Muscle metabolism during fatiguing exercise

In humans af ter short-term maximal exercise muscle glycogen levels are reduced to 75-80% of the initial muscle content. However, data obtained from maximally activated rat muscles show th at after only a few seconds of exercise total glycogen depletion had occurred in many fast fibres . It is argued that during short maximal exercise, effects of glycogen depletion in some fibres may lead to the inability to sustain the required high level of exercise. During short maximal exercise and at the end of prolonged heavy exercise large amounts of IMP and ammonia are produced. In experiments with rat muscles it was found that the extent of loss of force (and power) at the end of all kinds of exercise, was related to the amount of IMP produced. It is suggested that the IMP produced may help to con trol the energy flux through the muscle cells by inhibiting contractile function and thus proteet the muscle against an energy crisis. The high rate of production of IMP by muscles which are glycogen depleted indicate that IMP rnight he a linkage between glycogen depletion and fatigue. Glycogen and endurance In the late 1960's several reports by Scandinavian investigators showed that glycogen stored in skeletal muscles was a major determinant for endurance of long-term heavy exercise. First it was shown th at performance time was linearly related to the amount of stored glycogen in the muscle before exercise (Ahlborg et al. 1967). In a subsequent paper it was demonstrated th at work time was affected by manipulation of the amount of glycogen storage by different diets (Bergström et al. 1967). The conclusion from these papers, i.e. that glycogen was a lirniting factor for performance of long-term heavy exercise, was universally accepted and used by endurance athletes to improve performance by different "carbohydrate loading" regimes. In contrast to long-term exercise, relatively high muscle glycogen levels were found af ter fatiguing short-term high-intensity exercise (e.g. Boobis et al. 1983, Sahlin et al. 1975, 1976, 1989). It has further been demonstrated that human high-intensity exercise performance was not impaired by low muscle glycogen concentrations (varying from 153 to 4261lmol glucose units/gdw; Symons & Jacobs, 1989). Also the rate of glycogenolysis during short-term tetanic stimulation of rat muscles was not affeeted by glycogen levels between 80 and 165 Ilmol/gdw; Spriet et al. 1990). Based on these data glycogen depletion seems to play a non-significant role in fatigue during short-term high-intensity exercise. However, all these data were obtained from whole muscle samples. During high-intensity exercise it is necessary to recruit almost all the muscle fibres (e.g. V!1Illestad et al., 1985). Reduction in muscle performance may occur by fatigue of all fibres, but it is more rational to expect a selective fatigue of some muscle fibres which have a low resistance against fatigue. Since almost all fibres are active, fatigue of only a few fibres will already result in a decrease of performance, with only little effect on whole muscle metabolism. Therefore it is necessary to investigate metabolic changes in different fibre populations of muscles. In order to investigate changes in glycogen levels in different fibre types, histochemical techniques for fibre typing and substrate staining were used. The periodic acid-Shiff (PAS) stain is specific for glycogen (see e.g. V!1Illestad et al. 1984) and is used for detection of glycogen depletion in fibre types and in individual muscle fibres . Using these techniques, it was shown th at during long-term exercise the slow-oxidative fibres were depleted first, followed by the fast-oxidative and finally the fast-glycolytic fibres in man (Gollnick et al. 1973) and rat (Armstrong et al. 1974). In contrast, these reports showed that during high intensity exercise, fast-glycolytic fibres started to use glycogen immediately at the start of