0.180 ± 0.133 mmol.kg wet.(-1) .s(-1) , i.e. equivalent to 32% of leg VO2 peak (ion pumps expended energy). A greater degree of recovery is achieved, however, without occlusion.In conclusion, during incremental exercise task failure is not due to metabolite accumulation or lack of energy resources. Anaerobic metabolism, despite the accumulation of lactate and H(+) , facilitates early recovery even in anoxia. This points to central mechanisms as the principal determinants of task failure both in normoxia and hypoxia, with lower peripheral contribution in hypoxia. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.

Capturing the post-exertional exacerbation of fatigue following physical and cognitive challenge in patients with chronic fatigue syndrome

Article

Full-text available

May 2015
PHYSIOTHERAPY

Objective: To design and validate an instrument to capture the characteristic post-exertional exacerbation of fatigue in patients with chronic fatigue syndrome (CFS). Methods: Firstly, patients with CFS (N=19) participated in five focus group discussions to jointly explore the nature of fatigue and dynamic changes after activity, and inform development of a self-report instrument - the Fatigue and Energy Scale (FES). The psychometric properties of the FES were then examined in two case-control challenge studies: a physically-demanding challenge (moderate-intensity aerobic exercise; N=10 patients), and a cognitively-demanding challenge (simulated driving; N=11 patients). Finally, ecological validity was evaluated by recording in association with tasks of daily living (N=9). Results: Common descriptors for fatigue included 'exhaustion', 'tiredness', 'drained of energy', 'heaviness in the limbs', and 'foggy in the head'. Based on the qualitative data, fatigue was conceptualised as consisting of 'physical' and 'cognitive' dimensions. Analysis of the psychometric properties of the FES showed good sensitivity to the changing symptoms during a post-exertional exacerbation of fatigue following both physical exercise and driving simulation challenges, as well as tasks of daily living. Conclusion: The 'fatigue' experienced by patients with CFS covers both physical and cognitive components. The FES captured the phenomenon of a post-exertional exacerbation of fatigue commonly reported by patients with CFS. The characteristics of the symptom response to physical and cognitive challenges were similar. Both the FES and the challenge paradigms offer key tools to reliably investigate biological correlates of the dynamic changes in fatigue.

Disrupting the Supplementary Motor Area Makes Physical Effort Appear Less Effortful

Article

Full-text available

Jun 2015
J NEUROSCI

The perception of physical effort is relatively unaffected by the suppression of sensory afferences, indicating that this function relies mostly on the processing of the central motor command. Neural signals in the supplementary motor area (SMA) correlate with the intensity of effort, suggesting that the motor signal involved in effort perception could originate from this area, but experimental evidence supporting this view is still lacking. Here, we tested this hypothesis by disrupting neural activity in SMA, in primary motor cortex (M1), or in a control site by means of continuous theta-burst transcranial magnetic stimulation, while measuring effort perception during grip forces of different intensities. After each grip force exertion, participants had the opportunity to either accept or refuse to replicate the same effort for varying amounts of reward. In addition to the subjective rating of perceived exertion, effort perception was estimated on the basis of the acceptance rate, the effort replication accuracy, the influence of the effort exerted in trial t on trial t+1, and pupil dilation. We found that disruption of SMA activity, but not of M1, led to a consistent decrease in effort perception, whatever the measure used to assess it. Accordingly, we modeled effort perception in a structural equation model and found that only SMA disruption led to a significant alteration of effort perception. These findings indicate that effort perception relies on the processing of a signal originating from motor-related neural circuits upstream of M1 and that SMA is a key node of this network. Copyright © 2015 the authors 0270-6474/15/358737-08$15.00/0.

Self-pacing increases critical power and improves performance during severe-intensity exercise

Article

Full-text available

Mar 2015

The parameters of the power-duration relationship for severe-intensity exercise (i.e., the critical power (CP) and the curvature constant (W')) are related to the kinetics of pulmonary O2 uptake, which may be altered by pacing strategy. We tested the hypothesis that the CP would be higher when derived from a series of self-paced time-trials (TT) than when derived from the conventional series of constant work-rate (CWR) exercise tests. Ten male subjects (age, 21.5 ± 1.9 years; mass, 75.2 ± 11.5 kg) completed 3-4 CWR and 3-4 TT prediction trial protocols on a cycle ergometer for the determination of the CP and W'. The CP derived from the TT protocol (265 ± 44 W) was greater (P < 0.05) than the CP derived from the CWR protocol (250 ± 47 W), while the W' was not different between protocols (TT: 18.1 ± 5.7 kJ, CWR: 20.6 ± 7.4 kJ, P > 0.05). The mean response time of pulmonary O2 uptake was shorter during the TTs than the CWR trials (TT: 34 ± 16, CWR: 39 ± 19 s, P < 0.05). The CP was correlated with the total O2 consumed in the first 60 s across both protocols (r = 0.88, P < 0.05, n = 20). These results suggest that in comparison with the conventional CWR exercise protocol, a self-selected pacing strategy enhances CP and improves severe-intensity exercise performance. The greater CP during TT compared with CWR exercise has important implications for performance prediction, suggesting that TT completion times may be overestimated by CP and W' parameters derived from CWR protocols.

Psychological Determinants of Whole-Body Endurance Performance

Article

Full-text available

Mar 2015

No literature reviews have systematically identified and evaluated research on the psychological determinants of endurance performance, and sport psychology performance enhancement guidelines for endurance sports are not founded on a systematic appraisal of endurance-specific research. A systematic literature review was conducted to identify practical psychological interventions that improve endurance performance and to identify additional psychological factors that affect endurance performance. Additional objectives were to evaluate the research practices of the included studies, to suggest theoretical and applied implications, and to guide future research. Electronic databases, forward-citation searches and manual searches of reference lists were used to locate relevant studies. Peer-reviewed studies were included when they chose an experimental or quasi-experimental research design; a psychological manipulation; endurance performance as the dependent variable; and athletes or physically active, healthy adults as participants. Consistent support was found for using imagery, self-talk and goal setting to improve endurance performance, but it is unclear whether learning multiple psychological skills is more beneficial than learning one psychological skill. The results also demonstrated that mental fatigue undermines endurance performance, and verbal encouragement and head-to-head competition can have a beneficial effect. Interventions that influenced perception of effort consistently affected endurance performance. Psychological skills training could benefit an endurance athlete. Researchers are encouraged to compare different practical psychological interventions, to examine the effects of these interventions for athletes in competition and to include a placebo control condition or an alternative control treatment. Researchers are also encouraged to explore additional psychological factors that could have a negative effect on endurance performance. Future research should include psychological mediating variables and moderating variables. Implications for theoretical explanations for endurance performance and evidence-based practice are described.

Role of Ratings of Perceived Exertion during Self-Paced Exercise: What are We Actually Measuring?

Article

Jun 2015

Ratings of perceived exertion (RPE) and effort are considered extremely important in the regulation of intensity during self-paced physical activity. While effort and exertion are slightly different constructs, these terms are often used interchangeably within the literature. The development of perceptions of both effort and exertion is a complicated process involving numerous neural processes occurring in various regions within the brain. It is widely accepted that perceptions of effort are highly dependent on efferent copies of central drive which are sent from motor to sensory regions of the brain. Additionally, it has been suggested that perceptions of effort and exertion are integrated based on the balance between corollary discharge and actual afferent feedback; however, the involvement of peripheral afferent sensory feedback in the development of such perceptions has been debated. As such, this review examines the possible difference between effort and exertion, and the implications of such differences in understanding the role of such perceptions in the regulation of pace during exercise.

Skeletal Muscle Fatigue and Decreased Efficiency: Two Sides of the Same Coin?

Article

Feb 2015

During high-intensity submaximal exercise muscle fatigue and decreased efficiency are closely intertwined, and each contributes to exercise intolerance. Fatigue and muscle inefficiency share common mechanisms, e.g. decreased "metabolic stability", muscle metabolite accumulation, decreased free energy of ATP breakdown, limited O2 or substrate availability, increased glycolysis, pH disturbance, increased muscle temperature, ROS production, and altered motor unit recruitment patterns.SUMMARYDuring high-intensity submaximal exercise muscle fatigue and a decreased efficiency of contractions are strictly intertwined, and share several common mechanisms.

Fatigue in older populations

Article

Apr 2013

Kirsten Avlund

The aim of this paper is to give an overview of research on general fatigue, mobility-related fatigue, and fatigability in older adults, with a focus on fatigue as an early indicator of the aging process. Fatigue is a strong predictor of functional limitations, disability, mortality, and other adverse outcomes in young-old and old-old populations, in men and women, and in different geographic localities. Several biological, physiological, and social explanations are proposed: fatigue may be seen not only as a self-reported indicator of frailty, defined as a physiologic state of increased vulnerability to stressors which results from decreased physiologic reserves and even dysregulation of multiple physiologic systems, but this state may be accelerated by the cumulative impact of social, mental, and biological factors throughout life. Fatigue in older adults is the result of multiple potentially modifiable factors, of which some may be fully treated or at least alleviated, thus slowing down the speed of the aging process.

Translating Fatigue to Human Performance

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