Technical Issues in Quantifying Low-Frequency ...

Technical Report International Journal of Sports Physiology and Performance, 2006;2:__–__ © Human Kinetics, Inc.

Technical Issues in Quantifying Low-Frequency Fatigue in Athletes Jonathon R. Fowles A recent review by Cairns and colleagues published in Exercise and Sport Sciences Reviews (2005:33[1]:9-16)1 described experimental models used to study neuromuscular fatigue and explained the inherent strengths and weaknesses of applied versus reductionist approaches. This technical report addresses some of the recommendations made in that review, from the perspective of the applied sport scientist or practitioner in evaluating fatigue in elite athletes. The goal here is to highlight the inherent difficulties in assessing fatigue in the applied sport setting and to provide practitioners with future directions for fatigue research. A particular type of fatigue, called low-frequency fatigue (LFF), is of particular interest to the applied sport scientist or practitioner and could be the focus of future work. This report identifies some of technical challenges faced in developing a practical test of LFF for the use in the field setting. The outcome of further work in this area will lead to a better understanding of athlete monitoring, training and performance. Key Words: electrical stimulation, force, fitness testing, elite athlete, performance

It is well understood that elite athletes are required to train aggressively to perform at a high level. These athletes are in a cyclic state of training-fatigue adaptation and this cycle is often imperfect (ie, athletes often train fatigued). Monitoring fatigue is important because effective monitoring identifies the athletes’ recovery abilities and helps determine appropriate training loads to maximize performance. Recommendations from a recent review1 indicated that future work in fatigue should emphasize (1) quantifying fatigue in competitive sport, (2) evaluating the role of perceived exertion in limiting performance, (3) assessing power output rather than force, (4) moderate rather than severe force fatigue, (5) protocols with dynamic (and stretch-shortening cycle) contractions rather than static contractions, and (6) protocols with submaximal rather than maximal contractions. These recommendations suggest that additional research in the applied sport setting is required to further our understanding of fatigue. Low-frequency fatigue (LFF) is a multifactorial fatigue resulting from highintensity, moderate-to high-force, repetitive eccentric or stretch-shortening cycle (SSC) activities.2-5 Low-frequency fatigue is likely prevalent in competitive elite sport, although quantifying clinical LFF in many sports is difficult because of the technical challenges associated with assessing LFF in a field setting. Low-frequency fatigue is particularly insidious because of its long-lasting effects on low-frequency force-generating capacity resulting primarily from impaired calcium release in 97

98   Fowles

peripheral muscle.2 Submaximal isometric force is moderately affected (10% to 40%),2-5 although a reduced rate of force development in maximal exertions5 and reduced power output in sustained dynamic performance are more likely results, albeit not adequately reported at present. Many sporting activities require repeated submaximal dynamic contractions and may be affected by LFF. The primary laboratory or clinical method for assessing LFF in human muscle is by percutaneous nerve or muscle stimulation.4 Elicited low-frequency (10-Hz or 20-Hz force) relative to high-frequency (50 to 100 Hz) force is determined for a given muscle and a reduction in this ratio (typically 20:80 ratio) is indicative of LFF.4 The isolated muscle stimulation is not specific to compound movements normally employed in sport performance. Moreover, the stimulation technique is not practical to use in an applied setting on a regular basis because of the time and expertise required to complete individual assessments of fatigue. An applied LFF test should reliably and objectively measure LFF specific to the sporting task, be quick (able to assess multiple athletes and multiple time points), easy (so that nonlaboratory practitioners can complete assessments), and require little technology (to ensure a range of use across sports and levels) to be practical for use in a field setting. A number of methods show promise to indicate the presence of LFF in hard training or performing athletes. Recent research on athlete monitoring identifies that subjective tools of monitoring stress (Borg ratio scale or the profile of mood states, recovery-stress questionnaires) may be effective at identifying an early clinical state of an athlete (as discussed by Maestu and colleagues6). In the situation of LFF, actual force output is lower than “intended” force output and may elicit higher submaximal levels of perceived exertion—what athletes typically refer to as “heavy legs.” Given that LFF is typically long lasting and could influence recovery capacity, a recovery-stress questionnaire for athletes (eg, RESTQ-Sport) may identify the early onset and prevalence of clinical LFF. This questionnaire identifies fatigue through perceived ratings of tiredness, soreness, and lack of “vigor.” Although subjective assessments of fatigue have obvious limitations, future work could confirm a clearer relationship between practical psychometric assessments and the clinical evaluation of LFF. A performance task that assesses work capability at low-to-moderate activation levels is particularly relevant to assessing LFF and might include tests of (1) constant moderate power (20% to 70% activation)—variable time to fatigue, (2) constant moderate duration (30 seconds to 10 minutes)—variable power, or 3) or repeated explosive activities of moderate duration. The assessment technique should be specific to the sporting task, of high reliability to discern moderate changes in performance on an intermittent basis, and not so excessively taxing as to impair training. Constant power tests have the highest reliability, although many reliability factors need to be considered (such as pretest protocol, test conditions, equipment, and experimental technique)6 for accurate results. Field-based or ergometer tests for swimming, running, cycling, and rowing may be particularly suited. For sports involving more explosive SSC work that can result in fatigue (such as football, hockey, volleyball), modifications of the Bosco repeated jump test show promise in identifying reductions in repeat SSC performance that may be influenced by LFF5; however, use of a test of this type to assess fatigue has not been reported.

Quantifying Muscle Fatigue   99

A practical test assessing LFF in hard training athletes is warranted and needed to appropriately monitor responses to training loads and the development of specific fatigue types. This approach will allow practitioners to develop appropriate strategies to limit specific fatigue types for optimal performance. This technical report has highlighted the challenges of applying a classic laboratory test to a field setting and the need for collaborative work between sport scientists and practitioners in the future.

References 1. Cairns SP, Knicker AJ, Thompson MW, Sjogaard G. Evaluation of models used to study neuromuscular fatigue. Med Sport Sci Rev. 2005;33,1:9-16. 2. Jones DA. High- and low- frequency fatigue revisited. Acta Physiol Scand. 1996;156:265-270. 3. Lattier G, Millet GY, Martin A, Martin V. Fatigue and recovery after high-intensity exercise part I: neuromuscular fatigue. Int J Sports Med. 2004;25:450-456. 4. Martin V, Millet GY, Martin A, Deley G, Lattier G. Assessment of low-frequency fatigue with two methods of electrical stimulation. J Appl Physiol. 2004;97:1923-1929. 5. Strojnik V, Komi PV. Fatigue after submaximal intensive stretch-shortening cycle exercise. Med Sci Sports Exerc. 2000;32(7):1314-1319. 6. Hopkins WG, Schabort EJ, Hawley JA. Reliability of power in physical performance tests. Sports Med. 2001;31(3):211-234.