higher than the K + conductance (Bretag, 1987, Dulhunty, 1979, Franke et al., 1990). If the depolarizing effects of an increase in interstitial K +

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1 Introduction In this chapter the background for initiation of the studies that the present thesis is based on will be given. Study I-III investigates the effects of intensified training on muscular adaptations and performance in trained subjects. Study III further investigates muscle ion kinetics, with special emphasis on K + handling, in relation to alterations in muscle ion transport proteins and fatigue development during repeated intense exercise. Study IV investigates the effect of an optimized diet on muscle glycogen resynthesis in soccer players in the days after a soccer match, with special emphasis on fiber type specific muscle glycogen resynthesis. Through the 1980ies and 90ies muscle fatigue was referred to as failure to maintain the required or expected power output (Edwards, 1983), however a more appropriate definition of muscle fatigue has recently been introduced as any decline in muscle performance associated with muscular activity (Allen et al., 2008) including factors inducing a declining muscular function, despite a maintenance of the required and expected power output. Despite the possible involvement of the central nervous system and neuromuscular junction, the primary site of fatigue during intense exercise appear to be peripheral (Fitts, 1994). There are several possible peripheral sites thought to be involved in muscular fatigue including the surface membrane, T-tubules, coupling between ryanodine receptors and sarcoplasmatic reticulum (SR), SR calcium (Ca 2+ ) release and SR mediated uptake by the SERCA pumps, binding affinity of Ca 2+ to troponin and the myosin-actin binding involving ATP-hydrolysis, cross-bridge force development and rate of cross bridge cycles (Fitts, 1994). In a recent review a modified K + hypothesis is proposed during high-intensity exercise a rundown of the trans sarcolemmal K + gradient is the dominant cellular process around which interactions with other ions and metabolites occur, thereby contributing to fatigue (Allen et al., 2008). In the present thesis, the major focus on fatigue development during repeated short-term exercise is mainly on intramuscular K + handling, but also on the transport of H + and lactate across the muscle membrane. In Study III changes in K + handling and H + and lactate release during repeated short-term intense exercise is investigated in 8 trained cyclists after 7-wks of speedendurance training (SET) consisting of 30-s intervals in combination with a severe reduction in training volume (~70%). Despite the focus in the present thesis being mainly on the effects of changes in K + handling on fatigue development, the role of Cl - in relation to muscle fiber excitability and K + has to be mentioned. It has been suggested that the depolarizing effects of a K + build up in the muscle interstitium and the lumen of the t-tubules in vivo, possibly depressing excitability and force, is largely diminished by changes in the Cl - conductance being ~4-5 times 13

2 higher than the K + conductance (Bretag, 1987, Dulhunty, 1979, Franke et al., 1990). If the depolarizing effects of an increase in interstitial K + is greatly attenuated by the polarizing effects of a Cl - redistribution across the membrane (Cairns et al., 2004, Dulhunty, 1978, Hodgkin & Horowicz, 1959) then changes in the Na + -K + pump might not be of importance for the repeated short-term exercise capacity as previously suggested (Bangsbo et al., 2009, Iaia et al., 2008, Nielsen et al., 2003). However, the role of Na +, K +, Cl -, H + and lactate transport for muscle membrane excitability and fatigue development and the effects on the short- and long-term exercise capacity is unclear. It would have been of great interest to investigate the role of both K + and Cl - in fatigue development during intense exercise in the present thesis, but still today no specific anti-bodies aiming at the ClC-1 channel is available within the method of Western Blotting (personal communication with Dr Robyn Murphy and Dr. Martin Thomassen), and the ClC-1 channel is believed to be the most abundant and the one of biggest importance for Cl - fluxes (Bretag, 1987, Hodgkin & Horowicz, 1959). Thus, in the present thesis the role of muscle ion transport proteins involved in the transport of H +, Na +, K + and lactate across the sarcolemma is investigated. In addition, changes in K + kinetics and how it affects fatigue development during repeated short-term intense exercise is investigated in Study III. It is well established that untrained individuals have major muscle adaptations and increase in performance after a period of endurance training (Dubouchaud et al., 2000, Gettman et al., 1976, Krustrup et al., 2009, Madsen et al., 1994, Mohr et al., 2007, Nybo et al., 2010). Training at maximal or near maximal intensity induces muscular adaptations such as increasing the activity of oxidative enzymes, expression of Na + -K + pump subunits and lactate and H + transporters, and improves endurance performance in untrained individuals (Burgomaster et al., 2005, Mohr et al., 2007, Nielsen et al., 2004). However, in already trained individuals a different approach is needed in order to observe muscular adaptations and performance enhancements within a short period of time (weeks). SET consisting of 30-s intervals at a near maximal speed in combination with a reduced training volume has previously been shown to improve short- and long term exercise performance, associated with an increased amount of Na + -K + pump subunits ( 1 and 2) and an improved running economy in endurance trained athletes (Bangsbo et al., 2009, Iaia et al., 2008). Likewise, studies on well-trained subjects, who either performed strength training (Medbo et al., 2001) or increased their training intensity (Evertsen et al., 1997, Madsen et al., 1994), have reported increased Na + -K + pump concentrations as determined by the [ 3 H]ouabain-binding technique. In contrast, Aughey et al. (2007) did not find changes in the abundance of any of the Na + -K + pump 14

3 subunits when already trained subjects performed a period of intensified training. The lack of effect in the latter study may have been a result of the exercise intensity being below the one corresponding to V, O 2 -max. Nevertheless, changes in the expression of Na + -K + pump may affect K + handling and performance, since Nielsen et al. (2004) observed that elevated levels of Na + -K + pump 1 and 2 subunits after 8 wks of knee-extensor training at supramaximal exercise intensities were associated with a reduced muscle interstitial K + concentration during exercise as well as a better performance during intense exercise. In addition, changes in other muscle ion transport proteins, such as the Na + -H + exchanger isoform 1 (NHE1), monocarboxylate transporters 1 and 4 (MCT1 and MCT4), facilitating lactate and H + exchange across the muscle membrane, have also been reported with intense training and may contribute to an improved performance (Burgomaster et al., 2007, Iaia et al., 2008, Juel, 1997, Juel, 2006, Mohr et al., 2007). It should, however, be mentioned that the importance of a lowered muscle ph in fatigue development during high intensity exercise has been questioned in a few recent reviews (Allen et al., 2008, Bangsbo & Juel, 2006, Cairns & Lindinger, 2008). Performance at the end of a soccer match is impaired, which may be caused by lowered muscle glycogen and depletion of glycogen in some muscle fibers (Bangsbo et al., 2006, Krustrup et al., 2006b, Krustrup et al., 2011). In support of this stance, it has been demonstrated that elevating muscle glycogen before prolonged intermittent exercise using a high-carbohydrate (CHO) diet increases performance (Bangsbo et al., 1992, Balsom et al., 1999). In this context, an interesting observation is that the resynthesis of muscle glycogen after a match is slower than expected and that players even forty-eight h after a match may not have replenished muscle glycogen stores, even when a diet rich in CHO has been consumed (Jacobs et al., 1982, Bangsbo et al., 2006). This is in contrast to what is observed after prolonged concentric exercise where even super compensation of muscle glycogen stores may occur if adequate CHO is supplied in the immediate post exercise period (Bergstrom & Hultman, 1966, Kiens & Richter, 1998, McInerney et al., 2005, Sherman et al., 1981, Tarnopolsky et al., 1997). It is well supported that eccentric exercise can significantly impair post exercise muscle glycogen resynthesis (Asp et al., 1998, Costill et al., 1990, Pascoe & Gladden, 1996, Zehnder et al., 2004). Thus, it may be that many of the activities during match play, such as decelerations and sudden changes in direction, have a sufficiently high eccentric component which, together with the physical contact between the players, may lead to muscle damage and local inflammation impairing muscle glycogen resynthesis. In keeping with this, players often experience significant muscle soreness and reduced performance 15

4 in the days after a match (Krustrup et al., 2006b, Thompson et al., 1999, Thorlund et al., 2009). It is well known that a high CHO diet after endurance exercise enhances the rate of glycogen resynthesis (Kiens & Richter, 1998, Kimber et al., 2003, McInerney et al., 2005). However, in recent years there has also been focus on the importance of co-ingestion of protein on muscle glycogen rebuilding. Protein intake can increase the insulin response and stimulate muscle glucose uptake, leading to enhanced glycogen and protein synthesis (Berardi et al., 2006, Drummond et al., 2008, Ivy et al., 2002, Paolisso et al., 1997, Wolfe, 2006, Zawadzki et al., 1992). However, van Hall et al. (2000) found no additional effect on muscle glycogen concentration during the first hours of exercise by adding a hydrolyzate of whey proteins and apparently, the effect is not dependent on the type of amino acids and proteins (Brown et al., 2004). It is, however, unknown whether intake of proteins together with a carbohydrate-rich diet after a soccer match will have a positive effect on muscle glycogen resynthesis. In the present PhD thesis, Studies I-IV were conducted in order to answer the following questions: 1) Does 30-s SET training induce muscular adaptations and improve repeated short-term exercise capacity when implemented in trained soccer players in season? 2) Does a whey-protein and carbohydrate-enriched diet elevate the rate of glycogen resynthesis after a soccer match in trained soccer players compared to a normal diet, and is there a fibretype dependent glycogen resynthesis occurring? 3) Does 30-s SET in combination with aerobic high-intensity training and a reduced training volume induce muscular adaptations and alter intramuscular K +, H + and lactate handling in trained cyclists? - And what are the effects on the short- and long-term exercise performance? 4) Does training with short-intense exercise bouts and a reduced training volume induce muscular adaptations in moderately trained runners and improve the short- and long-term exercise performance? 5) Does training with short-intense exercise bouts and a reduced training volume affect the health profile of moderately trained runners? In the present PhD thesis the effects of 30-s intervals on muscular adaptations and performance were investigated in trained soccer players (Study I) and cyclists (Study III). Study I was conducted with soccer players in season implementing 30-s intervals once per week in the last 16

5 5 wks of the season, investigating the effects of intensified training on muscular adaptations and performance. In Study III the effects of 30-s intervals on muscular adaptations and short- and longterm performance was investigated in combination with a significant reduction in training volume (~70%). Furthermore, in Study III muscle ion kinetics, with special emphasis on K + handling, was investigated during repeated high-intensity exercise. In Study II the training concept was developed consisting of 5-min periods where subjects alternates between low speed for 30 s, moderate speed for 20 s and high speed running (>90% of maximal speed) for 10 s, and the effects on performance, muscular adaptations and health profile were investigated in moderately trained runners. In Study IV it was investigated whether an impairment of and a fiber type specific muscle glycogen resynthesis occurs in the days after a soccer match in trained soccer players. The players received a diet with a high content of CHO and whey protein believed to be optimal for muscle glycogen resynthesis, and the effects of the optimized diet on muscle glycogen resynthesis was compared to a normal diet. 17

SSE #126. Sports Science Exchange (2014) Vol. 27, No. 126, 1-9

SSE #126. Sports Science Exchange (2014) Vol. 27, No. 126, 1-9 SSE #126 Sports Science Exchange (2014) Vol. 27, No. 126, 1-9 PHYSIOLOGICAL BASIS OF FATIGUE RESISTANCE TRAINING IN COMPETITIVE FOOTBALL Magni Mohr Researcher Department of Food and Nutrition, and Sport