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The effect of metabolic alkalosis on central and peripheral mechanisms associated with exercise-induced muscle fatigue in humans.
Siegler, JC, Marshall, P
Experimental physiology. 2015;(5):519-30
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Abstract
What is the central question of this study? Does metabolic alkalosis affect central and peripheral mechanisms associated with exercise-induced muscle fatigue in humans? What is the main finding and its importance? Inducing metabolic alkalosis before exercise preserved voluntary activation, but not muscle excitation, after a 2 min maximal voluntary contraction (MVC) followed by ischaemia. An effect of pH was also observed in maximal rates of torque development, where alkalosis mitigated the reduction in maximal rates of torque development after the initial 2 min MVC. For the first time, these results demonstrate a differential effect of pH on voluntary activation as well as maximal rates of torque development after sustained, maximal voluntary knee extension in humans. The increased concentration of protons during fatiguing exercise may contribute to increased activation of group III and IV afferents and subsequently reduced central drive, but this has yet to be confirmed in exercising humans. Here, we determined whether inducing metabolic alkalosis differentially affects descending central drive after fatiguing exercise and whether this effect may, in part, be explained by attenuating group III and IV afferent firing. Eleven men performed a maximal 2 min voluntary knee extension (MVC) followed by a 2 min rest and subsequent 1 min MVC with an occlusive cuff either in placebo [PLA; 0.3 g (kg body weight)(-1) calcium carbonate] or alkalosis conditions [ALK; 0.3 g (kg body weight)(-1) sodium bicarbonate]. Femoral nerve stimulation was applied before exercise, after the 2 min MVC and at 40-60 s intervals throughout the remainder of the protocol to explore central and peripheral mechanisms associated with reductions in maximal force and rate of torque development. Although voluntary activation declined to a similar extent after the 2 min MVC, during the ischaemic period voluntary activation was higher during ALK (PLA, 57 ± 8%; ALK, 76 ± 5%). Maximal voluntary torque declined at similar rates during the task (203 ± 19 N m), but maximal rate of torque development was significantly higher in the ALK conditions after the 2 min MVC (mean difference of 177 ± 60 N m s(-1) ). These results demonstrate the effect of pH on voluntary activation as well as maximal rates of torque development after sustained, maximal voluntary knee extension in humans.
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Aspects of disordered eating continuum in elite high-intensity sports.
Sundgot-Borgen, J, Torstveit, MK
Scandinavian journal of medicine & science in sports. 2010;:112-21
Abstract
Dieting is an important risk factor for disordered eating and eating disorders. Disordered eating occurs on a continuum from dieting and restrictive eating, abnormal eating behavior, and finally clinical eating disorders. The prevalence of eating disorders is increased in elite athletes and for this group the cause of starting to diet is related to (a) perception of the paradigm of appearance in the specific sport, (b) perceived performance improvements, and (c) sociocultural pressures for thinness or an "ideal" body. Athletes most at risk for disordered eating are those involved in sports emphasizing a thin body size/shape, a high power-to-weight ratio, and/or sports utilizing weight categories, such as in some high-intensity sports. In addition to dieting, personality factors, pressure to lose weight, frequent weight cycling, early start of sport-specific training, overtraining, injuries, and unfortunate coaching behavior, are important risk factors. To prevent disordered eating and eating disorders, the athletes have to practice healthy eating, and the medical staff of teams and parents must be able to recognize symptoms indicating risk for eating disorders. Coaches and leaders must accept that disordered eating can be a problem in the athletic community and that openness regarding this challenge is important.
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Development of hydration strategies to optimize performance for athletes in high-intensity sports and in sports with repeated intense efforts.
Maughan, RJ, Shirreffs, SM
Scandinavian journal of medicine & science in sports. 2010;:59-69
Abstract
Hypohydration - if sufficiently severe - adversely affects athletic performance and poses a risk to health. Strength and power events are generally less affected than endurance events, but performance in team sports that involve repeated intense efforts will be impaired. Mild hypohydration is not harmful, but many athletes begin exercise already hypohydrated. Athletes are encouraged to begin exercise well hydrated and - where opportunities exist - to consume fluid during exercise to limit water and salt deficits. In high-intensity efforts, there is no need, and may be no opportunity, to drink during competition. Most team sports players do not drink enough to match sweat losses, but some drink too much and a few may develop hyponatremia because of excessive fluid intake. Athletes should assess their hydration status and develop a personalized hydration strategy that takes account of exercise, environment and individual needs. Pre-exercise hydration status can be assessed from urine markers. Short-term changes in hydration can be estimated from the change in body mass. Sweat salt losses can be determined by collection and analysis of sweat samples. An appropriate drinking strategy will take account of pre-exercise hydration status and of fluid, electrolyte and substrate needs before, during and after exercise.
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A lifespan perspective on the career of talented and elite athletes: perspectives on high-intensity sports.
Wylleman, P, Reints, A
Scandinavian journal of medicine & science in sports. 2010;:88-94
Abstract
Elite athletes will be confronted during as well as after their athletic career with transitional challenges that will impact the course and progress of their athletic development. This article provides in first instance a description of a lifespan model exemplifying a "whole career/whole person" conceptualization of career transitions in the elite athletic career. Second, four specific career transitions in the development of talented and elite athletes are detailed with special attention for high-intensity sports (HIS). Finally, perspectives are formulated on future lifespan research and the provision of career support services in HIS.
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Investigations of back muscle fatigue by simultaneous 31P MRS and surface EMG measurements.
Rzanny, R, Grassme, R, Reichenbach, JR, Scholle, HC, Kaiser, WA
Biomedizinische Technik. Biomedical engineering. 2006;(5-6):305-13
Abstract
Investigations of back muscle fatigue are important for understanding the role of muscle strain in the development of low back pain. The aim of this contribution is to review the two main techniques used for in vivo investigations of metabolic and electrophysiological changes, namely magnetic resonance phosphorous spectroscopy ((31)P MRS) and surface electromyography (SEMG), and to report some of our recent results on simultaneous measurements using these techniques during isometric back-muscle contraction in volunteers. Since it appears that electrophysiological and metabolic factors are simultaneously involved in the processes of fatigue and muscle recovery during load application, simultaneous acquisition of complete information is quite promising for obtaining new insights into the metabolic origin of electrophysiological changes or vice versa. Performing these measurements simultaneously, however, is more intricate owing to the occurrence of signal artifacts caused by mutual signal interferences of both techniques. Besides these mutual disturbances, further experimental difficulties are related to spatial limitations within the bore of clinical whole-body high-field magnetic resonance (MR) systems (1.5 T) and the sensitivity of MR measurements to motion-induced artifacts. Our own experimental results are presented, and problems that occur using both techniques simultaneously, as well as possibilities to resolve them, are discussed. The results shed light on the interrelation of electrophysiological and metabolic changes during fatigue of the back muscle while performing an exercise.
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Assessing voluntary muscle activation with the twitch interpolation technique.
Shield, A, Zhou, S
Sports medicine (Auckland, N.Z.). 2004;(4):253-67
Abstract
The twitch interpolation technique is commonly employed to assess the completeness of skeletal muscle activation during voluntary contractions. Early applications of twitch interpolation suggested that healthy human subjects could fully activate most of the skeletal muscles to which the technique had been applied. More recently, however, highly sensitive twitch interpolation has revealed that even healthy adults routinely fail to fully activate a number of skeletal muscles despite apparently maximal effort. Unfortunately, some disagreement exists as to how the results of twitch interpolation should be employed to quantify voluntary activation. The negative linear relationship between evoked twitch force and voluntary force that has been observed by some researchers implies that voluntary activation can be quantified by scaling a single interpolated twitch to a control twitch evoked in relaxed muscle. Observations of non-linear evoked-voluntary force relationships have lead to the suggestion that the single interpolated twitch ratio can not accurately estimate voluntary activation. Instead, it has been proposed that muscle activation is better determined by extrapolating the relationship between evoked and voluntary force to provide an estimate of true maximum force. However, criticism of the single interpolated twitch ratio typically fails to take into account the reasons for the non-linearity of the evoked-voluntary force relationship. When these reasons are examined, it appears that most are even more challenging to the validity of extrapolation than they are to the linear equation. Furthermore, several factors that contribute to the observed non-linearity can be minimised or even eliminated with appropriate experimental technique. The detection of small activation deficits requires high resolution measurement of force and careful consideration of numerous experimental details such as the site of stimulation, stimulation intensity and the number of interpolated stimuli. Sensitive twitch interpolation techniques have revealed small to moderate deficits in voluntary activation during brief maximal efforts and progressively increasing activation deficits (central fatigue) during exhausting exercise. A small number of recent studies suggest that resistance training may result in improved voluntary activation of the quadriceps femoris and ankle plantarflexor muscles but not the biceps brachii. A significantly larger body of evidence indicates that voluntary activation declines as a consequence of bed-rest, joint injury and joint degeneration. Twitch interpolation has also been employed to study the mechanisms by which caffeine and pseudoephedrine enhance exercise performance.
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7.
What is fatigue?
MacIntosh, BR, Rassier, DE
Canadian journal of applied physiology = Revue canadienne de physiologie appliquee. 2002;(1):42-55
Abstract
Fatigue and potentiation are two forms of force modulation. A general definition of fatigue is "a circumstance where less than the anticipated contractile response is obtained." Fatigue is associated with depressed Ca2+ release and possibly decreased Ca2+ sensitivity. Potentiation results from increased Ca2+ sensitivity due to regulatory light chain phosphorylation. Muscle fatigue and potentiation can coexist, making it difficult to quantify these processes. With repetitive 10Hz stimulation, the developed tension first increases, then decreases. Is fatigue present when developed tension first begins to decrease or when it falls below the developed tension of the first response? Intermittent incompletely fused tetanic contractions for which peak developed tension first decreases, then increases, is another unusual example of fatigue. A third example is when twitch contractions following a tetanic contraction decrease to a level below the pretetanic twitch amplitude, indicating that fatigue may have been coexistent with posttetanic potentiation. These observations illustrate the complexity of detecting fatigue, based on the simple, but commonly accepted definition presented above. Care must be taken in interpreting "before vs. after" contractile responses. Even when the contraction amplitude is greater than the initial response, there is no guarantee that mechanisms associated with fatigue are not present.