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This is the author’s version of a work that was submitted/accepted for publication in the following source:
Knicker, Axel J., Renshaw, Ian, Oldham, Anthony R.H., & Cairns, Simeon
P. (2011) Interactive processes Link the multiple symptoms of fatigue in
sport competition. Sports Medicine, 41(4), pp. 307-328.
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INTERACTIVE PROCESSES LINK THE MULTIPLE SYMPTOMS OF
FATIGUE IN SPORT COMPETITIONAxel J. Knicker1, Ian Renshaw2, Anthony R.H. Oldham3 and Simeon P. Cairns3 German Sports University, Institute of Motor Control and Movement Technique, Cologne, Germany.
School of Exercise and Sport Science, Faculty of Health Science, Queensland University of Technology, Brisbane, Australia.
School of Sport and Recreation, Faculty of Health and Environmental Sciences, AUT University, Auckland, New Zealand.
Running title: Fatigue symptoms in sport competition Text Word Count: 5997 words Corresponding Author: Simeon Cairns, PhD School of Sport and Recreation Faculty of Health and Environmental Sciences, AUT University Private Bag 90210, Auckland 1020, New Zealand e-mail: firstname.lastname@example.org Phone: 64 9 9179999 Ext 7125, Fax: 64 9 9179960
ACKNOWLEDGMENTSWe gratefully thank Gaye Bryham and Dr. Denis Loiselle for comments on the manuscript. No sources of funding were used in order to prepare this review. The authors have no conflict of interest that relates to the content of this review.
2. Quantifying the manifestations of fatigue
2.1 Muscle performance
2.2 Exercise performance 2.2.1 Physical abilities 2.2.2 Technical abilities 2.2.3 Subjective fatigue
2.3 Competition performance 2.3.1 Decision-making 2.3.2 Psychological aspects
3. Sport-specific symptoms/measures of fatigue
3.1 Racing sports
3.2 Team-game sports
3.3 Racquet sports
3.4 Contribution of symptoms
4. Are the symptoms of fatigue linked by common mechanisms?
4.1 Model to explain fatigue symptoms
4.2 Protective/compensatory mechanisms
5. Conclusion and recommendations
ABSTRACTMuscle physiologists often describe fatigue simply as a decline of muscle force and infer this causes an athlete to slow-down. In contrast, exercise scientists describe fatigue during sport competition more holistically as an exercise-induced impairment of performance. The aim of this review is to reconcile the different views by evaluating the many performance symptoms/measures and mechanisms of fatigue. We describe how fatigue is assessed with muscle, exercise or competition performance measures. Muscle performance (single muscle test measures) declines due to peripheral fatigue (reduced muscle cell force) and/or central fatigue (reduced motor drive from the central nervous system (CNS)). Peak muscle force seldom falls by 30% during sport but is often exacerbated during electrical stimulation and laboratory exercise tasks. Exercise performance (whole-body exercise test measures) reveals impaired physical/technical abilities and subjective fatigue sensations. Exercise intensity is initially sustained by recruitment of new motor units and help from synergistic muscles before it declines.
Technique/motor skill execution deviates as exercise proceeds to maintain outcomes before they deteriorate, e.g. reduced accuracy or velocity. The sensation of fatigue incorporates an elevated rating of perceived exertion (RPE) during submaximal tasks, due to a combination of peripheral and higher CNS inputs. Competition performance (sport symptoms) is affected more by decisionmaking and psychological aspects since there are opponents and a greater importance on the result. Laboratory based decision-making is generally faster or unimpaired. Motivation, selfefficacy, and anxiety can change during exercise to modify RPE and hence alter physical performance.
Symptoms of fatigue during racing, team-game or racquet sports are largely anecdotal, but sometimes assessed with time-motion analysis. Fatigue during brief all-out racing is described biomechanically as a decline of peak velocity, along with altered kinematic components. Longer sport-events involve pacing strategies, central and peripheral fatigue contributions, and elevated RPE. During match-play, the work rate can decline late in a match (or tournament) and/or transiently after intense exercise bursts. Repeated sprint ability, agility and leg strength become slightly impaired. Technique outcomes such as velocity and accuracy for throwing, passing, hitting and kicking can deteriorate. Physical and subjective changes are both less severe in realthan simulated-sport activities. Little objective evidence exists to support exercise-induced mental lapses during sport.
A model depicting mind-body interactions during sport competition shows that the RPE center-motor cortex-working muscle sequence drives overall performance levels and hence fatigue symptoms. The sporting outputs from this sequence can be modulated by interactions with muscle afferent and circulatory feedback, psychological and decision-making inputs. Importantly, compensatory processes exist at many levels to protect against performance decrements. Small changes of putative fatigue factors can also be protective. We show that individual fatigue factors including diminished carbohydrate availability, elevated serotonin, hypoxia, acidosis, hyperkalaemia, hyperthermia, dehydration, and reactive oxygen species, each contribute to several fatigue symptoms. Thus, multiple symptoms of fatigue can occur simultaneously and the underlying mechanisms overlap and interact. Based on this understanding we reinforce the proposal that fatigue is best described globally as an exercise-induced decline of performance as this is inclusive of all viewpoints.
Sport performance depends on the ability of an athlete to produce and then sustain high levels of physical, technical, decision-making and psychological skills throughout competition.
Deterioration of any of these skills could appear as a symptom of fatigue, yet the manner in which fatigue is best described and measured is controversial.[1-6] The phenomenon of fatigue is complex with the underlying processes developing as exercise proceeds to ultimately manifest as a decline of performance. By incorporating a holistic approach, fatigue can be described as an exerciseinduced impairment of performance during sport-events. But what exactly do we mean by impairment of performance? Figure1 shows that performance can be assessed at three different levels. At the simplest level there is a reduced force/power output by a single muscle cell or motor unit. Simultaneous detrimental effects in several motor units could impair function of a single whole-muscle, i.e. reduced muscle performance. A common assumption is that reduced muscle performance translates into reduced exercise performance. Test measures of the latter incorporate the force/power generated by several muscle groups, motor skill outcomes and fatigue sensations.
A diminished exercise performance usually causes a reduced competition performance during sport-events, which is assessed solely by performance symptoms. The inclusion of decisionmaking against competitors and greater psychological aspects feature in many sport-events.
Finally, the match result usually depends on the better overall competition performance on the day (Fig.1). However the result should not be used to assess competition performance, since the scoring system can have a role. Indeed, matches can be won through gaining the critical points in racket sports despite losing more points overall, or lost through a failure to convert periods of dominance into points or goals scored.
Limitations to understanding fatigue may have arisen in part from the belief that manifestations of fatigue obtained using a reductionist approach (e.g. stimulation of isolated muscles) or laboratory exercise models, relate directly to what happens in sport competition.[3,4] To enhance understanding beyond the in vitro and laboratory based approaches several recent reviews describe what happens during specific sports,[7-12] provide generalized fatigue mechanisms,[13-18] explore the integrative physiology of whole-body fatigue or focus on mindbody interactions during voluntary exercise.[20-24] However, the symptoms and mechanisms of fatigue during sport competition still need greater understanding. Indeed, muscle physiologists may be unaware of how altered muscle function impacts sport performance, and sport scientists may be unclear about which neuromuscular processes underpin fatigue symptoms during sport-events. Hence, the purpose of this review is to take a holistic and interdisciplinary approach to: 1) describe in general terms how fatigue is assessed at muscle, exercise or competition performance levels; 2) describe specifically how fatigue is manifested during sport-events; and 3) consider whether neuromuscular, motor skill and subjective symptoms of fatigue are linked through common mechanisms/processes. Literature was sourced through databases (PubMed, Web of Science), and from reference lists in related original research and review articles.
2. QUANTIFYING THE MANIFESTATIONS OF FATIGUE
Fatigue can be quantified using performance symptoms and/or test measures (Table1).
Performance symptoms are impairments of movement abilities/outcomes as they appear during sport-events. Symptoms are often obtained anecdotally from players or coaches as it is difficult to get such data without interfering with the competition. One objective approach used involves time-motion video analysis[25-31] which can reveal changes in work rate, technique, phases of play or the occurrence of errors. However, the uniqueness of each competition due to variations in the quality of opponents, match strategies, behaviors, environmental conditions, and terrain complicates interpretation of these data.[11,29] It is also unknown whether symptoms late in a match result from the physical exercise or other aspects such as anxiety due to mounting pressure.[32-34] Moreover standardized test measures are sometimes obtained before and after competitions (Table1).[8-11,27-31] These measures are commonly used to explore fatigue mechanisms but they also describe the components which determine overall performance.
Interestingly, several test measures are necessary in order to describe each performance symptom (Table2).
Sport activities can be assessed in the laboratory or field settings with test measures obtained. Simulated-sport activities involve replicating an entire match,[35-39] or component of a match.[40-43] For example, hitting skills can be studied using a ball projection machine,[34,41,44-46] although this leads to modified motor skills because visual cues are absent, with normal anticipation being restricted.[45,46] Laboratory exercise on treadmill, cycle or rowing ergometers allows work intensity and conditions to be controlled, with power output measured precisely.[47-51] Exercise components such as the force applied to pedals/oars, or pedal rate can also be evaluated.[48-51] However, some laboratory tasks do not adequately replicate sport-events. In particular the time-to-exhaustion tests[52-57] differ to racing sports where pacing strategies are employed.[56,57] Furthermore, test measures from exhausted individuals seldom mimic a performance symptom. For example, maximum isometric voluntary contractions (MVC) do not usually occur during sport.[4,6] To investigate mechanisms, stimulation-induced models of fatigue are often used since this permits analysis of muscle test measures independently of a variable motor drive, the muscle environment is controlled, and invasive interventions/measurements can be made.[13,17,58-62] But just how closely the stimulation regimes mimic the motor activation patterns which occur during sport is questionable.[4,17]
2.1 Muscle performance: Muscle performance test measures can be obtained after repeated activation of a single muscle, exercise tasks or sport-events (Table1). When processes originate in muscle cells and directly impair muscle contractile function the phenomenon is called peripheral fatigue.[1,2,15,17] This usually involves diminished peak force measures, but when combined with a slowed shortening velocity, can manifest as a reduced muscle power (power = force x velocity).